PRV Set Point Accuracy

PRV Set Point Accuracy
ASME NRC Pump & Valve Symposium
Presenter: Chad Dupill
July 2008

References
The American Society of Mechanical Engineers
(ASME) Pressure Relief Devices Performance
Test Code 25-2001 25 2001 (PTC 25-2001) 25 2001)
The National Board Inspection Code (NBIC)
Plant testimonials
– A special thanks to J. Alton Cox (Vice Chairman, NBIC
Sub-Group Sub Group Pressure Relief Devices) for his
contributions in guidance, and experience.

Definitions
Definitions
NOTE: For the purpose of this paper we are going to define the following as:
High Volume test bench – PRV Test device with at a minimum capacity volume of 28 Liter [1 [ 1
cubic foot] test vessel and a minimum piping (between the Test Vessel Vessel
and the Test Connection
for the Valve being tested) diameter of 5 cm [2 inches].
Low Volume test bench – PRV Test device with a test vessel less than 28 Liter [1 cubic feet] and
a piping diameter (between the Test Vessel and the Test Connection Connection
for the Valve being tested)
of less than 5 cm [2 inches].
Set Pressure – The value of increasing inlet static pressure at which a pressure pressure
relief device
displays one of the operational characteristics as defined under opening pressure, popping
pressure, start-to start to-leak leak pressure, burst pressure, or breaking pressure. (The applicable applicable
operating
characteristic for a specific device design is specified by the device manufacturer.) PTC 25-2001 25 2001
Opening Pressure – The value of increasing inlet static pressure of a PRV at which there is a
measurable lift, or at which the discharge becomes continuous as determined by seeing, feeling,
or hearing. (For liquid Service) PTC 25-2001 25 2001
Popping Pressure – The value of increasing inlet static pressure at which the disk moves in the
opening direction at a faster rate as compared with corresponding corresponding
movement at higher or lower
pressures. PTC 25-2001 25 2001
Start-to Start to-Leak Leak Pressure – The value of increasing inlet static pressure at which the first first
bubble
occurs when a PRV is tested by means of air under a specified water water
seal on the outlet. PTC 25- 25
2001
Chatter – abnormal rapid reciprocating motion of movable parts of a pressure pressure
relief valve in
which the disk contacts the seat. PTC 25-2001 25 2001
First Steady Stream – Set pressure for liquid Service Pressure Relief Valves. When gravity gravity
overcomes cohesion the water drops straight off the Pressure Relief Relief
Valve outlet; an observable,
repeatable Phenomenon. NB 18
Cohesion – the attraction of like molecules
Note: For the purposes of this paper Test Bench, Test Device, Test Test
Stand, and/or Test Unit are
interchangeable.

Introduction
Testing methods and devices vary widely
within the industry when testing for the
set point of Pressure Relief Valves.
Testing devices within Nuclear Plants
range from:
– Shop built devices
– Hand Pumps
– Manufacturer built test devices
– Others

Purpose
To discuss some of the different design
characteristics and how those design
characteristics can affect your set point
when testing for the set point of Pressure
Relief Valves in both gas and liquid
service.

Overview
Importance of following NBIC and ASME
PTC-25-2001 recommendations on
Internal Contours of Fittings, Adapter’s,
and Reducers between Test Vessel and
Test Device
Recommended Volume Capacity
– PRV Seat Alignment
Air Evacuation

Internal Contours of Fittings, Adapter’s, Adapter s, and
Reducers between Test Vessel and Test Device
ASME PTC 25-2001 25 2001 states that “The The pressure relief device to be
tested shall be installed on a test vessel with adapter fittings
(flanged, screwed, welded, etc.).” etc.).
The NBIC also states that “any any intervening piping between the test
vessel and the pressure relief valve should be as short and straight straight
as possible and be of adequate size to minimize inlet pressure
drop.” drop.
ASME PTC 25-2001 25 2001 goes on to state that “Other Other adapter fittings
may be used provided the accuracy of the test is not affected.” affected
ASME PTC 25-2001 25 2001 and the NBIC recommend that when testing a
PRV on a test bench the piping, adapters, and fittings leading up up
to
the test valve need be taken into account in order to minimize the the
pressure drop as well as minimize the affect of flow turbulence on
the final test results.

