HSE
Health & Safety
Executive
Examination of the effect of relief device
opening times on the
transient pressures developed within
liquid filled shells
Prepared by the
University of Sheffield
for the Health and Safety Executive
OFFSHORE TECHNOLOGY REPORT
2000/130
HSE
Health & Safety
Executive
Examination of the effect of relief device
opening times on the
transient pressures developed within
liquid filled shells
B C R Ewan, D Nelson
and P Dawson
Department of Chemical and Process Engineering
University of Sheffield
Mappin Street
Sheffield
S1 3JD
HSE BOOKS
© Crown copyright 2001
Applications for reproduction should be made in writing to:
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First published 2001
ISBN 0 7176 1985 0
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written permission of the copyright owner.
This report is made available by the Health and Safety
Executive as part of a series of reports of work which has
been supported by funds provided by the Executive.
Neither the Executive, nor the contractors concerned
assume any liability for the reports nor do they
necessarily reflect the views or policy of the Executive.
SUMMARY
A recent Joint Industry Project under the management of the Institute of Petroleum has been
concerned with the failure scenario in which a shell and tube heat exchanger with a low
pressure rated shell and with a high pressure tube-side suffers tube failure.
For water filled shells, the ensuing scenario is one in which a gas bubble rapidly grows
around the burst site generating a rising pressure wave within the shell. This pressure wave is
eventually relieved when the protective hardware operates leaving a residual lower pressure
tail as the water is driven out of the shell.
Tube failure for a shell protected by a relief device therefore gives rise to a transient pressure
pulse whose characteristics are influenced by the failure site geometry and location and the
locations and dimensions of the relief points.
The study revealed the importance of relief device opening times on the peak pressures which
could result and the present work being reported has been concerned with providing
information on such opening times for a range of devices including bursting discs and relief
valves.
The experimental work has been performed at University of Sheffield and the data from this
has been subjected to a hydraulic modelling analysis by PSI Ltd providing a good overview to
the sequence of events associated with wave propagation and relief operation.
Opening can be defined in a number of ways ranging from the instant at which pressure
begins to be relieved to the time at which the relief device is fully open. Ultimately, devices
must be chosen which meet the relief requirement on a timescale which is compatible with the
upstream system design and its failure characteristics. This represents an issue of sizing as
well as relief design choice.
The devices studied have not been chosen to meet any particular relief objective, and it
became clear that the bursting discs were oversized for the relief duty whilst the relief valves
were undersized. However, the data acquired enabled the times to full opening to be measured
with a high degree of confidence.
The burst discs ruptured in 1.9-10 msec and this is in line with (and therefore supportive of)
the values used in the IP study, which formed the basis of the IP Guidelines.
The study also shows that the pop-action of the relief valves (RVs) studied occurred in 2.5-4
msec but this finding, in particular, should be viewed with extreme caution and should not be
taken out of context. We believe it would be premature if these values were taken as typical
and applied across the industry in general, for all sizes of device and all operating conditions.
Overall, our reservations are as follows:
Firstly, the study implies that RVs are faster than a metal burst disc but this, potentially
misleading finding, has arisen by comparing a vastly oversized (8") disc against a severely
undersized, (2in) RV.
Secondly, the study has given the unexpected finding of very fast opening times for the RVs,
(4msec) almost 100 times faster than some of the values quoted by others. Although a quick
response can be expected from the test case (the valve is very small and it is the pop-action
type) this may not be typical in the field
ii
CONTENTS
page
SUMMARY
ii
1.
BACKGROUND AND OBJECTIVES
1
2.
PLAN OF WORK
2
3.
METHODOLOGY
2
3.1
Test Details
2
3.2
Test Characteristics
4
3.3
Experimental facilities
5
3.4
Experimental Method
7
RESULTS
8
4.1
Pressure Traces
8
4.2
Video Tape
8
ANALYSIS
9
Summary of modelling analysis
Preliminary Appraisal
Open Tube Tests
Graphite Burst Disc Tests
Stainless Steel Burst Disc Tests
Spring Loaded RV, High Pressure Test
Spring Loaded RV, Low Pressure Test
Pilot Operated relief Valve Test
9
14
17
20
26
31
35
39
6.
CONCLUSIONS AND RECOMMENDATIONS
43
7.
APPENDIX
45
Plates
Data Charts
Video tape record
45
55
73
4.
5.
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
7.1
7.2
7.3
iii
1.
BACKGROUND AND OBJECTIVES
A recent Joint Industry Project under the management of the Institute of Petroleum has been
concerned with the failure scenario in which a shell and tube heat exchanger with a low pressure
rated shell and with a high pressure tube-side suffers tube failure.
For water filled shells, the ensuing scenario is one in which a gas bubble rapidly grows around
the burst site generating a rising pressure wave within the shell. This pressure wave is eventually
relieved when the protective hardware operates leaving a residual lower pressure tail as the
water is driven out of the shell.
Tube failure for a shell protected by a relief device therefore gives rise to a transient pressure
pulse whose characteristics are influenced by the failure site geometry and location and the
locations and dimensions of the relief points.
The experimental study indicated that pulse widths could be in the range 1 - 10 msec for the
graphite bursting discs used, and this was comparable to results from recent numerical work by
Cassata et al1 for a longer shell, where pulse widths in the range
4 - 20 msec could be obtained with amplitudes of up to 25% of the tube-side pressure.
A recent parallel study2 was concerned with the response of the shell structure within the time
duration of the pulse loading, and concluded that engineering benefits could be secured by
ensuring the pulse widths were kept as short as possible compared with the fundamental period
of the shell structure. In this case, peak pressure amplitudes several times the yield pressure of
the shell could be tolerated. Since shells will typically have fundamental periods in the range 5 20 msec, design efforts should be directed at limiting pulse widths to 1/2 - 1/3 of this range. The
choice of relief device is important in this context as well as the number of such devices and
their distribution on the shell.
The objective of the work now being reported has been to measure a number of relief device
opening times and to consider their effect on the amplitude and duration of transient vessel
pressures arising from high pressure tube failures. This has been carried out through a series of
controlled experiments to monitor transient pressures in a water filled column combined with a
hydraulic analysis of the dynamic event.
1
2.
PLAN OF WORK
The programme of work undertaken had the following specific objectives :
1. To measure the opening times of a range of pressure relief devices when subjected to rapidly
rising pressure within a water medium,
2. To monitor the rates of pressure rise and peak pressures in the water column, which
from chosen combinations of gas discharge pressure and orifice size,
result
3. To provide an interpretation of the key features of the pressure behaviour using current
understanding of transient hydraulics.
3.
METHODOLOGY
The shock tube schematic shown in Figure 3.1 and described in greater detail below, was used
for the experiments.
3m
+
9m
bursting diaphragm
and orifice location
1
+
high pressure
driver section
+
+
+
water filled
driven section
2
+
relief
device
+ = pressure transducer
Figure 3.1
Outline of shock tube used for experiments
This is instrumented with a number of Kistler pressure transducers along its length.
The shock tube has a 100mm internal diameter and transition sections were available to extend
this to 200mm. It was intended that relief devices be examined whose diameters are
representative of those used offshore and that these should cover a range of types from the
fastest to the slowest operating and include bursting discs, spring loaded and pilot operated relief
valves.
3.1
TEST DETAILS
Since the key features of the effect of relief device opening time could be demonstrated over a
wide range of pressures, it was proposed that tests be carried out with relief devices which
operate at a nominal 15 bar gauge pressure. The shock tube gas driver pressure was set at 100
2
bar and pressurisation of the water column locating the relief device was by means of rapid
discharge through suitably sized orifices.
3
The key issues of interest are (i) the effect of opening times which cover the typical range for
devices in use, (ii) the relationship between the imposed rate of pressure rise from the source
and the resulting peak pressure when combined with these different opening times and (iii) the
demonstration of pressure interpretation and predictive capability through the use of hydraulic
modelling.
It was therefore proposed that the following relief devices be tested within the above water filled
shock tube :
Bursting discs :
Diameters
Materials
Relief valves :
4 in, 6 in and 8 in
Graphite and stainless steel
2” Spring loaded (pop-action), 2” Pilot operated, 2” bellows.
Note that relief device dimensions are given in inches to be consistent with manufacturers
specifications for the devices.
The range of discharge orifices to be used to achieve different rates of pressure rise in the water
column was 4mm, 8mm and 15mm in diameter.
The combinations available provided 24 sets of results, including those with an open ended tube
(thin plastic film). These are summarised in the test matrix shown in Table 1, where test
numbers refer to specific data sets generated.
Table 3.1.1
Summary of test conditions and test numbers
Relief device
↓
Open tube
Graphite disc
Stainless steel disc
(reversed dome)
2” Spring loaded RV
2” Bellows RV
2” Pilot operated RV
Relief
diameter
(in)
4
6
4
8
-
4mm orifice
8mm orifice
15mm orifice
Relief pressure
(bar)
39
51
55
41
48
59
62
66
38
50
54
42
47
58
61
65
37
49
53
40
46
57
60
64
0
10
10
15
15
15
15
15
= tests for detailed analysis
3.2
TEST CHARACTERISTICS
In simple terms, the tests are completely different and, as discussed later, these differences have
a significant impact on the action of the relief devices. Thus, the two tests that form the basis of
the mathematical modelling have the following characteristics:
The 15mm orifice gives a high pressure test which subjects the devices to a significant, rapidly
applied pressure which is far higher than the rupture pressure
The 4mm orifice givers a relatively low pressure test.
4
3.3
EXPERIMENTAL FACILITIES
The main facility used to generate a rapidly rising pressure transient was a shock tube of 100mm
internal diameter. This consisted of a driver section which could hold high pressure gas and
which was separated from a short buffer section containing atmospheric air by an aluminium
bursting disc. Discs could be designed to fail at any pressure and all tests were conducted at disc
failure pressures of 100 bar ± 10%. The buffer section was connected to the water filled driven
section of the tube via a plate, in the centre of which was the discharge orifice. To retain the
water during filling, the downstream side of the orifice plate was sealed with thin aluminium foil
or plastic film.
