Centrifugal Pump selection
Pump selection
Pump selection is based on the relevant parameters, characteristics and conditions of the conveying system. The following are the necessary conditions for selection:
1. Process parameters. To determine the flow and head of each pump, it is necessary to design the pipeline of the delivery system first, determine the total delivery volume, and then calculate the flow of each pump, and then calculate the head of each pump according to the pipeline system.
Self-balancing multistage centrifugal pump
2. The physical and chemical properties of the conveying medium. Including medium name, medium characteristics (such as corrosiveness, abrasiveness, toxicity, etc.), solid particle content and particle size density, viscosity, vaporization pressure, etc.
3. Other factors. When selecting the type, it is necessary to consider the restriction of site conditions, project cost, installation height, etc., so as to decide which pump type to use, such as vertical pump, horizontal pump, and submerged pump.
4. The reliability and energy consumption of the pump depend on whether the pump is operating at the best efficiency point
Water pump selection and design
(1), pump design selection elements
The main performance parameters of the pump The performance parameters of the pump mainly include flow and head, in addition to shaft power, efficiency, speed, and necessary cavitation allowance.
1. Flow rate refers to the amount of liquid output through the pump outlet per unit time, generally volume flow or mass flow is used. The volume of the liquid actually delivered by the pump per unit time is called the flow rate of the pump.
is usually represented by the letter Q. The unit is: m3/h, L/s
The flow rate of the water pump, that is, the water output, is generally not suitable to be too large, otherwise, the cost of purchasing the water pump will increase; the large flow rate and high power will also increase the cost of line electrical control. Specific issues should be analyzed in detail. 2. Head refers to the increased value of energy per unit weight of liquid flowing through the pump, that is, the difference between the water pressure of the liquid at the pump outlet and the inlet, usually represented by the letter H. The unit is: m. Standard use: Pa, hpa, KPa, Ⅶa. There are also: mmhg, kgf/cm2.
The so-called head refers to the total head required, not the lifting height, which is particularly important for choosing a pump. The pump lift is approximately 1.15~1.20 times the lifting height. For example, the vertical height from a certain water source to the place where the water is used is 20 meters, and the required lift is about 23 to 24 meters. When selecting a water pump, the lift on the pump sample (nameplate) should be close to the required lift. In this case, the pump has the highest efficiency and will be more economical to use. But it is not necessarily required to be absolutely equal. As long as the general deviation does not exceed 20%, the pump can work with more energy-saving conditions.
3. Inlet pressure Ps and outlet pressure Pd. Inlet and outlet pressure refer to the pressure at the flange of the pump inlet and outlet pipe. The size of the inlet and outlet pressure affects the pressure resistance of the shell and the requirements of the shaft seal.
4. Temperature T refers to the inlet medium temperature of the pump. Generally, the normal, minimum and maximum temperature of the pump inlet medium during the process should be given.
5. Device NPSH NPSHa, also known as effective NPSH.
6. Operating status: There are two types of continuous operation and intermittent operation.
7. Pump performance curve The curve representing the relationship between the main performance parameters is usually called the pump performance curve or characteristic curve. In essence, the pump performance curve is the external manifestation of the liquid movement in the pump, which is obtained through actual measurement.
The characteristic curve includes: flow-head curve (QH), flow efficiency curve (Q-η), flow-power curve (QN), flow-NPSH curve (Q-(NPSH)r), the function of the performance curve is For any flow point of the pump, a set of relative head, power, efficiency and NPSH values can be found on the curve. This set of parameters is called the working state, referred to as the working condition or working point. The centrifugal pump is the highest The operating condition of the efficiency point is called the optimal operating point, and the optimal operating point is generally the design operating point.
Pump performance curve function and type
The rated parameters of general centrifugal pumps are the design operating point and the best operating point coincide or very close. It is important to know the performance parameters of the pump to run in the practical efficiency range, which saves energy and can ensure the normal operation of the pump.
Including test data plotting curve, specified parameter plotting curve, multi-point parameter plotting curve, cutting (variable speed plotting curve), viscosity conversion curve plotting, axial flow pump plotting, similarity conversion plotting curve, parallel series curve device characteristic curve, general characteristics Curve etc. The graph can take many forms
The nameplate data is applied to a specific operating point. Different pumps have different performance curves. Different pumps have different shaft power. Power and flow diagrams. Different pumps have different shaft power curves. The power and flow curves of centrifugal pumps are operated in parallel. Type spectrum of performance curve pump
(2) Physical and chemical properties of conveying medium
The physical and chemical properties of the conveying medium directly affect the performance, material and structure of the pump, which is an important factor to be considered when selecting a model. The physical and chemical properties of the medium include: medium name, medium characteristics (such as corrosion, corrosion resistance, toxicity, etc.), solid particle content and particle size, specific gravity, density, viscosity, vaporization pressure, etc. If necessary, the gas content in the medium should also be listed, indicating whether the medium is easy to crystallize.
