Safety Relief Valves on Storage Tanks & Piping
The following information is designed to inform you of the requirements of this equipment.
Anhydrous Ammonia storage tanks are equipped with vapor safety relief valves to protect against over pressurization which could occur from the following conditions:
• Hydrostatic pressure due to overfilling or the trapping of liquid between two points.
• High pressure resulting from external heat impingement or process problems.
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.
Hydrostatic Relief Valves
Where liquid in piping systems may become trapped between two points and subsequent temperature increases cause expansion of this liquid, extremely high hydraulic pressures can result. To relieve this pressure, hydrostatic relief valves are used. Care must be taken to insure the placement of the hydrostatic relief valve is such that the isolated area of the piping is being protected. Some manufacturers equip their hydrostatic relief valves with a ¼” FNPT connection on the down stream side of the valve. The purpose of this connection is to allow for extension piping to convey the liquid to a safe location away from occupied areas in the event of a discharge. Should the valve discharge and not completely reseat, the valve will need to be replaced. Per manufacturer recommendation, a second hydrostatic relief valve should not be threaded into the affected valve in question.
Installation & Sizing:
The valves are typically ¼” MNPT valves that can be threaded into the body of a valve. Where long runs of larger piping are being protected, additional hydrostatic relief valves should be placed periodically in the runs of piping. Larger ½” MNPT hydrostatic relief valve are available in the market place for this application.
Inspection of Hydrostatic Relief Valves:
Hydrostatic relief valves should be inspected at the same intervals as discussed above for the vapor safety relief valves. The same criterion should be followed to complete the inspection process. It shall be noted that all piping being protected by these valves will need to be evacuated of liquid and residual vapor pressure to allow for replacement.
Hydrostatic relief valves follow the same guidelines as the vapor safety relief valves for replacement.
References
• OSHA 29 CFR 1910.111 – Storage and Handling of Anhydrous Ammonia, www.osha.gov
• ANSI K61.1 (CGA G-2.1) American National Standard Safety Requirements For The Storage And Handling Of Anhydrous Ammonia
While the information contained within this bulletin is believed to be true and accurate, a professional engineer should be consulted when designing any tank or piping system and nothing in this informational bulletin should substitute for such professional advice. Moreover, please refer to all manufacturer’s instructions and warnings in connection with the installation, use and replacement of these products. ANSI K61.1 and OSHA1910.111 should also be reviewed for additional information regarding the safe storage and handling of anhydrous ammonia. State and local codes may be more stringent and should also be reviewed.
Revision 2/08