The Most Comprehensive Analysis of Water Pump Failures in History, Summarized by an Expert!

1.The water pump fails to discharge water.
1.1 There is air in the inlet pipe and the pump body.
1.1.1 Before starting the self-priming pump, insufficient water is filled. Although it seems that water overflows from the air vent, the pump shaft is not rotated to completely expel the air, leaving a small amount of air remaining in the inlet pipe or the pump body.
1.1.2 The horizontal section of the inlet pipe in contact with the water pump should have a downward slope of more than 0.5% against the direction of the water flow. The end connected to the water pump inlet should be the highest and not be completely horizontal. Otherwise, air will remain in the inlet pipe, reducing the vacuum degree of the pipe and the water pump and affecting water absorption.
1.1.3 After long-term use, the packing of the single-stage centrifugal pump is worn out or too loose. A large amount of water sprays out from the gap between the packing and the pump shaft sleeve, and external air enters the interior of the water pump through the gap, affecting water lifting.
1.1.4 The inlet pipe has been submerged underwater for a long time, and holes are corroded on the pipe wall. After the water pump operates, the water level drops. Once the holes are exposed above the water surface, air enters the inlet pipe through the holes.
1.1.5 Cracks at the elbow of the inlet pipe and tiny gaps at the connection between the inlet pipe and the water pump may both allow air to enter the inlet pipe.
2.The rotation speed of the water pump is low.
2.1 Human factors. Some users' original motors are damaged, and they randomly replace them with another motor to drive the pump, resulting in a small flow rate, a low head, or even no water being pumped.
2.2 Mechanical failures of the water pump itself. The fastening nut of the impeller and the pump shaft becomes loose, or the pump shaft is deformed and bent, causing the impeller to shift and rub against the pump body, or the bearing is damaged, reducing the rotation speed of the water pump.
2.3 Improper repair of the power machine. The motor winding is burned out and loses its magnetism. During the repair, the number of turns of the winding, the wire diameter, and the wiring method are changed, or the fault is not completely eliminated, causing the rotation speed of the water pump to change.
3.The suction head of the water pump is too large.
In some places where the water source is deep or the surrounding terrain is flat, the allowable suction head of the water pump is overlooked, resulting in less water being absorbed or no water being absorbed at all. The vacuum degree established at the water inlet of the self-priming centrifugal pump is limited. The suction head under absolute vacuum is approximately 10 meters of water column height. The water pump cannot create an absolute vacuum, and if the vacuum degree is too high, the water in the pump is likely to vaporize, which is not conducive to the operation of the water pump. Each centrifugal pump has a maximum allowable suction head, generally ranging from 3 to 8.5 meters. When installing the water pump, one should not just seek convenience.
4.The resistance loss in the inlet and outlet pipes of the water flow is too large.
Some users have measured and found that the vertical distance from the reservoir or water tower to the water surface of the water source is slightly less than the head of the centrifugal pump, but the amount of water lifted is small or no water can be lifted. The reason is often that the pipeline is long and has many bends, resulting in a large resistance loss of the water flow in the pipeline. Generally, a 90-degree elbow has a greater resistance than a 120-degree elbow. The head loss of each 90-degree elbow is approximately 0.5 to 1 meter, and the resistance of every 20 meters of the pipeline causes a head loss of about 1 meter. In addition, some users randomly change the diameters of the inlet and outlet pipes of the water pump, which also affects the head.
5.Influences of other factors
5.1 The foot valve cannot be opened. Usually, because the water pump has been left idle for a long time, the gasket of the foot valve is stuck firmly, and a foot valve without a gasket may be rusted shut.
5.2 The filter screen of the foot valve is blocked; or the foot valve is buried in the sludge layer in the water, causing the filter screen to be blocked.
5.3 The impeller is severely worn. The impeller blades are worn out after long-term use, affecting the performance of the water pump.
5.4 The gate valve or check valve malfunctions or is blocked, resulting in a decrease in flow rate or even no water being pumped.
