Basic knowledge of pumps and selection principles and experience
Basic knowledge of pumps and selection principles and experience
Pump terminology
1, pump -- the prime mover mechanical energy into fluid energy machine or mechanism called pump. The pump is used to increase the potential energy, pressure energy and kinetic energy of the liquid (high speed liquid flow).
2. Flow rate -- the amount (volume or mass) of liquid flowing through a section per unit time. Let's call it Q. The unit is M3 /h, m3/s, and L /min.
3, head - unit mass of liquid from the pump inlet to the outlet of the increase in energy. Let's call it H. Unit: MPa, m.
4, speed -- the number of revolutions per unit time of the shaft. Let's call it n. The unit is r/min.
5. Belt power -- rated power of the prime mover. Let's call it N. The unit is kW.
6, shaft power -- the prime mover to the pump shaft power. Let's call it P. The unit is kW.
7, effective power -- also known as output power, unit time from the pump out of the liquid in the pump to obtain effective energy. Let's call it Pa. The unit is kW.
8. Efficiency - the ratio of effective power to shaft power. I'm going to use η.
9. NPSH - The amount of energy per unit mass of liquid at the pump inlet that exceeds the evaporation pressure of the liquid (expressed in meters of liquid column). This is denoted by NPSH. The unit is m.
10, must NPSH -- by the pump plant according to the test to determine the NPSH (meter liquid column). Let's call it NPSHr. The unit is m. Pump NPSH is determined by the characteristics of the pump itself, is the pump itself anti cavitation performance parameters. To improve the anti-cavitation performance of the pump itself, it is necessary to reduce the NPSH as much as possible.
11, suction vacuum height, also known as suction vacuum, from the pump datum level of the suction inlet vacuum (meter liquid column). Let's call it Hs. The unit is m. The old samples in China reflect the cavitation performance of pump by suction vacuum height, now this index has been eliminated. Conversion of Hs and NPSHr values is as follows:
10 - Hs NPSHr material
12, series - the pump series refers to the pump factory production of the same type of structure and use of the pump. Such as IS series clean water pump, IH series chemical pump, S series open pump.
13, viscosity - viscosity is the internal friction resistance of lubricating liquid. There are many units of viscosity, which are described below.
(1) dynamic viscosity (absolute viscosity) μ
In the SI system, a force of 1N is required to cause a relative movement of 1m/s between two parallel layers of fluid separated by 1m and each with an area of 1m2. The dynamic viscosity of the fluid is 1Pa•s (pa •s). 1Pa•s = 1N•s /m2, commonly used as mPa. In CGS system, the unit of dynamic viscosity is P (poise), and cP (centipoise) is often used because the unit of P is too large. The relation between it and international units is: 1cP = 0.001Pa•s.
(2) kinematic viscosity ν
Usually the viscometer measures not the dynamic viscosity μ, but μ/ρ. ρ is the density of the fluid, in units of kg/m2 or g/cm2. This ratio is called kinematic viscosity ν, i.e. ν = μ/ρ. Kinemastic viscosity is m2/s in si and (St) in CGS, 1St = 1cm2/s, and cSt = 1mm2/s, 1St = 100cSt.
(3) The conditional viscosity η
In addition to μ and ν, in practice also often encountered a variety of conditional viscosity, such as Engler viscosity (oE), Sayler viscosity (SUS, Sayler second) and Viscosity (RSS, Rayler second). The commonly used conditional viscosity in Our country are Engler viscosity and Sayler viscosity.
Pump classification
According to the working principle and structure of the pump classification is as follows:
The working principle of vane pump is to transfer the mechanical energy of the prime mover to the liquid by the action of the blade on the liquid.
The working principle of the positive displacement pump is to use the working volume to transport liquid periodically.
Vane pump flow changes with pressure, positive displacement pump in a certain speed or pump speed (reciprocating times) under the flow is certain, almost does not change with pressure; Vane pump generally does not have self-priming capacity, and positive displacement pump has self-priming capacity, pump can be pumped after the pipeline air suction liquid; In contrast, blade pump suitable for low pressure and large flow, positive displacement pump suitable for high pressure and small flow; In general, positive displacement pumps are more efficient than vane pumps.
Principles and steps of pump selection
Pump is a very wide range of applications, a wide variety of general machinery, the correct choice of pump type and model is very important. 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 of the life cycle. In reality, due to unreasonable selection, many pumps are in an abnormal state of operation, resulting in waste of energy, increased maintenance costs, and even normal use. Therefore, the selection of the pump needs to be carried out carefully, and the pros and cons of different schemes are repeatedly compared to ensure the normal operation of the pump. The following are the principles and steps of pump selection:
First, the principle of pump selection
1. Meet the requirements
The performance of the selected pump should meet the design requirements of the process flow.
