Liquid System Relief Valves and Spring Style Relief Valves:
A = (GPM x (G)1/2 ) / [28.14 x KB x KV x ( ▲P)1/2 ]
Liquid System Relief Valves and Pilot Operated Relief Valves:
A = (GPM x (G)1/2 ) / [36.81 x KV x (▲P)1/2 ]
Steam System Relief Valves:
A = W / (51.5 x K x P x KSH x KN x KB )
Gas and Vapor System Relief Valves (Lb/Hr.):
A = (W x (TZ)1/2 ) / [C x K x P x KB x (M)1/2 ]
Gas and Vapor System Relief Valves (SCFM):
A = (SCFM x (TGZ)1/2 ) / (1.175 x C x K x P x KB )
Where
• A = Minimum Required Effective Relief Valve Discharge Area (Sq- inches)
• GPM = Required Relieving Capacity at Flow Conditions (Gallons per Minute)
• W = Required Relieving Capacity at Flow Conditions (Lbs / hr)
• SCFM = Required Relieving Capacity at Flow Conditions (Standard Cubic Feet per Minute)
• G = Specific Gravity of Liquid, Gas, or Vapor at Flow Conditions Water = 1.0 for most HVAC applications; Air = 1.0
• C = Coefficient Determined from Expression of Ratio of Specific Heats; C = 315 if Value is Unknown
• K = Effective Coefficient of Discharge; K = 0.975
• KB = Capacity Correction Factor Due to Back Pressure; KB = 1.0 for Atmospheric Discharge Systems
• KV = Flow Correction Factor Due to Viscosity; KV = 0.9 to 1.0 for most HVAC Applications with Water
• KN = Capacity Correction Factor for Dry Saturated Steam at Set Pressures above 1500 Psia and up to 3200 Psia; KN = 1.0 for most HVAC Applications
• KSH = Capacity Correction Factor Due to the Degree of Superheat; KSH = 1.0 for Saturated Steam
• Z = Compressibility Factor; Z = 1.0 If Value is Unknown
• P = Relieving Pressure (Psia); P = Set Pressure (Psig) + Over Pressure (10% Psig) + Atmospheric Pressure (14.7 Psia)
• P = Differential Pressure (Psig); P = Set Pressure (Psig) + Over Pressure (10% Psig) - Back Pressure (Psig)
• T = Absolute Temperature (°R = °F. + 460)
• M = Molecular Weight of the Gas or Vapor
Notes:
1) When multiple relief valves are used, one valve shall be set at or below the maximum allowable working pressure, and the remaining valves may be set up to 5 percent over the maximum allowable working pressure.
2) When sizing multiple relief valves, the total area required is calculated on an overpressure of 16 percent or 4 Psi, whichever is greater.
3) For superheated steam, the correction factor values listed below may be used:
• Superheat up to 400 °F: 0.97 (Range 0.979–0.998)
• Superheat up to 450 °F: 0.95 (Range 0.957–0.977)
• Superheat up to 500 °F: 0.93 (Range 0.930–0.968)
Relief Valve Vent Line Maximum Length
L = (9 x P21 x D5) / C2 = (9 x P22 x D5) / (16 x C2)
Where
• P1 = 0.25 x [(PRESSURE SETTING x 1.1) + 14.7]
• P2 = [(PRESSURE SETTING x 1.1) + 14.7]
• L = Maximum Length of Relief Vent Line (Feet)
• D = Inside Diameter of Pipe (Inches)
• C = Minimum Discharge of Air (Lbs/Min)
