Control systems Safety valves
4
Safety valve sizing information
Formulas: For steam service:
For organic fluid vaporizers - lb/h:
Where: A =
Required orifice area in square inches
P = Relieving pressure in pounds per square inch absolute = set pressure (psi g) + overpressure + 14.7 where the overpressure is 3% or 2 psi, whichever is the greater. P = 1.03 x set pressure + 14.7 or P = set pressure + 2 psi + 14.7. T = Inlet temperature, °F. Absolute ( °F plus 460).
= Required steam capacity in pounds per hour. = Required vapor capacity in pounds per hour.
s
=
Coefficient of discharge, 0.955 for steam, air and vapour service.
K d
K sh
=
Steam superheat correction factor. See Table 1 below.
M = Average molecular weight of vapour. C = Gas or vapour flow constant. Z = Compressibility factor corresponding to T and P. If this factor is not available, compressibility correction can be safely ignored using a value of Z = 1.0.
4.7 111
Table 1 K sh superheat correction factor
Set pressure psi g 15 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 Saturated steam temperature °F 250 259 287 308 324 338 350 361 371 380 388 395 403 409 416 422
Total steam temperature °F
280
1.00 1.00
300
1.00 1.00 1.00
320
1.00 1.00 1.00 1.00
340 360
0.99 0.99 1.00 1.00 1.00
0.99 0.99 0.99 0.99 1.00 1.00 1.00
380
0.98 0.98 0.99 0.99 0.99 1.00 1.00 1.00 1.00
400
0.98 0.98 0.98 0.98 0.99 0.99 0.99 1.00 1.00 1.00 1.00 1.00
420
0.97 0.97 0.97 0.97 0.98 0.98 0.98 0.99 0.99 0.99 0.99 1.00 1.00 1.00 1.00
440
0.96 0.96 0.96 0.96 0.97 0.97 0.97 0.98 0.98 0.98 0.99 0.99 0.99 0.99 1.00 1.00
460
0.95 0.95 0.95 0.95 0.96 0.96 0.96 0.96 0.97 0.97 0.97 0.98 0.98 0.98 0.99 0.99
Do not exceed the maximum operating temperature of 446 °F.
TI-S13-30 CTLS Issue 16
Page 7 of 9
SV74 Carbon Steel Safety Valve
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