Water pump failure analysis

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Interesting Overview: 1. The pump does not deliver water. 2. The pump operates at a low speed. 3. The suction lift of the pump is too high. 4. Excessive resistance losses in the inlet and outlet pipes of the water flow. 5. Influence of other factors. 6. Common simple methods for diagnosing equipment failures. 7. Troubleshooting of pump tripping faults. 8. Handling and discussion of mechanical seal failures in pumps. 9. Pump failure diagnosis and corrective measures. Analysis of the reasons why a pump does not deliver water: Amplification is used, and workers listen to the vibration sounds of the operating equipment through headphones in order to qualitatively assess those sounds. By measuring the signals at the same measurement point at different times, under the same rotational speed and under the same operating conditions, and comparing them, it is possible to determine whether there is a fault in the equipment. When crisp and high-pitched noises are heard from the headphones, it indicates a high vibration frequency; generally, this is due to local defects or tiny cracks in components that are relatively small in size but have relatively high strength. When the headphones produce a muffled, low-frequency noise, it indicates that the vibration frequency is low; generally, this is due to large cracks or defects in parts that are relatively large in size and have relatively low strength. When the noise coming from the headphones increases compared to normal, it indicates that a fault is developing; the louder the sound, the more severe the fault. When the noise coming from the headphones appears in a chaotic and intermittent manner, it indicates that a component or part is loose. 2. Touch detection method: The tactile sense of the human hand can be used to monitor changes in the temperature, vibration, and clearance of equipment. The nerve fibers in the hands are sensitive to temperature, allowing them to detect temperatures down to 80°C with relatively high accuracy. When the temperature of the component is around 0°C, it feels icy to the touch; prolonged contact can cause a piercing pain. At around 10°C, it feels cool to the touch, but it is generally tolerable. At around 20°C, it feels slightly cool to the touch; as contact time increases, it gradually becomes warmer. At around 30°C, it feels slightly warm to the touch, providing a comfortable sensation. At around 40°C, it feels warm to the touch, with a slight burning sensation. At around 50°C, it feels quite hot to the touch; if one presses with the palm for an extended period, sweating will occur. At around 60°C, it feels very hot to the touch, but it is generally tolerable for up to 10 seconds. At around 70°C, the touch feels extremely hot and painful; generally, one can tolerate it for only about 3 seconds. Additionally, the area touched by the hand quickly turns red. When touching, touch it first and then feel it more carefully to estimate the temperature rise of the component. By shaking the component by hand, it is possible to sense the gap size of 0.1mm–0.3mm. By touching the components with your hand, you can sense changes in the intensity of vibration and whether shocks are generated, as well as the movement of the slide. Using a thermometer equipped with a surface thermocouple probe to measure the surface temperature of components such as rolling bearings, sliding bearings, spindle boxes, and motors offers the advantages of rapid identification of the location of thermal abnormalities, accurate data, and a convenient measurement process. 3. Observation method: Human vision can be used to check whether there is any looseness, cracks, or other damage to the components on the equipment ； It is possible to check whether lubrication is normal, and to detect any occurrences of dry friction or leaks ； It is possible to examine the quantity, size, and characteristics of metal particles in the fuel tank sediment in order to assess the wear level of related components ； It is possible to monitor whether the device is moving normally and if there are any abnormal phenomena ； It is possible to view the various gauges installed on the device, which indicate its operating status, in order to monitor changes in the data. Product quality can be checked, and the device’s operating condition can be assessed by using measurement tools and by directly observing the surface condition. By comprehensively analyzing various observed pieces of information, it is possible to determine whether there is a malfunction in the equipment, the location of the fault, its severity, and the cause. A simple method for monitoring wear conditions by using instruments to observe the wear particles collected from the equipment’s lubricating oil is the magnetic plug method. Its principle involves inserting a magnetic plug into the lubricating oil to collect the iron particles generated by wear; by using a reading microscope or simply observing with the naked eye, the size, quantity, and shape of these particles are examined in order to determine the degree of wear on the surface of mechanical parts. The magnetic plug method can be used to observe the larger particle sizes that appear in the later stages of wear of mechanical parts. During inspection, if small abrasive particles are found in small quantities, it indicates that the equipment is operating normally ； If large abrasive particles are found, it is necessary to pay close attention and monitor the equipment’s operating condition meticulously ； If large particles are detected