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The structure and application of large centrifugal pump

Posted by: steel world 2021-10-25 Comments Off on The structure and application of large centrifugal pump

Due to the topographical characteristics of northern China, the pumping station lifts are all high, generally exceeding 1 MPa, and some are multi-stage pumping stations, and most of them use double-suction centrifugal pumps. Some large cities also use large double-suction centrifugal pumps for water supply due to the large water supply and long pipelines. This kind of pump has good cavitation resistance and stress performance, stable operation and convenient maintenance. However, some problems often occur during operation, such as the contact surface of the pump shaft and the shaft sleeve, and the end face of the shaft sleeve are prone to fatigue damage. The shaft needs to be sprayed with alloy powder to repair it before it can be used, otherwise it will be scrapped. In addition, some pumping stations have replaced the seal ring made of cast iron with a seal ring made of low carbon steel in order to resist abrasion. From the start of the pump to the opening of the pump, the phenomenon of shaft holding between the outlet valves often occurs, and the pump body is sealed. For the bonding between the ring and the impeller sealing ring, the gap of the sealing ring must be increased to start normally, and the increase of the gap of the sealing ring will reduce the volumetric efficiency and affect the economic operation of the pumping station. The above-mentioned problems occur from time to time in large pumping stations, which bother the safe and normal operation and need to be solved fundamentally.

1 Cause analysis

It is found that almost all domestic large-scale double-suction centrifugal pumps are single-volute pumps, that is, they use spiral-shaped water pressure chambers. There are two reasons for the use of spiral pressurized water chamber. One is the design method. Generally, the reliable design method for large pumps is similar conversion, that is, select a small pump with excellent performance in all aspects, the same pump type and the same specific speed. Model pumps, similar to enlarged flow channel size, usually small pumps are single volutes, which results in large pumps also having single volutes; the second is to pursue high pump efficiency, because there is concern that the use of double volutes and other structures will increase the flow surface , Increase hydraulic loss and reduce efficiency. Of course, single-volute pumps have the advantages of being more convenient to manufacture, the high-efficiency area of ​​the pump performance curve is relatively wide, and the pump efficiency is relatively small after turning the impeller, so most large-scale centrifugal pumps still adopt the single-volute structure.

When the single-volute pump runs away from the design conditions, the hydraulic will generate a radial force perpendicular to the pump shaft. Especially when a large pump is started (zero flow), it will produce a large radial thrust acting on the impeller, causing excessive increase in shaft deflection, which can cause rapid wear of the seal ring, or for the seal ring using viscous materials. Occurrence of occlusal bonding, causing accidents. Excessive torsion will cause the pump shaft and the motor shaft to be eccentric, causing the motor to shift the shaft. At the same time, the radial force is an alternating load on the rotating shaft. In a double-suction pump with a large bearing span, due to the fatigue of the metal material, damage to the pump shaft often occurs.

According to the above analysis, the reason for the damage of the pump shaft and seal ring of the current large-scale centrifugal pump is the radial force formed when the single volute pump deviates from the design conditions or runs under zero flow.

2   Generation and calculation of radial force 2.1

Causes of radial force

When designing the spiral pressurized water chamber, the design idea is that the liquid flows out uniformly from the impeller under the design flow rate and moves at a constant velocity in the volute chamber, that is, the volute chamber only functions to collect the liquid, and the liquid is removed in the diffuser. Part of the kinetic energy becomes pressure energy. Therefore, the spiral pressurized water chamber is designed to match a certain impeller under a certain design flow (Qd). The cross-sectional area of ​​the designed volute chamber changes linearly. Under the designed flow rate, the volute chamber can basically ensure that the liquid moves at a uniform speed around the impeller. At this time, the pressure around the impeller is generally evenly distributed, and there will be no generation on the impeller. The radial force, the impeller and the worm chamber work in unison.

However, whether it is urban water supply or pumping irrigation, there are peaks and valleys in water demand. In addition, for the sake of reliability, the pump station designer leaves a margin for the pump parameters, and there is generally a margin during pump design. In this way, the actual pump operation often deviates from the design conditions (too large flow). Therefore, no matter the pump is running under large or small flow, the coordinated working state of the impeller and the volute will be destroyed. The flow velocity and pressure distribution of the liquid around the impeller become uneven, which forms the effect on the impeller. Radial force. If the pump flow rate is less than the design flow rate, the liquid flow speed in the volute must slow down. In addition, from the triangle diagram of the impeller outlet speed (see Figure 1), it can be seen that the impeller outlet speed has not decreased but increased, and the direction has also changed. . This liquid flow collides with the liquid flow in the volute chamber due to the difference in speed and direction. As a result, the speed of the liquid flowing out of the impeller drops to the flow speed of the liquid in the volute chamber, and at the same time, a part of the kinetic energy is transferred to the liquid in the volute chamber through the impact. The pressure of the liquid in the scroll chamber is increased, and another part of the kinetic energy is lost during the impact. Therefore, the liquid is constantly impacted in the flow from the pump diaphragm to the inlet of the diffuser, and the pressure is continuously increased, causing the pressure in the volute chamber to continue to rise from the diaphragm. The opposite is true when the pump flow rate is greater than the design flow rate. Therefore, the uneven pressure distribution caused by the deviation of the pump from the design conditions is the main reason for the formation of radial force.

