Page 475 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 441

Kb = 1.0 (Figure 7-26), constant backpressure with varia- capacity certified). A simplified equation based on the
tion not to exceed 10% of set pressure. ASME Pressure Vessel Code equations, Section VIII, Div.
a. for vapors and gases, in lb/hr; Kb = 1.0; "C" from 1, Mandatory Appendix XI uses K coefficient of discharge
Figure 7-25, P is relieving pressure absolute, psia in the equations, where K is defined as 90% of the average
K,i of certified tests with compressible or incompressible
fluids, see reference [68], pg 40.
vV) TZ .
A= , sq1n. For first trial, assume Ku for viscosity = 1.0
CKd PKb ,JM
( Effective net discharge area) (7 - 10) For final calculation use Ku from Figures 7-23 or 7-24
and substitute in the above equation. Determine the
b, For vapors and gases, in SCFM, Kb = 1.0
needed Reynolds number, Re, using the next size larger
orifice. Area is determined from that made in the first
v ,Jeri, . trial calculation [33].
J\ = , sq in. (7 - 11)
1.175 CKd PKb
(2800 G)
c. For steam, in lb/hr; Ki, = 1.0 and Ksh = 1.0 for satu- Re = V L t=: , or (7-16)
rated steam when backpressure is below 55% of µ-v A
absolute relieving pressure
Re= 12, 700VL/( u-...!A).
(Do not use when U < 100 SUS) (7- 17)
w
;\=----'----, sqin. (7- 12)
51.5 x , PKb K,h Kn
Re = Reynolds number

d. For air, in SCFM; I(i, = 1.0, when backpressure is µ = absolute viscosity@ flowing temperature, centipoise
below 55% of absolute relieving pressure P1 = set pressure, psig
U =viscosity@ flowing temperature, Saybolt Universal Sec-
Va ,iT onds (See Appendix A-12 and A-13)
A= (7 -13)
418 Kd PK 0 P 2 = total backpressure, psig
e. For liquids, GPM;
K,, = 1.0 @ 10% overpressure Calculations of Orifice Flow Area using Pressure Relieving
K,, = J _O at normal viscosities Balanced Bellows Valves, with Variable or Constant Back Pres-
LiP = P1 - P2 = upstream pressure, psig (set + overpres- sure. Must be used when backpressure variation exceeds
st.re) -- total backpressure, psig 10% of the set pressure of the valve. Flow may be critical
or non-critical for balanced valves. All orifice areas, A, in
ASME Code valves: Board Certified for liquids only. sq in. [68]. The sizing procedure is the same as for con-
ventional valves listed above (Equations 7-10 ff), but uses
VL fa equations given below incorporating the correction fac-
A=-------;:=== (7 - 14) tors K, and Kw. With variable backpressure, use maximum
value for P2 [33a, 68].

f. Non-ASME Code Liquid Valves [33a] non-board cer-
tified for liquids, but code acceptable for other ser- a. For vapors or gases, lb/hr
vices. KP from Figure 7-22, K,- 1 = 0.62, and 25% over-
pressure.
\V _! TZ .
A= , sq 111. (7-18)
CKct PK_. ,/ M
VL {c
A=----------- (7 - 15)
38 Kc1 K., Kr Ku � l.25P 1 -P 2 b. For vapors or gases, SCFM
To apply the viscosity correction K," a preliminary or
trial calculation should be made for the areas required
using the equation of paragraph (e) above or the modi- A= V ,/ GTZ , sq in. (7-19)
fied equation (still ASME conformance [33] but not 1-175 CKd PK,