repeatedly in a row, it is a sign that a failure is imminent; the machine should be stopped immediately for inspection to identify and resolve the issue. It was explained in great detail; accurate judgment using these diagnostic methods requires a long period of experience accumulation. As a supplementary note: For auscultation, you can use the tip of a screwdriver (or a metal rod) to point at the area to be examined; hold the screwdriver with your hand and listen carefully. Doing this can filter out some noise. Temperature touch sensation assessment training: Using a nodular thermometer, measure the metal surface at temperatures of 50 degrees, 60 degrees, 70 degrees, and 80 degrees. For lower temperatures, timing is used to determine how long one can touch the surface, and the temperature is inferred based on this duration. When it’s too hot to touch at higher temperatures, you can sprinkle a few drops of water to observe how the water evaporates, and then remember these states. When used with diagnostic equipment, it can yield relatively accurate judgments. I’ve seen the method of determining temperature by touch in the book “Comprehensive Guide to the Installation, Commissioning, Operation Monitoring, Fault Diagnosis, and Maintenance Management of Modern Electromechanical Equipment”. However, I believe that everyone’s tolerance levels may vary; therefore, it’s more accurate to make a judgment based on one’s own experience, using the method suggested by the general moderator. Troubleshooting of pump tripping fault 1: Fault symptoms Since its commissioning, the 125 MW unit in the power plant has experienced occasional instances where the pumps trip as soon as they are switched on, with no signal relays failing. After ruling out any faults in the switching mechanism, a routine inspection revealed that the cables, secondary circuit wiring, and all relays along with their settings were normal; subsequent attempts to restart were often successful. It is suspected that this was caused by a software malfunction in the DCS system; however, even when operations are performed on the control panel, this phenomenon still occurs. 2: Experiment to determine the cause. To identify the reason for this phenomenon, observe the changes in various gauges during the process of closing the switch, so as to determine what causes it to trip. In the test, the voltmeter monitors the microcomputer tripping circuit; the milliammeter monitors the operating conditions of differential relays 1CJ and 2CJ; and the ammeter monitors the thermal protection circuit. After connecting the meters properly, the feed water pump was started. After a period of testing, there was finally one instance in which the pump tripped immediately upon startup. At the same time, it was observed that the pointer of the milliammeter deflected slightly; no other monitoring instruments showed any response. The newly installed xjl-0025/31 type integrated-circuit signal relay 1xj also operated and its trip indicator activated, indicating that the trip was caused by the operation of the differential protection. 3: Root cause analysis. When a differential protection operates, it is first suspected that there is a fault within the equipment being protected. Through routine inspections, the pump motor and its cables are in good condition, the differential relay has been properly calibrated, and the polarity connections of the current transformers are correct. After ruling out causes such as equipment malfunction and wiring errors, the differential protection operated during the motor startup process, indicating that the differential current in the differential circuit exceeded the setting value of the differential relay during this process. Under normal conditions, there are mainly two reasons that cause a differential current in the differential circuit: first, the turns ratio errors of the current transformers on the upstream and downstream sides of the motor differ from each other, resulting in a small differential current; this differential current is less than 5% of the motor’s rated current Id. Secondly, the difference in the secondary loads of the current transformers at the beginning and end also leads to a difference in their transformation ratios, resulting in a differential current. The difference in load on the current transformers in the differential protection circuit of the pump motor is merely due to the difference in length of the secondary cables, which is about 50 m; at the rated current, the power consumption of the differential relay is no more than 3 VA, so the secondary load is not heavy. The inspection revealed that the current transformers used for the differential protection of the feedwater pump motor on both the primary and secondary sides are all of the LMZBJ-10 type; they are rated for 15 times the rated current, have a transformation ratio of 600/5, and a capacity of 40 VA, which is more than sufficient to meet the requirements of the secondary load. The above analysis is based on normal operating conditions; the situation is different when the motor starts. When the motor starts, the current is very high, and the current transformers on the left and right sides may become saturated. At this time, due to the different magnetization characteristics of the various current transformers, the secondary differential current can be quite large. According to the setting instructions for the LCD-12 differential relay manufactured by Acheng Relay Factory, the set value of the relay’s operating current is calculated as follows: izd = △i1 × kk × in/n = 0.06 × 3 × 356/120 = 0.534 