Vapor Relief Valves
Vapor safety relief valves typically have a spring loaded, rubber surfaced valve seat. Because of the seat material, these types of relief valves have a finite life. They are equipped with a manufacturer’s date stamped onto the body of the valve and have a service life of five (5) years; these valves must be replaced or re-certified prior to the five (5) year service life. There are safety relief valves in the market place that are equipped with other seating surfaces that are compatible with anhydrous ammonia. These valves also need to be re-certified or replaced on a five year interval.
Operation of Vapor Pressure Relief Valves:
Vapor safety relief valves are set and sealed by the manufacturer to function at a specific "start to discharge" pressure in accordance with regulations. Stationary storage tanks are equipped with valves that are rated for 250 PSIG. These valves will start to open prior to this pressure. As the pressure rises in the tank nearing the set pressure point, the valve will start to seep. As the pressure continues to raise the valve will continue to open to relieve pressures. The safety relief valve will be fully open at the 250 PSIG rating. Once the excess pressure has been relieved within the container, the valve will reseat. Be aware that once a valve does discharge and pressures subside, it may not fully reseat as designed. State and local codes should be verified as to the specific pressure rating required as some states may require a higher maximum working pressure vessel. The vapor safety relief valves will need to be sized accordingly to meet the specific criteria as set forth by the specific state and local agencies. Refrigeration systems are equipped with intermediate process vessels that may be equipped with a lower rated pressure relief valve. These vapor safety relief valves will need to be sized accordingly to meet the maximum working pressure of the vessel. The data plate on the vessel will provide the maximum allowable working pressure for the container. Should there be a question of the pressure rating; the manufacturer of the vessel should be consulted for additional information.
Installation & Sizing:
The rate of discharge required for a given sized tank is determined by a calculation of the surface area of the tank. Based on the tank's surface area, a certain flow rating for safety relief would be required and this would determine valve sizing as well as number of valves needed. This calculation can be found in OSHA regulations under 29 CFR 1910.111. Tanner Industries, Inc. can also provide assistance with proper sizing requirements.
Typically, smaller storage tanks (250 gallon through 1,250 gallon) are equipped with two (2) safety relief valves located on a safety manifold commonly called a two way or three way valve. One safety relief valve would be in operation while the second valve would be isolated; this provides back up protection for the vessel in case the valve in operation starts to leak or becomes damaged. The safety manifold is designed to allow for the operation of only one valve at a time and allows for the ability to remove and replace the safety relief valves at the end of their service life while maintaining protection for the tank during the exchange. It shall be noted that this type of valve is designed to protect one safety at a time and the valve stem must be kept in either the fully open (counter clockwise) of fully closed (clockwise) position. If the valve stem is placed in the partially open position, both safety relief valves will be active. Should the valves discharge, no back up protection would be available should leakage occur during the reseat process.
Tanks sized greater than 1,250 gallon typically require larger safety relief valves and are installed in the same manner using a larger safety manifold. Where tank surface area’s dictate, multiple safety relief valves can be installed to achieve the required relief capacity using either a multi-port style manifold or multiple safety manifolds.
All vapor safety relief valves are equipped with plastic rain caps. These caps provide protection against rain, snow, dirt and debris from coming in contact with the inner spring assembly. Should the valve discharge, the cap will be blown off the valve assembly and found nearby. Vapor relief valves should always have the rain cap in place.
Certain types of vapor safety relief valves have the ability to permit the installation of pipe away adaptors. These adaptors are installed into the discharge port of the safety relief valve and can be piped to convey the product discharge up and away from the tank to a remote location. The termination point should be placed in such a manner that rain, dirt and debris are shielded against entry into the piping. Flapper type rain caps can be used to cover the piping termination point; if no flapper style cap is available a double elbow can be installed to turn the termination point downward. Under this condition, a short length of pipe can be installed in the elbow to allow for the installation of a rain cap. The rain cap originally removed from the safety relief valve can be re-installed over the pipe termination. The remote piping should be sized, installed and adequately braced to allow for the inspection and removal/replacement of the safety relief valves. Unions should be installed as close as practical to minimize piping removal.
A rupture disk may be installed under a safety relief valve if good engineering practice is used to design the combined system, and there is a means of regularly checking for disk leakage by monitoring pressure in the space between the devices. A leaking disk should be replaced because pressure downstream of the disk will prevent rupture at the desired pressure. Under no circumstances shall a rupture disk be used as the sole, or as a supplemental, pressure relief device on an anhydrous ammonia tank.
Inspection of Safety Relief Valves:
Always wear proper protective equipment when performing inspections to safety relief valves.
- Inspect the rain cap for a secure fit, weathered caps should be replaced. Contact Tanner Industries, Inc. for replacement caps.
- Weep holes in the body of the valve should be cleared of any debris and foreign particles to insure proper drainage from the valve body. The valve should be replaced if the weep holes cannot be cleared.
- Valve springs should be inspected for deterioration and corrosion. Exposure to high concentrations of water, salt, industrial pollutants and chemicals can cause metal parts to prematurely fail. If the coating in the spring assembly is chipped or cracked, the valve should be replaced. Never look directly down the discharge port of the valve, consider using a small mirror to perform the inspection. Using litmus paper or sulfur sticks, test the valve for leaks by removing the protective rain cap and passing the litmus paper or sulfur stick across the opening. Should leakage be noted, isolate the valve in question (if equipped with a safety manifold) and replace. It should be noted that if the tank is not equipped with a safety manifold, the tank will need to be emptied and evacuated of all residual vapor pressure prior to replacing the safety relief valve in question.