Selecting the structure of centrifugal pump is an important step in the design of centrifugal pump.The so – called choice centrifugal pump structure form, is to determine whether single or double suction;Is it single level or multiple levels;Cantilever or double support;Is it segmented or horizontal.These problems are also related to the motor drive mode, pump speed, suction and efficiency factors.Because the complexity of the pump structure is different, the steps to determine the type of pump structure are not necessarily the same. Generally, the following steps can be followed.
ONE: Select the motor
Factors such as power source, price and maintenance management should be taken into account in selecting the motor.For agricultural water pumps, the natural conditions, technical and economic conditions and people’s requirements in different areas should also be considered.Generally speaking, motor drive should be considered first in areas with electricity, because motor drive cost is low, easy to operate, reliable to use, convenient for maintenance and maintenance, and no fuel is needed.However, the infrastructure investment of motor drive is large, and new transmission and transformation equipment is needed.Especially far from the power grid, infrastructure investment is greater.Gasoline engine, diesel engine driven infrastructure investment is less, in some parts of the fuel supply and convenient, especially now that the rapid development of high speed diesel engine have opened a new way for driven by diesel engine, for industrial pumps, due to the power supply is more convenient, generally adopt the motor drive, where there is steam source, can also be driven by steam turbine, steam turbine drive has the advantage of high speed, and can use the method of adjusting the rotational speed to change operation parameters.But maintenance management is complex.Recently, with the rapid development of gas turbines and the high speed of gas turbines, they can also be driven by gas turbines where conditions are suitable.
Using the environment condition is has a great influence on the choice of motor, for example, in mine, oil refinery are inflammable, explosive dangerous places, cannot use ordinary motor, gas turbine or should use explosion-proof motor or turbine driven, lest produce risk.
TWO: Determine the number of speed, specific speed, and stage speed
Speed, specific speed and series are three interrelated factors that directly influence the pump structure type.Therefore, to determine the speed, specific speed and series is to select two factors according to the actual situation, find out the third factor, and check whether the original pump efficiency and cavitation allowance can be achieved.
1. Speed n
At present, centrifugal pumps at home and abroad tend to adopt high speed, high speed after the pump, volume reduction, weight reduction, save labor and materials.However, provide the pump speed is limited by a pump cavitation performance, if the corresponding to the selected speed NPSH is greater than the given index, should try to improve pump cavitation resistance (such as using two-stage impeller impeller inlet diameter was increased, and the impeller entrance width, idler, etc.)Or adopt anti-cavitation materials to make impeller.If these measures are not enough to meet the original target, consideration should be given to lowering the speed of the low pump or adding the booster pump before the pump.
In general, it is not recommended to use gear gearbox or other transmission mechanism. It is recommended to use motor to drive directly.If the motor speed is too low or the operating conditions require speed regulation, the speed-increasing gear box or governor (such as hydraulic coupler, etc.) can be adopted.
The pump speed shall be selected according to the motor speed. If driven by the motor, the pump speed shall be selected according to the following table.
2. Specific speed
Specific speed is closely related to pump efficiency. When determining specific speed, it is necessary to consider whether the pump efficiency can reach the given target.
Should also be considered when determining the ratio of revolution, whether there is a suitable model, if there is a suitable model, should choose the model of the pump than revolutions, such not only simplified the experimental research and design work, and ensure the products with high technical and economic indicators.
The specific revolutions of centrifugal pump and mixed flow pump are within the range of 30~500.Under the condition of constant pump flow and head, the specific speed of the pump can be increased by increasing the speed of the pump or increasing the level of the pump.Under the condition of constant head and speed, the pump can be reduced by 1.4 times by adopting double suction impeller.
When determining the series, the possibility of structural arrangement should be taken into account first.Too many horizontal pump series will increase the span of the shaft, reduce the reliability of operation, and bring many difficulties to the use, maintenance and maintenance work.At present, horizontal multistage pumps generally do not exceed 10, up to 15.Although vertical multistage pumps can reach dozens of stages, it is necessary to seriously solve the balance of axial load and the supporting problems of vertical motors and vertical speed increasing gear boxes.
From the point of view of structure, it is desirable to have as few series as possible, that is, to have as high a head as possible.However, in the case of a certain speed, the impeller with a low specific speed must be adopted to improve the single stage head, and the smaller the specific speed, the lower the pump efficiency.;In addition, the impeller with a smaller number of revolutions has some difficulties from material selection (such as strength and erosion problems) to manufacturing process.
THREE: Determine pump structural and type
1.Is it single or double?
At present, the allowable suction vacuum height of general industrial and agricultural pumps is about 5~6 m water column.Therefore, when the suction diameter is less than 200 mm, it is mostly designed as a single suction pump, because the single suction pump is simple in structure, light in weight, few parts, and easy to manufacture.Pump for suction diameter over 200 mm, if choose general single suction structure, due to take care of not suction is too small, must be at a low speed, causing heavy unit, then can consider to design into a double suction pump.The double suction pump can adopt higher speed when the flow rate and anti-cavitation performance are the same.
2.Is it single-stage or multi-stage?
This problem has been described when determining the series and will not be repeated here.
3. Cantilever type or double support type?
Suction diameter at 200 mm below the single-stage pump and a part of the single suction pump can be designed as cantilever structure, because the light and small cantilever structure, easy processing and manufacturing, liquid flow condition is good (don’t wear axis).For multistage pumps and pumps with a suction diameter of more than 200 mm, the design of double support on both sides of the bearing is more appropriate from the structural point of view.
4. Is it a volute or segmented pump?
Generally china botou kemai pumps single-stage pump adopts spiral pressure chamber, which is commonly known as vortex shell pump.The multistage pump has two types of structure.The spiral shell multistage pump and the segmented multistage pump.The advantages and disadvantages of the two structures are as follows:
(1) when the performance is the same, the efficiency of the two types of pumps may be the same.
(2) with the same performance and speed, the weight of the segmented multistage pump is 20 to 50 percent lighter than that of the spiral shell multistage pump.
(3) the geometric shape of the volute multi-stage pump body is complex, and the casting time is more than that of the segmented multi-stage pump, and the precision is not easy to be guaranteed.The machine working hours of the spiral shell multistage pump and the segmented multistage pump can be processed on the general equipment, which is suitable for the production of small and medium-sized enterprises.
(4) the spiral shell multistage centrifugal pump is simple in structure and easy to maintain, which is popular with users. Moreover, the impeller is arranged symmetrically, which can automatically balance the axial force.The segmented multistage pump has complex structure, many parts and difficult maintenance.Therefore, nowadays, two-stage multistage pumps are often made into spiral shell multistage pumps, so the weight is similar to that of segmented multistage pumps, but the structure is simple, the operation is reliable, and the maintenance is convenient.If the series exceeds three levels, the manufacture of volute pump complex shape, weight increase.Therefore, it is made into multi – stage pump.
According to the above analysis, when determining the type of pump structure, we should make a comprehensive analysis and comparison according to the specific situation and the user’s requirements, and choose a reasonable structure.