Page 465 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 431

Retraction Bushing Index Shaft

Lessthan3%
Connection and pressure loss
Bleed Port for at rated flow
standbySRV

Spare

Isolation Disc
(Soft Seated)

(By others)
Figure 7-19 cont.

installed with the internals in the proper position. Discharge piping must be sized for low pressure drop
No special tools are necessary for switching. at maximum flow not only from any one valve, but for the
combined flow possibilities in the discharge collection
The alternate concept which has been in use for many manifold all the way to the vent release point, whether it
years is to fabricate or purchase a Tee connection upon be a Oare, incinerator, absorber or other arrangement
which the two safety relief valves can be mounted on top of [ 13). See Figures 7-20 illustrations.
their full-port plug or gate valve with required locking lugs. Conventional safety relief valves, as usually installed,
Rupture disks are often used in conjunction with safe- produce unsatisfactory performance when variable back
ty valves as shown in Figures 7-10, 7-l l, 7-12, and 7-18. pressure exists [10, 33). See Figure 7-6. The same variable
back pressure forces affect the set pressure release also. At
Inlet piping is held to a minimum, with the safety low back pressures, the valve flow falls rapidly as com-
device preferably mounted directly on the equipment and pared with the flow for a theoretical nozzle. See Figures
with the total system pressure drop loss to pressure relief 19 and 20 in Ref. [33a].
valve inlet not exceeding 3% of the set pressure in psig, of For conventional valves, pressure drop or variations in
maximum relief flowing conditions [JO]. To conform to back pressure should not exceed 10% of set pressure.
code (see ASME code, Sect, VIII, Div. l-UG-127 [l]) avoid Because most process safety valves are sized for critical
high inlet pressure drop and possible valve chatter:
pressure conditions, the piping must accommodate the
capacity required for valve relief and not have the pres-
1. Never make pipe connection smaller than valve or sure at the end of vent or manifold exceed the critical
disk inlet pressure. Designing for pressure 30% to 40% of critical
with balanced valves, yields smaller pipes yet allows prop-
2. Keep friction pressure drop very low, not over I to 2 er functioning of the valve. The discharge line size must
percent of allowable pressure for capacity relief [l, not be smaller than the valve discharge. Check the manu-
10, 25, 28, 33].
facturer for valve performance under particular condi-
3. Velocity head loss should be low, not over 2% of tions, especially with balanced valves which can handle up
allowable pressure for capacity relief [25]. to 70% to 80% of set pressure as back pressure.