Back to Basics Relief Valves

8/22/2019 Back to Basics Relief Valves

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Generically, a pressure relief valve (PRV) is a normally-closed spring-actuateddevice that automatically opens to relieve pressure from a system to prevent

overpressurization and catastrophic failure during abnormal or upset

conditions. When the overpressure situation recedes, the valve will again

seat.

The set pressure tolerances (plus or minus) of the relief valves shall not

exceed 2 psi for pressures up to and including 70 psi and 3% for pressures

above 70 psi.

A manufacturer that is certified by the National Board will place a UV stamp

on the valve body.


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In accordance with ASME Boiler and Pressure Vessel Code (Section VIII

Division 1), rupture or burst disks must break within 2 psi for disks rated to

30 psi and a tolerance of 5% is allowed for disks rated above 40 psi.

Rupture disks are also certified by the National Board. To determine whether

or not a manufacturers rupture disks are Board Certified, look for a UD

stamp on the device.


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Per ASME BPVC Section VIII Division 1, you need to de-rate the relief valve

capacity by 10% (i.e. a multiplier of 0.9) whenever a rupture disk assembly is

used upstream of a PRV. The ASME BPVC also requires that a gauge or other

sensing device (pressure gauge, try cock, free vent or other suitable sensing

device) be placed between the rupture disk and the PRV to provide

continuous indication as to whether or not the rupture disk has been

compromised.


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Dont leave the three-way valve in a middle position. During service, the

valve should be completely either front-seated or back-seated depending on

the relief valve being serviced. During normal operation, back-seat the valve

to minimize the likelihood of refrigerant leakage through the stem. In other

words, there should always only be one PRV active at any instant in time.


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The following is an excerpt from ASHRAE Standard 15-2010:

9.7.8.4 All pipe and fittings between the pressure-relief valve and

the parts of the system it protects shall have at least the area of the pressure-

relief valve inlet area.

Also required in 2010 ASME BPVC UG-135(b)(1).


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The following is an excerpt from ASHRAE Standard 15-2010:

9.7.8.4 The size of the discharge pipe from a pressure-relief device or

fusible plug shall not be less than the outlet size of the pressure-relief device

or fusible plug. Where outlets of two or more relief devices or fusible plugs

are connected to a common line or header, the effect of back pressure that

will be developed when more than one relief device or fusible plug operates

shall be considered. The sizing of the common discharge header downstream

from each of the two or more relief devices or fusible plugs that are expected

to operate simultaneously shall be based on the sum of their outlet areas

with due allowance for the pressure drop in all downstream sections.

Note that the standard says outlet size of the pressure relief device, the

above assumes that the pressure relief device is defined as the valve. One

could interpret that the outlet size would be the connected branch size. No

formal interpretations on this have been issued. Regardless of the

interpretation, the relief vent piping system design should result in a back

pressure at the valve outlet that is less than the maximum allowed by the

standard (15% of the set pressure for a conventional relief).


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Source: ANSI/ASHRAE 15-2010 Appendix E


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Back to Basics Relief Valves

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