Page 483 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 449

VARIABLE OR CONSTANT BACK PRESSURE SIZING FACTOR
Kw

Cl. - - --
·-
<I
z ; I•
0 1.00
0
LL.I
en .90 '.
a.; c( ' .
. (I) - '; -
ID . ........ _
>-
:Z:t- . 80
1-- J ·- ---··
-·-
-u
�� .70 EXAMPLE: - -- ·-
-
>- c( SET PRESSURE= 100 PSIG '.
1-u BACK PRESSURE= ZERO TO 40 PSIG ' . '
iJ:::) . 60 B.P.= � XI00=40% . '.
�w %GAUGE MAX . - - - . '
c( .... FOLLOW DOTTED LINE. Kw=0.88 (FR:>M CURVE)
u� .50 CAPACITY WITH B.P. =0.88 X RATED CAPACIT:t' ---
II . (FOR 10 % OVERPRESSURE, NON-CODE VALVES, MULTIPY BY Kp FACTOR 0.6 IN CAPACITY FORMULA)
� � : -r-:-,
.40 I I I I I I
'
0 10 20 30 40 50 60 70 80 90 100
% GAUGE BACK PRESSURE= BACK PRESSURE, p SIG X 100
SET PRESSURE,PSIG
Figure 7-28. Variable or constant backpressure sizing factor, Kw, for liquids only, BalanSeal® valves. Use this factor as a divisor to results of
constant backpressure equations or tables. By permission, Teledyne Farris Engineering Co.
Note: Reference of the A.PI gravity values to refinery /ZT(Sp
_
.
and petrochemical plant fluids will show that they corre- or, 1 \ - V Gr) , sq m. (7 - 27)
863. 63 F 2 Kd \ P(P - P 2)
spond to many common hydrocarbons.
Sizing Valves for Subcriiical Flow: Gas or Vap01; but not Steam Note: K,i = effective coefficient of discharge for valve= 0.975
[33AI for equations above and Equations 7-25, 26, 27
If the ratio of backpressure to inlet pressure to valve When using a balanced/bellows relief valve in the sub-
exceeds the critical pressure ratio, Pc/P 1, critical, use Equations 7-18 through 7-22; however, the
backpressure correction factor for this condition should
be supplied by the valve manufacturer [33A). For sub-crit-
ical, conventional valve:
(7 - 7)

/( . (r)(k-lJ/k]
J
k
the flow through the valve is subcritical. The required Fo = -- (r)2!k [ J - (7 - 28)
area (net, free unobstructed) is calculated for a conven- · � k-1 1-r
tional relief valve, including sizing a pilot-operated relief
valve [33A, Par 4.3.3): where: A = required effective discharge area, sq in.
F 2 = coefficient of subcritical flow, see Figure 7-29
0
T = relieving temperature of inlet gas or vapor, R
P = upstream relieving pressure, psia, = set pres-
(7 - 25)
sure + allowable overpressure + atmospheric
pressure, usually 14. 7 psia), psia
P 2 = backpressure on valve, psia
I W = required flow through valve, lbs/hr
or, A = V / ZTM sq in (7 - 26)
4645.2I�K<l V P(P-P 2) 1 • V = vapor flow required through valve, standard
cu ft/min at 14.7 psia and 60°F