For several years now, with OSHA and EPA have issued bulletins/alerts regarding Excess Flow Valves. These alerts/bulletins warned against placing too much faith in these valves when it comes to PREVENTING a “catastrophic release”. These valves do not have a history of failing because of defects, they have a history of failing due to IMPROPER DESIGN! In this article, I hope to explain the design errors we most often come across.
So how does an Excess Flow Valve work
Excess flow valves are essentially a spring loaded check valve that will close when the flow of liquid (or vapor) through the valve exceeds the flow to overcome the power of the spring holding it open. These valves will have a rating in gallons per minute and/or CFH/air. These valves allow the flow of liquid (and vapor) in either direction; HOWEVER, the excess flow is controlled in ONLY ONE direction! The valve will be stamped with an arrow indicating its PROTECTIVE FLOW. If the flow in that direction exceeds the predetermined flow rate, the valve will automatically CLOSE.
Most of these excess flow valves are either spring and float or a ball and magnet design. When the flow through the valve creates a pressure drop across the valve that exceeds the preset flow rate on the spring, the valve disk closes the opening to stop the flow of product. The valve 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, these disk has been drilled with an opening to allow for equalization. Once the pressures have equalized, the valve will automatically re-open.
Design errors
Most excess flow valves are positioned as close as possible to the bulk storage, often times they are even INSIDE the bulk tank. These excess flow valves may NOT close when a downstream break in a LONG run of pipe does not provide sufficient flow to close the excess flow valve. Where long runs of pipe are installed, elbows, branches, and vertical rises, as well as lengths of pipe all generate flow restrictions and these restrictions may not allow for the sufficient flow rates necessary to close the excess flow valve.
Excess flow valves MUST be installed throughout a pipe run to ensure adequate protection against the excess flow. To achieve the true protection factor of these valves, they MUST be installed where lines branch off of the main line. The excess flow valves MUST be sized accordingly to allow for proper flow rates to the process; undersized valves will cause premature closure; oversized valves will NOT provide the protection necessary to close when needed.
Pipe Specs
Excess Flow Valves should be specifically called out in our pipe specs when it comes to their location(s) and sizing. The specs should REQUIRE a new valve, properly sized, for any change in pipe diameter. This is the most common design failure we come across. For example, the bulk tank has a 3” excess flow valve and shortly after the material exits the tank, pipe size is reduced as it splits off before it reaches two (2) pumps in 1.5” diameter piping. Flows through a 1.5” pipe will most likely NOT be enough to close the 3” excess flow valve! This change in pipe diameter can virtually render the excess flow valve useless.
If we want to claim these excess flow valves as a “safety system” and use them to reduce our risks in our PHA’s (and real life) then we need to have some type of a design basis for how and where these devices will be utilized. Just claiming their presence is NOT ample evidence of an engineered “safety system”.