CONTROL & SMART VALVES
Control Valve Testing and
Qualification for PSA Service
In this article Emerson
process management talks
about Pressure Swing
Adsorption and how control
valves help to work with this
matter. The article provides
several solutions and tests to
illustrate the work of the
control valves.
By John Bessman, Global Sliding
Stem Test & Evaluation Engineer at
Emerson Process Management
PSA Process and Valve
Overview
Pressure Swing Adsorption (PSA) is a
process that separates a single product
gas from feed gas mixture. In the refining
industry, the feed gas mixture is typically
a waste gas from another process and
the desired product is hydrogen.The
hydrogen is then used for reducing sulfur
and compounds of nitrogen in fuels.The
gas is separated by cycling it through
series of modular beds which make up
the singular PSA skid. Product gas
recovery, purity and throughput are
highly dependant on the control valves,
which must provide tight bi-directional
shutoff, fast stroking speeds and high
cycle reliability.
PSA skids utilize both butterfly and globe
valves. Larger skids will tend to use more
butterfly valves for their higher
capacities, while smaller skids use more
globe valves for their advantageous
control range.The butterfly valves may
be as large as NPS 16.The valves utilize
soft seats in order to achieve Class VI
shutoff.They are typically actuated with
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spring return double acting piston
actuators. Solenoids are used for
actuation since they are typically on-off
applications, and volume boosters are
often used to decrease stroke times.
Larger skids are tending to use butterfly
valves to control flow as well –
Emerson’s Fisher® Valve Division is
introducing a butterfly valve with better
controllability for just such applications.
The globe valves are usually between
NPS 2 – 4.They also have soft seats for
Class VI shutoff.They have typically been
actuated with spring return double
acting piston actuators.The globe valves
are more likely to use a digital valve
controller because they are often used
in open-loop throttling applications.
Volume boosters are often used to
decrease stroke times on globe valves as
well.
Customer Experiences in PSA
Emerson talked with several customers
and licensors to determine what
features were important for PSA valves.
The incumbent Original Equipment
Manufacturer (OEM) for sliding stem
valves recommends maintenance at
100,000 cycle intervals. Assuming a PSA
valve cycles 100,000 times per year,
replacing that valve with a new valve
capable of 500,000 cycle maintenance
intervals would have a payback of 3.2
years.
Customers and licensors also discussed
common PSA valve failures.They have
experienced butterfly valve shafts failing
by fatigue at the pinned connection,
often causing the disc to go
downstream. Customers have
experienced issues with piston actuators
on globe valves; o-rings wear out, springs
break, spring tensioners loose tension
and actuator rods twist and break
mechanical position feedback linkages.
Positioner failures were typically caused
by wet instrument air freezing in the
small air passages. Occasionally a
customer would accidentally “fluff the
bed,” sending adsorbent media
downstream and destroying the soft
seals required for tight shutoff. Emerson
considered these failure modes and used
these examples to determine the best
valve butterfly and globe valve
Figure 1. Schematic of Helium Seat Leak Test
April 2009
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CONTROL & SMART VALVES
resistance, resulting in more robust seal
performance should the seal be exposed
to adsorbent media. The actuator was a
FieldQ® rack-and-pinion design. The
actuator did not have any options.
The positioner can be an ASCO
solenoid for purely on/off service, or a
FIELDVUE DVC can be used if accurate
intermediate stroking in necessary. The
DVC2000 offers a magnetic feedback
system with true linkage-less operation.
The DVC6000 is available if explosionproof is a requirement in the PSA skid. If
fast stroking speeds are required, a 2625
volume booster can be used as well.
Emerson Globe PSA Solution
The Fisher® GX globe valve was
selected as the globe valve solution. The
GX construction for PSA service is:
Figure 2. A41 Helium Seat Leak Test Results
constructions to test for PSA service.
