Parr Instrument Pressure Relief Valves User Manual
Pressure relief valves operating instructions, 323m, Figure 1

P a r r I n s t r u m e n t C o m p a n y
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Pressure Relief Valves
Operating Instructions
323M
Supplemental Protection
Although a safety rupture disc offers excellent protec-
tion against overpressure, it may be desirable in some
installations to supplement the rupture disc with a
spring-loaded relief valve, pre-set to open at a pres-
sure below the burst rating of the disk. Unlike rup-
ture discs, relief valves can be slightly higher that the
routine operating pressure and, since the valve will
reseal once the pressure falls below the relief setting,
valuable reactants will often be saved by this protec-
tive feature. Also, adding a relief valve will provide
positive protection for the pressure gage when using
gages less than 1000 psi since small diameter rupture
discs are not reliable at these lower pressures.
Two Different Styles
Parr spring-loaded relief valves are offered in two dif-
ferent styles, both with 1/4” NPT mounting threads.
They are commonly installed in the side port of the
gage adapter or by adding a 155VB street tee below
the pressure gage and attaching the relief valve to the
side opening on the tee, but other arrangements can
be used as well.
A140VB2 Values for Low Pressures
A140VB2 relief valves are offered in five ranges for
use at low pressures up to 600 psi. Since small diam-
eter rupture discs are not reliable at pressures below
1000 psi, adding a pre-set A140VB2 valve to a system
will protect low range pressure gages from acciden-
tal overpressure. The cracking pressure for A140VB2
valves is adjustable within the range prescribed for
each valve. To adjust the spring tension on the valve,
remove the green cap from the assembly. Remover
the outer locking screw with 5/32” Allen wrench.
Next, turn the inner 5/32” adjuster screw clockwise to
increase the cracking pressure, or counterclockwise
to lessen the cracking pressure. Replace the outer
locking screw until it comes in contact with the inner
screw. Place the green cap back onto the assembly.
A175VB Valves for High Pressures
A175VB relief valves are offered in multiple ranges
for pressures 50-5000 psi. When the upstream pres-
sure overcomes the force exerted by the spring in
these valves the poppet will open, allowing discharge
through a 1/4” NPT female opening to which extension
piping can be attached.
The cracking pressure in these valves is adjustable and
can be set by the user. To increase the cracking pres-
sure: release the lock nut under the adjustment cap
clockwise to increase the compression on the spring.
Figure 1
A140VB2
Figure 2
A175VB
Part No.
Preset Psi
Adj. Psi
Bar
A140VB2PA
100
50-150
3.45-10.3
A140VB2P2
145
50-150
3.45-10.3
A140VB2P1
150
50-150
3.45-10.3
A140VB2PB
200
150-350
10.3-24.1
A140VB2PC
600
350-600
24.1-41.4
A140VB2PD
300
150-350
10.3-24.1
Part No.
Adjustable Psi
Bar
A175VB
750-1500
52-103
A175VB2
1500-2250
103-155
A175VB3
2250-3000
155-207
A175VB4
3000-4000
207-276
A175VB5
4000-5000
276-345
A175VB7
350-750
24-52
A175VB8
50-350
3.4-24.1