5.5 The leakage of the outlet pipeline affects the amount of water lifted.
6.Common simple equipment fault diagnosis methods
6.1 The listening method
The sound of normal equipment operation has a certain rhythm and tempo. Once familiar with it, one can compare and identify abnormal noises of the equipment through listening, and determine potential hazards such as internal looseness, impact, and imbalance. Tap the parts with a hammer and listen for any cracking noises to determine if there are cracks. The electronic stethoscope converts the vibration of the equipment into an electrical signal and amplifies it, and workers use headphones to monitor the vibration sound for qualitative measurement. Judge the fault by comparing the signals measured at the same measuring point in different periods, at the same rotation speed, and under the same working conditions. If a clear and sharp noise is heard in the headphones and the vibration frequency is high, it generally indicates a local defect or a tiny crack in a small-sized and high-strength part; if it is a turbid and low-pitched noise and the vibration frequency is low, it generally indicates a larger crack or defect in a large-sized and low-strength part; if the noise increases, the fault is developing, and the louder the sound, the more serious the fault; if there is a chaotic and irregular intermittent noise, it means that some parts or components are loose. When listening, one can align the tip of a screwdriver (or a metal rod) with the diagnosed part, hold the handle of the screwdriver and put it to the ear to listen carefully to filter out the noise.
6.2 The touch measurement method
The sense of touch of human hands can be used to monitor the temperature, vibration, and gap changes of the equipment. The nerve fibers in the hand are sensitive to temperature and can distinguish temperatures within 80°C relatively accurately. The feel of mechanical parts at different temperatures is different. When touching, one should first make a tentative touch and then touch carefully to estimate the temperature rise. Shaking the mechanical parts with hands can sense the gap size between 0.1mm and 0.3mm, and touching can sense the strength of the vibration, the impact, and the creeping situation of the slide. Using a thermometer equipped with a surface thermocouple probe to measure the surface temperature of mechanical parts such as rolling bearings can quickly determine the location of thermal abnormalities, provide accurate data, and is convenient for touch measurement.
6.3 The Observation Method
Visually observe whether the mechanical components of the equipment are loose, cracked or otherwise damaged; check whether the lubrication is normal, and whether there is dry friction as well as leakage such as running, bubbling, dripping and leaking; check the metal wear particles in the sediment of the oil tank to determine the wear of the parts; monitor whether the movement of the equipment is normal; check the instruments of the equipment to understand the data changes; detect the product quality and determine the working condition of the equipment through measuring tools and observing the surface conditions. By comprehensively analyzing the observed information, the fault condition of the equipment can be judged. The magnetic plug method is a simple way to observe the wear particles in the lubricating oil of the equipment. Insert a magnetic plug head into the lubricating oil to collect the iron wear particles, and observe the wear particles with the help of a reading microscope or the naked eye to determine the wear degree of the parts. If small wear particles are found and the quantity is small, the equipment is operating normally; if large wear particles are found, attention should be paid; if large particles are continuously found several times, it is a precursor of a fault, and the equipment should be shut down for inspection and troubleshooting. Temperature determination training by hand feeling: Use a nodal thermometer to measure the metal surface at several states of 50 degrees, 60 degrees, 70 degrees and 80 degrees. When the temperature is low, touch it with your hand and determine the temperature according to the contact time. When the temperature is high, pour a small amount of water droplets, observe the evaporation state of the water and remember it for use when diagnosing the equipment.
7. Troubleshooting of the Water Pump Tripping Fault
7.1 Fault Phenomenon
Since the 125 mw unit of the power plant was put into operation, the water pump occasionally trips as soon as it is closed, and there is no signal relay dropping its indication. After excluding the fault of the switch mechanism, routine inspection shows that the cable, the wiring of the secondary circuit, and each relay and its setting value are normal, and subsequent startups are often successful. There is a suspicion of a soft fault in the DCS system, but this phenomenon still occurs even when the operation is changed on the control panel.
