Solid loss and prevention of mining centrifugal pumps

At present, among the drainage pumps used in coal mining working faces, there is no product specifically designed to discharge sediment. Regardless of the amount of sediment, the traditional segmented multi-stage clean water centrifugal pump is still commonly used. Therefore, the wear and tear is serious and the water pump is scrapped prematurely, which not only increases the cost, but also often directly affects the production schedule. Since it is impossible to set up a separate water tank for sedimentation in both the coal mining working face and the excavation working face, it is impossible to remove solid particles such as sediment on the spot. Therefore, to solve the problem of using a “clean water pump” to discharge sediment, the only way to solve the problem is to use a “clean water pump” to discharge sediment. Think of ways to improve the material performance of wearing parts, and ultimately make a big fuss about completely changing the structure of the drainage pump. Professional research work in this area has been going on for many years and is still being carried out intensively. Research has found that the wear of centrifugal pumps caused by solid particles follows certain rules in specific environments. By showing this law in front of us and prescribing the right medicine, we can find a more ideal solution. 1. Collection of working face water samples and determination of solid weight ratio. It has been observed that the content of solid particles in the inflow water from the coal mining working face changes with the change of the mining cycle operation content, and therefore has periodicity. ​

Generally, at the end of pre-mining preparations and before coal mining operations begin, the solid particle content is at its lowest. The highest levels occur towards the end of coal mining operations. When collecting water samples, in order to reduce the number of collections and explain the problem, they can be collected once within 30 minutes before the official mining, and again within 30 minutes before the end of the coal mining operation. The location for collecting water samples is at the lower end of the coal mining face, or at the separate outlet of the drainage pump installed on the slide. ​ Under normal circumstances, in the water inflow from the rock tunnel excavation working face, before multiple rock drills are drilled together, the solid particle content is the least. After blasting, the solid particle content is greatest during the last third of the rock excavation operation. Water samples were collected during these two periods. The collection location is within 10m behind the working site of the rake or rock loader, or at the separate outlet of the drainage pump on the working surface. ​

The water samples collected above weigh 2kg±0.2kg each time. After weighing, use filtration method to filter out all solid particles in the water sample with filter paper. After drying, weigh the solid particles, and the solid weight (percentage) ratio Cow of the water sample is: Solid weight Cow = —- H % Solid , the total weight of the liquid mixture The solid weight ratio Cow of the water sample is defined as the “weight ratio solid concentration” in “Hydraulics for Slurry and Granular Material Transport” (authored by Fei Xiangjun, published by Qinghua University Press in May 1994) , used to transport industrial slurries. This article refers to it as the solid weight ratio for short, and it is sometimes customary to call it the sediment weight ratio in the field. Generally speaking, when the water inflow from a working face is less than 10m3/h, the solid weight ratio Cow increases significantly; when the water inflow is greater than 80m3/h, the solid weight ratio Cow decreases significantly. In the same mine, in iconic coal seams and rock formations By collecting a large number of water samples in the mine, the general rules of the solid particle content in the mine water can be obtained. After further analysis and sorting based on water inflow, rock mass characteristics and degree of fragmentation, and the reaction of loose particles when exposed to water, it can be used to guide drainage design and drainage management. Table 1 is a comprehensive set of data obtained from the field. ​

Table 1 Project Coal mining working face Excavation working face Before the end of the coal mining operation Before the end of the rock extraction operation, the solid weight ratio Cow 0.83-5.48 %                   0.41-9.62 %

2. Heart pump wear parts and wear process

2.1 Wear parts of multi-stage centrifugal pumps At present, the most severely worn parts of multi-stage centrifugal pumps for mining are the balance plates, followed by the impeller mouth ring, packing sleeve, and middle shaft hole bushing, etc. Generally, solid particles will not wear through the pump shell, reduce the outer diameter of the impeller, or shorten the blades. The balance plate becomes thinner after wear, causing the rotor to move toward the motor direction (forward) beyond the limit, resulting in chain damage. After the impeller mouth ring and the middle shaft sleeve are worn, the sealing gap becomes larger, causing the centrifugal pump to produce insufficient or no water, and sometimes there is strong vibration.

