The basis for water pump selection should be based on the process flow and water supply and drainage requirements, and should be considered from five aspects, namely liquid delivery volume, device head, liquid properties, pipeline layout and operating conditions.
1. Flow rate is one of the important performance data for pump selection, which is directly related to the production capacity and delivery capacity of the entire device. For example, the design institute can calculate the normal, minimum and maximum flow rates of the pump in the process design. When selecting a pump, the maximum flow rate is used as the basis, while taking the normal flow rate into consideration. When there is no maximum flow rate, 1.1 times the normal flow rate can usually be taken as the maximum flow rate.
2. The head required by the installation system is another important performance data for selecting a pump. Generally, the head should be enlarged by 5%-10% to select the model.
3. Liquid properties, including the name of the liquid medium, physical properties, chemical properties and other properties. Physical properties include temperature c, density d, viscosity u, diameter of solid particles in the medium and gas content, etc. This involves the system head, effective cavitation margin calculation and suitable pump type: Chemical properties, mainly refer to the chemical corrosiveness and toxicity of the liquid medium, which is an important basis for selecting pump materials and which type of shaft seal to choose.
4. The piping arrangement conditions of the device system refer to the liquid delivery height, liquid delivery distance, liquid delivery direction, the lowest liquid level on the suction side, the highest liquid level on the discharge side and other data as well as the pipeline specifications and their length, materials, pipe fittings specifications, quantity, etc., so as to calculate the system head and check the cavitation margin.
5. The operating conditions include many aspects, such as the liquid operating T, saturated steam force P, suction side pressure PS (absolute), discharge side container pressure PZ, altitude, ambient temperature, whether the operation is intermittent or continuous, and whether the pump position is fixed or movable.
Specific operation of pump selection
According to the pump selection principles and basic conditions, the specific operations are as follows:
1. According to the layout of the device, terrain conditions, water level conditions, and operating conditions, determine the selection of horizontal, vertical, and other types of pumps (pipeline, submersible, submersible, non-clogging, self-priming, gear, etc.).
2. According to the properties of the liquid medium, determine whether to use a clean water pump, hot water pump, oil pump, chemical pump, corrosion-resistant pump, impurity pump, or non-clogging pump. Pumps installed in explosive areas should use corresponding explosion-proof motors according to the level of the explosive area.
3. According to the flow rate, determine whether to choose a single-suction pump or a double-suction pump; according to the head, choose a single-stage pump or a multi-stage pump, a high-speed pump or a low-speed pump (air-conditioning pump). The efficiency of a multi-stage pump is lower than that of a single-stage pump. If both single-stage and multi-stage pumps can be used, choose a single-stage pump first.
Determine the specific model of the pump
After deciding which series of pump to use, you can determine the specific model on the model spectrum or series characteristic curve based on the two main performance parameters: maximum flow (when there is no maximum flow, 1.1 times the normal flow can usually be taken as the maximum flow) and head after amplifying 5%-10% margin. The operation is as follows:
Using the pump characteristic curve, find the required flow value on the horizontal axis and the required head value on the vertical axis. Draw vertical or horizontal lines upward and to the right from the two values. If the intersection of the two lines falls exactly on the characteristic curve, then the pump is the one to be selected. However, this ideal situation is generally rare. Usually, the following two situations will occur:
The first type: The intersection 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%, it can still be selected. If the head difference is large, choose a pump with a larger head. Or try to reduce the pipeline resistance loss.
The second type: The intersection is below the characteristic curve, within the fan-shaped trapezoidal range of the pump characteristic curve, and the model is initially determined. Then, depending on the difference in head, it is decided whether to cut the impeller diameter.
If the head difference is small, no cutting is done. If the head difference is large, the impeller diameter is cut according to the required Q, H, and ns and the cutting formula. If the intersection does not fall within the fan-shaped trapezoidal range, a pump with a smaller head should be selected. When selecting a pump, sometimes the production process requirements must be considered and different shapes of QH characteristic curves should be selected.
1. After the pump model is determined, for water pumps or pumps whose physical and chemical media are similar to water, it is necessary to go back to the relevant product catalog or sample and make corrections based on the performance table or performance curve of the model to see if the normal working point falls within the priority working area of the pump? Is the effective NPSH greater than (NPSH). Conversely, the geometric installation height can also be corrected based on NPSH?
2. For pumps that transport liquids with a viscosity greater than 20mm2/s (or a density greater than 1000kg/m3), the pump characteristic curve tested with water must be converted into a performance curve for that viscosity (or density), and in particular, the suction performance and input power must be carefully calculated or compared.
Determine the number of pumps and the reserve rate:
a. For a pump that is in normal operation, generally only one is used, because a large pump is equivalent to two small pumps working in parallel (with the same head and flow rate), and a large pump is more efficient than a small pump. Therefore, from the perspective of energy saving, it is better to choose a large pump instead of two small pumps. However, in the following situations, two pumps can be considered to work in parallel: the flow rate is very large and one pump cannot reach this flow rate.
b. For large pumps that need a 50% standby rate, two smaller pumps can be used and two can be used as standby (three in total)
c. For some large pumps, pumps that meet 70% of the flow requirements can be selected for parallel operation without a spare pump. When one pump is under maintenance, the other pump can still take on 70% of the production.
d. For pumps that need to run continuously for 24 hours, three pumps should be kept in reserve, one for operation, one for reserve, and one for maintenance.
