Pump selection principles and procedures
The pump is a general-purpose machine with a wide range of applications and a wide variety of applications. It is important to choose the type and type of pump. This is directly related to whether the pump can meet system design requirements, pump reliability and service life, pump acquisition cost and total cost of life cycle. In reality, due to unreasonable selection, many pumps are operating in an abnormal state, resulting in wasted energy, increased maintenance costs, and even no normal use. Therefore, the selection of the pump needs to be carried out carefully, and the advantages and disadvantages of different schemes are repeatedly compared to ensure the normal operation of the pump. The following are the principles and steps for selecting a pump:
First, the principle of pump selection
Meet the requirements
The performance of the selected pump should meet the process design requirements.
2. Select the pump from Jane
Preference is given to pumps with a simple structure. Because the pump with simple structure is superior to the pump with complex structure, the former has the advantages of high reliability, convenient maintenance, and low total cost in the life cycle. For example: single-stage pumps and multi-stage pumps, vane pumps and reciprocating pumps.
3. Preferred centrifugal pump
The centrifugal pump has the characteristics of high speed, small volume, light weight, simple structure, no pulsation of infusion, stable performance, easy operation and convenient maintenance. Therefore, in addition to the following conditions, the centrifugal pump should be used as much as possible:
(1) When there is a measurement requirement, the metering pump is selected;
(2) When the small flow rate and high lift are used, the vortex pump and the reciprocating pump are selected;
(3) When the small flow rate and high lift are required, and the flow (pressure) is required to be pulsation-free, the vortex pump is selected;
(4) When the high flow rate and low lift are used, the axial flow pump and the mixed flow pump are selected;
(5) When the medium gas content is 75%, the flow rate is small and the viscosity is less than 37.4mm2/s, the vortex pump is selected;
(6) When the viscosity of the medium is large (greater than 650~1000mm2/s), the rotor pump and the reciprocating pump (gear pump, screw pump) are selected;
(7) For occasions with frequent start-up or inconvenient pumping, pumps with self-priming performance, such as self-priming centrifugal pumps, self-priming vortex pumps, diaphragm pumps;
(8) When there is special need, use other pumps, such as jet pump, hose pump, etc.
4. Special requirements
Pumps installed in explosive areas or special occasions should be equipped with explosion-proof motors or other effective measures depending on the level of the explosion area.
Second, the pump step
1. Confirm environmental conditions
Including ambient temperature, relative humidity, atmospheric pressure, air corrosivity, hazardous area level, dust and water resistance requirements.
2. Confirm the operating conditions
Refers to liquid suction side liquid level pressure (absolute), discharge side liquid level pressure, intermittent or continuous operation, fixed or moving position, installation and maintenance convenience.
3. Confirm the nature of the media
Including media name, temperature, density, viscosity, saturated vapor pressure, solid particle diameter and content, gas content, corrosivity, volatility, flammability, toxicity.
4. Select the material of the pump overcurrent part
According to the physical and chemical properties of the pumped medium, the following aspects are mainly considered:
(1) Corrosion of the medium
The corrosion resistance of the pump over-current component can meet the requirements of use, and it is not possible to pursue excessive corrosion resistance. Otherwise, the purchase cost of the pump will be greatly increased.
(2) Does the medium contain solid particles?
Because the hardness and content of solid particles are directly related to the durability of the pump overcurrent components.
(3) Temperature of the medium (pressure)
The higher the temperature (pressure) of the medium, the higher the material strength of the pump overcurrent components. Generally, when the temperature is >250 °C, steel or steel parts should be used.
(4) Media with sanitary or no pollution requirements
Select the appropriate overcurrent part material. In addition, the structure of the pump should be easy to clean.
5. Selected pump type
Refer to the article “I. Pump Selection Principles”.
6. Selected pump performance parameters
(1) Calculation method
A. Flow: It is directly related to the production capacity and delivery capacity of the entire unit. For example, the normal, minimum, and maximum flow rates of the pump can be calculated in the process design. When the pump is selected, the normal flow rate is based on the maximum flow rate. In the absence of the maximum flow rate, 1.1 times the normal flow rate can be taken as the maximum flow rate.
B. Head: Generally, the head should be selected by amplifying the 5%-10% balance. Special attention: The flow of the centrifugal pump, especially the head of the lift should not be too large. Otherwise, cavitation may occur during the operation of the pump, resulting in less flow and head. Once this occurs, generally, remedial measures can be taken to cut the outer diameter of the impeller or to close the pump outlet valve.
C. Power: Generally, the pump power and form are selected by the manufacturer and marked in the product sample.
D. Cavitation allowance
Check whether the NPSH of the pump is correct and whether the NPSH must match. When it is not met, effective measures should be taken to achieve it.
(2) Analogy method, test method
When the resistance value of the pump piping system cannot be accurately calculated, the performance parameters such as the flow rate and head of the required pump can be determined by analogy or test method.
7. Selected pump installation type
According to the pipeline layout and installation site, horizontal, direct, vertical and other types (right angle, variable angle, angle, double, and split) are selected.
8. Determine the number of pumps and the backup rate
For a normal operation of the pump, generally only one, because a large pump is equivalent to two small pumps working in parallel, (refer to the head, flow rate is the same), the efficiency of the large pump is higher than the small pump, so from the perspective of energy saving, choose one The Taida pump does not use two small pumps, but in the following cases, consider two pumps in parallel:
(1) The flow rate is very large, and one pump cannot reach this flow rate.
(2) For pumps that require a 50% reserve rate, they can be used in two configurations, for a total of three configurations.
(3) For some large pumps, the pumps with 70% flow requirements can be operated in parallel, without the standby pump. When one pump is overhauled, the other pump still bears 70% of the production.
(4) Pumps that must be operated continuously for 24 hours should be operated one, one spare, one repair, three in total. If necessary, the pump provider can assist with the selection.