2.2 The wear process of the balance plate The balance plate has axial end face runout, and the pump body balance plate also has axial end face runout. When the balance plate rotates once, when it turns to a certain angle, the maximum or minimum axial clearance will appear locally. The balance state of the balance plate is dynamic, and the pump rotor will pulsate axially back and forth at a certain balance position. When the operating point changes, the rotor will automatically move to the new equilibrium position for axial pulsation. After the superposition of this axial end face runout and axial pulsation, the axial clearance b0 will have a local dynamic maximum clearance b0max and a minimum clearance b0min. Assume that the particle diameter of a solid K is A, and it has reached the high-pressure area in front of the balance plate with the liquid, and b0max>A>b0min, then the solid K will be wrapped by the liquid from the axial gap of the balance plate near the maximum axial gap b0max. within. When the solid K has just stepped into a small distance S, the balance plate turns to the position where the axial gap b0 is less than A, or the balance plate is moving back to the position where the axial gap b0 is less than A. At this time, the balance plate will squeeze the solid K, and the two will move relative to each other. The end surface of the balance plate will be dented by the solid K and scraped into grooves until the solid K is ground. If the content of solid particles of this size is high, they will continue to be wrapped in one after another, and the first annular groove will soon appear on the end face of the balance plate. As the erosion intensifies, a second groove and a third groove will appear one after another slightly larger than the first annular groove… Despite this, the amount of movement of the pump shaft has not changed significantly at this time. When you take out the balance plate, you can see that these annular grooves are roughly concentric and arranged neatly. At the same time, similar grooves appeared on the balance board. The following phenomenon is worth noting: When the particle diameter of another solid W is B, it also reaches the high-pressure area of the balance plate, and B≥b0max, then the solid W will be blocked on the high-pressure area side of the balance plate. At this time, if solid particles of different sizes continue to flow toward the high-pressure area in front of the balance plate with the liquid, solid particles with a particle size less than or equal to A will pass through the axial gap with the liquid flow. Solid particles with a particle size greater than or equal to B will be separated here. We call this phenomenon “sieve retention phenomenon”. When the “sieved” solid particles reach a certain amount and block the entrance of the axial gap, the liquid pressure will increase, causing the rotor to pulsate backwards, causing the axial gap to instantly increase. At this time, these large-size solid particles took the opportunity to squeeze into the axial gap of the balance plate in large quantities, causing a “strong landing phenomenon”. If this phenomenon occurs continuously, the balance plate can be worn thin in a short period of time. ​

2.3 The wear process of the impeller orifice ring After the impeller is installed on the pump shaft, its orifice ring has radial runout; the pump body orifice ring hole also has radial runout. When the impeller rotates for one revolution, when it turns to a certain angle, the maximum or minimum gap of the radial gap of the mouth ring will appear locally. The rotating pump shaft will bend and deform, resulting in maximum deflection. After the superposition of this radial runout and the maximum deflection, the radial gap b of the mouth ring will have a local dynamic maximum gap bmax and a minimum gap bmin. Similarly, the sealing surface of the mouth ring will also be squeezed and scraped into grooves by solid particles. However, if the materials are the same, since the gap of the mouth ring will not cause “pulsation” and “strong landing phenomenon” of solid particles like the balance plate, the wear degree of the mouth ring will be significantly less than that of the balance plate. At this time, the wear of the mouth ring is not the main contradiction compared with the balance plate. However, it should be noted that in a multi-stage pump without a balance plate, the wear of the mouth ring will become the main contradiction and must be taken seriously. ​

