Page 211 - Valve Selection Handbook

198 Valve Selection Handbook

Secondary Back Pressure from Flow-Through Valve Body

A secondary back pressure develops from the flow-through valve body
between valve seat and valve outlet flange. For sonic flow in the valve
46
outlet branch, this back pressure is intrinsic to the valve.
The back pressure arising from this flow causes the lifting force to
decay in the same manner as back pressure that is present in the outlet
pipe. If the secondary back pressure is high enough, the valve may not be
able to achieve its rated capacity.
The back pressure that can develop in the valve body is related to the
area ratio of valve outlet size to nozzle size. For pressure relief valves of
API Std. 526, this area ratio varies considerably between valve sizes as
shown in Table 5-1. For example, the area ratio of valve size VA D 2M is
about 14 times larger than the area ratio of the valve 8T10. A convention-
al safety relief valve of size 8T10 used on steam has been reported to not
achieve its rated capacity at a set higher than 50 psig (3.5 barg), even
with no outlet piping. At this pressure level, the size of the valve ought to
be 8T12. Thus, when applying pressure relief valves with a low area ratio
of valve outlet size to nozzle size, caution should be applied. Consult
valve manufacturer when in doubt.

Recommended maximum non-recoverable pressure loss in piping
between protected equipment and inlet of direct-acting pressure
1 2
relief valves. ' The following recommendation has been universally
accepted:

• The non-recoverable pressure loss in the pipeline between the protected
equipment and the direct-acting pressure relief valves should not
exceed 3% of the set pressure of the valve.
• When the pressure relief valve is installed on a process line, the 3%
limit should be applied to the loss in the normally nonflowing pressure
relief valve inlet pipe and the incremental pressure loss in the process
line caused by the flow through the pressure relief valve.
• In both cases, the pressure loss should be calculated using the rated
capacity of the valve at an overpressure of 10% (see footnote on page
199).

1
Refer to API RP 520 Part II. Fourth Edition, paragraph 2.2.2.
2
For permissible pressure loss in inlet line to pilot-operated pressure relief valves with
direct-acting pilot, refer to "Stable operation of valves with on/off pilots" on page 209.