Page 495 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 461

Table 7-10 Q. = required flow, cu ft/min at standard conditions of I 4.7
continued psia and 6Q°F, SCFM

Example Actual flowing conditions, ACFM
'What is rupture pressure at 500CF of a nickel disk rated 300 psi
at 72cF?
1. Consult temperature conversion table. Correction 5.596 Q, 'M
I-
factor for nickel disk at 500°F is 86%. A= CKd " V TZ (7 - 39)
2. Multiply disk rating at 72°F by correction factor:
300 x 0.86 = 258. C2.A = required flow, cu ft/min at actual conditions, ACFM
Rupture pressure of a nickel disk rated 300 psi at 72°F
is therefore 258 psi at 500°F. Stearn: Rupture disk sonic flow; critical pressure = 0.55 and
If you require a disk for a specific pressure at elevated P 2/P 1 is less than critical pressure ratio of 0.55.
or cold temperature and want to determine if it is a stan- API reference [33a] [69] dry and saturated steam,
dard disk, convert the required pressure at elevated or pressure up to 1500 psig:
cold temperature to pressure at 72°F.

w .
A = _ • , sq m. (7 - 40)
P2 = backpressure or exit pressure, psia si.s s , Pb Kn K,h
P 1 = upsu·eam relieving pressure, psia
where W = flow, lbs/hr
For sonic flow conditions [69):
K.:i = coefficient of discharge = 0.62.
K,, = correction for Napier equation = 1.0 when P 1 �
(7-10) 1515 psig = (0.1906P1 - 1000)/ (0.2292P1 - 1061)
where P1 > 1515 psia and S 3215 psia, Table 7-6.
K. 11 = superheat correction factor, see Table 7-7. For sat-
where A = minimum net required flow discharge area after urated steam al any pressure, Rs1, = 1.0
complete burst of disk, sq in. Pb = stamped bursting pressure, psia
C = sonic flow constant for gas or vapor based on ratio
of specific heats, k, Figure 7-25, when k is not known AP! for subsonic flow: gas or vapor (not steam)
use k = 1.001, or C = 315
W = required flow, lb/hr For rupture disks, pressure ratio is greater than critical
M = molecular weight pressure; mass flow: pounds/hr
K.:i = coefficient of discharge, K = 0.62 for rupture
disks, except some coefficients are different. For PjP > critical pressure ratio [2/(k t l)Jk/k - 1
example, the Zook graphite standard ASME disks
when tested mono-style, Figures 7-9B and 7-13A
have a K0 of 0.888, and when inverted, Figure 7-
l 3B have a K0 of 0. 779. Consult manufacturer for ( 7 - 41)
special disks.
0
T = flowing relieving temperature, °F + 460 = R
absolute Where C2 = subsonic flow conditions based on ratio of specific
M = molecular weight of flowing fluid heats (See Table 7-11 and equation for C 2)
Z = compressibility factor for deviation from perfect
gas if known, otherwise use Z = 1 .0 for pressures Volumetric flow, SCFM conditions
below 250 psia, at inlet conditions.
Pb = stamped bursting pressure plus overpressure
allowance (ASME l 0% or 3 psi whichever is (7 - 42)
greater) plus atmospheric pressure ( 14. 7), psi a
Volumetric flow: SCFM standard conditions (14.7 psia
and 60°F) Actual flowing conditions, ACFM

Q, (MTZ) 112 .
A= -, so 111. (7- 38) (7- 43)
6.326 CKd Pb '