7.2 Test to Find the Cause
Observe the changes of each meter when the switch is closed to confirm the cause of the trip. Use a voltmeter to monitor the microcomputer trip circuit during the test, use a milliammeter to monitor the actions of the differential relays 1cj and 2cj, and use an ammeter to monitor the thermal protection circuit. Connect the meters and start the feed water pump. It trips as soon as it is started once. The pointer of the milliammeter deflects, and other meters show no response. The newly replaced XJL - 0025/31 integrated block type signal relay 1XJ acts and drops its indication, indicating that the differential protection acts and causes the trip.
7.3 Root Cause Analysis
When the differential protection acts, the internal fault of the equipment is first suspected. Through routine inspection, the water pump motor and its cable, the differential relay, and the polarity of the current transformer are all normal. After excluding the equipment fault and wiring error, the action of the differential protection during the motor start indicates that the differential current in the differential loop exceeds the setting value of the differential relay during the start process. The reasons for the differential current in the differential loop under normal conditions are as follows: First, the ratio error of the current transformers on both the head and tail sides of the motor generates a differential current less than 5% of the rated current Id of the motor; Second, the difference in the secondary loads of the current transformers on the head and tail sides leads to a difference in the transformation ratio and generates a differential current. The difference in the loads of the current transformers in the differential protection loop of the water pump motor is only due to the difference in the length of the secondary cables, which is about 50 m different. And under the rated current, the power consumption of the differential relay is not more than 3 va, and the secondary load is not heavy. The inspection finds that the models of the current transformers on the head and tail sides of the differential protection of the feed water pump motor are both LMZBJ - 10, B level, 15 times the rated current, the transformation ratio is 600/5, and the capacity is 40 va, which meets the requirements of the secondary load. When the motor starts, the current is large, and the current transformers on both the head and tail sides may be saturated. Since the magnetization characteristics of each current transformer are not consistent, the secondary differential current may be very large. According to the setting instructions of the LCD - 12 type differential relay from Acheng Relay Factory, the setting value of the relay operating current Izd =△i1×kk×In/n = 0.06×3×356/120 = 0.534a (In the formula: △i1— The maximum error of the current transformers at the head and tail ends during normal operation, 0.04 - 0.06; kk— Reliability coefficient, 2 - 3; In— Rated current of the motor; n— Transformation ratio of the current transformer), and it should be set at the position of 1.0a. When using the B-level transformer, even if the operating current of the differential relay is set at 1.5a and the braking coefficient is 0.4, it still occasionally acts when the motor starts, because the saturation point of the magnetization characteristics of the B-level current transformer is low, and its anti-saturation ability is low, which does not meet the requirements of the differential relay. Usually, the current transformers in the differential protection loop adopt the D level. The saturation point of the D-level transformers is high, and they are not easy to be saturated, which can reduce the differential current in the differential loop when the motor starts. After replacing it with the D-level current transformer, setting the operating current of the differential relay at 1.0a and the braking coefficient at 0.4, the fault that the switch trips as soon as it is closed has not occurred again.
8. Treatment and Discussion of the Mechanical Seal Fault of the Water Pump
8.1 Characteristics of the Mechanical Seal of the Water Pump
The main advantages of the mechanical seal of the water pump are reliable sealing, a long service cycle with less leakage; a long service life, generally about 5 years; and a long maintenance cycle. However, the structure is complex, the manufacturing and installation accuracy is high, the cost is high, and it has high technical requirements for maintenance personnel. The mechanical seals used in oil pipelines are mostly built-in types. When repairing, the oil pump needs to be disassembled, and the workload is large. It is very important to ensure the reliable operation of the mechanical seal and extend its service life.
8.2 Problems Prone to Occur in the Mechanical Seal of the Water Pump
During operation, the main problems of the mechanical seal are excessive leakage and overly high temperature. If you touch the gland of the mechanical seal by hand and can't keep your hand on it, it indicates that the temperature is too high. The leakage amount on each side should not exceed 60 drops per minute. If the liquid flows in a linear shape, it means the leakage is large, and the operation should be observed; if there is oil spraying outwards, the pump should be stopped immediately for inspection.
