Question About Pump Selection

The selection of the pump should be considered from five aspects according to the process flow, the water supply and drainage requirements, ie. the liquid’s delivery volume, the equipment’s head, the liquid’s properties, the pipeline arrangement and the operating conditions, etc.

1.Flow rate is one of the important performance data of the pump, which is  related to the production capacity and conveying capacity of the entire equipment directly .For example, the normal, minimum and maximum flow rates of the pump can be calculated by designing institute . To select a pump based on the maximum flow rate, taking into account the normal flow rate.If there is no maximum flow, it is usually possible to take 1.1 times of the normal flow rate as the maximum flow rate.

2. The head required for the system is another important performance data of the pump. Generally, it is necessary to use a head with a magnification of 5% to 10% to select the model.

3. Liquid properties, including liquid’s names, physical properties, chemical properties and other properties. The physical properties are temperature c density d, viscosity u, solid particle diameter and gas content in the medium, etc., which relate to the head of the system, the calculation of the effective cavitation allowance and the type of pump. The chemical nature mainly refers to the chemical corrosiveness and toxicity of the liquid medium. It is an important basis for selecting the pump material and selecting the shaft seal type.

4. The pipeline layout conditions of the equipment system refer to the data of the liquid feeding height, the liquid feeding distance, the lowest liquid level on the suction side, the highest liquid level on the discharge side, and other data and pipe specifications and their lengths, materials, pipe fittings specifications, quantities, etc. , In order to calculate the system head and check 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, ambient temperature, intermittent or continuous operation,fixed or removable position?

Specific Operation of Water Pump Selection

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

1.To determine whether to choose horizontal, vertical or other types (pipeline, submersible, submerged, non-blocking, self-priming, gear, etc.) according to the arrangement of the device, terrain conditions, water level and operating conditions.

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

3.To determine whether to choose single suction pump or double suction pump according to the flow rate; , to determine whether to choose single-stage pump or multi-stage pump,high-speed pump or low-speed pump (air-conditioning pump)according to the height of head. The efficiency of multistage pump is lower than that of single stage pump. Single-stage pump is preferred if both single-stage and multi-stage pump are applicable.

4,To determine the specific model of the pump

After determining which series of pump to use, you can determine the specific model on the characteristic curve according to the two main performance parameters,ie. a head with a magnification of 5% to 10% and the maximum flow rate(If there is no maximum flow, it is usually possible to take 1.1 times of the normal flow rate as the maximum flow rate).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.Then draw the vertical or horizontal line from the two values upward and rightward respectively. The intersection of the two lines falls on the characteristic curve. Above, the pump is the pump to be selected.The intersection of the two lines falls on the characteristic curve, and this pump is the pump to be selected.However, this ideal situation is rare. There are usually two situations: The first one: the intersection point is above the characteristic curve, which means that the flow meets the requirements, but the head is not enough.In this situation, if the head is about the same, or within 5%, it is still applicable.If the head difference is very large, choose a pump with a larger head. Or try to reduce the loss of pipeline resistance.

The second situation: the intersection point is below the characteristic curve, within the fan-shaped trapezoidal range of the pump characteristic curve.This model is preliminarily determined, then decided whether to cut the impeller diameter according to the difference of head.

If the head difference is small, cutting is unnecessary. If the head difference is big, the impeller diameter will be cut according to the required Q and H based on the required ns and cutting formula.If the intersection is not in the fan-shaped trapezoidal range, the pump with smaller head should be selected.When selecting a pump, it is necessary to consider the production process requirements sometimes and select Q-H characteristic curves of different shapes.

5.Regarding the water pump,or pump for delivering liquid which has similar physicochemical properties with water,after the pump model is determined, it is necessary to carry out the school modification according to the performance table or performance curve of the model in the relevant product catalog or sample.To check if the normal working point within the pump priority work area? Whether the effective NPSH is greater than (NPSH). Can also reverse the geometric installation height with NPSH? Geometric mounting height can also be changed according to NPSH.

6. For liquid pumps with a viscosity greater than 20mm2/s (or a density greater than 1000kg/m3),it is necessary to convert the water test pump characteristic curve into the performance curve at this viscosity (or this density), especially to calculate or compare the suction performance and input power carefully .

7. To determine the quantity of pumps and the backup rate:

For a normally operating pump, only one is used.Because one large pump is equivalent(the head and flow are the same) to two small pumps working in parallel, the efficiency of the large pump is higher than that of the small pump. Therefore, from the perspective of energy saving, it is better to choose one large pump instead of two small pumps. However, in the following cases, two pumps can be considered to work in parallel: The flow rate is very large, and one pump cannot reach this flow value.For large pumps that require a 50% reserve rate, two smaller pumps can be used (three in total).

For some large pumps, 70% flow pumps can be used in parallel, without backup pump. When one pump is being serviced, 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. Generally, customers can submit their “basic requirements for pump selection”, and we will select or recommend better pump products. If the design institute has determined the model of the pump when designing the equipment, it will be configured as required by the design institute.