In some cases, the discharge of a relief device may be directed to another point in the same process. For example, relief valves on the discharge of reciprocating pumps often have their outlets routed back to the suction of the pump. Relief valves installed to provide pressure relief for piping segments in the event of thermal expansion of a blocked-in liquid sometimes have their discharges routed to a storage vessel or another piping segment.
The advantages of this method of relief device discharge handling are that the process fluid is not transferred to a waste stream, and that the discharge handling “system” is minimal. However, there are some significant limitations on the application of this approach to discharge handling. The relieved material should be similar in composition to that at the location to which it is routed so that product purity is maintained. The normal pressure at the discharge location should be considered when establishing the differential or net set pressure of the device (see Sections 1231 and 1232). The net (or spring) set pressure of a conventional relief valve, or the burst pressure of a rupture disk, plus the normal pressure at the discharge should equal the desired set pressure (see Figure 1200-13). Therefore, a desired discharge location would not be appropriate if the pressure at that location is not known or is subject to significant fluctuations. Use of a balanced (e.g., bellows) relief valve can often alleviate these difficulties, but care should be taken to consider the effect of back pressure on the valve’s flow capacity and that the back pressure rating of the valve (particularly of the bellows) is not exceeded.