You must anticipate that all relief valves may leak and consider this possibility when choosing a method for disposing of effluent. For example, it is highly undesirable to connect discharges from relief valves on finished product piping to a common closed header connected to a tank or other location which could result in a back pressure on the system. Under such circumstances, discharge from, or leakage past, a relief valve on one line could leak backward through a relief valve on another line at lower pressure than the intended disposal location and contaminate product in the other line. The quantity of effluent expected during thermal relief, as well as the possibility of continual leakage, must also be considered when choosing a disposal method. Methods that have been used for disposal include the following.
Around Block Valve. The relief valve releases pressure around the block valve. This is feasible only if the fluid is the same on each side of the block valve so no contamination results, or if contamination is permissible. This is a common disposal practice for pressure relief at tank block valves. In other piping, valve back-pressure problems will probably rule out this method. An internally vented relief valve set for high back-pressure (low spring setting) could open on a low back-pressure condition, tending to nullify the intent of the block valve.
To a Sump or Sewer. This is a method to use when discharge around the block valve is impractical and where discharge to ground would create a hazard or be uneconomical. Collection in a sump with a pump to return accumulated effluent to the plant will be preferable to disposal into a sewer if lines are long and large quantities of effluent can be expected. If a common line is used to carry effluent from several relief valves to a sump or sewer, the system should be designed to prevent back-flow and possible contamination of the contents of different lines. Use of an open funnel at the point of discharge from each relief valve into an open gravity drain line has been found to be advantageous.
To Atmosphere or to Ground. This method should only be used when the quantity of effluent or leakage is small and when the release does not create a hazard or an undesirable condition. Relieved liquid should be directed away from piping or other equipment.
In Cascade. Cascaded relief valves are not recommended. If used, cascaded valves require careful consideration both from a safety and operating viewpoint, making due allowance for back-pressure effects.