Type selection of water pump

The principle of the pump is diverse, a wide range of, but also in the continuous development and innovation, different applications, the use of the pump is also different.

The rational selection of water treatment equipment is the knowledge that every designer needs to master. As the core equipment for conveying and upgrading, pumps are everywhere in water treatment projects. The principle of the pump is diverse, a wide range of, but also in the continuous development and innovation, different applications, the use of the pump is also different.

1. Principles and classification of pumps

By professional definition, a pump is a power plant that converts the mechanical energy of a prime mover into the pressure energy and kinetic energy of a fluid, thereby realizing the directional flow of fluid. In use, it is often named for its purpose, such as submersible sewage pump, sludge pump, metering pump, etc. The working principle is different. According to the working principle, it can be classified as follows:

1) vane pump

Vane pumps include centrifugal pumps (single stage, multistage), axial flow pumps, mixed flow pumps, vortex pumps and so on.

Centrifugal Pump - the use of rotating impellers to drive the fluid to rotate together, through the role of centrifugal force, so that the fluid pressure and kinetic energy increased.

Axial flow pumps use the lift generated by fluid rotation on the impeller blades to increase the energy of the fluid.

Mixed flow pumps, between centrifugal pumps and axial pumps, partially utilize centrifugal force and partly use lift.

2) positive displacement pump

Volumetric pump includes reciprocating pump (piston, plunger, diaphragm), rotary pump (gear, screw, slider, etc.)

Reciprocating pumps - Use periodic changes in working volume to deliver fluid and increase its pressure, including piston, plunger and diaphragm types.

Rotary Pump - The use of a pair or several special shapes of rotating bodies, such as gears, screw or other shapes of rotor in the shell for rotating motion to transport fluid and increase its pressure.

3) other types of pumps

Vane pumps and volumetric pumps basically include the types of pumps commonly used, and there are other types of pumps, such as:

Water-ring vacuum pump-water-ring vacuum pump injects a certain amount of water as working fluid before starting and forms a closed water ring by the rotation of star-shaped impeller. The space between impeller and water ring expands and decreases periodically, forming negative pressure, inhaling and discharging gas, thus achieving the purpose of vacuum pumping.

Jet Pump - the use of high-speed jet suction to pump and transport liquid, can play a role in vacuum.

2. Main performance parameters of pumps

1) flow and lift

Pump in the unit time of the flow called flow, pump flow generally refers to volume flow, expressed with Q. The increment of energy obtained by the liquid passing through the pump under the action of unit gravity is called the head, expressed in H and in M.

In the selection of model, the flow and head are the core parameters given by the designer according to the calculation results of the process, which is a fixed value.  For the performance of the pump itself, the flow and head is often a corresponding relationship within an interval, is a range. The ideal selection is that the fixed value of the technological parameters needs falls on the rate point of the pump performance curve. When there is a deviation, the possible adverse effects of the deviation should be evaluated comprehensively, and the result of the selection should be determined after weighing.

2) shaft power and efficiency

When the pump is running, the power transmitted by the prime mover to the pump shaft is called shaft power. It is expressed in P, and the unit is kW. The power gained by pumping fluid in unit time is called effective power, expressed in Pe. The efficiency of the pump is the ratio of effective power to shaft power, that is, et. =Pe/P.

The efficiency of the pump is related to the design level of the pump itself, the level of mechanical processing and so on. The efficiency directly affects the energy consumption. At the same time, the pump has its own efficiency range, so it is necessary to fall in this range in order to reduce energy consumption.

3) speed

The revolutions of the pump shaft per minute are called rotational speeds. They are expressed in N, and the units are r/min.

The rotational speed generally corresponds to the standard rotational speed of the motor. The rotational speed of the motor can be adjusted by frequency converter, and the performance curve of the pump also varies with strain. When the actual flow head changes, it can achieve good energy saving effect by adjusting frequency conversion.

4) cavitation and cavitation allowance

Cavitation refers to the operation of the pump, because some reasons make the local pressure in the pump to reduce to the vaporization pressure of water, water will produce vaporization and form gas-liquid flow, after reaching the high-pressure region, the bubble is extruded and destroyed to condense into water, in this process will produce a high water hammer pressure, so that the material is eroded and destroyed. Cavitation phenomenon is mainly for blade pump, which is avoided in pump selection design. The occurrence of cavitation is related to the performance of the pump, and is also related to the design of the suction device of the pump.

Cavitation allowance refers to the difference between the total head of the liquid at the inlet of the pump and the pressure head at the time of vaporization. The unit is marked in meters (water column) and expressed in NPSH (Net Positive Suction Head). In the performance parameters of vane pump, manufacturers will generally provide allowable cavitation allowance for designers to use in the selection design, cavitation allowance parameters are described as follows:

NPSHc - Critical cavitation allowance, obtained by cavitation test; [NPSH] - Allowable cavitation allowance, usually taken [NPSH] = (1.1-1.5) NPSHc or [NPSH] = NPSHc + K (generally K = 0.3).

Cavitation allowance mainly affects the installation conditions. The smaller the allowable cavitation allowance, the greater the negative pressure in front of the pump, the better the performance of the pump.