Four Kistler pressure transducers (K1 - K4) were located at positions along the driven section to
record the transient pressure profile. K1 being closest to the orifice was also used to trigger the
data acquisition. K4 was chosen to be as close as possible to the relief device at the opposite end
of the tube to minimise the transmission delay during device opening. A separate pressure
transducer, D1 was used to monitor the driver pressure and provided the starting pressure at the
point of rupture.
Slight variations in geometry were used for different groups of tests and these are represented in
Figures 3.3.1 - 3.3.4.
Air was supplied to the driver section from a compressor via an air reservoir and a number of
electrically operated isolation valves.
Raw voltage signals were acquired to a computer acquisition card at a rate of 20kHz on each
channel.
DRIVER
SECTION
WATER FILLED TUBE
ORIFICE
PRESSURISING
WATER COLUMN
150
2440
K1
RELIEF
DEVICE
1980
2140
K2
K3
90
K4
DIMENSIONS IN mm
Figure 3.3.1
51.
Shock tube geometry and dimensions used in tests 37 - 41, 49 K1 - K4 represent positions of Kistler pressure transducers.
5
DRIVER
SECTION
ORIFICE
PRESSURISING
WATER COLUMN
WATER FILLED TUBE
870
100
150
K1
DRIVER
SECTION
1205
2140
K2
K3
K4
WATER FILLED TUBE
600
100
Figure 3.3.3
150
150
2440
K1
65
Shock tube geometry and dimensions used in tests 46 - 48.
ORIFICE
PRESSURISING
WATER COLUMN
150
RELIEF
DEVICE
200
2440
Figure 3.3.2
150
935
2140
K2
K3
65
K4
Shock tube geometry and dimensions used in tests 53 - 55.
6
RELIEF
DEVICE
DRIVER
SECTION
WATER FILLED
TUBE
ORIFICE
PRESSURISING
WATER COLUMN
1100
4"
120
150
2440
920
2140
K4
110
K1
K2
K3
RELIEF
POINT
DIMENSIONS IN mm
Figure 3.3.4
66.
3.4
Shock tube geometry and dimensions used in tests 57 - 62, 64 -
EXPERIMENTAL METHOD
Aluminium bursting diaphragms could be produced in house and designed to rupture at any
prescribed pressure. These were designed for 100 bar operation and located so as to seal the
driver section of the shock tube. A 60mm thick spacer was located between this diaphragm and
a discharge orifice which fed into the water filled downstream section of the tube. The orifice
plate also held in place a plastic film to isolate the water column.
The removal of air bubbles from the water filled tube and connections before firing was an
important requirement to avoid contamination of the pressure traces by spurious reflections from
gas interfaces. Water inlets and outlets and the fabrication detail around relief devices were all
optimised such that no air pockets could remain during filling.
It was found initially that small air bubbles would remain immobile along the surface of the tube
during filling. The flow velocity of water during tube filling was important for their removal and
therefore water supply and outlet diameters were maximised whilst the shock tube was also
inclined at a gradient of 1:75. When the tube was full of water, surfactant was mixed with the
feed water to aid bubble flow and this feed was maintained until no further bubbles were
obtained in the outflow. Fresh water was finally flushed through the tube.
This procedure was ultimately judged to be satisfactory although the removal of the smallest
bubbles remained a central part of the experimental procedure before each test.
Each test then consisted of the slow pressurisation of the driver section until the aluminium
diaphragm ruptured. Data acquisition on all Kistler channels and the driver stagnation pressure
was then triggered by the voltage rise on K1. A small proportion of pre-triggering time was also
collected and allowed an exact determination of the pressure at rupture.
The pressure wave then took around 5.5 msec to arrive at K4 and it was therefore considered
sufficient to collect 250 msec of data. Only the early part of this is relevant to the relief device
opening and is represented on the enclosed charts.
Burst disc holders for both types used were fabricated according to manufacturer's specification.
7
4.
4.1
RESULTS
PRESSURE TRACES
The combined pressure - time traces for K1 - K4 for each test are shown in Charts A.1 - 33
within the Appendix section. In all cases K1 is the first to rise and K4, being furthest from the
source, is the last. Identification of the transducers can therefore be made on this basis. D1 is
not shown but instead the driver pressure at the moment of rupture is given on each trace. The
relief device set pressure is also indicated on each trace in the region of K4. Although pressure
data was collected over a 250 msec period, only the sections of this relevant to the discussion on
relief device opening time is included. For the bursting discs this is up to 15 msec after
diaphragm rupture. For the relief valves there is periodic behaviour and therefore for these, time
resolved traces up to 15 msec are presented initially and are followed by a longer duration trace
up to 50 msec.
4.2
VIDEO TAPE
A video tape record of the shock tube geometry and a number of components used has been
produced in conjunction with this work and the details of this are included in the Appendix. The
video record also includes a high speed sequence of the stainless steel and graphite bursting
discs opening.
8
5.
5.1
ANALYSIS
SUMMARY OF MODELLING ANALYSIS
Introduction
The results which have been produced provide an overview of behaviour with respect to
operating conditions and device types. To provide a greater insight into the relief device
operation, a detailed analysis has been undertaken for 10 of these tests. The following sections
therefore summarise the findings of this detailed analysis, with the particular objectives of:
• Determining the opening time of the relief devices
• Providing an interpretation of the key features of the pressure behaviour
Analysis methods
Fundamentally, the detailed analysis comprised a dynamic simulation study using a mathematical
model. A hydraulic model of the shock tube was configured with all the project data (i.e. tube
length, diameter, driver pressure, orifice diameter etc.) to produce an accurate mathematical
representation of the system. The model was then calibrated to provide good correlation
between the measured and predicted results.1 Finally, existing models were incorporated of the
burst disc and relief valves to reproduce the tests, as applicable, primarily using trend studies to
determine the opening time of the devices.
In addition to the dynamic simulation, wide ranging appraisals were undertaken. PSI have
extensive experience in the effects of surge pressure changes in pipes and piping systems and
this understanding was applied to the evaluation of the physical test results themselves, as well
as the findings from the dynamic simulation. 2
For example, one of the benefits of dynamic simulation is the ability to provide additional
information on the behaviour of a piping system, to supplement the data from SCADA systems,
transducers and pressure gauges.3 This information, equivalent to the output from virtual
instruments, provides further diagnostic information and this, together with PSI's experience,
means that evaluation of the test results was particularly useful.
The benefits of this approach were even evident during the initial testing phase. PSI were able to
interpret the test results and suggest that the presence air in the shock tube was generating 'nonstandard' behaviour. Sheffield University then identified the source of the problem and eliminated
it with revised test procedures. Subsequently, very good correlation was obtained between the
measured and simulated results for the Open Tube tests giving confidence that the dynamic
simulation phase with the relief devices would provide a meaningful outcome.
High and low pressure tests
1
Some parameters, such as the internal hydraulic roughness of the tube and the amount of free air in the
test-water are not unique data items and can vary between systems. These are therefore adjusted in the
calibration
2
Additional information is provided in Appendix A and B
3
This is widely used in industrial applications and is extremely helpful in troubleshooting; it is a noninvasive and low-cost way of investigating operating problems
9
In general two groups of tests results were looked at, differing because of the size of the orifice
on the driver end of the shock tube. The larger orifice (15mm) gave a high pressure test with
peak pressures of over 60 barg at the test-end of the tube in the Open Tube tests; this is over 4
times the nominal set pressure of the relief devices. In contrast the small orifice (4mm) gave a
low pressure test with pressures which were only a few bar above the set pressures.
These test-groups were therefore completely different and the study shows that these
differences had a significant impact on the action of the relief devices. In simple terms the relief
devices responded fully in the high pressure tests with the burst discs rupturing completely and
the relief valves lifting fully. In contrast, the low pressure tests gave 'marginal' conditions,4
tending to give slower (or incomplete) operation of the devices. However, this project arose from
the investigation into tube rupture within an industrial heat exchanger and, in that context, the
rapidly applied high pressure tests are far more relevant than the marginal, low pressure ones.
Behaviour of Relief Devices
The study examined two groups of devices (i.e. burst discs and relief valves) categorised by the
fact that the burst discs are 'non-closing' devices whilst relief valves (RVs) are designed to
automatically re-close and prevent the further flow of fluid. But, in practice, the findings
confirmed the need to further sub-divide these categories and hence the four different simulation
models that are available. Although there are many similarities between the graphite and metal
burst discs, the graphite discs continue to shatter even if the initial pressure surge decays.5 In
contrast, a metal disc only opens when there is a positive driving force; as seen by the
researchers, the metal discs were only partially opened in some of the tests. In turn, this tended
to impose an effective back pressure on the relief system.
Similarly, the study confirms that the two types of relief valve necessitate PSI's different
simulation models. Both of the valves are characterised by 'pop' action (whereby the design of
the internal forces within the valve means that they open rapidly when the inlet pressure is
slightly above the set pressure) but the pilot valve can have a modulated closure characteristic
while the spring loaded valve also re-seats rapidly. 6 As discussed below, these closure
characteristics are particularly significant in 'marginal' pressure conditions.
Accuracy of Modelling
The overall accuracy of the modelling and the reason for the high level of confidence in the
study as a whole is best shown by example. Figure 5.1.1 demonstrates the excellent level of
correlation that was obtained, in this case with the high pressure test on the 4" graphite disc. The
simulated result is the dark line, superimposed over the measured result.
4
The term 'marginal' is used in this report to indicate cases where the surge pressure in the shock tube was
only slightly higher than the set pressure of the relief device
5
Under marginal conditions, discs did not always rupture fully, leaving a narrow annulus at the edge. But
this had little effect on the disc capacity and can be classified as materially complete rupture
6
The full RV operating cycle is briefly outlined in Section 5.6. More comprehensive information is given in
manufacturers' catalogues.