8.3 Control Measures Adopted
8.3.1 Ensure the Quality of Components
The mechanical seal should undergo a sealing performance test before leaving the factory and come with a certificate of conformity. After long-term operation, the wear of the rotating ring and the stationary ring, the corrosion and wear of the spring and the shaft, and the wear, aging and deformation of the sealing rubber ring will lead to sealing leakage, and repair or replacement with new parts is required. There should be no cracks, chipping, scratches, pitting, burrs or uneven wear on the sealing surfaces of the rotating ring and the stationary ring, and the scratches and pitting should not penetrate the sealing end face. For the repaired rotating and stationary rings, the sum of the heights of the bosses should be no less than 3mm, and the height of a single boss should be no less than 1mm to ensure heat dissipation. After the rotating ring is installed, it should be able to move flexibly on the shaft, and it should be able to bounce back freely after being pressed against the spring, maintaining the perpendicularity and parallelism of the rotating and stationary rings. The specifications of the sealing rubber rings for the rotating and stationary rings should conform to the drawings, and there should be no damage, uneven thickness or uneven hardness on the surface. They should be replaced during major overhauls. The outer surface of the spring should be clean and free of rust. Before use, conduct length and shape inspections and pressure tests. The pressure difference and pressure error at the specified compression length of each group of springs should meet the requirements, the tolerance of the free length should not exceed 0.5mm, and the error of the compression amount should be ±2mm. The materials of the sealing sleeve and the pump shaft are different. The tolerance of the parallelism of the two end faces and the tolerance of the non-perpendicularity to the axis should not exceed ±0.20mm.
8.3.2 Ensure Sufficient Cooling and Lubrication
Adjust the opening of the regulating valve of the cooling pipeline to ensure that the cooling pipeline of the mechanical seal is unobstructed. When filling the water pump, open the vent valve to exhaust the gas in the sealing cavity.
8.3.3 Ensure Installation Accuracy
When disassembling and assembling the mechanical seal of the water pump, clean the rotating and stationary rings thoroughly, and apply a small amount of clean lubricating oil to the friction pair surface. Take both the high-pressure end and the low-pressure end into account, and avoid any bumping or knocking. When installing the stationary ring gland, apply force evenly to prevent it from being pressed unevenly. Check with a feeler gauge, and the deviation of the up, down, left and right positions should not be greater than 0.05mm; check the matching clearance between the gland and the outer diameter of the shaft, which should be uniform around, and the deviation at each point should not be greater than 0.1mm. The radial runout of the pump shaft at the installation position of the mechanical seal of the water pump should not exceed 0.05mm. Before installing the pump cover and the sealing end cover, recheck the installation positioning dimensions of the mechanical seal. If they do not meet the requirements, they can be adjusted with steel gaskets between the shaft sleeves. The steel gaskets should have high precision, and the difference in thickness should not exceed 0.01mm. Measure that the radial runout of the mechanical seal sleeve and the end face runout of the sealing surface meet the requirements. For the mechanical seal that has been in operation, if the gland is loosened and the sealing surface moves, the rotating and stationary ring parts must be replaced and cannot be tightened again for continued use, because the movement track of the friction pair changes, and the sealing performance is likely to be damaged.
8.3.4 Adjust the Specific Pressure of the End Face
The specific pressure of the end face is related to the sealing performance and service life, and is related to the sealing structure type, the size of the spring and the medium pressure. If the specific pressure of the end face is too large, it will damage the friction pair; if it is too small, leakage is likely to occur. The manufacturer will provide an appropriate range, generally taking 3 - 6kg/cm². Adjusting the specific pressure means adjusting the compression size of the spring. The free length A of the spring, the stiffness k, and the specified specific pressure P are parameters given by the manufacturer. After compression, the size is B, and P/(A - B) = k, from which B = A - P/k, that is, the size of the spring after installation and compression. If the size of the spring after installation is too large, increase the thickness of the adjusting shim between the spring seat and the spring; if it is too small, reduce the thickness, and measure the thickness of the adjusting shim with a micrometer.
