Positive displacement pump
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; the side is transferred to the discharge side.
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.
Relying on the force of the rapidly rotating impeller on the liquid, the mechanical energy is transferred to the liquid to increase its kinetic energy and pressure energy, and then most of the kinetic energy is converted into pressure energy through the pump cylinder to achieve transportation. Power pumps are also known as impeller pumps or vane pumps. Some power pumps have both the main impeller and the auxiliary impeller, and the centrifugal pump is the most common power pump.
The heat produced by the power pump at a certain speed has a limited value and the head changes with the flow; the work is stable, the conveying is continuous, and the flow and pressure have no pulsation; generally, there is no self-priming ability, and the pump needs to be filled with liquid first or the pipeline is pumped. It can start to work after being victimized; it has a wide range of applicable properties; it is suitable for conveying clean liquids with low viscosity.
In addition to differences in principle, dynamic pumps and positive displacement pumps also have great differences in working characteristics and applications.
The main features of the power pump are:
① The lift generated by a certain pump at a certain speed has a limited value. The operating point flow and shaft power depend on the conditions of the plant system (head difference, pressure difference, and line losses) to which the pump is connected. Head varies with the flow.
② Stable work, continuous delivery, no pulsation in flow and pressure.
③ Generally, there is no self-priming ability, and the pump needs to be filled with liquid or the pipeline can be evacuated before starting to work.
④The centrifugal pump is started when the valve of the discharge pipeline is closed, and the vortex pump and the axial flow pump are started when the valve is fully opened to reduce the starting power.
⑤The centrifugal pump is suitable for direct drive by a high-speed electric motor and steam turbine, with a simple structure, low manufacturing cost, and convenient maintenance.
⑥ It has a wide range of application performance. The flow rate of centrifugal pumps can be from several to hundreds of thousands of meters per hour, and the lift can be from several meters to several thousand meters. Axial flow pumps are generally suitable for large flow and low lift (below 20 meters). The efficiency of centrifugal pumps and axial flow pumps is generally below 80%, and the highest can reach 90%.
⑦Suitable for conveying clean liquids with low viscosity (such as clean water), specially designed pumps can convey mud, sewage, etc, or water conveying solids. Power pumps are mainly used for water supply, drainage, irrigation, process liquid transportation, power station energy storage, hydraulic transmission, and ship jet propulsion.
The main features of positive displacement pumps are:
① The flow rate of a certain pump at a certain speed or a number of reciprocations is constant, and it hardly changes with the pressure. The operating point pressure and shaft power depend on the condition of the device system connected to the pump, so when the pump runs with a blocked discharge line (equivalent to infinite system resistance), its pressure and shaft power will increase to make the pump Or the prime mover is destroyed, so a safety valve must be installed to protect the pump (except for pumps that are directly acted by steam or driven by compressed air).
② The flow and pressure of the reciprocating pump have large pulsation, and corresponding measures to reduce the pulsation need to be taken; the rotary pump generally has no pulsation or only small pulsation.
③ It has a self-priming ability, and the air in the pipeline can be sucked into the liquid after the pump is started.
④When starting the pump, the valve of the discharge pipeline must be fully opened.
⑤ A reciprocating pump is a low-speed machine with a large size and high manufacturing and installation costs; the rotational speed of the rotary pump is high, up to 3000 rpm.
⑥ The reciprocating pump is suitable for high pressure (up to 350 MPa) and small flow (below 100 m3/hour); the rotary pump is suitable for small and medium flow (below 400 m3/hour) and higher pressure (below 35 MPa) ). In general, the efficiency of positive displacement pumps is higher than that of dynamic pumps, and the efficiency region of the efficiency curve is wider. The efficiency of the reciprocating pump is generally 70 to 85%, and the high one can reach more than 90%.
⑦ The reciprocating pump is suitable for conveying clean liquid or gas-liquid mixture. Some pumps such as diaphragm pumps can convey mud, sewage, etc., mainly used for water supply, high-pressure liquid sources, and metering transportation. Rotary pumps are suitable for conveying lubricating clean liquids and liquid-gas mixtures, especially liquids with high viscosity. They are mainly used in the transportation of oil, food liquids, and hydraulic transmission.