2.2  Calculation of radial force

There is no precise formula for calculating the radial force. In order to calculate the radial thrust, the deflection of the shaft is measured and the deflection of the shaft is scaled by static load. The magnitude of the radial force can be determined. The radial force can be determined by pressing Formula calculation:

P=0.36[1-(Q/Qd)2]HD2B2γ(1) where  D2——the outer diameter of the impeller, m B2——the width of the impeller outlet including the cover plate, m γ——the volume and mass of the liquid 3

3.1 Practical problems

A large-scale double-suction centrifugal pump in a pumping station in Gansu, with design conditions Q=3m3/s, H=560 kPa, n=600r/min, and η=88%, adopts a single spiral pressure water chamber. The actual pump head is 520 kPa and the actual flow rate is 3.5 m3/s. In addition, the cavitation and sand abrasion damage caused by transporting the sandy water of the Yellow River makes the scrap cast iron sealing ring into fish scale pits and honeycomb pits, the depth is 4~ 10mm, the deepest point can reach more than 12mm, and the pump efficiency drops from 88% to 68%. The main reason is that the gap between the seal ring and the approximate volumetric efficiency decrease.

In order to solve the serious wear problem of the seal ring, a steel ring made of steel plate is added to the pump body seal ring. After one year of operation, the maximum wear amount is less than 0.4 mm (cast iron seal ring up to 5 mm), and the damage marks are significantly reduced , It can be seen that the resistance of steel plate to sand abrasion is obviously stronger than that of cast iron. However, when the impeller and the pump body seal ring are both steel plates, the shaft holding phenomenon often occurs from the start of the pump to the opening of the pump outlet valve. Because the pump seal ring and the impeller seal ring are occluded and bonded, the impeller seal gap is changed from the pump for safety. 0.64 mm (the design gap is 0.36~0.64 mm) can be started normally when it is increased to 1.2 mm. When the pump is started, large motors with sliding bearings often have the problem of axial movement of the motor rotor. In the case of severe shaft movement, the end face of the motor bearing bush and the shaft shoulder will be rubbed, causing the bearing temperature to be too high and unable to operate normally.

At the same time, the pump and other double-suction pumps in the pumping station deviated from the design conditions for a long time, causing fatigue damage on the surface of the pump shaft and causing the pump shaft to be scrapped.

3.2 “Theoretical calculation and analysis”

①Deflection of the pump shaft when starting

The resultant radial force at zero flow is calculated by formula (1) as:

P=0.36×56×1.15×0.274×1040=6606.5 kg=64810 N

Calculate the deflection at A and B of the shaft according to the symmetrical support (see Figure 2):

yA=P·l3/48EJ=0.949 mm   yB=P·x/48EJ(3l2-4×2)=0.38 mm

Where  E——material elastic modulus     J——shaft section moment of inertia

It can be seen from the calculation that the single volute pump produces a large radial force at zero flow, causing the deflection of the pump shaft at the seal ring to be greater than the maximum design value of 0.64 mm (the mass of the rotor and the shaft sleeve of about 2000 kg are not considered in the calculation. Etc.), resulting in the adhesion of the steel plate sealing ring. In actual operation, it has been increased to 1.2 mm. It can be seen that the radial force has exceeded the bearing capacity of the shaft’s design stiffness. Even if no adhesion occurs at the cast iron seal ring, the calculation shows that wear is inevitable and this should also be prevented. .

In addition, a 0.38 mm eccentricity of the pump shaft and the motor shaft was caused at the coupling (location B). Because the original nylon pin coupling, and the nylon pin has very small compressibility, it cannot compensate for the eccentricity caused by the radial force during startup, which causes the axial thrust to the motor rotor and causes friction between the end face of the motor bearing bush and the shaft shoulder. After changing to the elastic ring pin coupling, because the rubber elastic ring can compensate a certain eccentricity, no axial force is generated after the test, and the motor no longer shifts the shaft. Of course, only the eccentricity is compensated to solve the motor shaft shifting problem, and the eccentricity is not fundamentally eliminated. The eccentricity problem will still affect the smooth operation of the pump.

