Page 471 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 437

Before performing any calculations, a thorough exami- Sizing Safety Relief Type Devices for Required Flow
nation of the possible causes and flow conditions of tem- Area at Time of Relief
perature and pressure should be evaluated. From this list,
select the most probable and perhaps the worst case possi- Before initiating any calculations, it is necessary to
bility and establish it as a design basis, Figure 7-14. See [80]. establish the general category of the pressure relief
valve being considered. This section covers conven-
When the possibilities of internal explosion or run- tional and balanced spring-loaded types.
away chemical reaction exists, or are even suspected, they Given the rate of fluid flow to be relieved, the usual
must also be rigorously examined and calculations per- procedure is to first calculate the minimum area
formed to establish the magnitude of the flow, pressure, required in the valve orifice for the conditions con-
and temperature problems. Select the worst condition tained in one of the following equations. In the case
and plan to provide for its proper release to prevent rup- of steam, air or water, the selection of an orifice may
ture of equipment. This latter situation can only be han- be made directly from the capacity tables if so desired.
dled by application of rupture disks and/ or remote sens- In either case, the second step is to select the specif-
ing and predetermined rupture of the disks (see Figures ic type of valve that meets the pressure and temper-
7-5A, 7-8K and 7-8L) or remote sensing and application of ature requirements.
quenching of the reaction/ developing explosive condi- General equations are given first to identify the
tion by automatic process action and/ or commercial
application of quenching medium. See later discussion basic terms which correlate with ASME Pressure Ves-
sel Code, Section VIII.
under Explosions.
It is recommended that computations of relieving
loads avoid cascading of safety factors or multiple
Standard Pressure Relief Valves Relief Area Discharge contingencies beyond the reasonable flow required
Openings Lo protect the pressure vessel."
*Extracted by permission from Teledyne-Farris Engineering Catalog.
The "orifice" area of these devices (see illustrations) is
at the outlet end of the SRV nozzle through which the dis- Effects of Two-Phase Vapor-Liquid Mixture on Relief
charging vapor/ gases/liquids must pass. These values are Valve Capacity
identified in industry as: (valve body inlet size in.) X (ori-
fice letter) X (valve body outlet size, in.). For example, a Many process systems when at conditions for safety
valve would be designated 3E4. relief valve discharge also are not single phase of all liquid
(through the valve) or all vapor, but a mixture either
inside the "containing" vessel or quite often as the fluid
The standard orifice area designations are (also refer
to mechanical illustrations of valves, previously shown this passes through the valve orifice and the liquid flashes to
chapter): partial vapor, or the flashing starts just ahead of the ori-
fice. Here a mixture attempts to pass through the orifice,
and the size must be sufficient or a restriction will exist
Orifice letter D E F G H J and pressure will build up in the vessel clue to inadequate
Area, sq. in. 0.11 0.196 0.307 0.503 0.785 1.287 relief. This problem was of considerable concern to the
Design Institute for Emergency Relief of the American
Orifice letter K L M N p Q Institute of Chemical Engineers during their studies [67].
Area, sq. in. l.838 2.853 3.600 4.340 6.380 11.05 As a result, considerable research was performed leading
Orifice ietter R T V* w W2* X* to design techniques to handle this problem. The details
Area, sq. in. 16.0 26.0 42.19 57.26 93.6 101.8 are more than can be presented here; therefore, the
60.75 designer is referred to the references in the bibliography
of this chapter. Also see Leung [77] for detailed proce-
Orifice letter Y* Z* Z2* AA BB BB2 dure and additional references.
Area, sq. in. 128.8 159.0 ------ The API-RP-520 [10] recommends caiculating the
82.68 90.95 108.86 136.69 168.74 185.00 amount of vapor flashed and the amount of residual liq-
uid (unflashed) and then sizing valve orifices for each
*Note: These lcuers and orifice areas are not consistent for these large condition. Select a valve (s) area that has a total area at
orifices between various manufacturers. Some sizes go to 185 sq in.,
which is a very large valve. When two values are shown, they represent least equal to the sum of these two areas. Before settling
two different published values by manufacturers. for this approach, this author recommends examining