Ice Breaker: Relief Valves 101
Safety Device Helps Protect Equipment, Workers
A safety device used on many refrigeration systems is the relief valve. Relief valves are used to release abnormally high pressure inside a vessel before that
pressure causes the vessel to erupt. There are three general types of relief valves:
• One-Time Relief;
• Ruptured Disc; and
• Pressure Relief.
A one-time relief valve, sometimes called a “fusible plug,” is normally constructed from a fitting with a drilled hole filled in with a low-temperature, solder. At a specific temperature the solder will soften and pressure within the vessel will cause the solder plug to “blow out.”
The ruptured disc contains a thin disc of metal designed to rupture at a certain pressure. Neither the fusible plug nor the ruptured disc reseal after opening. Consequently, all the refrigerant is discharged from the storage vessel or protected portion of the system, should they open. These relief devices would then have to be replaced.
Pressure relief valves are spring-loaded valves normally encased in a brass body with a neoprene seal. They are designed to automatically reset once the pressure inside the vessel reaches a safe level. They are located on a section of the vessel where refrigerant vapor is located. This allows only the vapor to be released from the vessel rather than any liquid refrigerant. Some pressure relief valves will also have a thread connection on the top to allow piping to be attached to it so the released refrigerant can be vented out of the building or mechanical room where the vessel is located.
A popular type of pressure relief valve is the spring-loaded “pop” type. When pressure rises above the spring setting, refrigerant will initially begin to seep through. When enough flow develops, the piston will pop open, allowing full discharge. The pop-type relief valve has its advantages, including simple design, low initial cost, and high discharge capacity.
There are limits on the length of discharge pipe from a safety pressure relief valve. The limits are based on pipe size and relief valve discharge capacity in pounds of air per minute. Pressure relief valves are designed to re-close as the pressure is reduced. However, the valve may not completely reseal. Manufacturers generally recommend replacement after it has been opened. Failure to reseal tightly is generally due to an accumulation of dirt and foreign matter that attaches to the valve seat disc while the valve is discharging. For this reason, it is impossible to predict the reliability of the relief valve resealing after it has discharged in service.
All relief valves must comply with the ASME code for unfired pressure vessels. Discharge rates are certified by the National Board of Boiler and Pressure Vessel Inspectors. A code symbol is stamped on relief valves indicating this certification. It includes the letters “UV” in a clover leaf design. The letters “NB” are stamped directly below this symbol. The pressure setting and capacity are also stamped on the valve. The exact number, location, and type of relief devices required are set forth in detail in the American Standard Safety Code for Mechanical Refrigeration. Local codes vary somewhat in this respect, and should also be considered in designing an installation.
Never eliminate or seal off a relief valve, as it serves a very important safety function in the systems we service, maintain, and install. Without these safety devices, serious harm could occur to those working around these systems.
Publication date: 11/4/2013