Excess Flow Valves on Storage Tanks & Piping
The following information is designed to inform you of the requirements of this equipment.
An excess flow valve is a spring loaded check valve which will close only when the flow of liquid or vapor through the valve generates sufficient flow or force to overcome the power of the spring holding it open. Each valve has been fabricated with a closing rate in gallons per minute and CFH/air. These valves come in a variety of different sizes and flow rates depending on their intended use. It is critical to understand what the intended service of the valve will be prior to installation.
Excess flow valves permit the flow of liquid or vapor in either direction. This flow is controlled in only one direction (each valve is stamped with an arrow). If the flow in that direction exceeds the predetermined flow rate, the valve will automatically close.
The valve disk is held in the open position by a spring. When the flow through the valve creates a pressure drop across the valve disc that exceeds the preset flow rate on the spring, the valve disk closes against the opening to stop the flow of product. The valve disk will remain closed until the pressure between the upstream side and downstream side of the valve equalize in pressure. To allow for this to occur, the disk has been drilled with a .040 inch (#60 drill bit) opening to allow for equalization. Once the pressures have equalized, the valve will automatically re-open. When a line has been ruptured or broken, the down stream pressure will not equalize, thus the valve will remain closed until the line has been repaired. The bleed port equipped on the excess flow valve, though useful for equalization, also hinders the valves closure ratio making it less than 100%.
Per OSHA 29CFR 1910.111 and ANSI K61.1 standards, all openings on an anhydrous ammonia stationary storage tank larger than .055 inch (#54 drill bit) orifice must be equipped with excess flow protection. Exceptions to this standard are given for the installation of a mechanical float gage and for the safety relief valves. The 85% outage gage is typically a 1/4” port installed at the point of manufacture that has been internally reduced to the .055 inch (#54 drill bit) opening. A tube is welded to the end of the coupling and fitted to the proper length required to allow for maximum fill of 85%. In instances where the tank has not been equipped with this assembly, a maximum fill tube can be created. Call Tanner Industries, Inc. for additional assistance for this requirement. The pressure gage assembly is typically mounted into a 1/4” threaded port on the tank that has been plug welded at the point of manufacture and drilled with a .055 inch (#54 drill bit) opening. If a port is not provided by the manufacturer of the tank, a reduced port bushing (commonly referred to by industry as a snubber) is installed between the vessel and the pressure gage assembly.
Excess flow valves come in different sizes, configurations, and flow rates depending on how the vessel has been constructed and the desired rate of flow necessary for process. These valves may be incorporated into the actual valve, or may be an appurtenance installed between the vessel and the tank isolating valve. In either case, the excess flow valves should not be relied entirely upon for protection. Secondary isolating valves and emergency shut off valves are recommended in addition to the excess flow protection.
It must be noted that a downstream break in a long run of pipe may not provide sufficient flow to close an excess flow valve. Where long runs of pipe are installed, elbows, tee’s, branches and vertical rises, as well as lengths of pipe all generate flow restrictions. These restrictions may not allow for the sufficient flow rates necessary to close the excess flow valve in the case of a downstream break. It is recommended that in-line excess flow valves be installed periodically to insure adequate protection is being obtained. Inline excess flow valves should also be installed where lines branch off of the main header line to insure adequate protection is achieved. The excess flow valves will need to be sized accordingly to allow for proper flow rates to the process. Under sizing of the valves will cause premature closure and interrupt service; over sizing of the valves may not provide the protection necessary to close in an emergency. Contact our Manager of Technical Services at Tanner Industries, Inc. for assistance with sizing requirements.
All isolating valves must be completely opened to ensure adequate flow rates are obtained. A partially opened valve may not provide sufficient flow to overcome the required flow rate necessary to close the excess flow valve in the case of a downstream rupture. It should also be noted that a crack or tear in a pipe may not allow sufficient flow rates necessary to close the excess flow valve.
Bushings should not be used to reduce the port size of a valve as this will reduce the flow rate through the valve and may not allow for a sufficient flow rate to close the excess flow valve in the case of a broken or ruptured line downstream.