355 IAC 3-8-5 Pressure relief valves

Sec. 5. (a) Pressure relief valves shall be set to start-to-discharge at a pressure not in excess of the design pressure of the tank and shall have a total relieving capacity sufficient to prevent a maximum pressure in a tank of more than one hundred twenty percent (120%) of the design pressure. (b) The total relieving capacity shall be the larger requirement of the following: (1) Possible refrigeration system upset, such as the following: (A) Cooling water failure. (B) Power failure. (C) Instrument air or instrument failure. (D) Mechanical failure of any equipment. (E) Excessive pumping rates. (2) Either of the following formulas for fire exposure: (A) For valve manufacturers who use weight of vapors to be relieved as a basis for classifying valves: (B) For valve manufacturers that classify valves on the basis of air flow: Where: W = Weight of vapors to be relieved in pounds per hour at relieving conditions. Qa = Air flow in cubic feet per minute at standard conditions (sixty (60) degrees Fahrenheit and fourteen and seven-tenths (14.7) psia). F = Fireproofing credit. Use F = 1.0 except when an approved fireproofing material of recommended thickness is used, then use F = 0.2. A = Total surface area in square feet up to twenty-five (25) feet above grade or to the equator of a sphere, whichever is greater. Z = Compressibility factor of ammonia at relieving conditions (if not known, use Z = 1.0). T = Temperature in degrees R (460 + temperature in degrees F of gas at relieving conditions). M = Molecular weight = 17 for ammonia. L = Latent heat of ammonia at relieving conditions. C = Constant based on relation of specific heats. (C may be obtained from the following table.) (If K is not known, use C = 315.) K C K C K C 1.00 315 1.26 343 1.52 366 1.02 318 1.28 345 1.54 368 1.04 320 1.30 347 1.56 369 1.06 322 1.32 349 1.58 371 1.08 324 1.34 351 1.60 372 1.10 327 1.36 352 1.62 374 1.12 329 1.38 354 1.64 376 1.14 331 1.40 356 1.66 377 1.16 333 1.42 358 1.68 379 1.18 335 1.44 359 1.70 380 1.20 337 1.46 361 2.00 400 1.22 339 1.48 363 2.20 412 1.24 341 1.50 364 Where: K = CP = Specific heat of vapor at constant pressure. Cv = Specific heat of vapor at constant volume. If the relieving capacity required for fire exposure is greater than that required by subdivision (1), the additional capacity may be provided by weak roof to shell seams in tanks operating at essentially atmospheric pressure and having an inherently weak roof to shell seam. The weak roof to shell seam is not to be considered as providing any of the capacity required by subdivision (1). (c) All pressure relief devices shall comply with the following: (1) The discharge from pressure relief valves shall be vented away from the tank at any desired angle above the horizontal using a vent stack suitably designed for weather protection. The size of discharge lines from pressure relief valves shall not be smaller than the nominal size of relief valve outlet connections. Suitable provisions shall be made for draining condensate that may accumulate. (2) Discharge lines from two (2) or more pressure relief devices located on the same unit may be run into a common discharge header. Where pressure relief valves from two (2) or more units vent into the same discharge header, the header shall be designed for maximum back pressure of ten percent (10%) of the lowest set pressure for conventional pressure relief valves and fifty percent (50%) of the lowest set pressure for balanced valves when the maximum possible quantity of gas is flowing in the header.