The positive displacement pump relies on the reciprocating or rotating motion of the working element in the pump cylinder to make the working volume increase and decrease alternately to realize the suction and discharge of the liquid. The positive displacement pump whose working element performs reciprocating motion is called a reciprocating pump, and the one that performs rotary motion is called a rotary pump. The suction and discharge processes of the former are carried out alternately in the same pump cylinder and are controlled by the suction valve and the discharge valve; side is transferred to the discharge side. The screw pump is a positive displacement rotor pump
The flow rate of the positive displacement pump at a certain speed or the number of reciprocations is constant, and it hardly changes with the pressure; the flow rate and pressure of the reciprocating pump have large pulsation, and corresponding measures to reduce pulsation need to be taken; the rotary pump generally has no pulsation or only small Pulsation; with self-priming ability, the air in the pipeline can be sucked out after the pump is started; the discharge pipeline valve must be fully opened when starting the pump; the reciprocating pump is suitable for high pressure and small flow; the rotary pump is suitable for medium and small flow and higher pressures; reciprocating pumps are suitable for conveying clean liquids or gas-liquid mixtures. In general, positive displacement pumps are more efficient than dynamic pumps.
The screw pump is a commonly used pump for conveying