A. Here, △i1 represents the maximum error between the current transformers at the beginning and end under normal operating conditions; this value ranges from 0.04 to 0.06 ； kk—reliability coefficient, 2–3 ； in—Rated current of the motor ； n—Turns ratio of the current transformer. It should be set at 1.0a. When using class B current transformers, with the operating current of the differential relay set at 1.5 A and a braking coefficient of 0.4, the differential protection still occasionally trips during motor startup. This is because class B current transformers have a low saturation point in their magnetization characteristics and poor resistance to saturation, which fails to meet the requirements of the differential relay. It is generally required that the current transformers in the differential protection circuit use class D transformers. Class D transformers have a higher saturation point and are less likely to saturate, which helps to reduce the differential current flowing through the differential circuit during motor startup. After replacing it with a current transformer of class D and setting the operating current of the differential relay at 1.0 A with a braking coefficient of 0.4, there have been no more instances of tripping as soon as the switch was closed. Treatment and Discussion on Mechanical Seal Failures in Eight Water Pumps. A mechanical seal, also known as an end-face seal, relies on the pressure of springs and the sealing medium to generate an appropriate compressive force on the contact surfaces of the rotating moving ring and the stationary ring, thereby ensuring that these two end faces fit tightly together. A very thin layer of oil film is maintained between the end faces; the resistance to the flow of the medium is high, which prevents liquid leakage and thus achieves sealing. At the same time, it provides lubrication for both the moving ring and the stationary ring. With proper adjustment, there can be absolutely no leaks. 1 Characteristics of pump mechanical seals. The main advantage of pump mechanical seals is their reliable sealing performance, with very little leakage over a long service life ； It has a long service life, typically lasting around 5 years ； The maintenance cycle is long. However, mechanical seals have a complex structure, require high precision in manufacturing and installation, are costly, and demand high technical skills from maintenance personnel. Since the mechanical seals used in oil pipelines are of the internal type, repairing them often entails disassembling the oil pump, which is a time-consuming task. Therefore, it is very important to ensure the reliable operation of mechanical seals and extend their service life. 2 Common problems with the mechanical seal of water pumps During use, the main issues that arise with mechanical seals are excessive leakage and high temperatures. Touch the mechanical seal gland with your hand; if you cannot keep your hand on it, it indicates that the temperature is too high. The leakage rate on each side should not exceed 60 drops per minute; if liquid flows in a linear pattern, it indicates an excessive leakage rate, and it is then possible to determine whether to continue monitoring the operation ； If oil is spraying outward, the machine should be stopped immediately for inspection. 3 Control measures taken
3.1 Ensuring the quality of components
Before leaving the factory, mechanical seals must undergo seal performance tests and must be accompanied by a certificate of conformity. After long-term operation, mechanical seals experience wear of the rotating ring and the stationary ring, corrosion and wear of the springs and shaft, as well as wear, aging, and deformation of the sealing rubber rings – all of which can lead to leakage. In such cases, the seal must be repaired or replaced with new parts. The sealing surfaces of the rotating ring and the stationary ring must be free from cracks, chipped corners, scratches, pitting, burrs, and uneven wear; scratches and pitting must not extend across the entire sealing surface. When using the repaired stationary and rotating rings, the total height of their bosses should be no less than 3 mm, and the height of each individual boss should be no less than 1 mm, so as not to affect heat dissipation. After installation, the moving ring must be able to move freely on the shaft; when pressed against the spring, it should rebound freely, maintaining both the perpendicularity and parallelism between the moving and stationary rings. The specifications of the seal rings for the stationary and rotating rings comply with the requirements specified in the drawings; the surface must be free from any damage, uneven thickness, or uneven hardness. The seal rings should be replaced during major overhauls. The outer surface of the springs is clean and free from rust. Before use, they should undergo length and shape inspections as well as pressure tests; the pressure difference at the specified compression length for each set of springs must meet the required standards, and the pressure error at that same compression length also needs to be within acceptable limits. The tolerance for the free length shall not exceed 0.5 mm, and the compression amount should not be too large or too small; the allowable error is ±2 mm. The sealing sleeve and the pump shaft must not be made of the same material. The tolerance for the parallelism of their two end faces and the tolerance for their non-perpendicularity to the shaft axis must not exceed ±0.20 mm. 3.2 Ensure adequate cooling and lubrication: Adjust the opening degree of the control valves in the cooling circuit to guarantee unobstructed flow in the mechanical seal cooling circuit; when operating the tank pump, open the drain valve to remove all gas from the seal chamber. 