10
1. 10
Test 49p 1.9ms Rupture
Pressure (bar)
45
40
35
30
25
20
15
10
5
0
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K4 Pressure
Max = 40.87
Sheffield Model - Re-Calibrated after Test 39p
Min = 0
Figure 5.1.1 Correlation for Graphite Disc, High Pressure Test
Overall, the study therefore confirmed the suitability of the simulation models of the different
relief devices and so the trend studies were used to investigate their response time. In this, all the
test parameters remained the same with the exception of one (i.e. the overall rupture time of the
burst disc), which was changed systematically.
Graphite and Metal Burst Disc
The trend studies showed that the burst time for the 4" graphite disc was 1.9 msec compared
with 10 msec for the 8" stainless steel disc. These compare favourably with the values of 0.1-10
msec given in the recent IP Guidelines for the Design of Heat Exchangers.7 However, the
analysis identified other factors.
In the context of the 4" shock tube, the 4" burst disc was oversized, presenting a massive amount
of relief and this was even more marked with the 8" disc. The effect of this was that, even in the
high pressure tests, the pressure wave was relieved within about 10% of the opening time and
this means, subsequently, the test became insensitive to the increasing disc capacity. In practice
therefore the overall burst time of the metal disc represents an effective rupture period of about
1 msec, extrapolated to give an overall best estimate of 10 msec.
Spring Loaded and Pilot Relief Valves (RVs)
The correlation for the spring loaded relief valves was also excellent (e.g. Figure 5.1.2, where
again the simulated result is the dark line, superimposed over the measured result). It is also
interesting to note that, in the high pressure tests, the 2in RVs do not exhibit any of the oversizing problems seen with the burst discs. In the test below, the RV remains open after the initial
pressure wave is relieved; the system pressure remains above the valve's set pressure of 15
barg.
7
Guidelines for the Design and Safe Operation of Shell and Tube Heat Exchangers to Withstand the
Impact of Tube failure, The Institute of Petroleum, London, 2000
11
5. 8
Test 57p 10% Capacity in 4ms
Pressure (bar)
120
100
80
60
40
20
0
-20
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K4 Pressure
Max = 100.75
Sheffield Model - Taken from Test 51p
Min = 0
Figure 5.1.2 Correlation for Spring RV, High Pressure Test
Strictly, the tested RV is defined by API RP 520 as a 'safety' valve which means that it opens
by pop-action. In this way it differs from valves defined by API RP 520 as 'relief valves' in
which the lift is proportional to the inlet pressure. Similarly, the pilot configuration offers the
same options of either pop-action or modulating-action; there is also a combined option of popopen and modulate-closed which was the configuration studied in these tests. And overall these
different options are significant because they mean that the test valves were the fastest
available.
The 'rated' capacity8 of a pop-action valve is reached when it has popped open (although a slight
increase in capacity can subsequently occur on a further increase in pressure). We have
therefore defined the opening time for the RV as the time taken for it to reach the rated capacity
i.e. the capacity for 110% of the set pressure, or 10% over-pressure (10% OP). And on this
basis the spring loaded valve opened in 4 msec and the pilot valve in 2.5 msec.
These values are supported by the high level of correlation shown above and, by inspection,
particularly of the low pressure tests discussed later. However, these results were completely
unexpected; they do not agree with the data given in the IP Guidelines of 80-350 msec and are
faster even than the tested value of 25 msec by Kruisbrink, 1990. 9 The finding must therefore be
treated with extreme caution until further work has been undertaken to investigate this further
and to establish points of comparison. In the interim however, we do make the following
observations:
The RV is small (2H3), it is subjected to a significant over-pressure and it is the pop-action type
and, subjectively, we would therefore expect the opening times to be at the fast end of any
performance range. Further testing (or test analysis) is needed to establish whether the findings
are size or pressure related i.e. whether these findings are representative of larger valves at
different over-pressure conditions.
We were also surprised by the finding that the pilot valve opened more quickly than the spring
loaded valve; again, subjectively, a slower response was expected. However, we noted that the
8
The 'rated' capacity is in accordance with international codes
9
Modelling of Safety and Relief Valves in Waterhammer Computer Codes, Kruisbrink, A.C.H., Proc. 3rd
Int. Conf. Valves and Actuators, BHR Group, STI 1990
12
pilot operated RV and the spring loaded RV are not directly similar valves. The physical mass of
the moving parts is smaller for the pilot operated valve and so the inertia effects would be lower.
Additionally, the study suggests that the pilot is the type that is characterised by pop action,
followed by a modulating action. This means that the opening time would not be adversely
slowed by the pilot
Behaviour of Devices at Low Pressure
The over-sizing issue mentioned earlier was even more evident in the low pressure tests because
these only created 'marginal' conditions for all of the devices. The most significant case arose
with the spring loaded RV, generating the classic case of valve chatter (Figure 5.1.3).
2in Spring Relief - 4mm - Smoothed
Pressure
(barg)
50
K4
40
30
20
10
0
0
0.05
0.1
0.15
Time (s)
Figure 5.1.3 Measured Test Showing RV Chatter
API RP 520 notes that RVs "operating at low pressures tend to chatter; therefore overpressures
of less than 10% should be avoided". And these are the very conditions that exist in this low
pressure test. It is a serious problem in the field; as shown above, it can lead to pressure
oscillations (with the attendant problems of vibration, pipe movement and damage) as well as
damage to the valve itself and galling of the guiding surfaces. And, as a result valve chatter has
been the subject of other industrially oriented research (e.g. Kruisbrink, 1990 and Auble, 198310).
The detailed study of this phenomenon was therefore well outside the scope and the aims of this
study but we still obtained good correlation with the opening phase. Moreover, the results support
our estimation of the opening time for the RV of 4 msec. Figure 5.1.3 shows an oscillating
frequency of about 80 cycles per sec, giving an opening/closing sequence within 12.5 msec.
The detailed analysis of the performance of the pilot under the 'marginal' conditions was also
beyond the study scope although again we showed good correlation with the opening phase. The
effects of over-sizing were still a potential problem although, in this case, the modulating action
of the pilot totally eliminated the chatter seen previously with the spring loaded RV. However,
this is not the only way of eliminating chatter; for example, it can be avoided by using dynamic
simulation methods to ensure that the spring loaded RV is correctly sized in the design stage.
10
Full Scale Pressurised Water Reactor Safety Valve Test Results, Auble T.E., Testing and Analysis of
Safety/Relief Valve Performance, 4th National Congress on Pressure Vessel and Piping Technology,
Portland Oregon, 1983
13
Conclusions
The overall aim of the study was to determine the opening time of relief devices that may be
used on industrial heat exchangers to provide protection in the event of a tube rupture. And this
has been achieved with the study showing that fast-acting protection devices are available. The
burst discs ruptured in 1.9-10 msec and this is in line with (and therefore supportive of) the
values used in the IP study, which formed the basis of the IP Guidelines.
The study also shows that the pop-action of the RVs occurred in 2.5-4 msec but this finding, in
particular, should be viewed with extreme caution and should not be taken out of context. We
believe it would be premature if these values were taken as typical and applied across the
industry in general, for all sizes of device and all operating conditions.
Overall, our reservations are as follows:
Firstly, the study implies that RVs are faster than a metal burst disc but this, potentially
misleading finding, has arisen by comparing a vastly oversized (8") disc against a severely
undersized, (2in) RV.
Secondly, the study has given the unexpected finding of very fast opening times for the RVs,
(4msec) almost 100 times faster than some of the values quoted by others. Although a quick
response can be expected from the test case (the valve is very small and it is the pop-action
type) this may not be typical in the field
Further work is therefore essential to determine whether the action of the RV is affected by
size, pressure, onsite variables (such as the settings of blowdown rings) etc.
5.2
PRELIMINARY APPRAISAL
Introduction
Our preliminary appraisal of the test results raised questions that might affect the mathematical
modelling, the test program and/or the test procedures that were being used. The first section of
the report therefore reproduces a document which was issued to Sheffield University with the
aim of raise these questions and thereby increasing the likelihood that, between us, we could
identify and eliminate (or control) the phenomenon that was initially presenting as 'rogue' (i.e.
non-standard) behaviour.
Test Data
This review uses the basic numbering system from the University tests.
Table 5.2.1
Summary of Test Numbers and Conditions
Type
Open
Tube
4mm
Orifice
Test No. 1
Test No. 1b
8mm Orifice
15mm Orifice
Test No. 2
Test No. 3
Test No. 3b
Sample Results
From our basic experience and from our involvement in a similar study in the past, we would
expect the results to show a basic surge pattern,11 with test-specific effects superimposed. But
11
For reference, the basic wave and the variations are outline in Appendix A
14
comparison between the predicted and measured results for K2 during Test 3b (Figure 5.2.1)
shows several departures. Most notably:
the period of the measured wave is longer than the predicted one
the measured wave shows an inflection - an early decrease and subsequent recovery12
2. 6
Sheffield Test 3b - 15mm orifice and film
Pressure (bar)
70
60
50
40
30
20
10
0
0
0.005
0.01
0.015
K2 Pressure
0.02
0.025
0.03
Time (seconds)
Basic Test
Max = 59.87
Standard Model
Min =-0.03
Figure 5.2.1 Idealised and Measured Pressures, High Pressure Test
Other features we noted are:
the wavespeed is not apparently constant down the tube - the transmission time between K1/K2
and K2/K3 gives a wavespeed of between 1500-1200m/s but there is a step change between
K3/K4 down to a minimum of 350m/s the apparent friction loss in the tube (indicated by the
pressure offset between K1 and K4 as the pressure wave decays) is significantly higher than
expected
Wavespeed Changes
To investigate this further, the mathematical model was arbitrarily calibrated to match the
measured wavespeeds and the results immediately mimicked the M-shape wave.
However, we had no reasonable hypothesis to support this calibration:
in the absence of any air bubbles in the water, the wavespeed in this test rig would be constant
because it is a solid steel tube
the wavespeed would vary if the water contains a typical distribution of free air (as occurs when
taking water from the mains, for example). But, under these circumstances, the wavespeed
would vary with pressure (as the bubbles compress) rather than distance and so this would not
give the sudden step change between K3/K3
Localised Phenomena
In view of the fact that the difference appeared to lie between K3 and K4 we investigated the
potential impact of two possible options, namely a discrete air bubble or pipe distension. These
12
This can be visualised as an M-shape
15
were localised at the site of the orifice/baffle 13 because we were unaware of any other suitable
location on the tube.
The results show that, as with the calibrated wavespeed, the approach of using a short section of
distensible pipe gives the right trends at K2 but K3/K4 are poor.
In contrast, the inclusion of a small air pocket gives fair correlation at all of the transducers,
replicating both the initial M-shape and also the amplitude and phasing as the wave decays
(Figure 5.2.2).
8. 6
Sheffield ST3 with Air at Baffle Site (0.2 litre)
Pressure (bar)
90
80
70
60
50
40
30
20
10
0
-10 0
0.01
0.02
K2 Pressure
0.03
0.04
0.05
Time (seconds)
Air Bubble Test
Max = 76.35
Standard Model
Min =-.91
Figure 5.2.2 Effect of Air Pocket on High Pressure Test
We did not attempt to optimise the correlation and so, as shown on Figure 5, the prediction overestimates the second part of the M-wave. But the overall trend is sufficiently close that we feel
it clearly illustrates the phenomenon.
Additional Data
We also assessed the additional data (Test 2 and the re-tested results, Test 1b and 3b) and
although these show some differences, the overall trends are materially unchanged; most
significantly, both the M-shape wave and the wavespeed variations still remain.
For completeness, the additional data shows:
more conformity between tests 1b and 3b, differing as expected only by test-specific factors. In
contrast, the pressures originally recorded at K3 and K4 during Test 1 had been very different
from the pressures at the same sites in Tests 2 and 3. But this disparity is eliminated by the retesting (i.e. Test 1b)
some difference between the pressures at K1 in both Test 1b and 3b compared against the
original testing. But this is to be expected - this site is the one that is most likely to be affected by
blowby from the driver section, one of the reasons for re-testing
Summary
In summary therefore, the results suggest that the new testing procedure has eliminated one
source of unpredictable variation but a further source still remains.
13
We understood that there had been a baffle in the tube for the initial JIP testing
16
We felt that the presence of a small air pocket was the most likely cause of the 'non-standard'
behaviour, firstly because of the correlation shown above and secondly because the volume of
air that gives good results differs between the tests. Sheffield University therefore expended a
great deal of time and effort on identifying the 'rogue' phenomenon and eliminating it with revised
test procedures.
5.3
OPEN TUBE TESTS
Introduction
Our preliminary appraisal of the initial test results suggested that small air pocket(s) were the
most likely cause of the 'non-standard' pressure and flow transients. And, as a result, a
significant amount of work was undertaken by Sheffield University to isolate and then eliminate
the air.
This section therefore compares the final set of measured results for the Open Tube tests with
the mathematical modelling results.
Test Data
Table 5.3.1
Test Summary
Type
Open Tube
4mm Orifice
Test No. 39p
8mm Orifice
-
15mm Orifice
Test No. 37p
Calibration of the Hydraulic Model
The hydraulic model of the shock tube was firstly configured with all the project data (i.e. tube
length, diameter, driver pressure, orifice diameter etc.) to produce an accurate mathematical
representation of the system. However, some parameters, such as the internal hydraulic
roughness of the tube and the amount of free air in the test-water are not unique data items and
can vary between systems. The model was therefore calibrated to provide good correlation
between the measured and predicted results.
Using the methodology outlined in the Preliminary Appraisal, the measured data was reviewed
for consistency, smoothed14 and then the wavespeed and friction losses were calibrated.
The low pressure test (39p) was then simulated and, using transducer K2 as an example, Figure
5.3.1 shows that a reasonable correlation was obtained:
The pressure rises at the same time
The phasing (periodicity) of the waves is the same
The peak pressure is similar
14
A limited amount of data smoothing (5 point average) was used to eliminate the most severe of the
measured oscillations
17
1. 6
Test 39p - Calibrated
Pressure (bar)
20
15
10
5
0
0
0.005
0.01
0.015
0.02
Time (seconds)
K2 Pressure
Max = 18.14
Sheffield Model - Calibrated from Test 39p
Min = 0
Figure 5.3.1 Preliminary Correlation for Open Tube, Low Pressure Test
Air in the Tube
Although Sheffield University had made significant changes to their test procedures and thereby
reduced the amount of air that was trapped within the tube, they noted that some could remain
on the downstream side of the filling outlet, on the tube soffit. A small volume was therefore
included in the hydraulic model and this immediately introduced the type of pressure inflections
seen on the measured traces.
The volume was therefore calibrated at about 0.025 litres (under initial, atmospheric conditions)
and the resulting correlation is given on Figure 5.3.2. For comparison, the high pressure test
(37p) was also run on the same model (Figure 5.3.3).
6. 8
Test 39p - Calibrated with Air Pocket and Free Air
Pressure (bar)
20
15
10
5
0
0
0.005
0.01
0.015
0.02
-5
Time (seconds)
K3 Pressure
Max = 16.90
Sheffield Model - Calibrated from Test 39p
Min = -0.7
Figure 5.3.2 Correlation for Open Tube, Low Pressure Test
18
7. 8
Test 37p - Calibrated from 39p
Pressure (bar)
80
70
60
50
40
30
20
10
0
0
0.01
0.02
0.03
0.04
0.05
Time (seconds)
K3 Pressure
Max = 62.47
Sheffield Model - Calibrated from Test 39p
Min = 0
Figure 5.3.3 Correlation for Open Tube, High Pressure Test
Criteria for Acceptance
Our criteria for acceptance of the accuracy of the correlation are based only on the first wave.
This generates the pressure conditions that opens the relief device and so the subsequent
pressure oscillations are not relevant.
Additionally, we have taken position K3 to be the most important for this particular correlation.
Although K4 is closer to the end of the shock tube (and therefore closest to the relief device) it
provides very little information in the Open Tube tests because the duration of the pressure wave
at this point is too short. K3 is therefore used this correlation.
Review of Results
The hydraulic model used to generate Figure 5.3.2 and Figure 5.3.3 was therefore accepted for
the second phase of the study (i.e. the analysis of the relief device tests) on the basis that:
The initial pressure rises are coincident with the measured tests
The initial rates of pressure rise are the same as the tests
The duration of the pressure waves is the same as the tests
The magnitude of the pressure waves is similar
Summary
Our analysis of the hydraulic modelling and measured results for the Open Tube tests shows that
a high level of correlation can be obtained for both of the available tests. Some air remained in
the shock tube under test conditions but the volume was now very small and could be adequately
accommodated by calibration.
We were therefore confident that the calibrated model could be used for the analysis of the test
results obtained from the relief devices.
19
5.4
GRAPHITE BURST DISC TESTS
Introduction
This section of the report describes the next phase of the modelling study, investigating the
response times of relief devices; in this case two sets of results from the 4in graphite burst disc
tests are studied.
Test Data
Table 5.4.1
Test Summary
Type
4in Graphite
Burst Disc
4mm Orifice
Test No. 51p
8mm Orifice
-
15mm Orifice
Test No. 49p
The nominal burst pressure for the discs is 15.4 barg.
Test Characteristics
As noted earlier, the tests are completely different in simple terms and this study shows that
these differences have a significant impact on the action of the relief devices:
The 4mm orifice givers a relatively low pressure test. In the Open Tube tests, the peak
pressures were only a few bar above the nominal burst pressure of the discs (15.4 barg)
The 15mm orifice gives a high pressure test with the disc subjected to a significant, rapidly
applied pressure which is far higher than the rupture pressure
It is also interesting to note that the university researcher observed a physical manifestation of
these differences; the graphite discs did not always shatter completely with low pressure tests,
occasionally leaving an graphite annulus at the edge of the disc holder.
Mathematical Model of the Burst Discs
Under steady state conditions, the mathematical model used by PSI validates against API RP
520 and manufacturers' catalogue data. The aim here was therefore to calibrate the opening
time and thereby confirm the model under dynamic conditions.
Trend Study for High Pressure Test (49p)
The 15mm orifice test was investigated first because this is the 'high' pressure test which
subjected the disc to a significant pressure wave. This means that the disc ruptured completely
with no likelihood of a residual annulus.
Firstly, the hydraulic model of the shock tube was re-configured from the Open Tube tests to
include a burst disc. Then, a trend study was undertaken to investigate the burst time of the test
disc. In this, all the test parameters remained the same with the exception of one (i.e. the overall
burst time of the disc) which was changed systematically.
The burst time was taken as the time for the disc to shatter completely. And the results on
Figure 5.4.1 and Table 5.4.1 show the effect of parameter on the pressure at K4. The dark line
20
is the measured result and the 3 lighter ones are the simulated result with a disc burst time of 1, 2
and 3 msec, respectively.
Table 5.4.1
Peak Pressure at K4 from Trend Study
Disc Burst
Time (msec)
1
2
3
Peak Pressure (Barg)
29.99
41.93
51.60
Test 49p Trend Curve for 1,2, 3 millisec
Pressure (bar)
60
50
40
30
20
10
0
0.004
0.0045
0.005
0.0055
K4 Pressure
0.006
Time (seconds)
Sheffield Model - Re-Calibrated after Test 39p
Figure 5.4.1 Trend Study for Graphite Disc, High Pressure Test
Optimising the Burst Time
From these results, the optimum burst time of 1.9 msec was selected and the high pressure test
(49p) was re-simulated with this burst time.
The results are given for K4 and K2 (Figures 5.4.2 and 5.4.4) and, to demonstrate the high level
of correlation, K4 is also repeated at a very small time scale (Figure 5.4.3). The dark line is the
simulated result.
