The pump is double -acting, that is, liquid is admitted to either side of the piston where it is alternately drawn in and discharged. As the piston moves upwards,suction takes place below the piston and liquid is drawn in, the valve arrangement ensuring that the discharge valve cannot open on the suction stroke. Above the piston, liquid is discharged and the suction valve remains closed. As the piston travels down, the operations of suction and discharge occur now on opposite sides.A relief valve is always fitted between the pump suction and discharge chambers to protect the pump should it be operated with a valve closed in the discharge line.
Reciprocating displacement pumps are self-priming, will accept high suction lifts, produce the discharge pressure required by the system and can handle large amounts of vapor or entrained gases. They are, however, complicated in construction with a number of moving parts requiring attention and maintenance.When starting the pumps the suction and discharge valves must be opened. It is important that no valves in the discharge line are closed, otherwise either the relief valve will lift or damage may occur to the pumps when it is started.