3.3 Ensuring installation accuracy: When disassembling and reassembling the mechanical seal of the water pump, the stationary and rotating rings must be cleaned thoroughly. A small amount of clean lubricating oil should be applied to the surfaces of the friction pair. Attention must be paid to both the high-pressure and low-pressure sides; any impact or collision is strictly prohibited. When installing the static ring gland, apply even force to avoid misalignment; use a feeler gauge to check that the deviation in the up, down, left, and right directions is no more than 0.05 mm ； Check the fit clearance between the gland and the outer diameter of the shaft; it should be uniform around the perimeter, with the allowable deviation at each point not exceeding 0.1 ram. The radial runout of the pump shaft at the location where the mechanical seal is installed must not exceed 0.05 mm. Before installing the pump cover and the sealing end cap, it is necessary to carefully verify the installation positioning dimensions of the mechanical seal. If these dimensions do not meet the requirements, steel shims can be used for adjustment between the shaft sleeves; however, the precision of these shims must be high, with a thickness difference of no more than 0.01 mm. The radial runout of the mechanical seal sleeve and the end-face runout of the sealing surface meet the requirements. For mechanical seals that have been in use and in which the gland loosening has caused the sealing surfaces to shift, the stationary and rotating ring components must be replaced; it is absolutely not acceptable to retighten them and continue using the seal. Because after such loosening, the original motion path of the friction pair changes, and the sealing performance of the contact surface is easily compromised. 4.4 Adjusting the face pressure ratio. The face pressure ratio is an important parameter affecting sealing performance and service life; it is related to the structural type of the seal, the size of the spring, and the medium pressure. Excessive end-face pressure will damage the friction pair ； If the specific pressure is too low, leakage is likely to occur; manufacturers usually specify an appropriate range, with the specific pressure at the end faces typically being between 3 and 6 kg/cm2. Adjusting the specific pressure means adjusting the compression size of the spring. The free length of the spring is denoted by A, the load that the spring stiffness can withstand for a unit amount of compression is denoted by k, and the required specific pressure is denoted by P; these are all parameters specified by the manufacturer. If the dimension after compression is denoted by B, then P/A – 13 = k, which gives 13 = A – e/k; this is the dimension of the spring after it has been compressed and installed. If the dimensions after spring installation are too large, increase the thickness of the adjustment shim between the spring seat and the spring; if they are too small, decrease this thickness. The thickness of the adjustment shim should be measured using a micrometer. Diagnosis and Remedial Measures for Pump Failures: During maintenance, the diagnosis of pump failures is a crucial step. The following outlines several common failure types along with their corresponding remedial measures, to help in carrying out accurate diagnoses of pump failures. 1. No liquid is available, or the supply of liquid is insufficient or the pressure is low. (1) The water pump is not filled with water or has not been properly vented. Remedial action: Check whether the pump casing and inlet pipes are completely filled with liquid. 2) The water pump speed is too low. Remedy: Check whether the motor’s wiring is correct, whether the voltage is normal, or whether the steam pressure of the turbine is normal. 3) The head pressure in the water pump system is too high. Remedy: Check the head pressure of the system (especially friction losses). 4) The water pump’s suction head is too high. Solution: Check the existing net head (very small or long inlet pipes can cause significant friction losses). 5) Clogging of the water pump impeller or pipelines. Remedy: Check for any obstructions. 6) The rotation direction of the water pump is incorrect. Solution: Check the rotation direction. 7) Air is generated by the water pump or there is a leak in the inlet pipeline. Remedy: Check the inlet pipeline for cavitation and/or air leaks. 8) Wear of the packing or seal in the water pump’s stuffing box allows air to leak into the pump casing. Remedial measures: Check the packing or seal and replace it as necessary, and check whether the lubrication is proper. 9) Insufficient suction head when the water pump pumps hot or volatile liquids. Remedial measure: Increase the suction head; consult the manufacturer. 10 Water pumps) The bottom valve is too small. Solution: Install a bottom valve of the correct size. 11) The submergence depth of the water pump’s bottom valve or inlet pipe is insufficient. Solution: Consult the manufacturer for the correct submergence depth. Use baffles to eliminate vortices. 12) The clearance of the water pump impeller is too large. Remedy: Check whether the clearance is correct. 13) Damaged water pump impeller. Remedy: Inspect the impeller and replace it as required. 14) The diameter of the water pump impeller is too small. Solution: Consult the manufacturer for the correct impeller diameter. 