21
1. 10
Test 49p 1.9ms Rupture
Pressure (bar)
45
40
35
30
25
20
15
10
5
0
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K4 Pressure
Max = 40.87
Sheffield Model - Re-Calibrated after Test 39p
Min = 0
Figure 5.4.2 Correlation for Graphite Disc, High Pressure Test
1. 10
Test 49p 1.9ms Rupture
Pressure (bar)
45
40
35
30
25
20
15
10
5
0
0.004
0.0045
0.005
0.0055
0.006
Time (seconds)
K4 Pressure
Standard
Max = 40.87
Sheffield Model - Re-Calibrated after Test 39p
Min = 0
Figure 5.4.3 Correlation at K4 over Short Time Scale
22
1. 3
Test 49p 1.9ms Rupture
Pressure (bar)
80
70
60
50
40
30
20
10
0
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K2 Pressure
Max = 67.30
Sheffield Model - Re-Calibrated after Test 39p
Min = 0
Figure 5.4.4 Correlation for K2
Discussing the High Pressure Test (49p)
The correlation between the measured and simulated results is exceptionally high for Test 49p,
giving a high level of confidence that the mathematical model accurately reflects the action of
the bursting disc when a 1.9msec burst time is adopted.
However, a detailed appraisal of the results shows that, in fact, the duration of the wave at K4 is
only about 0.3 msec from the time when it starts to rise until the time that the pressure peaks.
This means that, in this particular test, the results are only sensitive to the disc action for the first
15% of the burst time. Thereafter, the disc must have continued to shatter but, with the pressure
wave already decaying, this further increase in the relief capacity had no impact.
Low Pressure Test (51p)
The same burst disc model was adopted for the low pressure test i.e. with a burst time of 1.9
msec and the results are given on Figure 5.4.5.
Initially, these results show good correlation but the modelled pressure wave then decays almost
as soon as the burst disc ruptures whereas the measured wave takes about 2 msec to decay.
(The dark line is the simulated result).
23
3. 12
Test 51p Linear 1.9 ms Rupture
Pressure (bar)
20
15
10
5
0
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K4 to Burst Disc Pressure
Max = 17.31
Sheffield Model - Re-Calibrated for Test 49p
Min = 0
Figure 5.4.5 Graphite Disc, Low Pressure Test
A trend study was therefore undertaken, to determine whether the correlation would improve
with a longer burst time but in fact there was less overall agreement, (Figure 5.4.6, where the
dark line is measured).
For example, the duration of the simulated wave is similar to the measured one if the burst time
is 10 msec, but the peak pressure is over 30% higher than the measured one.
3. 12
Test 51p Trend Curve for 5, 10, 20 millisec
Pressure (bar)
25
20
15
10
5
0
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K4 Pressure
Sheffield Model - Re-Calibrated for Test 41p
Figure 5.4.6 Trend Study for Graphite Disc, Low Pressure Test
Effect of Incomplete Rupture
In the light of this poor correlation, the measured and simulated results were completely reappraised. This indicated that the burst disc model we were using (which had proved highly
suitable for the high pressure test) did not reflect the performance of a graphite disc under
marginal, low pressure conditions which were likely to result in incomplete rupture.
24
Obviously, we were not in a position to develop a new model based solely on one set of results;
nor do we feel that we would use such a model very often. However, we interrogated the
measured results diagnostically to gain insight into the possible performance of the disc.
Firstly, this review showed that the burst disc is able to provide some relief capacity almost
instantly (i.e. within 0.1 msec). This capacity only represents about 2% of the overall capacity
(and may therefore be provided by the initial cracks) but, interestingly, this was enough to limit
the pressure rise in this particular low pressure test.
Subsequently, for our own interest, we developed a bespoke capacity-model for the low pressure
test, giving the correlation shown on Figure 5.4.7. This was interesting as a correlation exercise
but overall, did not provide any further information about the way in which a graphite disc
shatters in normal conditions.
3. 12
Test 51p
Pressure (bar)
20
15
10
5
0
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K4 Pressure
Max = 17.52
Sheffield Model - Re-Calibrated for Test 49p
Min = 0
Figure 5.4.7 Bespoke Correlation for Graphite Disc, Low Pressure Test
Summary
Our analysis of the hydraulic modelling and measured results for the Graphite Burst Disc tests
again shows that a high level of correlation can be obtained. When the disc was subjected to a
significant, rapidly applied pressure then the (overall) burst time was 1.9 msec. However, results
are only sensitive to the disc action for the first 15% of the burst time. Thereafter, the disc must
have continued to shatter but, with the pressure wave already decaying, this further increase in
the relief capacity had no impact.
In contrast, the findings from the low pressure test were not conclusive. The results suggest that
the disc provides a small relief capacity almost instantly; although we have no proof, we feel that
this is possibly in the form of the initial cracks. In turn a small relief flow developed and this was
sufficient to reduce the pressures. Thus, for most of the test, the pressure and flow changes in
the shock tube were insensitive to the performance of the burst disc.
Overall these results suggests that further work would be beneficial to investigate the 'marginal'
conditions, i.e. low pressure cases where the natural system pressures are only slightly higher
than the setting of the relief device. But this is probably low priority when put in context with the
aims of the study.
This project arose from the investigation into tube rupture within an industrial heat exchanger
and, in that context, the rapidly applied high pressure wave is far more important than the
25
marginal, low pressure wave. For such high pressure cases, the results obtained from the high
pressure test could be adopted, within an overall burst time of 1.9 msec.
5.5
STAINLESS STEEL BURST DISC TESTS
Introduction
This section continues the examination of the burst discs, this time looking at the stainless steel
discs.
Test Data
Table 5.5.1
Test Summary
Type
4in Stainless
Steel
8in Stainless
Steel
4mm Orifice
Test No. 41p
8mm Orifice
-
15mm Orifice
-
-
-
Test No. 46p
As noted previously, the tests are completely different in simple terms:
The 4mm orifice givers a relatively low pressure test
The 15mm orifice gives a high pressure test
The nominal burst pressure for the discs is:
14.7 barg for the 4in stainless steel disc
14.8 barg for the 8in stainless steel disc
Mathematical Model of the Stainless Steel Discs
There is a significant difference between the metal discs and the graphite ones in the way in
which they rupture and this difference is incorporated in the mathematical models:
the graphite discs will continue to shatter even if the upstream pressure subsequently drops away
with the exception of inertia effects, there is no mechanism to continue opening a metal disc if
the pressure on the upstream face is falling and is less than the downstream pressure. As seen
by the researchers, the end results is that a metal disc can finish only partially open
As noted for the graphite burst discs, the mathematical model used by PSI for the stainless steel
discs validates against API RP 520 and manufacturers' catalogue data under steady state
conditions. Additionally, the dynamic model was upgraded to include the feature noted with the
graphite tests i.e. that a small relief capacity develops almost instantly, within 0.1 msec.
Trend Study for High Pressure Test with 8" Disc (46p)
Firstly, the hydraulic model of the shock tube was re-configured to reflect the physical changes
needed to study an 8" disc e.g. the expansion piece was added. Then, a trend study was again
undertaken with the metal burst disc model, to investigate the burst time of the test disc.
26
The results on Figure 5.5.1 and Table 5.5.2 show the effect of the burst time on the pressure at
K4. The dark line is the measured result and the 3 lighter ones are the simulated result with a
disc burst time of 5, 10 and 15 msec, respectively.
Table 5.5.2
Peak Pressure at K4 from Trend Study
Disc Burst
Time (msec)
5
10
15
Peak Pressure
(Barg)
18.95
19.44
21.50
Test 46p Trend Curve for 5, 10 and 12 millisec
Pressure (bar)
25
20
15
10
5
0
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K4 Pressure
Sheffield Model - Taken from Test 51p
Figure 5.5.1 Trend Curve for 8" Metal Disc, High Pressure Test
Optimising the Burst Time
From these results, the optimum burst time of 10 msec was selected and the results are given for
K4 (Figure 5.5.2). The dark line is the simulated result.
27
5. 10
Test 46p 10ms Rupture
Pressure (bar)
25
20
15
10
5
0
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K4 Pressure
Max = 19.44
Sheffield Model - Taken from Test 51p
Min = 0
Figure 5.5.2 Correlation for 8" Metal Disc, High Pressure Test
28
Discussing the High Pressure Test (46p)
The test rig had been re-configured for this test to include a 4in-8in expansion and this, in itself,
introduced additional pressure changes in the shock tube. But despite this, we show good
correlation for a metal disc by incorporating the features shown with the graphite disc (i.e. of an
initial, immediate opening of a few percent) and an effective time of 10 msec.
Low Pressure Test with 4in Disc (41p)
The final correlation for the burst discs was Test 41p which studies a 4in metal disc. The model
used in the previous cases was therefore simulated and the results are shown on Figure 5.5.3.
This again shows reasonable correlation based on an initial immediate opening of a few percent
followed by an effective opening time of 10 msec. But as seen with the previous test, relief of
the pressure wave is achieved with less than 10 percent of the total disc capacity and so an
accurate estimation of the overall opening time is difficult.
4. 8
Test 41p 10ms Rupture
Pressure (bar)
25
20
15
10
5
0
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K4 Pressure
Max = 19.96
Sheffield Model - Taken from Test 51p
Min = 0
Figure 5.5.3 Correlation for 4in Metal Disc, Low Pressure Test
Disc Performance: Graphite versus Metal
In addition to investigating the opening times for the different devices we undertook a
comprehensive appraisal of each set of results, with comparison against the original open tube
tests and also comparing different relief devices (see Appendix B). And this was particularly
interesting when examining the burst disc results.
For example, we earlier outlined the need for two different disc models, one for graphite and one
for metal discs and this difference is apparent when examining the test results for the K4
transducer (Figure 5.5.4).
