Pump selection principle, basis and specific operation mode

First, understand the principle of pump selection

1. Make the type and performance of the selected pump meet the requirements of process parameters such as flow, head, pressure, temperature, cavitation flow, and suction.

2, must meet the requirements of the media characteristics.

For pumps that transport flammable, explosive, toxic or valuable media, shaft seals are required to be reliable or leak-free pumps, such as magnetic drive pumps, diaphragm pumps, and shield pumps

For pumps that transport corrosive media, corrosive materials such as AFB stainless steel corrosion resistant pumps and CQF engineering plastic magnetic drive pumps are required for convection components.

For pumps containing solid particulate media, it is required that the convection component be made of wear-resistant material, and if necessary, the shaft seal should be flushed with a cleaning liquid.

3. High mechanical reliability, low noise and low vibration.

4. The economic consideration should be based on the lowest total cost of equipment, operating, maintenance and management fees.

5. The centrifugal pump has the characteristics of high speed, small volume, light weight, high efficiency, large flow, simple structure, no pulsation of infusion, stable performance, easy operation and convenient maintenance.

Therefore, in addition to the following conditions, the centrifugal pump should be used as much as possible:

a. When there is a measurement requirement, the metering pump is selected.

b. The head is very high, the flow rate is very small, and there is no suitable small flow. The high lift centrifugal pump can be used. The reciprocating pump can be used. If the cavitation requirement is not high, the vortex pump can also be used.

c, the head is very low, when the flow is very large, the axial flow pump and the mixed flow pump can be selected.

d. When the viscosity of the medium is large (more than 650~1000mm2/s), consider the choice of rotor pump or reciprocating pump (gear pump, screw pump)

e. When the medium contains 75% gas, the flow rate is small and the viscosity is less than 37.4mm2/s, a vortex pump can be selected.

f. For frequent start-up or inconvenient pumping, pumps with self-priming performance, such as self-priming centrifugal pumps, self-priming vortex pumps, pneumatic (electric) diaphragm pumps.

Second, know the basic basis of pump selection

The selection of the pump should be based on the process flow, water supply and drainage requirements, from five aspects, including liquid delivery, device head, liquid properties, piping layout and operating conditions, etc.

1. Flow rate is one of the important performance data of pump selection, which is directly related to the production capacity and conveying capacity of the entire device. For example, the normal, minimum and maximum flow rates of the pump can be calculated in the design of the design institute. When the pump is selected, the normal flow rate is based on the maximum flow rate. When there is no maximum flow rate, 1.1 times the normal flow rate can be taken as the maximum flow rate.

2. The head required for the system is another important performance data of the pump. Generally, it should be selected by amplifying the 5%-10% balance.

3, liquid properties, including liquid media name, physical properties, chemical properties and other properties, physical properties are temperature c density d, viscosity u, solid particle diameter and gas content in the medium, etc., which involves the lift of the system, effective gas Calculation of the amount of eclipse and the type of suitable pump: chemical properties, mainly refers to the chemical corrosiveness and toxicity of the liquid medium, is an important basis for the selection of the pump material and the type of shaft seal.

4. The piping arrangement conditions of the equipment system refer to the data of the liquid feeding height, the liquid feeding distance, the minimum liquid level, the highest liquid level on the discharge side, and other data and pipe specifications and their lengths, materials, pipe fittings specifications and quantities. Etc., in order to carry out the calculation of the combing process and the checking of the NPSH.

5, operating conditions have a lot of content, such as liquid operation T saturated steam force P, suction side pressure PS (absolute), discharge side container pressure PZ, altitude, ambient temperature operation is gap or continuous, pump position is fixed Still removable.

Third, the specific operation of the pump

According to the pump selection principle and the basic conditions of selection, the specific operations are as follows:

1. According to the layout of the device, terrain conditions, water level conditions and operating conditions, determine the choice of horizontal, vertical and other types (pipeline, submersible, submerged, non-blocking, self-priming, gear, etc.) Pump.

2. According to the nature of the liquid medium, determine the clean water pump, the hot water pump or the oil pump, the chemical pump or the corrosion-resistant pump or the impurity pump, or use the non-blocking pump.
Pumps installed in the explosion area should be equipped with corresponding explosion-proof motors according to the explosion area.