Emerson Butterfly PSA
Solution
The Fisher® A41 High Performance
Butterfly Valve was selected as the
butterfly valve solution. The A41
construction for PSA service is:
•Body Size & ANSI: Application
suitability
• Trim Materials of Construction:
– 316SST disc with chrome-plate
– UHMWPE seal technology
– PTFE-lined PEEK Bearings
– Emission control PTFE live-loaded
packing
Figure 3. A41 Helium Seat Leak Test Results
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April 2009
• Solenoid or Positioner:
– ASCO high capacity solenoid
– DVC2000 FIELDVUE® digital valve
controller – linkage-less (eliminates
linkage to stem)
– DVC6000 FIELDVUE® digital valve
controller – explosion proof
• Accessories (to facilitate fast stroking
speeds):
– Fisher® 2625 volume booster
The chrome plating on the 316SST disc
is a standard option, as well as the UltraHigh Molecular Weight PolyEthylene
(UHMWPE) seal. The UHMWPE seal
was chosen over the standard PTFE seal
because it has greater abrasion
• Body Size & ANSI: Application
suitability
• Trim materials of construction:
– 316L / PTFE seat ring
– 316L / ion-nitrided plug
– 316L strain hardened stem
– Emission control PTFE live-loaded
packing
• Positioner:
– DVC2000 FIELDVUE® digital valve
controller – linkage-less (eliminates
linkage to stem)
– DVC6000 FIELDVUE® digital valve
controller – explosion proof
• Accessories (to facilitate fast stroking
speeds):
– Fisher® 2625 volume booster
– High Cv Fisher® 95 regulator
The PTFE seat and ion-nitrided plug are
standard options. The PTFE seat provides
Class VI shutoff, while the ion-nitrided
plug is a harder, more wear resistance
guide surface. The actuator is a GX
spring and diaphragm actuator. During
product development, extensive
research and testing showed that this
spring and diaphragm actuator design is
more reliable than a piston actuator
since it does not rely on a large
diameter, sliding piston o-ring for
pressure integrity.
As with the butterfly valve, a DVC2000
positioner is available with linkage-less
feedback and the DVC6000 is available
for customers with explosion-proof
requirements. If stroke times of less than
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CONTROL & SMART VALVES
two seconds are necessary, Emerson
offers the Fisher® 2625 volume booster
and Fisher® 95 regulator.
Testing Fisher® Valves in PSA
Service
Helium Seat Leak Test
Test Setup and Procedure
Emerson Test Engineers worked with
some of the worlds leading licensors of
PSA technology in an attempt to qualify
Fisher® valves for use in PSA
applications. The test required that the
valve assembly be stroked against a 350
psid pressure differential. Pressure was
to be applied to the disc face of a
butterfly valve and above the plug of a
globe valve. At 0 cycles and every 5,000
cycles thereafter a bidirectional seat leak
test was performed. The seat leak test
was performed per ANSI/FCI 70-2 with
the following exceptions:
• Helium would be used as the test
medium because the molecule size
most closely approximated hydrogen
• Seat leak tests would be repeated at
an elevated pressure differential (450
psid) closer to process conditions
After 100,000 cycles had accumulated,
seat leak testing was no longer required.
However, the valve assembly continued
cycling through 500,000 cycles. After
500,000 cycles had been accumulated, a
final seat leak test was performed and a
complete teardown and inspection was
performed. These results were
communicated to PSA licensors for their
review.
Figure 4. GX Helium Seat Leak Test Results
but no leakage was observable. Overall
the valve appeared to be in very good
working order after 500,000 cycles. A
final leak test at 500,000 cycles revealed
that leakage continued at the same
levels observed at 100,000 cycles.
The 3” GX leakage, in all flow directions
and at all pressure drops, increased up
to approximately 30,000 cycles at which
point the plug and seat appear to have
worn to create a matched set of parts.
After this wear in period, leakage at 50
psid remained below Class VI, and was
often bubble tight. At 450 psid, leakage
was approximately Class VI from 30,000
cycles through 100,000 cycles with
pressure above the plug. At 450 psid
with pressure below the plug, seat load
was reduced and leakage predictably
increased to approximately Class V
through 100,000 cycles. After 500,000
cycles, all leakage measurements were 0
bubbles per minute regardless of
pressure drop or flow direction.