15) The water pump pressure gauge is in the wrong position. Remedy: Check whether the position is correct, and inspect the outlet nozzle or pipe. 2. The water pump stops operating after running for a while. 1) The suction lift is too high. Solution: Check the existing net head (very small or long inlet pipes can cause significant friction losses). 2) Clogging of the impeller or pipeline. Remedy: Check for any obstructions. 3) Air is generated or there is a leak in the inlet pipeline. Remedial action: Check the inlet pipeline for cavitation and/or air leaks. 4) Wear of the packing or seal in the stuffing box allows air to leak into the pump casing. Remedial measure: Inspect the packing or seal and replace it as necessary. Check whether the lubrication is normal. 5) Insufficient suction head when pumping hot or volatile liquids. Solution: Increase the suction head; consult the manufacturer. 6) Insufficient immersion depth of the bottom valve or inlet pipe. Remedial measure: Consult the manufacturer for the correct immersion depth, and use baffles to eliminate vortices. 7) Damaged pump casing gasket. Remedy: Check the condition of the gasket and replace it as required. 3. Excessive power consumption by the water pump 1) Incorrect rotation direction. Solution: Check the rotation direction. 2) Impeller damage. Remedy: Inspect the impeller and replace it as required. 3) Seized rotating parts – Remedy: Check whether the clearance of the internally worn parts is normal. 4) Shaft bending – Remedy: Straighten the shaft or replace it as required. 5) Too high speed. Remedy: Check the winding voltage of the motor or the steam pressure supplied to the turbine. 6) The head is below the rated value. Too much liquid is being pumped out. Solution: Consult the manufacturer. Install the throttle valve and cut the impeller. 7) The liquid density is higher than expected. Countermeasure: Check the density and viscosity. 8) The stuffing box does not have proper packing (insufficient packing, improper installation or wear, packing that is too tight). Remedy: Inspect the packing and refill the stuffing box. 9) Incorrect bearing lubrication or bearing wear. Remedial action: Inspect and replace as required. 10) The operating clearance between the wear rings is incorrect. Remedy: Check whether the clearance is correct. Replace the pump casing and/or the wear rings of the impeller as required. 11) The stress on the pipes on the pump casing is too high. Remedy: Relieve the stress and consult the manufacturer’s representative. After stress relief, check the alignment. 4. Excessive leakage in the pump’s stuffing box. 1) Bent shaft. Remedial measure: Straighten the shaft or replace it as required. 2) Misalignment of the coupling or between the pump and the drive unit. Remedial action: Check the alignment; realign if necessary. 3) Incorrect bearing lubrication or bearing wear. Remedy: Inspect and replace as required. 5. Excessively high bearing temperature 1) Bent shaft. Remedy: Straighten the shaft or replace it as required. 2) Misalignment of the coupling or between the pump and the drive unit. Remedial action: Check the alignment; realign if necessary. 3) Incorrect bearing lubrication or bearing wear. Remedy: Inspect and replace as required. 4) The stress on the pipes on the pump casing is too high. Remedy: Relieve the stress and consult the manufacturer’s representative. After stress relief, check the alignment. 5) Excessive lubricant. Remedy: Remove the plug to allow the excess grease to drain out automatically. If it is an oil-lubricated pump, drain the oil to the correct level. 6. Overheating of the water pump stuffing box 1) The packing or seal in the water pump stuffing box is worn, allowing air to leak into the pump casing. Solution: Inspect the packing or seal and replace it as necessary. Check whether the lubrication is normal. 2) The water pump stuffing box does not have the proper packing (insufficient packing, not properly inserted or not properly seated, or the packing is too tight). Remedial action: Check the packing and refill the stuffing box. 3) There are design issues with the water pump packing or mechanical seal. Remedy: Consult the manufacturer. 4) Damage to the mechanical seal of the water pump. Remedy: Inspect and replace it as required. Consult the manufacturer. 5) Scratches on the water pump shaft sleeve – Remedial measures: Repair, re-machine, or replace as required. 6) The pump packing is too tight or the mechanical seal is not properly adjusted. Remedy: Check and adjust the packing, and replace it as required. Adjust the mechanical seal (refer to the manufacturer’s instructions provided with the water pump or consult the manufacturer). 7. Difficult rotation or friction in rotating components 1) Bent water pump shaft. Remedy: Straighten the shaft or replace it as required. 2) The operating clearance between the wear rings of the water pump is incorrect. Remedy: Check whether the clearance is correct. Replace the wear rings of the pump casing or impeller as required. 3) The stress on the pipes on the water pump casing is too high. Remedy: Relieve the stress and consult the manufacturer’s representative. After stress relief, check the alignment. 4) Excessive swinging of the water pump shaft or impeller ring. Remedy: Inspect the rotating parts and bearings, and replace any worn or damaged components as required. 5) There is dirt between the water pump impeller and the wear ring of the pump casing, as well as inside the wear ring of the pump casing. Remedial action: Clean and inspect the wear rings, and replace them as required. Isolate and eliminate the source of dirt.