3. Remove sedimentation at the bottom of drainage pipes: Solid particles will settle at the bottom of drainage pipes, especially large-diameter permanent pipes borrowed separately. These solid particles settled through repeated water separation have a larger specific gravity, and their surfaces are spherical and relatively smooth. When the water output of the centrifugal pump mentioned above is insufficient, the flow rate decreases and the flow rate in the pipeline slows down, which will increase the settlement of solid particles. If the flow rate of the centrifugal pump is reduced to a certain critical value, the settled solid particles will flow back down, pass through the valve, and fill the pump chamber. In just a few dozen minutes, the pump will be scrapped. We can conduct experiments on site or conduct simulation experiments in the laboratory to verify this phenomenon. To this end, it is necessary to calculate the flow velocity value in the pipeline at the minimum flow rate, which should generally be greater than 1m/s. To increase the flow velocity, drainage pipes with smaller diameters should be given priority. Taking the pipeline installation of a sand discharge submersible pump as an example (see Figure 1), a short pipe can be connected to the pipeline between the centrifugal pump check valve 1 and the gate valve 4, or at the lowest elevation at the bottom of the pipeline – except Sand pipe. The sand removal pipe is controlled by another gate valve 5. When there is a large content of solid particles in the water, it will be discharged underground on the spot. The amount of discharge each time is about 0.2-0.3m3. In this way, the settled solid particles in the drainage pipe can be removed in time, which can prevent accidental wear of the centrifugal pump. Experience shows that during rock excavation, regular discharge of sediment during rock excavation operations can generally extend the service life of the centrifugal pump by about 3-10%, and make the retired water pump still valuable for overhaul, or make overhaul easier. In automated drainage, auxiliary equipment should also be added to regularly remove the sediment at the bottom of the drainage pipe. If it can cooperate with the collection of water samples, timely analysis and sorting of data, more scientific water pump use and maintenance methods can be formulated to maximize the life of the centrifugal pump. life. Of course, this measure is best adopted at the design stage. ​

4. The new sand discharge submersible pump that eliminates the balance plate uses a multi-stage clean water centrifugal pump to directly transport mine water containing solid particles. The main focus has been on the balance of axial force. Therefore, in order to extend the service life of the water pump, it has become an inevitable trend to cancel the balance plate or balance drum). ​

4.1 Selection of wear-resistant materials Depending on different uses, various types of water pump parts use different wear-resistant materials. For the sake of longevity, the wear-resistant material of the clean water pump is copper alloy; the sewage pump starts from discharging paper pulp, and the wear-resistant material is only ordinary cast iron. The slurry pump is a single-stage pump, and the wear-resistant material is high-chromium cast iron; the sand pump can mine sand in the river, and the wear-resistant material is rubber. It can be seen that the definition of “wear-resistant materials” is very wide. When researching and developing multi-stage sand discharge water pumps, we must deal with hard, angular “sand” and study the rules of sand abrasion on water pumps under high pressure and high speed. Therefore, it is even more important to choose “wear-resistant materials”. The following parts are made of the same material and are arranged in descending order of damage degree: balance plate, impeller mouth ring, packing sleeve, shaft sleeve, guide vane, pump body, and impeller. From this, there is a reference and basis for selecting wear-resistant materials for sand discharge water pumps, from which the following tips can be obtained: 4.1.1 Various wearing parts should use high-quality materials with different hardnesses. Among them, materials with high hardness are used for parts that are easy to wear, and materials with low hardness are used for parts that are not easy to wear. In this way, weak links can be prevented and the cost can be reduced. ​

4.1.2 Cancel the balance plate that requires the highest material hardness. The highest hardness material currently available – tungsten-cobalt carbide is used on the impeller mouth ring and shaft sleeve to obtain the longest service life and the best economic benefits. ​

4.2 One of the ways to cancel the balance plate is to increase the lift of the single-stage pump and replace the multi-stage pump. It is simple and easy to use the lift of the single-stage pump to replace the previous two-stage and three-stage segmented multi-stage pumps. In 1988, this wish was realized with a single-stage double-suction sand discharge submersible pump with a lift of 70m, a flow rate of 100m3/h, and a power of 37kw. This pump was designed for the drainage well of the Quaternary quicksand layer, and was later used for remote remote recycling of wastewater from the northern coal washing plant in winter (3km away, using old open-air sedimentation of coal slime). In 1989, this wish was once again realized with a single-stage single-suction sand discharge submersible pump with a lift of 50m, a flow rate of 12m3/h, a solid particle diameter of 10mm, and a power of only 4.0kw. Since then, the impeller and the inner and outer rings of the pump body have been made of carbide. For the first time, it replaced the multi-stage centrifugal pump for drainage in the downhill excavation working face, and its “life span was increased by 5-8 times” (extracted from the 1991 ministerial-level “New Product Appraisal Certificate” of this pump). The replaced multi-stage centrifugal pump has a power of 40kw, and its high-pressure water is used to drive a jet pump (commonly known as a belt pump or water pump) to drain water down the mountain. Therefore, this small pump quickly became popular and won many awards. Ten years of on-site inspections in hundreds of coal mines with different geological conditions across the country have proven that this technical solution for sand discharge submersible pumps is feasible. At present, the varieties and uses of single-stage sand discharge submersible pumps have been greatly expanded, such as: ① When used in coal mining faces, the technical parameters can reach: head 95m, flow rate 168m3/h. ② Used in excavation working faces, the technical parameters can reach: lift 85m, flow rate 158m3/h. The efficiency can reach 62.1%. Motor power is 75kw.

