MAKING MODERN LIVING POSSIBLE
Pressure transmitter design
Liquid (water) hammer phenomenon
How to protect your system from damage caused by water hammering
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Liquid (water) hammer
Pressure peak is a well-known phenomenon in many systems with sudden changes in the liquid
flow, for example, by closing of a valve or by stopping of a pump.
What is not widely acknowledged is
that the situation before the pressure
peak is the actual reason for the possible damage of the components which
form part of the system - and that this
Fig. 1. Test setup.
The test setup comprises a liquid container, a pump, a 25 m pipeline to the
valve, a pressure transmitter in front of
the valve (B), a pressure transmitter behind the valve (C) and a 25 m pipeline
from the valve back to the container.
Please note that this setup corresponds
to the setup illustrated in fig. 5 - 8.
Valve
also applies to pressure transmitters.
If the pressure is measured before and
after a valve in a system with a given
liquid flow, sudden closing of the valve
will cause pulsating pressure on the
inlet as well as the outlet of the valve.
This can be demonstrated, both by
simulation as well as in practice, on a
system as outlined on Fig. 1.
Fig. 2. Measured and simulated
pressure transients in front of the
valve when closing
Pos. B
Fig. 3. Measured and simulated
pressure transients behind the
valve when closing
Liquid
container
Pos. C
Pump
!
Pressure transients
by closing of the valve.
It appears from Fig. 3 that the pressure
on the outlet of the valve will, however, reach absolute zero, i.e., vacuum,
immediately after closing of the valve.
If this vacuum creates a formation of
gas pockets in the dead volume in
front of the pressure transmitter sensor
(see Fig. 4), by cavitation or in other
ways, the subsequent rise in pressure
will have the effect that liquid will be
hammered against the sensor causing possible damage to the pressure
transmitter.
Cavitation:
Liquid hammer: Liquid backlash.
Formation of “gas pockets” when the
pressure gets below the vapour pressure of the liquid. If these “gas pockets”
are imploded (exploded) at a material surface, this surface is exposed to
heavy mechanical stress (pressure) in
the implosion area.
Due to the kinetic energy, a deceleration
of liquid, for example by closing of a valve,
will cause the liquid to collapse when the
pressure gets below the vapour pressure of
the liquid. The liquid flow continues until
the “suction” power in the cavitation area
has reached a size which turns the liquid
flow. When the liquid returns it will
result in high pressure peaks that are
non-uniformly spread on the surface
that it hits.
Fig. 2 and Fig. 3 show the measured
and simulated pressure transients in
front of and behind the valve immediately after closing of the valve.
The pressure transients or the variation
of pressure in front of the valve (Fig. 2)
correspond to what could be expected
Separation
diaphragm
Piezoresistive
sensor
Oil filling
Design of Pressure Transmitters type MBS
Danfoss pressure transmitters type MBS are based on a piezoresistive, monolithic silicon pressure sensor, protected
by a stainless steel separation diaphragm and silicone oil
filling between the diaphragm and the sensor (fig. 4).
Fig. 4. Cross section of MBS
pressure connection
The effects of pressure transients
The two figures 5 and 6 show a cross
section of the pressure transmitter
(B) mounted in front of the valve, the
valve itself and a cross section of the
pressure transmitter (C) mounted
behind the valve.
Fig 5 shows the liquid condition at the
separation diaphragm of the pressure
transmitters during the vacuum situation at transmitter C (immediately after
closing of the valve).
Solving pressure peak problems
Fig 6 shows the liquid condition at the
transmitters at the first pressure peak
after closing.
It appears that the separation diaphragm is deformed on the transmitter (C) that is mounted after the valve.
This is due to a non-uniform pressure
distribution on the diaphragm when
the liquid hammering hits.
It is not the pressure peak
itself that causes damage to
the sensor, but the condition
before the peak occurs.
This conclusion only applies to sensors
with a very high degree of safety towards
static overload pressure, as for example,
the monolithic silicon pressure sensor.
A sensor with a low degree of safety
towards static overload can be damaged
by the pressure peak itself.
valve, but,
on the contrary, it occurs on the
out­let (Fig.
3). In practice,
it is therefore
often difficult to find the right place to
look for the problem.
The phenomenon has been observed
on various applications, thus it is now
possible to point them out in advance
and propose a solution.
The solution consists of a
specially designed nozzle with
a precision laser drilled orifice
The conclusion also explains why the
problem can be observed at a pressure mounted in the pressure port
range of 1 bar as well as at a pressure
of the transmitter.
range of 400 bar.
In the test system, the actual problem does not occur on the inlet of the
C
B
Valve
Fig. 5. Vacuum and cavitation occurs at the sensor and in the pipe after closing the valve
due to liquid kinetic energy.
C
B
Valve
Fig. 6. Liquid backlash can create high pressure peaks of a non-uniform nature at the
sensor diaphragm that may destroy the diaphragm.
The function of the nozzle is not to
protect against pressure peaks. To a
very high degree this is secured by the
nature of the silicon pressure sensor
(overload pressure: 10 - 20 × measuring
range).
The function of the nozzle is to prevent the gas pockets that may result in
the creation of the dangerous nonuniform pressure peaks (cavitation and
liquid hammer) shown in Fig. 5 and
Fig. 6.
A benefit of the nozzle design solution
is that it does not increase the total
time constant perceptibly:
Mechanical + electronic time constant
is typically 1 mS for Danfoss pressure
transmitters.
For viscosity larger than standard
hydraulic oil (32 cSt), it can be stated as
a rule-of-thumb that the time constant
in mS increases by approximately
2 - 3% of the viscosity increase in cSt
(mm2/s).
Conclusion
All pressure transmitters in the HEAVY
DUTY series based on the piezo resistive design are delivered with the
specially designed nozzle and thus
comply with our philosophy that
pressure must not be a problem for a
pressure transmitter (gauge) regardless
of how erratic the pressure appears, as
long as the stated maximum limits for
the pressure range in question have
been complied with.
The problem does of course not occur
in all liquid systems, but our experience show that the phenomenon can
occur in a wide range of plant, such
as hydraulic presses, mobile cranes,
braking systems for windmills, diesel
engines, refrigeration systems with
NH3, etc.
Prevention
Prevention of “the liquid hammer
phenomenon” must take place during
dimensioning of the plant.
In situations where it is not possible
to eliminate liquid hammer problems
by dimensioning it is necessary to use
components that resist the operating
and environmental influences they are
exposed to.
By using Danfoss pressure transmitters
of the HEAVY DUTY series a reliable
pressure measurement is obtained,
also during critical conditions.
C
B
Fig. 7
After closing the valve vacuum and
cavitation only occurs in the pipe due
to the restriction in the nozzle.
Valve
C
B
Fig. 8
Liquid backlash creates high pressure
peaks. Due to the uniform pressure
profile at the diaphragm and the high
degree of safety of the silicon chip towards overpressure, the pressure peak
will cause no damage of the sensor.
Valve
Danfoss A/S, Industrial Automation
DK-6430 Nordborg · Denmark · Tlf: +45 7488 2222 · [email protected] · www.danfoss.com/ia
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IC.PB.P20.L1.02 / 520B5038
Danfoss A/S, April 2012 /hat