Water pump selection is the main design basis for the supporting power machine, transmission and auxiliary equipment and pump-related buildings. The pump selection manual needs to determine the appropriate pump type, model and number, etc., as well as the required lift, flow and change law of the project. , These are the design parameters determined in the engineering planning and design, so what principle should be used to select the pump? What is the basis?
Principles of pump selection
1. Make the type and performance of the selected pump meet the requirements of process parameters such as device flow, head, pressure, temperature, cavitation flow, suction lift, etc.
2. Must meet the requirements of medium characteristics
For pumps that transport flammable, explosive, toxic, or precious media, reliable shaft seals or leak-free pumps, such as magnetic drive pumps, diaphragm pumps, and canned pumps, are required
For pumps that transport corrosive media, the convection parts are required to use corrosion-resistant materials, such as DFP multi-stage corrosion-resistant centrifugal pumps
For pumps that transport media containing solid particles, the convection parts are required to be made of wear-resistant materials, and when necessary, the shaft seal is flushed with a clean liquid
3. High mechanical reliability, low noise and low vibration
4. Economically, the total cost of equipment costs, operating costs, maintenance costs and management costs should be considered as the lowest
5. The centrifugal pump has the characteristics of high speed, small size, light weight, high efficiency, large flow, simple structure, no pulsation of infusion, stable performance, easy operation and convenient maintenance, etc. Therefore, except for the following conditions, centrifugal pumps should be used as much as possible
a When there is a metering requirement, choose a metering pump
b. The head requirements are very high, the flow is small and there is no suitable small flow. When high-head centrifugal pumps are available, reciprocating pumps can be used, and vortex pumps can also be used when the cavitation requirements are not high.
c When the head is very low and the flow rate is large, an axial flow pump and a mixed flow pump can be used.
d When the medium viscosity is large (greater than 650~1000mm2/s), you can consider using a rotor pump or a reciprocating pump (gear pump screw pump)
When the air content of e medium is 75%, the flow is small and the viscosity is less than 37.4mm2/s, a vortex pump can be used.
f When starting frequently or pumping is inconvenient, pumps with self-priming performance should be selected, such as self-priming centrifugal pumps, self-priming vortex pumps, pneumatic (electric) diaphragm pumps
Water pump selection basis
The basis for the selection of water pumps should be considered from five aspects according to the process flow and water supply and drainage requirements, including liquid delivery volume, device head, liquid properties, piping layout, and operating conditions, etc.
1. Flow rate is one of the important performance data of pump selection, it is directly related to the production capacity and conveying capacity of the entire device. For example, in the process design of the design institute, the normal, minimum and maximum flow rates of the pump can be calculated. When selecting the pump, take the maximum flow rate as the basis and take into account the normal flow rate. When there is no maximum flow rate, usually 1.1 times the normal flow rate can be taken as the maximum flow rate.
2. The head required by the installation system is another important performance data for pump selection. Generally, the head should be selected by magnifying the head of 5% and 10%.
3. Liquid properties, including the name of the liquid medium, physical properties, chemical properties and other properties. The physical properties include temperature c, density d, viscosity u, solid particle diameter and gas content in the medium, etc., which involve the lift of the system, the effective gas Calculation of residual allowance and suitable pump type: chemical properties, mainly refer to the chemical corrosiveness and toxicity of the liquid medium, which is an important basis for the selection of pump materials and the selection of which type of shaft seal
4. The piping layout conditions of the device system refer to some data such as the liquid delivery height and the liquid delivery distance from the liquid delivery direction, the lowest liquid level on the suction side, the highest liquid level on the discharge side and other data and the pipe specifications and their length, materials, and pipe fitting specifications. , Quantity, etc., in order to calculate the comb head and check the cavitation margin
5. There are many operating conditions, such as liquid operation T saturated steam power, suction side pressure PS (absolute), discharge side container pressure PZ, altitude, ambient temperature operation is intermittent or continuous, and the position of the pump is fixed The ones are still removable.