Upon disassembly, the actuator
diaphragm and springs looked as new.
The actuator rod showed very little
wear as well. The wear surface of the
plug, which was ion-nitrided for
increased hardness, showed almost no
wear. The guiding surface of the seat ring
on the downstream side showed some
wear where the plug had made contact.
However, the wear particles remained
below the seat line as designed. The
packing appeared to be in good
condition, and no leaks were detected
Test Results
The 8” A41 appeared to have a break in
period of approximately 10,000 cycles.
After 10,000 cycles the leak rate held
fairly constant irrespective of flow
direction or pressure drop. It was noted
the leakage was higher with pressure to
the face of the disc. This makes sense
because a pressure drop across the disc
from the face to the shaft side tends to
unload the seat. The reduction in seat
load leads to increased leakage.
Upon disassembly, the valve seat and
bearings showed acceptable amounts of
wear. The rack-and-pinion assembly in
the actuator appeared almost as new.
The piston o-rings did show some wear,
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Figure 5. GX Seat Leak Test Results
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CONTROL & SMART VALVES
Figure 6. Helium Seat Leak Test of 3” GX after 1.3 million cycles
using a soap water solution called
Snoop.
Finally, a PSA licensor requested a
helium seat leak test of the 3” GX valve
from an Emerson High Cycle PSA Test
since it had accumulated many more
cycles. The results of the helium leak test
can be seen in Figure 6. At the time of
the test, the valve had accumulated 1.3
million cycles. Leakage at 50 psid was
still well within the Class VI limits.
The leakage data and post test
inspection indicated that the GX and
A41 valves would be strong choices for
a PSA process. The leakage data showed
that the valves would provide a high
purity product gas and that a large
percentage of the desired product could
be recovered from the feed gas. The low
wear observed on seals and mechanical
contact surfaces showed that the valves
would provide years of trouble free
operation.
DVC6030 digital valve controller and
ASCO solenoid
• (1) A41, NPS 6, FieldQ E600 Actuator,
DVC6030 digital valve controller and
ASCO solenoid
The A41 valves were accessorized with
ASCO solenoids to perform the typical
day to day cycling. The DVC6030 digital
valve controllers were plumbed in a
custom configuration to allow diagnostic
data to be gathered opportunistically.
The valves were intended to operate
under normal process conditions for five
years or until a failure brought the skid
down.
All three GX valves accumulated 25
months of continuous runtime with zero
maintenance issues. The A41 valves also
accumulated 25 months of continuous
runtime with zero maintenance issues.
The valve technicians on site provided
valve diagnostics to Emerson test
engineers on a regular basis, as well as
updates on the general operation of the
skid. Again, after 25 months of service
the valves were still operating as new.
No leakage was detectable through the
skids monitoring software. No significant
changes in valve friction or spring rates
were observed. At the end of the 25
month test, the customer was pleased
with the Fisher® valve performance.
They have since decided to replace all of
the OEM valves on their PSA skid with
Fisher® valves
Emerson has compiled extensive data
on Fisher® valves in PSA service. Over
one million cycles have been
accumulated in the Fisher® Technology
Development Center, and several years
of experience have been gained in real
process lines. For more information on
Emerson’s PSA valve solutions, please
contact you local Emerson Process
Management sales office.
Field Trial Testing
A North American customer in the
USA agreed to install Fisher® GX and
A41 valves on a trial basis in their PSA
skid. The skid had six beds and had given
them continued problems over its six
years of service. The following valves
were installed on a single bed in the
skid:
• (1) GX Valve and Actuator, NPS 2, and
DVC6030 digital valve controller
• (2) GX Valve and Actuator, NPS 3, and
DVC6030 digital valve controller
• (1) A41, NPS 4, FieldQ E350 Actuator,
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April 2009
Figure 7. Fisher® Valves installed in a North American refining customer’s PSA skid. These valves were
tested in process for 25 months, then returned to Emerson for analysis.
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