29
Position K4 for 4in Graphite and SS Discs
20
K4, 51p, 4in
Graphite
Pressure (bar)
15
K4 41p, 4in SS
10
5
0
0
0.002
0.004
0.006
0.008
0.01
0.012
-5
Time (s)
Figure 5.5.4 Measured Test at K4 - Graphite and Metal Discs
Figure 5.5.4 shows the low pressure tests for both 4" discs and hence are directly comparable
but their performance is different:
The capacity15 of the graphite disc is high enough that the device offered no restriction to the
flow in the shock tube after it had ruptured. This means that the pressure at K4 dropped to
atmospheric pressure by about 8 msec
The metal disc only opened partially and offered significant restriction to the flow in the shock
tube. This means that the pressure at K4 remained at almost 10 bar. Subsequently, the pressure
throughout the shock tube dropped, as discussed in Appendix A
Summary
We have two different models for burst discs, one for graphite discs (in which the relief capacity
continues to increase to the maximum once it starts to rupture) and one for metal discs (which
only opens when there is a positive driving force). And the this study has confirmed the need for
two different models.
The results also show that the metal discs opened more slowly than the graphite discs (nominally
in 10 msec compared with only 1.9 msec for the graphite discs). And, subjectively a slower time
is expected because of the difference in the disc material and the manner in which they
open/shatter. However, the discs studied in the high pressure tests16 are also different sizes (the
slower disc is also the larger) and so it is not possible to state whether the difference in opening
time is only attributable to the material or whether it is also a function of size.
Additionally, we have some reservations about the accuracy of the burst time for the metal disc
(10 msec). As seen with the graphite disc, the pressure wave was relieved within about 10% of
the opening time and this means, subsequently, the test became insensitive to the disc capacity.
15
Although it is impossible to say whether the graphite disc shattered completely (or whether a small
annulus remained), either way, it gave materially the full capacity
16
The high pressure tests give a more reliable burst time
30
In practice therefore the overall burst time represents an effective rupture period of about 1
msec, extrapolated to give an overall estimate of 10 msec.
Overall, the study again shows good correlation between the measured and simulated results and
we have a high level of confidence in the model. But we suggest that further work is undertaken
to examine this in more detail:
with a wider range of tests that are directly comparable,
under tests conditions that sensitive to the disc capacity and hence give a more accurate
estimation of the overall opening time
5.6
SPRING LOADED RV, HIGH PRESSURE TEST
Introduction
This section continues the examination of the relief devices, starting to look at the spring loaded
relief valves (RVs).17
Test Data
As noted previously, the 15mm orifice gives a high pressure test. The equivalent low pressure
test is discussed later in the section.
Table 5.6.1
Test Summary
Type
2in Spring
Loaded
4mm Orifice
See Section
5.7
8mm Orifice
-
15mm Orifice
Test 57p
The set pressure for the RV is 15 barg.
Mathematical Model of the Spring Loaded RVs
As noted for the burst discs, the mathematical model used by PSI for the spring loaded relief
valve validates against API RP 520 and manufacturers' catalogue data under steady state
conditions. However, the dynamic model also incorporates the unique characteristics of the valve
action, described in the manufacturers' catalogues with the specific terms 'pop open' and
'blowdown'.
In brief the cycle of an RV is:
Closed
Valve starts to 'simmer' when local pressure exceeds set pressure
Valve pops open at popping point (i.e. a pressure slightly above the set pressure). When the
valve has popped open, the rated capacity is in accordance with the international codes i.e. at
110% of set pressure
Further lift is proportional to pressure to give slightly more capacity
In the initial closing phase, lift is proportional to pressure
17
The simple difference between a burst disc and a relief valve (as defined in API RP 520) is the fact that
the burst disc is a non-closing device whilst an RV is designed to automatically re-close and prevent the
further flow of fluid.
31
Then the valve blows down, with a smaller pop action, down to closure at a lower (blowdown)
pressure. The cycle is now complete with the valve re-closed.
32
Opening Time for RVs
As noted above, the 'rated' capacity18 of an RV is reached when it pops open and this is the
most important feature of the valves. We have therefore defined the opening time for RVs as
the time taken to reach the rated capacity i.e. the capacity for 110% of the set pressure, or 10%
over-pressure (10% OP).
Trend Study for High Pressure Test (57p)
Firstly, the hydraulic model of the shock tube was re-configured to incorporate the RV and then,
a trend study was again undertaken to investigate the opening time.
The results are given on Table 5.6.2 and Figure 5.6.1, the dark line is measured.
Table 5.6.2
Peak Pressure at K4 from Trend Study
RV 10% OP
Time (msec)
1.5
3.0
4.5
6.0
Peak Pressure
(Barg)
87.0
97.2
102.1
104.9
Test 57p Trend Curve for 1.5, 3 4.5 & 6millisec
Pressure (bar)
120
100
80
60
40
20
0
0
0.002
0.004
0.006
0.008
K4 Pressure
0.01
0.012
Time (seconds)
Sheffield Model - Taken from Test 51p
Figure 5.6.1 Trend Curve for Spring RV, High Pressure Test
Optimising Opening Time
18
The 'rated' capacity is in accordance with international codes
33
From these results, the optimum opening time of 4 msec was selected and the high pressure test
was re-simulated with this time.
The results are given for K4 and K3 (Figures 5.6.2 and 5.6.3, the dark lines are the simulated
result) and these again show very good correlation.
5. 8
Test 57p 10% Capacity in 4ms
Pressure (bar)
120
100
80
60
40
20
0
-20
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K4 Pressure
Max = 100.75
Sheffield Model - Taken from Test 51p
Min = 0
Figure 5.6.2 Correlation for Spring RV, High Pressure Test
5. 12
Test 57p 10% Capacity in 4ms
Pressure (bar)
120
100
80
60
40
20
0
-20
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K3 Pressure
Max = 101.28
Sheffield Model - Taken from Test 51p
Min = 0
Figure 5.6.3 Correlation for K2
Evaluation of the Results
Our evaluation of the tests results also highlighted two other features:
RV remains fully opening for the test period. As shown on Figure 5.6.4, the pressure in the
shock tube remains well above the set pressure of the RV (15 barg)
As might be expected, the capacity of the 2in RV is far lower than the capacity of the 4in burst
disc. Additionally, the burst disc reacts more quickly. For example, in comparable tests, the
34
graphite burst disc limited the peak pressure to about 41 barg and reduced the pressure at K4
towards atmospheric pressure. In contrast the peak pressure is over 100 barg with the 2in RV
and the pressure at K4 remains above 60 barg for the remainder of the test.
2in Spring Relief - 15mm
Pressure (barg)
120
100
K4
80
60
40
20
0
0
0.05
0.1
0.15
Time (s)
Figure 5.6.4 Measured Test at K4 - Spring Loaded RV
Summary
The mathematical model for the RV is far more complex than the burst disc model as it needs to
reflect the complete performance cycle of pop open and then blowdown. But despite this we are
again able to show good correlation when with an opening time of 4 msec. However, it must be
remembered that this opening time is defined very specifically for RVs as the time taken to
reach the rated capacity i.e. the capacity for 110% of the set pressure, or 10% over-pressure
(10% OP).
5.7
SPRING LOADED RV, LOW PRESSURE TEST
Introduction
This section discusses the findings of the low pressure test on the spring loaded RV.
Test Data
The test and number is that used by Sheffield University. As noted previously, the 4mm orifice
gives a low pressure test.
Table 5.7.1
Test Summary
Type
2in Spring
Loaded
4mm Orifice
Test 59p
8mm Orifice
-
35
15mm Orifice
Section 5.6
The set pressure for the RV is 15 barg.
36
Evaluation of Test Results
The main feature of this low pressure test was immediately apparent when the test results were
initially appraised. Unlike the previous tests, where the pressure in the shock tube dropped after
the relief device is active (see Figure 19, for example) this test showed unstable pressure
oscillations, commonly termed valve chatter (Figure 5.7.1). This behaviour shows:
A pressure frequency of about 80 cycles per sec
The initial pressure peak was not the highest; a later pressure peak is 36% higher
2in Spring Relief - 4mm - Smoothed
Pressure
(barg)
50
K4
40
30
20
10
0
0
0.05
0.1
0.15
Time (s)
Figure 5.7.1 Measured Test Showing RV Chatter
RV Chatter
Chatter is associated with over-sizing of an RV. In the context of the system, a relatively high
relief flow develops when the valve pops open and this tend to reduce the inlet pressure. But the
pressure can drop below the blowdown pressure if the valve is too big and the relief flow is too
high. This means that the valve will close, generating a surge pressure rise when the relief flow
stops suddenly. And so a repetitive cycle can develop where opening is followed by immediate
closure, resulting in the chatter effect shown on Figure 5.7.1.
Low Pressure Test
To date, the mathematical modelling of the shock tube and the relief devices has shown very
good correlation, being able to take into account the differences in the devices themselves and
also re-configuration of the shock tube (with the expansion piece for example). However, the
accurate study of a chattering RV was well outside the scope of this project.
As a matter of interest, we therefore took a simplistic approach and looked only at the first
opening phase; we used an opening time based on the cycle frequency. 19 The results are shown
19
The valve frequency was 80 cycles per second i.e. 12.5 msec and so an opening time of 6 msec was used,
slightly slower than the opening time in the high pressure test
37
on Figure 5.7.2 and Figure 5.7.3 and the overall correlation is good. But it already underestimates the oscillation seen at K4, i.e. near the RV.
Summary
The low pressure test on the spring loaded relief valve showed all the characteristics of valve
chatter, consistent with the performance of an over-sized valve. The detailed study of this
phenomenon was well outside the scope of this study and so only the initial pop-open phase was
studied. And this suggested that the RV opened slightly slower in this test than in the high
pressure test (6 msec compared with 4 msec). However, some pressure oscillation is still
apparent, even within this opening stage.
