When we select or design backwash pumps and flush pump equipment, we must first determine where the pump is used and the performance parameters of the pump, then select the type and form of the pump, and then determine the pump model, then in the backwash pump, flush pump What basic data are needed when selecting a model?
Principles for the selection of backwash pumps and flushing pumps
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, and suction.
2. Must meet the requirements of media characteristics.
For pumps that transport flammable, explosive, toxic, or valuable 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 AFB stainless steel corrosion-resistant pumps, and CQF engineering plastic magnetic drive pumps.
For pumps that transport solid particles, the convection parts are required to be made of wear-resistant materials, and the shaft seal should be flushed with clean liquid when necessary.
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 taken into account in the lowest.
5. 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.
Therefore, except for the following conditions, centrifugal pumps should be used as much as possible:
A. When metering is required, use metering pump
B. High head requirements, small flow and no suitable small flow, high head centrifugal pumps can be used, reciprocating pumps can be used, if cavitation requirements are not high, vortex pumps can also be used.
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)
E. When the medium has 75% air content, the flow is small and the viscosity is less than 37.4mm2/s, a vortex pump can be used.
F. For occasions with frequent start-up or inconvenient pumping, pumps with self-priming performance should be selected, such as self-priming centrifugal pumps, self-priming vortex pumps, and pneumatic (electric) diaphragm pumps.
Basic basis for the selection of backwash pumps and flushing pumps
Pump selection basis 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, which 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 a pump, take the maximum flow as the basis and take into account the normal flow. When there is no maximum flow, 1.1 times the normal flow can usually be taken as the maximum flow.
2. The head required by the device system is another important performance data for pump selection. Generally, the head should be selected after magnifying 5%-10% of the margin.
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. This relates to 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 type of shaft seal.
4. The piping layout conditions of the device system refer to the liquid delivery height, the liquid delivery distance and the liquid delivery direction, the lowest liquid level on the suction side, the highest liquid level on the discharge side, and some data and pipe specifications and their length, materials, pipe fitting specifications, and quantity Etc. in order to carry out the combing head calculation and cavitation margin check.
5. There are many operating conditions, such as liquid operation T saturated steam power P, suction side pressure PS (absolute), discharge side container pressure PZ, altitude, ambient temperature operation is intermittent or continuous, and the pump position is fixed The ones are still removable.
Specific operation of the selection of backwash pump and flush pump
According to the pump selection principle and basic conditions, the specific operation is 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. Determine the clean water pump, hot water pump, oil pump, chemical pump or corrosion-resistant pump or impurity pump according to the nature of the liquid medium, or use a non-clogging pump.
The pump installed in the explosion area should adopt 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 pump, high-speed pump or low-speed pump (air-conditioning pump), multi-stage pump efficiency is lower than single-stage pump, such as When both single-stage pumps and multi-stage pumps can be used, first select single-stage pumps.
4. Determine the specific model of the backwash pump and flush 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 two performances of the head after amplifying 5%-10% of the margin For the main parameters, determine the specific model 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, and find the required head value on the ordinate, draw a vertical or horizontal line from the two values upwards and to the right respectively, and the intersection of the two lines just falls on the characteristic curve, then This pump is the pump to be selected, but this ideal situation is generally rare, and the following two situations are usually encountered:
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 differs greatly, choose a pump with a larger head. . Or try to reduce the pipeline resistance loss.
The second type: the intersection point is below the characteristic curve and is within the fan-shaped trapezoid range of the pump characteristic curve. This model is preliminarily determined, and then according to the difference in head, whether to cut the impeller diameter,
If the head difference is very small, do not cut. If the head difference is large, cut the impeller diameter according to the required Q, H, and according to the ns and cutting formula. If the intersection point does not fall within the range of the fan-shaped trapezoid, choose a higher head. Small pump. When selecting a pump, sometimes it is necessary to consider the requirements of the production process and choose different shapes of Q-H characteristic curves.
5. After the pump model is determined, if the physical and chemical medium of the pump or the transport medium is similar to water, you need to go to the relevant product catalog or sample and make corrections according to the performance table or performance curve of the model to see if the normal operating point falls within this Pump priority working area? Is the effective NPSH greater than (NPSH). Can the geometric installation height be modified 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 into a performance curve of the viscosity (or density), especially the suction performance and The input power is carefully calculated or compared.
7. Determine the number of backwash pumps and flushing 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. A large pump is used instead of two small pumps, but in the following situations, two pumps can be considered in parallel cooperation:
The flow is very large, and a pump cannot reach this flow.
