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Control systems Safety valves
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.
K d = Coefficient of discharge, 0.955 for steam, air and vapour service. K sh = Steam superheat correction factor. 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. Table 1 Ksh superheat correction factor
4.7 102
Set pressure psi g 15 20 40 60 80 100 120 140 160 180 200 220 240 250 Saturated steam temperature °F 250 259 287 308 324 338 350 361 371 380 388 395 403 406
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
TI-S13-25 CTLS Issue 12
Page 6 of 8
SV73 Cast Iron Safety Valve
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