6. 8
Test 59p 10% Capacity in 6ms
Pressure (bar)
50
40
30
20
10
0
0
0.002
0.004
0.006
0.008
0.01
0.012
-10
Time (seconds)
K4 Pressure
Max = 27.38
Sheffield Model - Taken from Test 51p
Min = 0
Figure 5.7.2 Preliminary Correlation for RV, Low Pressure Test
6. 12
Test 59p 10% Capacity in 6ms
Pressure (bar)
50
40
30
20
10
0
0
0.002
0.004
0.006
0.008
0.01
0.012
-10
Time (seconds)
K3 Pressure
Max = 27.39
Sheffield Model - Taken from Test 51p
Min = 0
Figure 5.7.3 Correlation for K3
38
5.8
PILOT OPERATED RELIEF VALVE TEST
Introduction
This section discusses the final set of results, namely for the pilot operated relief valve.
Test Data
As noted previously, the 4mm orifice gives a low pressure test and the 15mm is a high pressure
test.
Table 5.8.1
Test Summary
Type
2in Pilot RV
4mm Orifice
Test 66p
8mm Orifice
-
15mm Orifice
Test 64p
The set pressure for the RV is 15 barg.
Valve Characteristics
Although the pilot operated RV is nominally the same size as the spring loaded RV discussed in
the preceding section, the capacity of a pilot operated valve is slightly greater. It is also worth
noting that, physically, this pilot valve is more compact and most importantly the moving parts of
the valve are smaller than the spring loaded RV. And, overall, these factors mean that we
expected the pilot operated valve to react slightly more quickly than the spring loaded valve.
Additionally, the pilot configuration offers either pop-action or modulating-action; there is also a
combined option of pop-open and modulate-closed which was the configuration studied in these
tests. And again, this means that the test valves were the fastest available.
Opening Time for RVs
As noted earlier, the 'rated' capacity20 of an RV is reached when it pops open and this is the
most important feature of the valves. We have therefore defined the opening time for RVs as
the time taken to reach the rated capacity i.e. the capacity for 110% of the set pressure, or 10%
over-pressure (10% OP).
Trend Study for High Pressure Test (64p)
As before, the hydraulic model of the shock tube was re-configured to incorporate the pilot
operated RV and a trend study was again undertaken to investigate the opening time.
The results are given on Table 5.8.2 and Figure 5.8.1, where the dark line is measured.
Table 5.8.2
Peak Pressure at K4 from Trend Study
RV 10% OP
Time (msec)
2.0
3.0
20
Peak Pressure
(Barg)
81.04
86.66
The 'rated' capacity is in accordance with international codes
39
4.0
90.04
Test 64p Trend Curve for 2, 3 & 4millisec
Pressure (bar)
100
80
60
40
20
0
0
0.002
0.004
0.006
0.008
K4 Pressure
0.01
0.012
Time (seconds)
Sheffield Model - Taken from Test 51p
Figure 5.8.1 Trend Study for Pilot RV, High Pressure Test
Optimising Opening Time
From these results, the optimum opening time of 2.5 msec was selected and the high pressure
test was re-simulated with this time.
The results are given for K4 (Figure 5.8.2, the dark line is the simulated result) and again this
shows very good correlation.
7. 8
Test 64p 10% Capacity in 2.5ms
Pressure (bar)
100
80
60
40
20
0
0
0.002
0.004
0.006
0.008
0.01
0.012
-20
Time (seconds)
K4 Pressure
Standard
Max = 84.07
Sheffield Model - Taken from Test 51p
Min = 0
Figure 5.8.2 Correlation for Pilot RV, High Pressure Test
Low Pressure Test
Examination of the test results for the low pressure test shows that, within 2 msec of the valve
opening, the local pressure drops back below the set pressure. This means that the valve will pop
40
open and then immediately start to modulate closed. However, this is a function of the type and
configuration of the pilot system and the accurate study of its performance was well outside the
scope of this project.
Again, as a matter of interest, we therefore took a simplistic approach and looked only at the
first opening phase; the opening time was taken as 4 msec, based on earlier findings of this study
that the devices tend to act more slowly in the marginal, low pressure tests, compared with the
high pressure ones. The results are shown on Figure 5.8.3, where the dark line is simulated.
8. 8
Test 66p 10% Capacity in 4ms
Pressure (bar)
30
25
20
15
10
5
0
-5
0
0.002
0.004
0.006
0.008
0.01
0.012
Time (seconds)
K4 Pressure
Max = 22.11
Sheffield Model - Taken from Test 51p
Min = 0
Figure 5.8.3 Correlation for Pilot RV, Low Pressure Test
RV Performance: Spring Loaded versus Pilot
Pilot operated RVs are not widely used in industry. As stated in API RP 520, "since the main
valve and pilot contain non metallic components, process temperature and fluid compatibility limit
their use. In addition, fluid characteristics such as a susceptibility to polymerisation, fouling,
viscosity, the presence of solids and corrosiveness may affect pilot reliability".
Despite this, as shown on Figure 5.8.4, their modulating action may by beneficial. The severe
chatter exhibited by the spring loaded RV is completely eliminated from the low pressure test
and replaced by stable performance.
41
Position K4 Relief Valve
K4, 66p, Pilot
50
K4 59p, Spring
45
40
Pressure (bar)
35
30
25
20
15
10
5
0
-5 0
0.05
0.1
Time (s)
Figure 5.8.4 Measured Test at K4 - Spring Loaded and Pilot RV
Summary
The study shows that the 2in spring loaded RV popped open in 4 msec compared with 2.5 msec
for the pilot operated valve. And this finding, was unexpected. Subjectively, a slower response
was expected. However, we note that:
The pilot operated RV and the spring loaded RV are not directly similar valves. The physical
mass of the moving parts is smaller for the pilot operated valve and so the inertia effects would
be lower
We do not have details of the pilot system for the valve but the findings of the study suggest that
it is the type that is characterised by pop action, followed by a modulating action. 21 This means
that the opening performance will not be adversely affected by the pilot
The study also shows that the modulating action is particularly beneficial in eliminating the severe
valve chatter seen previously with the spring loaded RV. However, this is not the only way of
eliminating chatter. It can also be avoided by other means, for example, by using dynamic
simulation methods to ensure that the spring loaded RV is correctly sized in the design stage.
21
This is supported by the valve performance in the low pressure test
42
6. CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER
WORK
The study was initiated as a result of concerns over the failure of pressure vessels holding liquids
when subjected to internal pressure pulses whose amplitudes and durations were above certain
limiting values. Whilst pressures higher than yield pressure may be tolerated for durations which
are short compared to some characteristic time of the structure, the effect of delayed pressure
relief is expected to extend the period of higher pressure exposure and hence may move the
transient into a hazard range.
The study emphasises a few key features associated with the pressurising source and
subsequent wave propagation, and these can be summarised as follows :
1. following gas release there is a constant pressure which develops at the liquid/gas interface
and this pressure propagates at sound speed in the liquid, being experienced by the structure
following its passage. The constant pressure will depend on the rate of flow of gas into the
interface region and the compressibility of the downstream liquid. This is characterised by a
plateau region on the upstream transducers (K2, K3) in the study.
2. On arrival at the relief device, the pressure wave opens the device after a certain time and
this then propagates an expansion wave upstream which cancels the high pressure. For
instantaneous opening, the duration of the high pressure exposure upstream is solely
dependent on the distances involved and the sound speed in the liquid. For water filled vessels
this corresponds approximately to 0.7msec/meter pathlength, where the pathlength is the
round-trip distance from any point to the relief device.
3. Any delay in the opening of the device will result in the reflection of a pressure wave of
twice the amplitude upstream and therefore the exposures which should be of concern are the
initial plateau pressure, the reflected wave pressure and the duration associated with each. The
plateau duration has been referred to above. The reflected wave duration will depend on the
opening time of the device, and a number of data traces show this effect.
4. The analysis has indicated that for the devices tested, times to full opening are 1.9 - 10 msec
for the bursting discs and 2.5 - 4 msec for the RVs. The differences in the relative scale of
these two families of devices has already been emphasised and is reiterated here.
5. Whilst the study used oversized discs and undersized RVs, the quoted times should strictly be
applied to pressurising systems for which the devices would represent the correct sizes.
6. It is considered that the timescales for opening are significant and are comparable with the
response times for structures as they move toward yield when exposed to a step pressure
rise. A transient analysis is therefore considered a worthwhile exercise during the design of
low pressure vessels which may be exposed to high pressure transients, whose relief is
43
governed by RV or bursting disc behaviour. In summary, the opening times to full relief capacity
for a number of the devices tested are collected together in Table 6.1.
Table 6.1
Measured relief times for selected relief devices
Relief device
Graphite 4"
Stainless steel 8"
Stainless steel 4"
Relief valve
Relief valve
Pilot operated relief valve
Pilot operated relief valve
Conditions
high pressure
high pressure
low pressure
high pressure
low pressure
high pressure
low pressure
Relief time - msec
1.9
10
10
4
6
2.5
4
We recommend that further work is undertaken to determine the dependency of these findings
on pressure conditions. Although we have nominally undertaken a detailed analysis of two sets
of results (high and low pressure), only the high pressure tests were effective in terms of the full
opening of the devices. We therefore suggest that we take advantage of the fact that Sheffield
University already have another set of measured data available (8mm tests) and repeat the
detailed analysis on this information.
To determine whether the apparently slow opening of the metal disc is due predominantly to its
size we also suggest that we analyse the existing test data for the 4" metal disc.
Turning to the RVs, we feel that there are two areas of investigation available. The first, and
potentially the most conclusive, would be to undergo further physical testing on different valve
sizes and types. The second would combine the skills of detailed analysis and literature review to
explore the basis for the longer opening times for the RVs that were given in the IP guidelines
and in other research papers. The work should focus on points of comparison e.g. establishing
whether the valves were the same type (pop action) and size.
44
7.
7.1
APPENDIX
PHOTOGRAPHS
A number of photographs illustrate the key components used in carrying out the experimental
study.
45
Plate 1.
Working area for shock tube control and data collection
46
Plate 2
Aluminium driver diaphragm controlling driver operating pressure before and after rupture
Plate 3.
Driver diaphragm in place after firing - driver section on the right.
47
Plate 4.