For large pumps that need a 50% standby rate, two smaller pumps can be changed to work, and two standby (three in total)
For some large pumps, pumps with 70% flow requirements can be used in parallel operation, instead of standby pumps. When one pump is overhauled, the other pump still bears 70% of the production.
For pumps that need to run continuously for 24 hours, three pumps should be backed up, one for operation, one for backup and one for maintenance.
8. Under normal circumstances, customers can submit their "basic conditions for pump selection", and our company will select models 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.
9. Determine the number of backwash pumps and flushing pumps and the standby 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. A large pump is used instead of two small pumps, but in the following situations, two pumps can be considered in parallel cooperation:
The flow is very large, and a pump cannot reach this flow.
For large pumps that need a 50% standby rate, two smaller pumps can be changed to work, and two standby (three lifts in total)
For some large pumps, pumps with 70% flow requirements can be used for parallel operation, without the need for standby pumps. When one pump is overhauled, the other lift pump still bears 70% of the production.
For pumps that need to run continuously for 24 hours, three pumps should be backed up for operation, one for backup and one for maintenance.
Classification basis of backwash pumps and flushing pumps
There are many types of pumps, which can be divided into: ① Power pumps, also called impeller pumps or vane pumps, rely on the dynamic action of the rotating impeller on the liquid to continuously transmit energy to the liquid, so that the kinetic energy of the liquid ( Mainly) and pressure energy increase, and then the kinetic energy is converted into pressure energy through the extrusion chamber, which can be divided into centrifugal pump, axial flow pump, partial flow pump and vortex pump. ② Positive displacement pumps, relying on the periodic change of the volume of the sealed working space containing the liquid, transfer energy to the liquid periodically, and increase the pressure of the liquid to force the liquid to be discharged. According to the movement form of the working element, it can be divided into reciprocating pumps. And rotary pump. ③Other types of pumps transfer energy in other forms. For example, the jet pump relies on the high-speed injection of working fluid to suck the fluid to be transported into the pump and mix it to exchange momentum to transfer energy; the water hammer pump uses part of the water flowing during braking to be raised to a certain height to transfer energy; the electromagnetic pump makes The energized liquid metal flows under the action of electromagnetic force to realize transportation. In addition, pumps can also be classified according to the nature, driving method, structure, and use of the conveyed liquid.
Backwash pump, flushing pump selection data
The basic parameters that characterize the main performance of the pump are as follows:
1, flow rate Q
Flow is the amount of liquid (volume or mass) delivered by the pump in a unit time.
The volume flow rate is expressed by Q, and the unit is: m3/s, m3/h, l/s, etc.
The mass flow rate is expressed by Qm, and the unit is: t/h, kg/s, etc.
The relationship between mass flow and volume flow is:
Qm=ρQ
where ρ——the density of the liquid (kg/m3, t/m3), clean water at room temperature ρ=1000kg/m3.
2, head H
The head is the increase in energy of the unit weight of liquid pumped by the pump from the pump inlet (pump inlet flange) to the pump outlet (pump outlet flange). That is, the effective energy of one Newtonian liquid through the pump. Its unit is N·m/N=m, that is, the height of the liquid column of the pumped liquid, which is often referred to as meter for short.
3, rotation speed n
Rotational speed is the number of revolutions per unit time of the pump shaft, represented by the symbol n, and the unit is r/min.
4, NPSH
The cavitation margin is also called the net positive tip, which is the main parameter indicating the cavitation performance. The NPSH has been expressed by Δh in China.
5. Power and efficiency
The power of the pump usually refers to the input power, that is, the power on the shaft of the prime mover to support the pump, so it is also called the shaft power, represented by P;
The effective power of the pump is also called the output power, expressed by Pe. It is the effective energy obtained in the pump by the liquid delivered from the pump per unit time.
Because the head refers to the effective energy obtained from the pump by the unit heavy liquid output by the pump, the product of the head, mass flow and gravitational acceleration is the effective energy obtained from the liquid output from the pump per unit time-namely the pump Effective power:
Pe=ρgQH(W)=γQH(W)
in the formula ρ——the density of the liquid delivered by the pump (kg/m3);
Γ——The gravity of the liquid delivered by the pump (N/m3);
Q——The flow rate of the pump (m3/s);
H——the head of the pump (m);
G——Acceleration of gravity (m/s2).
The difference between shaft power P and effective power Pe is the power loss in the pump, and its magnitude is measured by the efficiency of the pump. The efficiency of the pump is the ratio of the effective power to the shaft power, expressed by η.