2. Choose a simple pump
The pump with simple structure is preferred. Because of the simple structure of the pump compared with the complex structure of the pump, the former has high reliability,
Convenient maintenance and low total cost within the life cycle. For example: single stage pump and multistage pump, vane pump and reciprocating pump.
3. Preferred centrifugal pump
Centrifugal pump has high speed, small volume, light weight, simple structure, transfusion without pulsation, smooth performance, easy to operate
And convenient maintenance and other features. Therefore, in addition to the following circumstances, the centrifugal pump should be selected as far as possible:
(1) when there are metering requirements, choose metering pump;
(2) small flow and high head, the use of vortex pump, reciprocating pump;
(3) small flow and high head, and the flow (pressure) without pulsation, choose vortex pump;
(4) large flow and low head, axial flow pump, mixed flow pump;
(5) when the medium contains 75% gas, the flow is small and the viscosity is less than 37.4mm2/s, the vortex pump is selected;
(6) medium viscosity is larger (more than 650~1000mm2/s), the selection of rotor pump, reciprocating pump (gear pump, screw pump);
(7) to start frequently or inconvenient irrigation pump occasions, choose self-priming performance of the pump, such as self-priming centrifugal pump, self-priming vortex pump, diaphragm pump;
(8) When there is special need, choose other pumps, such as jet pump, hose pump, etc.
Special requirements
Explosion-proof motors or other effective measures shall be adopted for pumps installed in the explosion area or special occasions according to the class of the explosion area.
Two, pump selection steps
1. Check the environmental conditions
The requirements include ambient temperature, relative humidity, atmospheric pressure, air corrosiveness, hazardous zone level, dust proof and water proof requirements.
2. Confirm the operating conditions
It refers to the liquid level pressure on the suction side (absolute), the liquid level pressure on the discharge side, intermittent or continuous operation, fixed or shifted position
Convenient operation, 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, corrosive, volatile, flammability, toxicity.
4. Select the material of pump flow parts
According to the physical and chemical properties of pumping medium, the following aspects are mainly considered:
(1) Corrosive medium
The corrosion resistance of the flow parts of the pump can meet the use requirements. It is not possible to pursue too high corrosion resistance, otherwise, it will greatly increase the purchase cost of the pump.
(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 the medium
The higher the temperature (pressure) of the medium, the higher the material strength of the pump flow parts. Generally, when the temperature is >250℃, cast steel or steel parts should be used.
(4) media with hygienic grade or prohibited pollution requirements
Select the appropriate material for the flow parts. In addition, the pump structure should be easy to clean.
5. Set pump performance parameters
(1) Calculation method
A. Flow rate: it is directly related to the production capacity and transmission capacity of the whole device. Such as process design can calculate the pump normal, minimum, maximum three kinds of flow. When selecting the pump, take the maximum flow as the basis and take into account the normal flow. In the absence of the maximum flow, 1.1 times the normal flow is usually desirable as the maximum flow.
B. Head: generally, the head should be enlarged by 5%-10% margin to select the type. Special attention: centrifugal pump flow, especially the head allowance should not be too large, otherwise, the pump may occur cavitation, resulting in flow, head can not meet the requirements. Once this happens, generally, can take cutting impeller diameter or turn down the pump outlet valve remedial measures.
C. Power: Generally, the pump power form and size are selected by the manufacturer and marked on the product sample.
D. NPSH
Check the device of the pump NPSH, NPSH must match. When can not meet, should take effective measures to achieve.
(2) analogical method and experimental method
When the resistance value of the pump pipeline system cannot be calculated accurately, the performance parameters such as flow rate and head of the pump can be determined by analogy or test method.
7. Select the installation type of pump
According to the pipeline layout, installation site to choose horizontal, direct, vertical and other types (right Angle type, variable Angle type, corner type, double type, disassembly type).
8. Determine the number of pumps and reserve rate
For the normal operation of the pump, generally only one, because a large pump and parallel work of two small pumps, (refers to the same head, flow), the efficiency of the large pump is higher than the small pump, so from the point of view of energy saving would rather choose a large pump, and not two small pumps, but in the following circumstances, can consider two pumps in parallel cooperation:
(1) the flow is very large, a pump can not reach the flow.
(2) For the need for 50% of the standby rate of the pump, according to the dual-use one standby, a total of three configurations.
(3) for some large pumps, 70% flow requirements of the pump parallel operation can be selected, no standby pump, in one pump maintenance, another pump is still responsible for 70% of the throughput of the production.
(4) must be 24 hours continuous operation of the pump, should run a, a standby, maintenance, a total of three. If necessary, the pump provider can assist in the selection.