4.3 The second method to cancel the balance plate – the impellers are installed back to back to offset the axial force. The number of impellers is an even number, and every two impellers are installed back to back to offset each other’s axial force. This is a good way to completely eliminate the balance plate. In 1995, this idea was realized with a four-stage segmented multi-stage sand discharge submersible pump. This kind of pump once restored a flooded mine in Gansu. The ground of this mine is permeable, carrying surface sediment from the northwest into the mine, and ordinary centrifugal pumps have been used to restore it many times, but to no avail. Technical parameters of the pump: head 200m, flow rate 80m3/h, efficiency 59.3%, power 185kw. Today, through unremitting efforts, a number of practical technologies have been added, almost completely improving the original design. The technical parameters have been modified to: head 320m, flow 80m3/h, power 185kw. 4.4 Introduction to the structure of the sand discharge submersible pump The above-mentioned sand discharge submersible pump is vertical and can be placed on site or suspended. The pump body is coaxial with the motor, with the motor at the top and the pump body at the bottom (lower pump type). The main features of the sand discharge submersible pump structure are:

a. The motor is placed in the “air chamber”. The “air chamber” is like an open “tea cup”. Place the “tea cup” with the bottom facing up and the mouth of the cup facing down, and put it into the water to fix it. At this time, the air in the “cup” will not escape. Place the stator, rotor and upper and lower bearing supports of the motor in the “tea cup” in advance, and then extend the motor shaft downward from the mouth of the “cup”. This is the motor model of the sand discharge submersible pump. With the protection of the “air chamber”, the stator, rotor, bearings and shaft seals of the motor will not be in direct contact with water and harmful substances such as sediment, acid, alkali and salt in the water. Therefore, even if you dive into water containing a lot of sediment, the motor will not be damaged. In addition, the motor windings are cast with heat-resistant epoxy resin and solidified in order to adapt to the requirements of the underground environment for a long time. The motor shell is covered in a “water guide sleeve”, and all the water discharged by the water pump flows between the motor and the water guide sleeve. The motor realizes “external water cooling”. In this way, the sand discharge submersible pump can work out of the water. The above-mentioned sand discharge submersible pumps are available in three forms: single-stage single-suction type, multi-stage single-suction type, and single-stage double-suction type. All of them eliminate the important wearing parts of the pump shaft on the pump body shell-the shaft seal of the pump shaft. This is because:

① The suction port of the single-stage single-suction impeller faces upward, and the lower end of the pump body is closed. ​

② The first-stage impeller of the multi-stage single-suction type is at the top, the suction port of the first-stage impeller faces upward, and the lower end surface of the pump body is closed. ​

③ The single-stage double-suction impeller has upper and lower suction inlets. ​

b. It is allowed to run without water – it will not cause any damage to the motor and pump body. After there is water, drainage can continue without the need for a dedicated person to guard it. This requirement comes from the front line of coal mines and was put forward by the Shanghai Branch of the General Institute of Coal Science and Technology after investigation. The sediment discharge submersible pump was designed based on this valuable topic at the beginning of its design. With its unique structure, the first product It is realized at birth. When leaving the factory, the idling test must be carried out every time, including 75kw and 185kw. ​

Today, when coal mines are equipping themselves with modernization and mechanization, people have begun to envision drainage automation. There is a need for water pumps for “clean water warehouses”, water pumps for tunneling along the roof and automatic drainage in the recesses of the tunnel, and large sand-discharging submersible pumps for use in explosive hazardous environments, etc., etc., and the calls are getting louder and louder. Therefore, the research on “multi-stage sand discharge pump” has received widespread attention. Since the advent of the dewatering submersible pump, many design units and large and medium-sized coal mines have cooperated in research and bold attempts to contribute to this new thing. Detailed information such as the structure, technical application parameters and on-site feedback information of single-stage, multi-stage “sand removal submersible pumps” and horizontal multi-stage “sand removal centrifugal pumps” will be introduced to readers in the future.