SWV-R11,12,22 (System Pressure Relief Valve):

This valve has a function to maintain the inlet side pressure through a diaphragm type relief pilot installed on the main valve. When the flow of water inside pipe that holds large amount of water is blocked or if the consumption is suddenly reduced, pressure inside the pipe will build up rapidly, causing water hammer or breakage of the pipe. 

This would create high resistance against the impeller of the pump, possibly resulting in power loss and shortening the life span of the pump.

In order to solve this problem, the valve and the by-pass piping are installed around the pump, allowing excessive pressure to be relieved. 

Since this is a pilot acting type valve, the capacity is relatively larger than direct acting type valves.