3, according to the flow size, determine the choice of suction pump or double suction pump; according to the height of the lift, choose a single-stage pump or a multi-stage pump, high-speed pump or low-speed pump (air-conditioning pump), multi-stage pump efficiency is lower than single-stage pump, such as When the single-stage pump and the multi-stage pump are also available, the single-stage pump is first used.

4. Determine the specific model of the pump. After determining which series of pumps to use, you can press the maximum flow rate. (In the absence of the maximum flow rate, you can usually take 1.1 times of the normal flow rate as the maximum flow rate.) After taking the 5%-10% balance. The main parameters of the two performances of the head are determined on the type spectrum or series characteristic curve. The operation is as follows:
Using the pump characteristic curve, find the required flow value on the abscissa, find the required lift value on the ordinate, and draw the vertical or horizontal line from the two values ​​up and to the right respectively. The intersection of the two lines falls on the characteristic curve. This pump is the pump to be selected, but this ideal situation is generally rare, usually encounter the following two situations:
The first type: the intersection point is above the characteristic curve, which means that the flow meets the requirements, but the lift is not enough. At this time, if the lift phase is similar, or the difference is about 5%, it can still be used. If the lift difference is much different, select the pump with larger lift. . Or try to reduce the loss of pipeline resistance.
The second type: the intersection point is below the characteristic curve. In the fan-shaped trapezoidal range of the pump characteristic curve, the model is preliminarily determined, and then according to the difference of the head, it is determined whether to cut the impeller diameter.
If the head difference is small, it will not be cut. If the head difference is very large, the impeller diameter will be cut according to the required ns and cutting formula according to the required Q, H, and if the intersection point does not fall within the fan-shaped trapezoidal range, the head should be selected. Small pump. When selecting a pump, it is sometimes necessary to consider the production process requirements and select QH characteristic curves of different shapes.

5. After the pump model is determined, the pump that approximates the water in the physicochemical medium of the pump or the transport medium needs to go to the relevant product catalog or sample, and make a correction according to the performance table or performance curve of the model to see if the normal working point falls on the Pump priority work area? Whether the effective NPSH is greater than (NPSH). Can also reverse the geometric installation height with NPSH?

6. For liquid pumps with a viscosity greater than 20mm2/s (or a density greater than 1000kg/m3), the performance curve of the water test pump must be converted to the viscosity (or the density) performance curve, especially for the suction performance and The input power is carefully calculated or compared.

7. Determine the number of pumps and the backup rate:
For a normal operation of the pump, generally only one, because a large pump is equivalent to two small pumps working in parallel, (refer to the head, flow rate is the same), the efficiency of the large pump is higher than the small pump, so from the perspective of energy saving, choose one The Taida pump does not use two small pumps, but in the following cases, consider two pumps in parallel:
The flow is very large and one pump does not reach this flow.
For large pumps that require a 50% reserve rate, two smaller pumps can be operated, and two spares (three in total)
For some large pumps, pumps with 70% flow requirements can be operated in parallel without a backup pump. When one pump is overhauled, the other pump still carries 70% of the production.
For pumps that need to run continuously for 24 hours, three pumps should be used, one for operation, one for standby, and one for maintenance.

8. Under normal circumstances, customers can submit their “basic conditions for pump selection”, and we will select or recommend better pump products. If the design institute is designing the equipment, the model of the pump has been determined and configured according to the requirements of the design institute.

9. Determine the number of pumps and the backup rate:

For a normal operation of the pump, generally only one, because a large pump is equivalent to two small pumps working in parallel, (refer to the head, flow rate is the same), the efficiency of the large pump is higher than the small pump, so from the perspective of energy saving, choose one The Taida pump does not use two small pumps, but in the following cases, consider two pumps in parallel:

The flow is very large and one pump does not reach this flow.

For large pumps that require a 50% reserve rate, two smaller pumps can be operated, and two spares (three lifts)

For some large pumps, pumps with 70% flow requirements can be operated in parallel without a backup pump. When one pump is overhauled, the other lift pump still carries 70% of the production.

For pumps that need to run continuously for 24 hours, three pumps should be used for operation, one for standby and one for maintenance.