Principles and basis for pump selection
Specific operation of pump selection
According to the principle of pump selection and the basic conditions of selection, the specific operations are as follows
1. According to the arrangement of the device, terrain conditions, water level conditions, and operating conditions, determine the choice of horizontal, vertical and other types (pipeline, submersible, submerged, non-clogging, self-priming, gear, etc.) Pump
2. According to the nature of the liquid medium, determine the clean water pump, hot water pump, oil pump, chemical pump, or corrosion-resistant pump or impurity pump, or use a non-clogging pump. The pump installed in the explosion area should use the corresponding explosion-proof motor according to the explosion area level. .
3.According to the flow rate, determine whether to choose a single-suction pump or a double-suction pump; according to the height of the head, choose a single-stage pump or a multi-stage centrifugal pump, a high-speed pump or a low-speed pump (air conditioning pump), and the efficiency of multi-stage pumps is lower than that of single-stage pumps. If both single-stage pumps and multi-stage pumps can be used, first select single-stage pumps
4. Determine the specific model of the pump
After determining what series of pumps to choose, you can press the maximum flow rate (when there is no maximum flow rate, usually 1.1 times the normal flow rate can be taken as the maximum flow rate), and take the main performance of the two performances of the head after amplifying 5% and 10%. Parameters, determine the specific model on the type spectrum or series characteristic curve. The operation is as follows
Use the pump characteristic curve to find the required flow value on the abscissa, and find the required head value on the ordinate, draw a vertical or horizontal line from the two values upwards and to the right, and the intersection of the two lines just falls on the characteristic curve, then This pump is the pump to be selected, but do not draw vertical or horizontal lines up and to the right. The intersection of the two lines just falls on the characteristic curve, then the pump is the pump to be selected, but this ideal situation is generally rare, usually Encountered 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 head is not enough.At this time, if the head is similar, or the difference is about 5%, it can still be used. If the head difference is much, the pump with the larger head should be selected. . Or try to reduce the pipeline resistance loss
The second type: the intersection point is below the characteristic curve, and within the fan-shaped trapezoid range of the pump characteristic curve, this model is initially determined, and then according to the difference in the head, it is decided whether to cut the impeller diameter. If the difference in the head is very small, it will not cut. If the difference is large, cut the impeller diameter according to the required Q, H, and ns and the cutting formula. If the intersection does not fall within the range of the fan-shaped trapezoid, a pump with a smaller head should be selected. When selecting a pump, sometimes it is necessary to consider the requirements of the production process and choose different shapes of QH characteristic curves
5. After the pump model is determined, the physical and chemical medium of the water pump or the transport medium is similar to water pumps, and the relevant product catalogs or samples need to be corrected according to the performance table or performance curve of the model to see whether the normal operating point falls on this Pump priority working area, whether the effective NPSH is greater than (NPSH). The geometric installation height can also be adjusted by NPSH in reverse
6.For liquid pumps with a viscosity greater than 20mm2/s (or a density greater than 1000kg/m3), the characteristic curve of the water test pump must be converted to the performance curve of the viscosity (or the density), especially the suction performance and The input power is carefully calculated or compared
7. Determine the number of pumps and reserve rate:
Generally, only one pump is used for normal operation, because a large pump is equivalent to two small pumps working in parallel (referring to the same head and flow), and the efficiency of the large pump is higher than that of the small pump, so it is better to choose one from the perspective of energy saving. A large pump does not need two small pumps, but in the following situations, it can be considered that the flow of two pumps in parallel is large, and one pump cannot reach this flow.
For large pumps that require a 50% standby rate, two smaller pumps can be changed to work, and two standby (three in total). For some large pumps, pumps with 0% flow requirements can be selected for parallel operation, without the need for standby pumps. When one pump is overhauled, the other pump is still responsible for 70% of the production. For pumps that need to run continuously for 24 hours, three pumps should be spared, one running, one standby, and one repair
8. Under normal circumstances, customers can submit their "basic conditions for pump selection", and our company will select or recommend better pump products. If the design institute has determined the pump model when designing the installation equipment, configure it according to the requirements of the design institute.
