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Ammonia Refrigeration Relief Valve Calculator mechanical
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Ammonia Refrigeration Relief Valve Calculator

Size safety relief valves for industrial ammonia (R-717) pressure vessels to determine minimum required air discharge capacity per ANSI/IIAR 2 and ASHRAE 15.

Vessel Geometry & Dimensions

Outside diameter in feet (e.g. 3.5 ft = 42 in)
Tangent-to-tangent or overall length

PRV Set Pressure & Header

Vessel Design Pressure MAWP

Required Relief Discharge Capacity

Minimum Required Air Capacity (C)
-- lb/min air
-- SCFM Air Equivalent
Ammonia Mass Flow
-- lb/min NH3
-- kg/min
Recommended Valve Size
-- Orifice
Each valve rating
Formula: C = f · D · L : --
Vessel Internal Gross Volume: -- ft³ (-- gallons)
Max Allowable Backpressure (15%): -- psig
ANSI/IIAR 2 requires vessels with gross volume exceeding 10 ft³ to be protected by dual relief valves on a three-way manifold valve.

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Frequently Asked Questions

What is the C = f·D·L formula in ANSI/IIAR 2 and ASHRAE 15?

The formula calculates the minimum required discharge capacity of a pressure relief valve in pounds of air per minute (C) protecting a refrigeration pressure vessel: f is the refrigerant factor (0.5 for ammonia R-717), D is the outside diameter of the vessel in feet, and L is the overall length of the vessel in feet.

Why does IIAR 2 require dual relief valves on vessels over 10 cubic feet?

Industrial ammonia refrigeration systems are closed pressurized circuits that cannot be easily de-pressurized without complex pump-out procedures. A dual relief valve assembly connected to a three-way changeover valve ensures one relief valve is always operational while the second valve can be safely isolated, tested, or replaced without releasing toxic ammonia.

What is the 15% backpressure limit on refrigeration relief valves?

Conventional spring-loaded pressure relief valves suffer reduced lift and reduced relieving flow when discharge backpressure builds up in the relief header. ASHRAE 15 and IIAR 2 mandate that total developed backpressure in common relief vent headers must not exceed 15% of the lowest PRV set pressure.