9. Fault Diagnosis and Elimination Measures of the Water Pump
9.1 No liquid is supplied, the supplied liquid is insufficient, or the pressure is insufficient.
9.1.1 The water pump has not been filled with water or has not been properly vented. Elimination measure: Check whether the pump casing and the inlet pipeline are filled with liquid.
9.1.2 The speed of the water pump is too low. Elimination measure: Check whether the motor wiring, voltage or the steam pressure of the turbine is normal.
9.1.3 The head of the water pump system is too high. Elimination measure: Check the system head (especially the friction loss).
9.1.4 The suction head of the water pump is too high. Elimination measure: Check the net head (a too small or too long inlet pipeline will cause a large friction loss).
9.1.5 The impeller of the water pump or the pipeline is blocked. Elimination measure: Check for any obstacles.

9.1.6 The rotation direction of the water pump is wrong. Elimination measure: Check the rotation direction.
9.1.7 The water pump generates air or there is a leak in the inlet pipeline. Elimination measure: Check whether there are cavitation and/or air leakage in the inlet pipeline.
9.1.8 The packing or seal in the stuffing box of the water pump is worn, allowing air to leak into the pump casing. Elimination measure: Check the packing or seal and replace it as needed, and check whether the lubrication is normal.
9.1.9 When the water pump pumps hot or volatile liquid, the suction head is insufficient. Elimination measure: Increase the suction head and consult the manufacturer.
9.1.10 The foot valve of the water pump is too small. Elimination measure: Install a foot valve of the correct size.
9.1.11 The immersion depth of the foot valve or the inlet pipe of the water pump is insufficient. Elimination measure: Consult the manufacturer about the correct immersion depth and use a baffle to eliminate the eddy current.
9.1.12 The clearance of the impeller of the water pump is too large.
Elimination measure: Check whether the clearance is correct.
9.1.13 The impeller of the water pump is damaged. Elimination measure: Check the impeller and replace it as required.
9.1.14 The diameter of the impeller of the water pump is too small. Elimination measure: Consult the manufacturer for the correct impeller diameter.
9.1.15 The position of the pressure gauge of the water pump is incorrect. Elimination measure: Check whether the position is correct and inspect the outlet nozzle or pipeline.
9.2 The water pump stops operating after running for a while.
9.2.1 The suction head is too high. Elimination measure: Check the net head (a too small or too long inlet pipeline will cause significant friction loss).
9.2.2 The impeller or pipeline is blocked. Elimination measure: Check for any obstacles.
9.2.3 Air is generated or there is a leak in the inlet pipeline. Elimination measure: Check whether there are cavitation and/or air leakage in the inlet pipeline.
9.2.4 The packing or seal in the stuffing box is worn, allowing air to leak into the pump casing. Elimination measure: Check the packing or seal and replace it as required, and check whether the lubrication is normal.
9.2.5 When pumping hot or volatile liquid, the suction head is insufficient. Elimination measure: Increase the suction head and consult the manufacturer.
9.2.6 The immersion depth of the foot valve or the inlet pipe is insufficient. Elimination measure: Consult the manufacturer for the correct immersion depth and use a baffle to eliminate the eddy current.
9.2.7 The sealing gasket of the pump casing is damaged. Elimination measure: Check the condition of the sealing gasket and replace it as required.
9.3 The power consumption of the water pump is too high.
9.3.1 The rotation direction is incorrect. Elimination measure: Check the rotation direction.
9.3.2 The impeller is damaged. Elimination measure: Check the impeller and replace it as required.

9.3.3 The rotating parts are seized. Elimination measure: Check whether the clearance of the internally worn parts is normal.
9.3.4 The shaft is bent. Elimination measure: Straighten the shaft or replace it as required.
9.3.5 The speed is too high. Elimination measure: Check the voltage of the motor winding or the steam pressure supplied to the turbine.
