Installing relief valves

Question: We received a notice from a Chicago inspector who wanted us to install relief valves on a walk-in freezer and cooler at a KFC. We all are aware of the system's characteristic that if the water pressure drops and head pressure rises, there goes the charge. This is a major release! So what is the truth? Do we or don't we? Should we or should we or shouldn't we?

­-Tom Reynolds, Heating & Cooling, Lansing, Ill.

Answer: To prevent nuisance refrigerant loss through pressure relief valves during high ambient or abnormal operating conditions, it is necessary that the relief valve setting be substantially higher than the system operating pressure.

For relief valves:

•  The minimum setting should be at least 25 percent above the maximum system operating pressure (MSOP). Example: MSOP = 280 psi. Minimum relief valve setting = 1.25 × 280 = 350 psi.

•  The maximum allowable setting should be equal to the design working pressure of the vessel.

These factors of safety will provide sufficient spring force on the valve seat to maintain a tight seal and still allow for setting tolerances and other factors, which cause settings to vary.

A refrigerant safety valve is designed to prevent pressure in a vessel from rising above a safe level when operating controls fail or, when the vessel is exposed to excessive heat.

With a properly sized relief valve installed on the vessel, the refrigerant is released at a controlled rate so that a safe pressure is maintained in the vessel. If sized correctly, the relief valve has enough discharge capacity to prevent the pressure in the vessel from rising more than 10 percent above its design working pressure.

Experience has shown that although relief valves are originally set to within a few pounds of the stamped setting, the variation may be as much as 10 percent of the stamped setting after the valves have been stored, or placed in service for prolonged periods of time.

If vessel pressure increases within 10 percent of set pressure of the relief valve, the valve may begin to “seep.” With increasing pressure, gas flow will increase to the “pop” point when the valve goes fully open providing full discharge. By Code, the valves must be fully open within 10 percent of the valves stamped setting. However, under certain system conditions, it is possible a valve could start to relieve, but not open fully. Under these conditions, valves would either continue to “seep” or “chatter.”

Relief valves are designed to reclose as the pressure is reduced. However, it is very unlikely that the valve will reseal – and it should be replaced.

Failure to reseal tightly is generally due to an accumulation of dirt and foreign matter that impinges on the valve seat disc while the valve is discharging. For this reason it is almost impossible to predict with any degree of certainty the reliability of the relief valves resealing after they have discharged in service. This condition also allows the relief valve to “seep” refrigerant after reclosing, but not necessarily reseal.

Although relief valves are designed to reclose after discharging, this feature is intended only to minimize the loss of refrigerant until the valve can be replaced.

Service complaints concerning the loss of refrigerant are greatest with refrigerants having high saturation pressures such as R-22and R-502. Complaints are noted particularly during the summer with very high ambient temperatures.

Table 2: Pressure Temperature Valves R-22 and R-502

*Potential R.V. “seep” points valve set at 350 psi.

** Potential R.V. “seep” points set at 400 psi.

Problems may arise with valves subjected to high ambient temperatures that might be encountered in an equipment room or on a rooftop. Failure to take advantage of the maximum allowable relief valve setting may contribute to relief valves seeping:

Consider a system with:

•  Maximum system operating pressure: 280 psi.

•  Pressure vessel maximum working pressure: 400 ps.

•  Minimum relief valve setting: 350 psi.

Check Table 1, example A: Relief valve set at 350 psi.

Check Table 1, example B: Relief valve set at 400 psi.

By comparing the potential relief valve “seep” point from Table 1 with the pressure shown in Table 2, we note the added safety zone provided by the properly sized relief valve.

Failure to consider the above factors when sizing relief valves can lead to valves that are set at too low a pressure. The result can be valves that “seep” in extremely hot weather with the possibility they will continue to “seep” after reclosing.

If the recommendations in the Tech Tip “Needless Loss of Refrigerant Through Relief Valves During Abnormal Operating Conditions” are applied, you should not experience a release of refrigeration through a relief valve.

-Paul Schwartz, marketing manager, Henry Valve Co.