9.3 Safety Relief Valves and Charging

Pressure Relief Valves (PRVs): Code Requirements and Operation

Ammonia refrigeration systems operate under high pressures and contain anhydrous ammonia, which is toxic and mildly flammable (safety classification B2L). To prevent catastrophic vessel failures from overpressurization due to external fires, thermal expansion of trapped liquid, or control failures, pressure relief valves (PRVs) must be installed on all pressure vessels.

Governing Codes

The design, construction, and certification of pressure relief valves are governed by the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1. In addition, the IIAR 2 Standard specifies the design, selection, and installation requirements of relief systems in ammonia refrigeration systems.

• Set Pressure: The relief valve is calibrated to open at a specific set pressure, which is equal to or less than the Maximum Allowable Working Pressure (MAWP) of the protected vessel. High-side vessels typically have a set pressure of 250 or 300 psig.
• Operation: The relief valve is a direct-spring-loaded valve. When the vessel pressure reaches the set pressure, the force of the ammonia overcomes the spring force, causing the valve disc to lift off its seat and discharge vapor to relieve the pressure.

Dual Relief Valve Assemblies and Three-Way Valves

To comply with IIAR and ASME codes, any vessel containing liquid ammonia with a gross volume of 10 cubic feet or more must be protected by a dual relief valve assembly.

• The Dual Configuration: This assembly consists of two identical pressure relief valves mounted on a specialized three-way manifold valve.
• Three-Way Valve Operation: The three-way valve is a manual divert valve with one inlet (connected to the vessel) and two outlets (connected to relief valve A and relief valve B). The valve stem is designed such that it is physically impossible to isolate both relief valves at the same time. The stem can be adjusted to close off the port leading to relief valve A (allowing it to be safely serviced or replaced) while the port leading to relief valve B remains wide open to the vessel.
• Safety Benefit: This allows continuous overpressure protection. An operator can replace an expired relief valve without having to pump down or decommission the vessel.

The Five-Year Replacement Rule

Safety relief valves must be replaced or recalibrated every 5 years from the date of installation.

• Mechanism of Aging: Over time, the internal spring can suffer from material fatigue, the valve guide can corrode, and the elastomeric seat disc can stick to the metal nozzle. These issues can cause the valve to lift at a pressure higher than its set point, or fail to open at all.
• Discharge Policy: If a relief valve ever discharges (even momentarily), it must be replaced immediately. During a discharge, the high-velocity vapor carries particulate matter and compressor oil across the valve seat. When the pressure drops and the valve attempts to close, these particles prevent the seat from sealing tightly, resulting in a continuous, hazardous ammonia leak.

Vent Piping Design

Discharged ammonia must be routed to a safe location through dedicated vent piping.

• Atmospheric Venting: The discharge pipe must extend outdoors. The outlet must terminate at least 15 feet above grade, at least 7.2 feet above any adjacent roof or walkway, and at least 20 feet away from any window, door, building exit, air intake, or mechanical ventilation intake.
• Weather Protection: The vent pipe outlet must be equipped with a rain cap or be turned downward to prevent water or snow from entering the pipe.
• Weep Holes: A small weep hole or drain plug must be provided at the lowest point of the discharge piping. If rainwater or condensation accumulates in the pipe, it can corrode the relief valve or freeze. An ice plug in the discharge line would block the flow of venting ammonia, causing the vessel to rupture.
• Diffusion Tanks: In some urban areas, relief valves discharge into a water diffusion tank. Because ammonia is highly soluble in water, the water absorbs the ammonia vapor. The tank must contain a minimum of 1 gallon of clean water for every pound of ammonia in the relief capacity.

Refrigerant Charging Procedures

Adding anhydrous ammonia to a system is a high-risk operation. Operators must follow strict procedures to ensure safety and prevent equipment damage.

Required Personal Protective Equipment (PPE)

• Ammonia-rated neoprene or rubber gloves (insulated for cold protection).
• Chemical splash goggles and a full-face shield.
• An ammonia-rated respirator (with green canisters) or a Self-Contained Breathing Apparatus (SCBA) nearby.
• A pressurized water hose must be active and running near the charging station to knock down any vapor leaks.

Thermodynamics of Charging: Liquid vs. Vapor

The boiling point of ammonia at atmospheric pressure is -28°F.

• The Danger of Direct Liquid Charging: If liquid ammonia is introduced directly into an evacuated, dry system, the low pressure (vacuum) will cause the liquid to boil instantly. The temperature of the liquid will drop to -28°F. If the system contains water-cooled chillers or shell-and-tube evaporators, the water inside the tubes will freeze. Since water expands when it freezes, the tubes will rupture, flooding the refrigeration system with water and causing catastrophic mechanical damage.
• The 47 psig Rule: To prevent freezing, the operator must first charge vapor ammonia into the evacuated system. Vapor charging is continued until the system pressure rises above the saturation pressure corresponding to a temperature above freezing (32°F / 0°C). For ammonia, this is 47 psig. Once the system pressure exceeds 47 psig, the internal temperature cannot drop below 32°F, making it safe to charge liquid.

Step-by-Step Liquid Charging Protocol

1. Set Up Safety Station: Put on all required PPE, ensure the water hose is active, and barricade the charging area.
2. Secure and Weigh Cylinder: Place the ammonia cylinder on a digital scale. Note the tare weight and total weight.
3. Connect Charging Hose: Connect a high-pressure charging hose to the system charging valve (usually downstream of the King Valve) and the cylinder.
4. Purge Air from Hose: Crack the cylinder valve slightly to allow a small amount of ammonia to sweep air out of the hose, then tighten the fitting at the system charging valve. This prevents air (a non-condensable gas) from entering the system.
5. Open Valves: Fully open the system charging valve, then slowly open the cylinder valve.
6. Control Flow: Position the cylinder to discharge liquid (cylinders have an internal curved diptube; laying the cylinder horizontally with the valve outlet pointing in the designated direction ensures liquid flow).
7. Monitor Scale: Watch the weight decline to prevent overcharging. Monitor system pressures.
8. Draw Down and Disconnect: When the desired weight is reached, close the cylinder valve. Allow the system compressor to draw down the pressure in the hose. Close the system charging valve. Slowly loosen the hose connection to vent any residual pressure safely, and reinstall the protective caps.

Charge Diagnostics

Operators monitor system parameters to evaluate the refrigerant charge.

• Undercharged System: Indicated by low liquid level in the receiver, bubbles in the liquid line sight glass, high suction superheat, and longer compressor run times.
• Overcharged System: Indicated by a high liquid level in the receiver (exceeding 80%), high head pressure (liquid backs up into the condenser coils, reducing the surface area available for condensation), and high risk of liquid carryover (slugging) to the compressor.