At {HEAD ~ the pressure side} of the pump unit, there is a valve which opens during certain times to form a connection between {TAIL ~ the pump} and the pump station to create a circulation 
At { HEAD ~ the pressure side } of the pump unit , there is a valve which opens during certain times to form a connection between { TAIL ~ the pump } and the pump station to create a circulation
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH