By installing two relief valves in sequence, you add back pressure above the first relief valve piston, causing a change in the pressure setting. For example, the estimated relieving pressure of a valve stack could be:

300-psi setting (first relief valve) + 300-psi setting (second relief valve) = 600-psi setting (total valve stack)

As the relief flow then passes through the second valve, the stack also experiences a change in relieving capacity. If any of these conditions exist, the valve should be replaced.

The condition of the discharge piping should also be inspected. Valves should be piped to ensure that they do not collect dirt and debris. The vent pipes should be protected to prevent the entrance of rain water, which would inhibit valve operation.

Any problems with the vent line should be corrected.

When to replace

After maintenance, the major question concerning relief valves is, when and how often should the system relief valves be changed?

Relief valves should be changed out after discharge to ensure safeguarding a system with a properly set relief valve. Most systems are subject to accumulations of piping debris (i.e., metal shavings and solder impurities) as the system is fitted for installation.

These impurities are generally blown into the relief valve seats at the time the valve is discharged. The impinged debris then inhibits the relief valve from reseating at its original set pressure.

Valves are then found to relieve at considerably lower pressure settings than the stamped valve setting, due to the force of the reclosing action.

Replacement intervals for valves that have not discharged may be dictated by city, state, or federal regulations. In addition, they may also be regulated by industry standards, company policies, insurance requirements, or unwritten, accepted standards of good practice.

In the case of city, state, or federal regulations and insurance regulations, there appear to be no written rules covering the replacement schedule. However, these agencies do govern by verbal requirements requesting that system operators-owners provide proof of the reliability of existing relief valves.

Industry standards

The International Institute of Ammonia Refrigeration (IIAR), in its Bulletin 109, IIAR Minimum Safety Criteria for a Safe Ammonia Refrigeration System, recommends that the relief valve be replaced or inspected, cleaned, and tested every five years.

ANSI STD K61.1-1989, Safety Requirements for the Storage and Handling of Anhydrous Ammonia, is very specific in its requirements. Paragraph 6.8.15 states:

“No container pressure relief devices shall be used after the replacement date as specified by the manufacturer of the device. If no date is specified, a pressure relief valve shall be replaced no later than five years following the date of its manufacture.”

In industrial refrigeration, the current recommendation is to replace the relief valve on a five-year cycle. Be sure to check with other agencies to verify that a more stringent regulation is not applicable.

Beyond these recommendations, there are a number of factors to be considered when selecting and installing relief valves:  

• Provide a pressure vessel that will permit the relief valve to be set at least 25% above the maximum system pressure. However, the relief valve setting cannot exceed the maximum allowable working pressure as stamped on the vessel the relief valve is protecting.

•  Select a relief valve that has sufficient capacity.

•  Select a relief valve suitable for the refrigerant.

•  Install the relief valve so that it will be exposed to refrigerant vapor (e.g., above the liquid level).

•  Install the relief valves where they are accessible for inspection and replacement.

•  Do not discharge relief valves prior to installation or when pressure testing the system.

•  Use the proper size and length of discharge tube or pipe. Correct sizing is required to prevent back pressure from building up in the discharge line, preventing the relief valve from discharging at its rated capacity.

•  The use of a three-way valve with two relief devices, which complies with the code requirements for vessels 10 cu ft or more in gross volume, is recommended for any installation containing a large quantity of expensive refrigerant.

•  When the outlets of two or more relief devices are connected to a common header, the size of the header must be large enough to remain unaffected by back pressure, even if all the relief devices were to discharge simultaneously.

By following these simple rules and recommendations, you can be assured that your system is protected by a properly functioning relief valve.

For more information, contact Henry Valve Co., 3215 North Ave., Melrose Park, Ill. 60160; 708-344-1100; 708-344-0026 (fax).