Superior initial concept and benchmark starting point of CQB15-15-65F Magnetic drive chemical pump design.
When designing devices and equipment, the design engineers shall determine the use and performance of the magnetic centrifugal pump and select the collapse type. This kind of selection starts with the selection of the type and form of the magnetic pump, so what is the principle for selecting the centrifugal water pump? What is the basis?
1.Understand the principle of mag drive pump selection
1. Make the type and performance of the selected mag drive pump meet the requirements of process parameters such as pump flow rate,pump Head, discharge pressure,water temperature, cavitation flow, suction head,speed,power etc.
2. It must meet the requirements of medium characteristics.
For pumps conveying flammable, explosive, toxic or valuable media, reliable shaft seals or leak free pumps are required, such as magnetic drive pumps, chemical pumps and canned pumps
For pumps conveying corrosive media, it is required that the convection parts should be made of corrosion-resistant materials, such as AFB stainless steel corrosion-resistant pumps and CQB engineering plastic magnetic drive pumps.
For pumps conveying medium containing solid particles, the convection parts shall be made of wear-resistant materials, such as high-chrome alloy metal,stainless steel,and the shaft seals shall be flushed with clean liquid when necessary.
3. High mechanical reliability, low noise and low vibration.
4. Economically, the total cost of equipment cost, operation cost, maintenance cost and management cost should be considered comprehensively.
5. Centrifugal pump is characterized by high speed, small volume, light weight, high efficiency, large flow, simple structure, no pulsation in infusion, stable performance, easy operation and maintenance.
Therefore, centrifugal pump shall be selected as far as possible except for the following situations:
a. The metering pump shall be selected when there are metering requirements
b. Reciprocating pumps can be selected when the requirements for lift are very high, the flow is very small, and there is no suitable small flow high lift centrifugal pump available. Vortex pumps can also be selected if the requirements for cavitation are not high
c. When the head is very low and the flow is large, axial flow pump and mixed flow pump can be selected.
d. When the medium viscosity is large (more than 650~1000mm2/s), rotor pump or reciprocating pump (gear pump, screw pump) can be considered
e. When the medium contains 75% air, the flow is small and the viscosity is less than 37.4mm2/s, the vortex pump can be selected.
f. In case of frequent startup or inconvenient pump filling, pumps with self-priming performance shall be selected, such as self-priming centrifugal pumps, self-priming vortex pumps, and pneumatic (electric) diaphragm pumps.
2、 Know the basic basis of mag drive chemical pump selection
The basis for magnetic pump selection shall be considered from five aspects according to the process flow and water supply and drainage requirements, including liquid delivery capacity, device lift, liquid property, pipeline layout and operating conditions
1. Flow rate(pump capacity) is one of the important performance data of pump selection, which directly affects the production capacity and delivery capacity of the whole unit. For example, the process design of the design institute can calculate the three flow rates of normal pump, superior small pump and superior large pump. When selecting the pump, take the superior large flow as the basis, and give consideration to the normal flow. When there is no superior large flow, usually 1.1 times of the normal flow can be taken as the superior large flow.
2. The Pump Head required by the device system is another important performance data for pump selection, which is generally selected by enlarging the head after 5% – 10% allowance.
3. The liquid properties include the name, physical properties, chemical properties and other properties of the liquid medium. The physical properties include temperature c, density d, viscosity u, solid particle diameter and gas content in the medium. This involves the head of the system, the calculation of effective cavitation allowance, and the type of appropriate pump. The chemical properties mainly refer to the chemical corrosivity and toxicity of the liquid medium. They are important basis for selecting pump materials and shaft seal types.
4. The pipeline layout conditions of the device system refer to the data such as the liquid feeding height, the liquid feeding distance, the liquid feeding direction, the superior low liquid level on the suction side, and the superior high liquid level on the discharge side, as well as the pipe specifications and their lengths, materials, pipe fitting specifications, and quantities, so as to facilitate the calculation of the system comb head and the check of the NPSH.
5. There are many operating conditions, such as liquid operation T saturated steam force P, suction side pressure PS (absolute), discharge side vessel pressure PZ, altitude, whether the ambient temperature operation is intermittent or continuous, and whether the pump position is fixed or movable.
3、 Specific operation of pump selection
According to the principles and basic conditions of pump selection, the specific operations are as follows:
1. Horizontal, vertical and other types (pipeline type, submersible type, submerged type, non clogging type, self-priming type, gear type, etc.) shall be selected according to the layout, terrain conditions, water level conditions and operation conditions of the unit.
