Refrigeration Pressure-Relief Discharge Capacity (ASHRAE 15)

The relieving capacity a refrigerant pressure vessel requires, and whether the valve and its discharge piping together actually deliver it. ASHRAE 15 and IIAR write the requirement as C = f x D x L in pounds per minute of air, with D the vessel diameter and L the length in feet and f a constant that depends on the refrigerant. The D x L term is the vessel's external surface in disguise, because the governing case is a fire heating the shell and boiling the contents -- which is why a long thin vessel and a short fat one of the same VOLUME need different relief, and why relief sizing never asks how much refrigerant is inside. The constant f carries the refrigerant's latent heat and vapor properties, so ammonia, R-22, and CO2 give different answers for identical vessels; it is entered from the standard's own table rather than assumed here. The second half is the discharge piping, and it fails more often than the valve does. A relief valve's rated capacity is achievable only if the downstream piping does not build enough back pressure to choke it, and on a plant where several reliefs share a header the equivalent-length arithmetic decides whether the valve can pass what it is stamped for. A correctly sized valve on undersized discharge piping is an undersized relief system -- with a valve that is stamped correctly, inspected annually, and still will not do its job. That check belongs in the sizing rather than after it, which is why both verdicts are reported and the system passes only if both do. The maximum equivalent length is ENTERED from the valve manufacturer's published table at the set pressure. This does not select the valve, size the header for simultaneous relief of several vessels, compute back pressure from first principles, or address the hydrostatic relief a liquid-full line needs. ASHRAE 15, IIAR 2, the applicable pressure-vessel code, and the valve manufacturer govern.

required capacity C = f x D x L in lb/min of air, with D the vessel diameter and L the length in feet; the margin is the entered valve rating over that; the discharge equivalent length is the straight run plus the fittings, compared against the maximum the valve supports.

The required relieving capacity as ANSI/ASHRAE 15 and IIAR 2 write it, the D x L product standing for the shell area the fire case heats, with f taken from the standard's own table and ENTERED. The maximum discharge equivalent length the valve supports at its set pressure is ENTERED from the valve manufacturer's published table. It does not select the valve, size a common header for simultaneous relief, compute back pressure from first principles, or address hydrostatic relief.

One product and one length comparison; no table from the standard is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: vessel_diameter_ft, vessel_length_ft, f_constant, valve_rated_lb_min, pipe_straight_length_ft, fitting_equivalent_length_ft, max_allowable_equivalent_length_ft, dl_product_ft2, required_lb_min, margin_ratio, equivalent_length_ft, length_margin_ft

• f is entered from the standard's table it carries the refrigerant's latent heat and vapor properties, so ammonia, R-22 and CO2 differANSI/ASHRAE 15
• The maximum discharge length is entered it comes from the valve manufacturer's table at the set pressurethe valve manufacturer
• It does not size a common header simultaneous relief of several vessels is a separate analysisIIAR 2 and the pressure-vessel code