PTC 25-2001 25 2001 Recommended Internal
Contours of fittings, adapters, and reducers

PTC 25-2001 25 2001 Recommended Arrangements
for Testing Valves with Incompressible fluids

Test Plate
Test Plate
Tapered Inlet Adaptor
Sharp Corner Adaptor

Test Plate
Sharp Corner
Air Flow
Sharp Corner on Inlet may
cause turbulence in flow
path and result in poor
performance from SRV
including Flutter or Chatter.

Test Plate
Tapered Inlet
Air
Flow
Radius on Inlet eliminates
turbulence in flow path and
results in consistent
performance from SRV
without Flutter or Chatter.

Recommended Volume Capacity
Test vessels, piping size, and the size of test port vary to
a large degree
– DeLuca Test standard model test benches are offered with 56
Liter [2 cubic foot] stainless steel test vessel, 7.6 cm [3 inch] inch]
piping and test port
– well known Test bench manufacturer’s manufacturer s PRV test benches is
offered standard with a 4 liter [less than .2 cubic feet], 1.3 cm cm
[½ inch piping] and test port

Recommended Volume Capacity
The size of the vessel, piping, and test
port affects the volume that the test
bench has the capability to act on the
valve which can affect the results of the
test.
– may affect the results of the test specifically
when conducting liquid service PRV test’s test s and
possibly damage the PRV seat by inducing
chatter when testing with air.

Recommended Volume Capacity
Liquid Service PRV Test
– Visually determining when you have achieved a first steady
stream
– If the test bench being utilized does not have sufficient volume it
can be difficult for the operator to determine when they are
actually achieving set pressure.
– it can be very difficult, on a low volume test bench, to
differentiate between a “Pre Pre-leak leak” where cohesion causes the
water to curve back toward the PRV inlet (which is not the PRV
opening pressure) and a first steady stream.

INLET
Liquid Service PRV Testing
*It *It is is necessary necessary to to have have sufficient sufficient volume volume and and flow flow path path
to to accurately accurately perform perform liquid liquid service service PRV PRV Testing Testing
““First First Steady Steady Stream”” Stream
Preleak
LIQUID SERVICE PRV
OUTLET
PIPE NIPPLE
When Gravity overcomes
Cohesion, the Water drops
Straight off the PRV outlet.
This is Set Pressure. It is
an observable, repeatable
Phenomenon.
Opening Pressure

Recommended Volume Capacity
Testimonials
– Rich Booth, with Vermont Yankee, has experience with both Low and and
High volume test benches and stated that:
“Accuracy Accuracy and repeatability of new relief valve test equipment (High (High
Volume) exceeds the capabilities of the old equipment (low volume volume
test
unit). A three (3) inch test bench port and accumulator now provides provides
the
volume to control relief valve lift at set point more accurately versus the
previous reciprocating pump test bench and 3/8 inch tube. The low low
volume
test bench could in some instances only “burp burp” the valve before dropping
below the lift point. Determination of manufacturer’s manufacturer s set points (i.e. first
steady stream) was subjective, susceptible to inconsistencies and and
sometimes
not achievable”.
achievable
– Gary Caudill with VC Summer is also a strong proponent of high volume volume
testing:
Has seen improved results by using his high volume test bench (3 inch
diameter piping along with a 4 cubic foot Stainless Steel test vessel) vessel)
when
compared to the previous method of testing with a low volume test test
bench.