The three discharge orifices used in the tests
48
Plate 5.
Entrance to water filled section showing water filling inlet and
surfactant feed reservoir.
49
Plate 6.
Samples of 4" and 8" stainless steel bursting discs before firing
Plate 7.
8" disc and holder
50
Plate 8.
Expansion section for 8" stainless bursting disc.
51
Plate 9.
Graphite disc holder in place at outlet end of shock tube.
52
Plate 10.
Samples of graphite and stainless bursting discs after firing
53
Plate 11.
2" relief valve body used in valve tests.
54
7.2 DATA CHARTS
The data charts represent up to 50 msec of pressure data following driver disc rupture.
Full records are actually 250msec in duration.
55
Pressure - bar
25
4mm orifice, plastic
film at outlet
Test 39
20
15
10
5
K1
K3
K2
0
K4
-5
-2
-1
0
1
2
3
4
5
6
7
8
10
msec
CHART A.1
60
Pressure - bar
9
8mm orifice, plastic
film at outlet
Test 38
50
40
30
20
10
K1
K3
K2
0
K4
-10
-2
-1
0
1
2
3
4
CHART A.2
5
6
7
8
9
msec
56
10
Pressure - bar
120
15mm orifice, plastic
film at outlet
Test 37
100
80
60
40
20
K1
K2
K3
K4
0
-20
-2
-1
0
1
2
3
4
5
6
7
8
10
msec
CHART A.3
30
Pressure - bar
9
4mm orifice, 4"
graphite disc
Test 51
25
20
15
10
5
K3
K2
K1
K4
0
-5
-2
-1
0
1
2
3
4
CHART A.4
5
6
7
8
9
msec
57
10
Pressure - bar
70
8mm orifice, 4"
graphite disc
Test 50
60
K4
50
40
30
20
10
K1
K3
K2
0
-10
-2
-1
0
1
2
3
4
5
6
7
8
CHART A.5
10
msec
120
Pressure - bar
9
15mm orifice, 4"
graphite disc
Test 49
100
80
60
40
K3
20
K1
K4
K2
0
-20
-2
-1
0
1
2
3
4
CHART A.6
5
6
7
8
9
msec
58
10
Pressure - bar
30
4mm orifice, 6"
graphite disc
Test 55
25
20
15
10
K2
5
K3
K1
K4
0
-5
-2
-1
0
1
2
3
4
5
6
7
8
CHART A.7
10
msec
80
Pressure - bar
9
8mm orifice, 6"
graphite disc
Test 54
60
40
20
0
-20
-40
-2
-1
0
1
2
3
4
CHART A.8
5
6
7
8
9
msec
59
10
Pressure - bar
120
15mm orifice, 6"
graphite disc
Test 53
100
80
60
40
K1
K3 K4
K2
20
0
-2
-1
0
1
2
3
4
5
6
7
8
CHART A.9
10
msec
30
Pressure - bar
9
4mm orifice, 4"
stainless steel disc
Test 41
25
20
15
10
5
K1
K3
K2
K4
0
-5
-2
-1
0
1
2
3
4
5
CHART A.10
6
7
8
9
10 11 12
msec
60
Pressure - bar
80
8mm orifice, 4"
stainless steel disc
Test 42
70
60
50
40
30
20
10
K2
K1
0
K3
K4
-10
-2
-1
0
1
2
3
4
5
6
7
8
10
msec
CHART A.11
120
Pressure - bar
9
15mm orifice, 4"
stainless steel disc
Test 40
100
80
60
40
20
K1
K3
K2
K4
0
-20
-2
-1
0
1
2
3
4
CHART A.12
5
6
7
8
9
msec
61
10
Pressure - bar
40
4mm orifice, 8"
stainless steel disc
Test 48
30
20
10
K1
K4
K3
K2
0
-10
-2
-1
0
1
2
3
4
5
6
7
8
CHART A.13
10
msec
50
Pressure - bar
9
8mm orifice, 8"
stainless steel disc
Test 47
40
30
20
10
K2
K1
0
K4
K3
-10
-2
-1
0
1
2
3
4
CHART A.14
5
6
7
8
9
msec
62
10
120
15mm orifice, 8"
stainless steel disc
Pressure - bar
Test 46
100
80
60
40
20
K1
0
K3
K2
K4
-20
-2
-1
0
1
2
3
4
5
6
7
8
CHART A.15
10
msec
50
4mm orifice, 2"
relief valve
Test 59
Pressure - bar
9
40
30
20
10
K1
K2
0
K3
K4
5
7
-10
-20
-1
1
0
3
2
4
6
9
8
CHART A.16
11
10
13
12
15
14
msec
63
Pressure - bar
50
4mm orifice, 2"
relief valve
Test 59
40
30
20
10
0
-10
-20
0
5
10
15
20
25
30
35
40
CHART A.17
50
msec
70
Pressure - bar
45
8mm orifice, 2"
relief valve
Test 58
60
50
40
30
20
10
K1
0
K4
K3
K2
-10
-1
1
0
3
2
5
4
7
6
9
8
CHART A.18
11
10
13
12
15
14
msec
64
Pressure - bar
70
8mm orifice, 2"
relief valve
Test 58
60
50
40
30
20
10
0
-10
0
5
10
15
20
25
30
35
40
CHART A.19
50
msec
120
Pressure - bar
45
15mm orifice, 2"
relief valve
Test 57
100
80
60
40
20
0
K1
K3
K2
K4
-20
-1
1
0
3
2
5
4
7
6
9
8
CHART A.20
11
10
13
12
15
14
msec
65
Pressure - bar
120
15mm orifice, 2"
relief valve
Test 57
100
80
60
40
20
0
-20
0
5
10
15
20
25
30
35
40
CHART A.21
Pressure - bar
40
45
50
msec
Test 62
4mm orifice, 2"
bellows relief valve
30
K4
20
10
0
K2
K1
K3
-10
-1
1
0
3
2
5
4
7
6
9
8
CHART A.22
11
10
13
12
15
14
msec
66
Pressure - bar
40
Test 62
4mm orifice, 2"
bellows relief valve
5
15
30
20
10
0
-10
0
10
20
25
30
35
40
CHART A.23
Pressure - bar
70
45
50
msec
8mm orifice, 2"
bellows relief valve
Test 61
60
50
40
30
20
10
K1
K2
K3
0
K4
-10
-20
-1
1
0
3
2
5
4
7
6
9
8
CHART A.24
11
10
13
12
15
14
msec
67
Pressure - bar
70
8mm orifice, 2"
bellows relief valve
Test 61
60
50
40
30
20
10
0
-10
-20
0
5
10
15
20
25
30
35
40
50
msec
CHART A.25
120
Pressure - bar
45
Test 60
15mm orifice, 2"
bellows relief valve
100
80
60
40
K1
20
K3
K2
K4
0
-20
-1
1
0
3
2
5
4
7
6
9
8
CHART A.26
11
10
13
12
15
14
msec
68
120
15mm orifice, 2"
bellows relief valve
Pressure - bar
Test 60
100
80
60
40
20
0
-20
0
5
10
15
20
25
30
35
40
45
50
msec
CHART A.27
30
Pressure - bar
Test 66
4mm orifice, 2"
pilot assisted
relief valve
20
10
K1
K3
K2
K4
0
-10
-1
1
0
3
2
5
4
7
6
9
8
11
10
13
12
15
14
msec
CHART A.28
69
Pressure - bar
30
Test 66
4mm orifice, 2"
pilot assisted
relief valve
20
10
0
-10
0
5
10
15
20
25
30
35
40
CHART A.29
50
msec
60
8mm orifice, 2" pilot
assisted relief valve
Test 65
Pressure - bar
45
50
40
30
20
K1
K2
10
K3
K4
0
-10
-1
1
0
3
2
5
4
7
6
9
8
CHART A.30
11
10
13
12
15
14
msec
70
Pressure - bar
60
Test 65
8mm orifice, 2" pilot
assisted relief valve
25
35
50
40
30
20
10
0
-10
0
5
10
15
20
30
40
CHART A.31
50
msec
100
15mm orifice, 2" pilot
assisted relief valve
Test 64
Pressure - bar
45
80
60
40
20
K1
K3
K2
K4
0
-20
-1
1
0
3
2
5
4
7
6
9
8
CHART A.32
11
10
13
12
15
14
msec
71
100
Pressure - bar
Test 64
15mm orifice, 2" pilot
assisted relief valve
80
60
40
20
0
-20
0
5
10
15
20
25
CHART A.33
30
35
40
45
msec
72
50
7.3
VIDEO TAPE RECORD
The following is a brief review in sequence of the content of the 13 minute video tape
accompanying the report.
1. Panning view of shock tube with driver section to the left and water filled section at the
right. At the right hand end can be seen the conical expansion and holder for the 200 mm
stainless steel bursting disc.
2. Close-up of the driver aluminium diaphragm after firing followed by one of the Kistler
transducer amplifier boxes.
3. Close-up of 200 mm expansion section showing the bursting disc in situ.
4. Close-up of burst diaphragm region showing 'petalled' diaphragm after firing, the water inlet
used for tube filling, the driven section with a discharge orifice in place and the surfactant
reservoir.
5. Panning view of high speed camera set-up and associated computer for image downloading.
6. A panning view of the control area and pressure data acquisition system.
7. A sequence of views showing the materials used in tube firing and including :
aluminium diaphragms before and after firing
graphite bursting discs before and after rupture
100 mm and 200 mm stainless steel bursting discs before and after rupture
the set of 3 discharge orifices used at the entrance to the water filled section.
one of the relief valves used for testing showing the outlet flange of the valve.
8 Two sequences follow showing the operation of the 200 mm stainless steel and 150 mm
graphite bursting discs at normal video rates.
9 The final sequences show the same discs bursting but taken at a framing rate of 4000 frames
/sec.
73
Printed and published by the Health and Safety Executive
C0.35
4/01
ISBN 0-7176-1985-0
OTO 2000/130
£25.00
9 780717 619856