(3) Site conditions
1. The site conditions include the installation location of the pump (indoor and outdoor), ambient temperature, relative humidity, atmospheric pressure, atmospheric corrosion and the classification of hazardous areas.
Hazardous areas include: flammable, explosive, toxic, high temperature, high altitude and other special occasions.
The selection of the pump is based on the working environment and conditions of the pump, the performance parameters necessary for the normal operation of the pump, and the physical and chemical properties of the conveyed medium. The technical performance indicators of the pump in the pump system, the selection of pump materials, and the matching of the motor are fully considered. , Seal reliability, energy saving, use and maintenance and other comprehensive economic indicators, select the most suitable pump type and model specifications from the finalized pump products.
The principle of rational selection of pumps requires comprehensive consideration of comprehensive technical and economic indicators such as the investment and operating costs of pump units and pumping stations, so that they conform to the principles of economy, safety and application. Specifically, there are the following aspects:
2. The selected water pump should not only be small in size, light in weight, low in cost, but also have good characteristics and high efficiency; low investment, low operation and maintenance costs. It is a product with advanced technology, reasonable economy, mature and reliable, and has high flexibility, which can meet the needs of operation mode.
3. It has good anti-cavitation performance, which can reduce the excavation depth of the pump house without causing cavitation of the pump, stable operation, long life, high reliability, low noise and low vibration.
4. The total cost is the lowest considering the initial purchase cost, operation cost, maintenance cost and management cost.
5. Must meet the requirements of media characteristics.
①For pumps that transport flammable, explosive, toxic or precious media, reliable shaft seals or leak-free pumps, such as magnetic drive pumps, diaphragm pumps, and canned pumps, are required
②For pumps that transport corrosive media, the flow parts are required to use corrosion-resistant materials. A. The flow parts and shaft seals should meet the corrosive (pH value) requirements of the liquid to be transported; B. The liquid flow parts with strong corrosiveness should be made of non-metallic materials, such as: fluoroplastics, ceramics, glass fiber reinforced plastics , Graphite, etc.; C. In general, metal pumps have better wear resistance, pressure resistance, and high temperature resistance than non-metal pumps, while non-metal pumps such as fluoroplastic pumps have better corrosion resistance than metal pumps.
③For pumps that transport corrosive liquids containing solid particles, the flow part should be made of corrosion-resistant and wear-resistant metal corrosion-resistant materials, such as high-silicon cast iron, duplex stainless steel, nickel-chromium alloy, special alloy steel, etc. The shaft seal is flushed with clean liquid.
④ According to the ratio of the size and content of the fixed particles contained in the liquid to be transported, different flow parts are selected. A. When the conveyed liquid contains large and many solid particles, you can choose a non-clogging design of the flow component pump, such as a swirl pump, a single-channel impeller pump; B. According to the hardness of the solids contained in the conveyed liquid , Content and corrosiveness to choose the material of the wetted parts.
⑤For toxic, flammable, explosive, and peculiar odor liquids, precious media, aseptic transportation, vacuum transportation, etc., the principle of not allowing leakage should be complied with. The sealing part of the pump is required to be safe and reliable, or canned pump, magnetic pump, diaphragm should be selected Pump etc.
Containing long-fiber liquid: A. The flow part adopts a structure with tearing and cutting to ensure that the impeller is not entangled by long fibers; B. Choose a channel-type, open-type or single-blade impeller without clogging; C. Consider using a motor Mechanical characteristics (overload capacity, etc.).
⑦High temperature and high pressure liquids: The flow parts are made of high temperature and high pressure resistant materials, which can be made of ductile iron, cast steel and chrome steel in sequence. A. Consider the requirements of mechanical strength and thermal expansion of the flow parts under high temperature and high pressure; B. Choose high temperature and high pressure shaft seals and cooling devices
After the pump medium characteristics are determined, the specific pump series, structure and materials can be selected according to the process parameters and medium characteristics
6. Must meet the installation requirements of the site environment:
① For pumps installed in places where corrosive gas exists, measures to prevent atmospheric corrosion are required.