PRV Seat Alignment
One of the primary issues with performance within a PRV design is is
Seat alignment
Seat and Disc alignment is critical for PRV performance
OEMs routinely employ radial surfaces called “bearing bearing points” points
Allows for movement between the disc and the disc holder
(commonly referred to as “disc disc rock”) rock
Following a bench test or system pressure excursion, movement
allows disc to re-align re align with the nozzle upon reseat forming a tight
seal
Low Volume Test Benches do not generate the lifting forces
required to permit the disc to realign upon closure
Consequently result in a leaking PRV
PRV may have come out of the system tight, but as a result of a low
volume test bench, the “as as found” found condition is “leaking leaking below set
point”
point

PRV Seat Alignment
Alton Cox (NBIC Sub-Group Sub Group Pressure Relief
Devices) recently participated in a liquid test
with an ASME Consultant for a “UV UV” Certificate
Holder which highlights the importance of
volume in regards to PRV seat alignment.
– Alton stated that they “had had to gush a liquid PRV …. . to
get it to stop leaking. We had 31 drops per minute
(allowable was 2.5 dpm) dpm)
after three successful lifts.
Then we gushed the valve (sent into overpressure)
and the PRV had zero drops after that.”
that.

Air Evacuation
The “Air Air Bubble” Bubble trapped in the nozzle between the disc and the
water
Causes the first Test to give an inaccurate reading
The trapped air beneath the seat of a PRV installed on a test bench bench
skews the first “As As Found” Found test
Goal: Utilize the first pop as an actual documented Set Point
Reading
Eliminate the Air pocket
Compressed Air generates a “burp burp” of compressible fluid just ahead
of the liquid which follows it out of the PRV (Pre Conditioning)
Has the effect of yielding something other than a steady stream as
required by PTC 25
“As As Found” Found set point may be different from expected

Liquid Test –
Trapped Air in PRV Nozzle
Liquid Test
Media
Test Plate
Air compressed by
liquid entering flow path
will exhibit “pop” action
on PRV initial lift

Air Evacuation
There are a couple methods commonly used to evacuate
the compressed air out of the PRV prior to initiating the
test.
– One of these methods is to literally rotate the entire valve and
clamping station 180 degree’s degree s in order to allow the air to escape,
sometimes referred to as pre-conditioning pre conditioning the valve.
This method is effective but can be tedious and time consuming.
– Another method commonly used is to implement a test plate
adapter with a bleed valve that is placed in between the test
plate itself and the test valve. One then would pressurize the
system with the bleed valve open until all of the air is evacuated. evacuated.
After all air is evacuated one would then close the bleed valve
and initiate the test.
– DeLuca incorporates a standard air evacuation tube design
developed by Gary Caudill at VC Summer

Method 3:
Air Evacuation
Incorporates a standard Air Evacuation
tube design
Completely eliminating the air pocket
Saves time
Validates the first “As As Found” Found test
*Gary Caudill developed idea of the Air Evacuation method currently
used by DeLuca to validate the “As Found” Set Pressure Test

J-Tube
Vent
Water
Fill
Valve
NITROGEN
SUPPLY
WATER
SUPPLY
Liquid Test Air Evacuation
Water When Open Close Install Air J-Tube water fills Water Test Evacuation test flows PRV, Vent Fill connection Valve, out Clamp and Valve, tube, Water Open in and remove Insert Position Fill forces Bypass Valve Plastic tube, air Valve out close Tube tubeair
evac valve
Green Arrow OPEN
Red Arrow CLOSE
INSERT REMOVE ¼”
PLASTIC PLASTIC
TUBE TUBE
Air Evac
Valve

Conclusion
The National Board Inspection Code along with The
ASME PTC 25-2001 25 2001 provide input and recommendations
in regards to bench testing Pressure Relief Valves and
gives a basic guide on test bench layout and
characteristics. Although these recommendations
provide a base to follow they can still be open to
interpretation. These interpretations will be evident in
the differences in both the design’s design s and capabilities of
PRV test benches found throughout the industry.
PRV test bench users should be at least aware of these
differences and the impact they could have on the
accuracy of their set point results.

Acknowledgements
Richard Booth, Vermont Yankee
Gary Caudill, V.C. Summer
J. Alton Cox, Vice Chairman, NBIC Sub- Sub
Group Pressure Relief Devices
Curt Dupill, Dupill Group / DeLuca Test
Equipment
Dean Mathews, DeLuca Test Equipment

Questions

“Our Pressure Point is in the Volume”
Deluca Test Equipment
Test & Reconditioning Equipment for
Industrial valves
Tel: 724-468-3271 Fax: 724-468-5896
Email: solutions@dupillgroup.com
Internet: www.delucatest.com