Pump is a general-purpose machine with a wide range of applications. It is very important to correctly select the type and model of pump. This is directly related to whether the pump can meet the system design requirements, the reliability and service life of the pump, the purchase cost of the pump and the total cost in the life cycle. In reality, due to unreasonable model selection, many pumps are in abnormal operation, resulting in waste of energy, increased maintenance costs, and even can not be used normally at all. Therefore, the pump selection needs to be carried out carefully, and the advantages and disadvantages of different schemes should be compared repeatedly to ensure the normal operation of the pump.
The following are the principles and steps of pump selection:
1、 Pump selection principle
1. Meet the requirements
The performance of the selected pump shall meet the requirements of process flow design.
2. Simplify pump selection
The pump with simple structure is preferred. Because compared with the pump with complex structure, the pump with simple structure has the advantages of high reliability, convenient maintenance and low total cost in the life cycle. For example: single-stage and multi-stage pumps, vane pumps and reciprocating pumps.
3. Preferred centrifugal pump
Centrifugal pump has the characteristics of high speed, small volume, light weight, simple structure, no pulsation in infusion, stable performance, easy operation and maintenance. Therefore, centrifugal pumps should be selected as far as possible except for the following cases:
(1) When there are metering requirements, select metering pump;
(2) Vortex pump and reciprocating pump shall be selected for small flow and high head;
(3) When the flow is small and the head is high, and the flow (pressure) is required to have no pulsation, the vortex pump is selected;
(4) Axial flow pump and mixed flow pump shall be selected for large flow and low head;
(5) When the medium air content is 75%, the flow is small and the viscosity is less than 37.4mm2/s, the vortex pump is selected;
(6) When the medium viscosity is large (greater than 650~1000mm2/s), rotor pump and reciprocating pump (gear pump and screw pump) shall be selected;
(7) For occasions with frequent startup or inconvenient pump filling, select pumps with self-priming performance, such as self-priming centrifugal pumps, self-priming vortex pumps, and diaphragm pumps;
(8) When there is special need, choose other pumps, such as jet pump, hose pump, etc.
4. Special requirements
For pumps installed in explosive areas or special occasions, explosion-proof motors or other effective measures should be adopted according to the level of explosive areas.
2、 Pump selection steps
1. Confirm environmental conditions
Including ambient temperature, relative humidity, atmospheric pressure, air corrosivity, dangerous area grade, dust and water proof requirements.
2. Confirm the operating conditions
It refers to the liquid level pressure at the liquid suction side, the liquid level pressure at the discharge side, intermittent or continuous work, fixed position or movement, and the convenience of installation and maintenance.
3. Confirm the nature of the medium
Including medium name, temperature, density, viscosity, saturated steam pressure, solid particle diameter and content, gas content, corrosivity, volatility, flammability and toxicity.
4. Select the material of the flow passage parts of the pump
According to the physical and chemical properties of the pumped medium, the following aspects are mainly considered:
(1) Corrosivity of medium
The corrosion resistance of the flow passage parts of the pump can meet the use requirements. Do not pursue too high corrosion resistance, otherwise, the purchase cost of the pump will be greatly increased.
(2) Whether the medium contains solid particles
Because the hardness and content of solid particles are directly related to the durability of pump flow parts.
(3) Temperature (pressure) of medium
The higher the temperature (pressure) of the medium, the higher the material strength of the flow passage parts of the pump. Generally, when the temperature is >250 ℃, steel castings or steel parts should be selected.
(4) Media with sanitary grade or prohibited pollution requirements
Select the corresponding flow passage component material. In addition, the structure of the pump should be easy to clean.
5. Select pump type
Refer to "I. pump selection principle" in this article.
6. Select pump performance parameters
(1) Calculation method
A. Flow: it is directly related to the production capacity and transportation capacity of the whole device. For example, in the process design, three flow rates of normal pump, small pump and large pump can be calculated. When selecting a pump, it is based on large flow and takes into account the normal flow. In the absence of large flow, it is usually 1.1 times the normal flow as the large flow.
B. Head: generally, the head after enlarging the margin of 5%-10% shall be used for model selection. Special attention: the margin of centrifugal pump flow, especially head, should not be too large, otherwise, cavitation may occur when the pump is working, resulting in the flow and head failing to meet the requirements. Once this happens, generally, remedial measures such as cutting the outer diameter of the impeller or turning down the pump outlet valve can be taken.
C. Power: generally, the power form and size of the pump are selected by the manufacturer and marked in the product sample.
D. NPSH
Check whether the NPSH and necessary NPSH of the pump device match. When it cannot be met, effective measures should be taken to achieve it.
(2) Analogy method, test method
When the resistance value of the pump pipeline system cannot be accurately calculated, the flow, head and other performance parameters of the required pump can be determined by analogy or test method.
7. Select the pump installation type
According to the pipeline layout and installation site, horizontal, direct connection, vertical and other types (right angle type, variable angle type, angle type, double connection type, removable type) shall be selected.
8. Determine the number of pumps and 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, so from the perspective of energy conservation, it is better to choose a large pump instead of two small pumps, but in case of the following situations, the parallel cooperation of two pumps can be considered:
(1) The flow is large, and one pump cannot reach this flow.
(2) For pumps requiring 50% standby rate, three pumps can be configured as two-purpose and one standby.
(3) For some large pumps, the pumps with 70% flow requirements can be selected for parallel operation without standby pumps. When one pump is overhauled, the other pump still undertakes 70% of the production delivery.
(4) One pump that must operate continuously for 24 hours should be operated, one for standby and one for maintenance. There are three pumps in total. If necessary, the pump supplier can assist in the selection.