②For pumps installed in an outdoor environment with a temperature lower than -20℃, it is required to consider the cold and brittleness of the pump, use low-temperature resistant materials, and use cylindrical double-shell centrifugal pumps for the structure.
③Environmental noise: The pump has low noise requirements, and low noise design of sliding bearing structure and hydraulic power should be considered, and a low noise motor should be used.
④Relative humidity: when used in a high humidity environment, anti-rust measures should be taken into consideration for the pump; the protection and insulation grade of the equipped motor should also be considered
⑤Power grid conditions: whether the capacity is sufficient; the frequency of the power grid (domestic 50Hz, foreign 60Hz and frequency conversion usage); AC and DC power supply;
The grid voltage is three-phase (380V), single-phase (220V), or different voltage
(including high voltage above 660v), the range of power supply voltage change; the voltage drop between the power supply and the pump location (long-distance cable transportation).
⑥ For pumps that are overhauled once a year, the continuous operation period of the pump should generally not be less than 8000 hours, and should comply with GB5656-2008 (|s05199) or ANSB73.1M standards. Petroleum, petrochemical, chemical and other industries require pumps to be overhauled once every three years. They should meet the requirements of API610 (Tenth Edition) and operate continuously for at least 3 years.
⑦ For pumps installed in explosive areas, explosion-proof motors of corresponding levels should be used according to the level of the explosive area
⑧ Special pumps should be selected for applications with special requirements, such as vacuum transportation, high temperature and high pressure transportation, and low temperature transportation.
7. When determining the pump model and manufacturer, the pump performance, efficiency, quality, energy consumption, reliability, installation and maintenance, price and manufacturing specifications should be considered comprehensively. Choose a pump with high speed, small size, light weight, high efficiency, large flow, simple structure, no pulsation in infusion, stable performance, easy operation and convenient maintenance. When all parameters meet the selection criteria and there are more than two specifications, the model with a high comprehensive index shall be the final selected model. Specifically, the following parameters can be compared: the higher efficiency is better, the lighter weight is better, and the lower price is better
8. Except for the following conditions, centrifugal pumps should be used as much as possible
① When there is a metering requirement, choose a metering pump.
②. High head requirements, small flow and no suitable small flow high head centrifugal pumps can be used, reciprocating pumps can be used; vortex pumps can also be used when cavitation requirements are not high.
③. When the head is very low and the flow is large, the axial flow pump and the mixed flow pump can be used
④ When the medium viscosity is large (greater than 6501000mm2/s), rotor pump or reciprocating pump (gear pump, screw pump) can be considered.
⑤. When the medium gas content is greater than 5%, the flow rate is small, and the viscosity is less than 37.4m2/s, a vortex pump can be used.
⑥. For occasions with frequent start-up or inconvenient pump filling, pumps with self-priming performance should be selected, such as self-priming centrifugal pumps, self-priming vortex pumps, pneumatic (electric) diaphragm pumps
Fourth, the basis for pump selection
Pump selection principle should be considered from five aspects according to the process flow and water supply and drainage requirements, including liquid delivery volume, device head, liquid properties, pipeline layout and operating conditions.
1. Flow rate is one of the important performance data of pump selection, which is directly related to the production capacity and conveying capacity of the entire device. For example, in the process design of the design institute, the normal, minimum and maximum flow rates of the pump can be calculated. When selecting the pump, take the maximum flow as the basis and take into account the normal flow. When there is no maximum flow, 1.1 times the normal flow can usually be taken as the maximum flow.
6. When giving the rated, minimum and maximum temperature of the pump inlet medium during the process, the maximum temperature should be used as the basis.
7. The inlet and outlet pressures refer to the pressure at the flange of the pump inlet and outlet. The inlet and outlet pressure affects the pressure resistance of the shell, the selection of the shaft seal and the cooling device.
8. Matching power inspection. When the pump is in normal operation, use a clamp meter to capture the current. If ≤70%, it means that the motor is equipped with a large amount, if ≥30%, it means that the motor is equipped with a small amount. Due to unreasonable model selection, many pumps are in unreasonable operating conditions, low operating efficiency, and a lot of energy wasted. Some pumps cannot be used at all due to unreasonable model selection, or increase in maintenance costs and low economic benefits. It can be seen that a reasonable selection of pumps can save energy.
