Maintenance points of multistage pump

The multi-stage centrifugal pump is the main equipment in coal mine drainage, its performance directly affects the safety and production of the mine, and has an important impact on the economic benefits of the mine. Combining with the actual work, the author puts forward the following views on the repair technology of centrifugal pump.

Maintain tightness

Because in most cases, the pump adopts negative pressure drainage, that is: the difference between the negative pressure at the inlet of the impeller and the atmospheric pressure on the water surface makes the water rise and sucks the water into the pump. This requires that the pump itself must form a closed space, which can be divided into external seal and internal seal. The external seal mainly refers to the seal between the pump casings of each section. When it is required to repair and assemble, the internal and external laps of each section of the pump must be sealed in place, and the stress between the pull rod bolts shall be balanced as far as possible. There are two aspects of internal sealing:

1. Seal between water pump shaft (shaft sleeve), water suction section and water outlet section: this kind of seal is completed by stuffing box (packing) and water seal ring together, that is to say, before the water pump starts to work, the cut packing and water seal ring installed on the shaft must be pressed into the stuffing box in order to ensure the axial seal (as shown in Figure 1).

1-packing box; 2-packing; 3-water seal ring; 4-gland; 5-water diversion pipe

It is worth noting that the function of the water seal ring cannot be ignored. When some people repair the water pump, they often do not install the water seal ring and seal it completely by packing, which is unfavorable to the operation of the water pump. Because the water seal ring introduces the pressure water into the ring and overflows the water to both sides with the help of the water pressure to form a drop like leakage. After the water seal ring is adopted, water curtain can be formed to prevent the air from entering, cool the packing, prevent the packing from overheating and wearing, extend the service life of the packing, so as to better ensure the normal drainage of the water pump. 2) The clearance between the sealing ring (large and small mouth rings) and the impeller is sealed, and the large and small mouth rings are respectively installed on both sides of the impeller (see Fig. 2).

1-impeller; 2-large ring; 3-small ring; 4-pump housing; 5-shaft sleeve

From the perspective of sealing, the smaller the clearance, the better. The utility model can prevent the water flowing out due to the unequal water pressure on both sides of the sealing gap and consume energy. The larger the clearance is, the larger the volume loss is, but too small clearance will affect the normal operation of the impeller. Therefore, the clearance between the large and small ring and the impeller should not be too large. The principle of assembling the ring is to ensure the flexible operation of the impeller, the smaller the clearance, the better, so as to achieve the purpose of not only ensuring the sealing, but also not affecting the operation.

Stay flexible

The pump itself transfers energy by the rotation of the impeller. When the impeller is running, it should be flexible and stable. Therefore, during the maintenance and assembly process of the pump, it should always pay attention to the middle section of each installation section and the main shaft of the pump should be rotated by hand to check whether it is flexible. If it is found that the pump shaft does not move flexibly or at all during the assembly of a certain link, the reasons for the failure must be found out, dealt with and solved (the possible reasons are that the coaxiality of the pump shaft is not high, the clearance between the ring and the impeller is too small, and the impeller and the pump shell are in friction, etc.). The main part of the pump is formed by adopting the method of section by section installation and section by section inspection until the last section is installed, and tightening the pump body with the pull rod bolt.

Maintain proper axial movement

After the assembly of each section of the water pump, it is necessary to ensure that the axial displacement of the water pump shaft assembly (hereinafter referred to as the total axial displacement), and the total axial displacement is the basis for adjusting the balance cross clearance. Strictly speaking, for - water pump, its maximum axial total string is a fixed value, but often because of the differences in the installation position of the shaft sleeve, the length of the shaft sleeve and the thickness of the impeller during the assembly process, the total axial string is always less than this value. When it is small to a certain degree, it is impossible to adjust the balance cross clearance. This requires that in the process of assembly and repair, the actual total string should be close to this constant value as much as possible, so as to leave enough adjustment space for the adjustment of the balance disk gap. The commonly used method is to measure the axial string of the first stage impeller from the beginning of installation. For each installation - stage measurement - time, try to make the total string of the lower stage not too different from that of the upper stage until all the impellers are assembled. At this time, the water pump shaft still has enough axial total string (specific data depends on different types of water pumps).

Adjustment of balance plate clearance

The purpose of assembling the balance plate is to balance the axial thrust of the water pump. Because most of the mining pumps are single suction pumps, they are driven by the axial thrust during operation, which forces the impeller and rotor to move towards the suction side. The thrust of the single-stage pump is several hundred kg, and the thrust of the multi-stage segmented pump can reach several tons. If it is not balanced, the impeller and pump shell will be crushed, let alone normal operation, so the axial thrust must be balanced. For multi-stage pump, the commonly used balance device is the balance plate (see Figure 3).

1-impeller; 2-water ring; 3-pump body; 4-balance ring; 5-balance plate

It is one of the key technologies in the repair and assembly of water pump to determine the clearance between the balance disk and the balance ring. Generally speaking, the clearance between the balance disk is about 1 / 3 of the total axial amount. If the clearance is too large and the balance force is not enough to offset the thrust of the shaft, the pump shaft will still move towards the suction side, which is easy to wear the impeller and the jacking motor. If the clearance is too small, it is easy to wear the balance plate or the balance ring, shorten the normal use time of the pump, and increase the maintenance cost. Therefore, the adjustment of the balance plate clearance is good or bad, which is very important in the maintenance of the water pump