9.3.6 The head is lower than the rated value, and too much liquid is pumped. Elimination measure: Consult the manufacturer, install a throttle valve, and cut the impeller.
9.3.7 The liquid is heavier than expected. Elimination measure: Check the specific gravity and viscosity.
9.3.8 The stuffing box is not properly packed (insufficient packing, not properly inserted or run-in, the packing is too tight). Elimination measure: Check the packing and refill the stuffing box.
9.3.9 The bearing lubrication is incorrect or the bearing is worn. Elimination measure: Check and replace as required.
9.3.10 The operating clearance between the wear rings is incorrect. Elimination measure: Check whether the clearance is correct and replace the wear rings of the pump casing and/or the impeller as required.
9.3.11 The stress on the pipeline of the pump casing is too high. Elimination measure: Relieve the stress and consult the manufacturer's representative, and check the alignment after relieving the stress.
9.4 The leakage of the stuffing box of the pump is too large.
9.4.1 The shaft is bent. Elimination measure: Straighten the shaft or replace it as required.
9.4.2 The coupling or the pump and the driving device are not aligned. Elimination measure: Check the alignment and realign as required.
9.4.3 The bearing lubrication is incorrect or the bearing is worn. Elimination measure: Check and replace as required.
9.5 The temperature of the bearing is too high.
9.5.1 The shaft is bent. Elimination measure: Straighten the shaft or replace it as required.
9.5.2 The coupling or the pump and the driving device are not aligned. Elimination measure: Check the alignment and realign as required.
9.5.3 The bearing lubrication is incorrect or the bearing is worn. Elimination measure: Check and replace as required.
9.5.4 The stress on the pipeline of the pump casing is too high. Elimination measure: Relieve the stress and consult the manufacturer's representative, and check the alignment after relieving the stress.
9.5.5 There is too much lubricant. Elimination measure: Remove the plug to let the excess grease drain out automatically; for oil-lubricated pumps, drain the oil to the correct oil level.
9.6 The stuffing box of the water pump is overheated.
9.6.1 The packing or seal in the stuffing box of the water pump is worn, allowing air to leak into the pump casing. Elimination measure: Check the packing or seal and replace it as required, and check whether the lubrication is normal.
9.6.2 The stuffing box of the water pump is not properly packed (insufficient packing, not properly inserted or run-in, the packing is too tight). Elimination measure: Check the packing and refill the stuffing box.
9.6.3 There are design problems with the packing or mechanical seal of the water pump. Elimination measure: Consult the manufacturer.
9.6.4 The mechanical seal of the water pump is damaged. Elimination measure: Check and replace as required, and consult the manufacturer.
9.6.5 The shaft sleeve of the water pump is scratched. Elimination measure: Repair, re-machine or replace it as required.
9.6.6 The packing of the water pump is too tight or the mechanical seal is not properly adjusted. Elimination measure: Check and adjust the packing, and replace it as required; adjust the mechanical seal (refer to the manufacturer's instructions or consult the manufacturer).
9.7 It is difficult for the rotating parts to rotate or there is friction.
9.7.1 The shaft of the water pump is bent. Elimination measure: Straighten the shaft or replace it as required.
9.7.2 The operating clearance between the wear rings of the water pump is incorrect. Elimination measure: Check whether the clearance is correct and replace the wear rings of the pump casing or the impeller as required.
9.7.3 The stress on the pipeline of the water pump casing is too high. Elimination measure: Relieve the stress and consult the manufacturer's representative, and check the alignment after relieving the stress.
9.7.4 The shaft or the impeller ring of the water pump swings too much. Elimination measure: Check the rotating parts and the bearings, and replace the worn or damaged parts as required.
9.7.5 There is dirt between the impeller of the water pump and the wear ring of the pump casing, and there is dirt in the wear ring of the pump casing. Elimination measure: Clean and check the wear ring, replace it as required, and block and eliminate the source of the dirt.