②The effect of seal ring clearance on efficiency

The influence of different seal ring clearances on pump efficiency is shown in Table 1. Due to the advantages of fine steel structure, good toughness, and resistance to falling off, the resistance to cavitation and abrasion is significantly better than that of cast iron, and calculations from Table 1 show that the use of steel plate sealing rings can reduce leakage, increase volumetric efficiency by 16.5%, and increase pump efficiency by 14.7 %. Table 1   Comparison of volumetric efficiency and pump efficiency under different seal ring gaps. Leakage b(mm) η(%) ηv(%) ηmηh(%) q(m3/s) 0.64 (design value) 88.0 98.8 89.1 0.03580 0.4+0.64 86.8 97.4 89.1 0.08024 5.0+0.64 72.1 80.9 89.1 0.58000 1.2+0.64 84.3 94.7 89.1 0.16200 Note: Use the calculation formula q/2=Dm·π·b/(1+0.5φ) 0.5(2gHm) 0.5+λL /2b   ηv=(Q-q+q1)/(Q+q1) (q1=0.035 8m3/s)   η=ηvηmηh” In the calculation, the hydraulic efficiency ηh and the mechanical efficiency ηm are unchanged, but they are actually reduced.

Due to the excessive radial force at start, in order to prevent the steel plate seal ring from biting, the seal ring gap was increased from 0.64 mm to 1.2 mm, but the volumetric efficiency was reduced by 4.1%, and the pump efficiency was reduced by 3.7%. Generally speaking, it is quite difficult to increase the efficiency of large pumps by 1%, but the economic benefits are very significant. For example, there are more than 100 such large pumps used in the irrigation project. Due to the use of single-volute pumps, the sealing gap is enlarged, resulting in a decrease in pump efficiency.

③Radial force when deviating from design conditions

When the flow rate Q=3.5m3/s, the radial force is calculated by formula (1) as P=23 379.7 N, plus the mass of the rotating parts, these forces are alternating loads on the pump shaft, which can cause fatigue damage to the pump shaft .

Therefore, the damage of the pump shaft and the seal ring and the motor shaft shifting are all caused by the excessive deflection of the pump shaft caused by the radial force. For single-volute pumps, avoid deviations from the design conditions. If the pump head is too large, the outer diameter of the impeller can be turned to ensure that the pump runs under the design flow. Frequent pump start should be minimized to avoid excessive radial force. Damage to the pump shaft and seal ring.

4 Structural design of large centrifugal pumps

From the above calculation and analysis, it can be seen that the disadvantages of large radial force generated by the large single volute pump during operation cannot be completely eliminated. The fundamental solution should be considered in the design of the pump, such as thickening the pump shaft and Choose better materials to increase the rigidity of the shaft and increase the rigidity of the fulcrum, but this does not completely eliminate the radial force. The best way is to use hydraulic methods to balance the radial force under various working conditions. The structure of the guide vane and the volute chamber can be used to balance the radial force acting on the impeller (see Figure 3). This structure is relatively complex and Maintenance is inconvenient, and the volume of the pump is increased, and the digging depth of the pump station will be increased. The simpler one is to use a symmetrical double volute chamber to balance the radial force (see Figure 4). In the past, due to the limited level of casting technology, it was relatively difficult to align the runners and clean the sand on the opening surface. With the advancement of science and technology, it is no longer a problem now. The efficiency of a well-designed double volute pump is close to that of a single volute pump (within 1%), the efficiency curve is flatter, and when the flow exceeds the design flow value, the improvement of pump efficiency is more than that under low flow conditions Significantly. The reason why the double volute pump body can improve efficiency under large and small flow conditions is that the pressure distribution around the impeller outlet is more uniform than that in the single volute chamber, and the impeller outflow is better. In the double volute chamber, the conversion of the velocity head into pressure energy occurs in the diffuser, and the flow channel that guides the liquid in the first volute chamber (outside the second volute chamber) is of equal section (see Figure 4), otherwise The efficiency reduction will exceed 1%. Therefore, the design of large-scale centrifugal pumps with double volute chambers can create conditions for efficient and safe operation.

5 Conclusion

  • ① Large-scale single-volute pumps will form a large radial force when starting, which can cause problems such as seal ring wear or adhesion and motor shaft shifting. This cannot be ignored. Frequent starting and deviation from design work should be avoided as much as possible. To prevent the formation of excessive radial force.
  • ②When designing a large-scale centrifugal pump, a double volute pressure chamber with a symmetrical structure can be used, which can basically balance the radial force, eliminate problems caused by shaft deflection, expand the pump’s operating range, and improve volumetric efficiency.

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