2. According to the nature of the liquid medium, determine whether the clean water pump, hot water pump or oil pump, chemical pump, corrosion resistant pump or impurity pump, or use the non clogging pump.
Pumps installed in explosive areas shall adopt corresponding explosion-proof motors according to the grade of explosive areas.
3. Single suction pump or double suction pump shall be selected according to the flow; Single stage pump or multi-stage pump shall be selected according to the lift, and high speed pump or low speed pump (air conditioning pump) shall be selected. The efficiency of multi-stage pump is lower than that of single stage pump. If both single stage pump and multi-stage pump can be used, single-stage pump shall be selected first.
4. Determine the specific model of the pump
After determining the series of pumps to be selected, the specific model can be determined on the type spectrum or series characteristic curve according to the superior large flow (when there is no superior large flow, 1.1 times of the normal flow can usually be taken as the superior large flow), and the main parameters of the two performances, namely, the head after amplification of 5% – 10% margin, can be taken. The operation is as follows:
Use the pump characteristic curve to find the required flow value on the abscissa and the required lift value on the ordinate. From the two values, draw the vertical line or horizontal line up and to the right respectively. The intersection point of the two lines just falls on the characteristic curve, then the pump is the pump to be selected. However, this ideal situation is rarely seen, and the following two situations usually occur:
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 difference is not much, or the difference is about 5%, the pump can still be selected. If the head difference is large, the pump with larger head is selected. Or try to reduce the pipeline resistance loss.
Second, the intersection point is below the characteristic curve, within the fan trapezoid range of the pump characteristic curve, the model is preliminarily determined, and then whether to cut the impeller diameter is determined according to the difference in head,
If the head difference is very small, do not cut. If the head difference is very large, cut the impeller diameter according to the required Q, H, and ns and the cutting formula. If the intersection point does not fall within the range of the fan-shaped trapezoid, select the pump with smaller head. When selecting the pump, it is sometimes necessary to consider the production process requirements and select Q-H characteristic curves with different shapes.
5. After the pump model is determined, if the pump or the physical and chemical medium of the transmission medium is similar to water, it is necessary to go to the relevant product catalog or sample again and correct it according to the performance table or performance curve of the model to see whether the normal working point falls in the priority working area of the pump? Whether the valid NPSH is greater than (NPSH). The geometric installation height can also be corrected by NPSH in reverse?
6. For liquid pumps conveying viscosity greater than 20mm2/s (or density greater than 1000kg/m3), the characteristic curve of water test pump must be converted into the performance curve of the viscosity (or density), especially the suction performance and input power must be carefully calculated or compared.
7. Determine the number and standby rate of pumps:
Generally, only one pump is used for normal operation. Because one large pump is equivalent to two small pumps working in parallel (the head and flow are the same), and the efficiency of the large pump is higher than that of the small pump, so from the perspective of energy conservation, one large pump is preferred to two small pumps. However, in the following cases, the parallel cooperation of two pumps can be considered:
The flow is very large, and one pump cannot reach this flow.
For large pumps with 50% standby rate, two smaller pumps can be used for operation and two for standby (three in total)
For some large pumps, pumps with 70% flow requirements can be selected for parallel operation without standby pump. When one pump is overhauled, the other pump still undertakes 70% of the transportation in production.
For pumps that need to operate continuously for 24 hours, three pumps shall be standby, one for operation, one for standby and one for maintenance.
8. In general, customers can submit their “basic conditions for pump selection”, and our company will give model selection or recommend better pump products. If the design institute has determined the pump model when designing the device and equipment, the pump shall be configured according to the requirements of the design institute.
9. Determine the number and standby rate of pumps:
Generally, only one pump is used for normal operation. Because one large pump is equivalent to two small pumps working in parallel (the head and flow are the same), and the efficiency of the large pump is higher than that of the small pump, so from the perspective of energy conservation, one large pump is preferred to two small pumps. However, in the following cases, the parallel cooperation of two pumps can be considered:
The flow is very large, and one pump cannot reach this flow.
For large pumps with 50% standby rate, two smaller pumps can be used for operation and two for standby (three lifts in total)
For some large pumps, pumps with 70% flow requirements can be selected for parallel operation without standby pump. When one pump is overhauled, the other pump still undertakes 70% of the transportation in production.
For magnetic pumps that need to operate continuously for 24 hours, three pumps shall be standby for operation, one for standby and one for maintenance.
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