The service life of a water pump cannot be separated from regular inspections. The inspection process mainly judges the condition of the water pump based on its external operating performance, in order to discover any abnormalities in the water pump. Most abnormalities are not caused by irreversible damage to the water pump. If the fault can be diagnosed and maintained in a timely manner, the water pump can be restored to normal operation.
There are five main manifestations of abnormal water pumps:
1. Abnormal noise
2. Abnormal vibration
3. Abnormal performance
4. Abnormal temperature rise
5. Other abnormalities
Abnormal performance is mostly not detected by the water pump itself, but is manifested through other components upstream and downstream of the water pump system, such as low water flow from the faucet at the end of the water pump system, high temperature and high pressure alarms from the upstream heat source host, poor heating effect of the downstream fan or underfloor heating, and so on. For performance abnormalities detected externally, the final manifestation is that the flow rate or head on the water pump does not match the design. The reasons for this situation are usually:
1. The water pump has not been vented
Exhaust is a necessary step for the initial installation of a water pump. Failure to exhaust or incomplete exhaust can lead to mixed discharge of air and water inside the pump body. When there is continuous gas in the pump body that cannot be discharged, it will cause the performance curve of the water pump to decay, and the flow rate and head to decrease.
When the pump is stopped, the exhaust screw can be opened. If there is gas escaping or gas escaping after filling with water, it can be determined that there is gas in the pump body. In this case, the pump body should be completely exhausted or filled with water, and the exhaust screw should be closed to run the water pump.
In some cases, there may be gas in the suction pipe of the water pump, which requires multiple exhausts or pump refills to solve the problem.
2. Cavitation
As mentioned in previous content, cavitation not only causes vibration and noise in the water pump, but also affects its performance. This is because during the process of cavitation, the suction inlet of the impeller presents a mixed state of air and water. The presence of bubbles causes a decrease in the cross-sectional area of the inlet flow channel, resulting in an increase in local flow velocity and the generation of eddies, which affects the performance of the water pump.
Due to the characteristic of cavitation that changes with the flow rate of the water pump, gradually closing the outlet valve will narrow the gap between the measured performance and the curve performance of the water pump, until it is closed to a certain angle or fully closed, and the water pump performance will be consistent with the curve. The characteristic curve can be used to determine cavitation.
There are many methods to solve cavitation, but they are difficult to implement, such as reducing the medium temperature, increasing the inlet pipe diameter to reduce resistance, reducing the inlet pipe length to reduce resistance, and reducing the outlet valve opening.
3. Air blockage
The problem of gas blockage often occurs in sewage pump systems. When the sewage pump stops, the liquid level drops below the impeller. During the secondary water supply, the water pump and outlet pipeline are blocked by gas, causing the water level inside the pump body to not rise to the height of the impeller. At this time, starting the pump will cause the impeller to be unable to contact the water and run idle.
In this case, the operating current of the water pump is relatively small, and the problem of air blockage can be determined by the current.
To solve the gas blockage, a vent hole needs to be opened on the pipe section from the pump outlet to the check valve to discharge the gas inside the pump body.
4. Pump body cavitation
The similarity between pump body cavitation and pump non exhaust lies in the phenomenon of mixed air and water discharge inside the pump body. However, the key difference lies in the internal structure and installation angle of the pump body, which results in some air inside the pump body being unable to be discharged through pumping or exhaust. This can be analyzed and confirmed through the system structure.
When the water pump is trapped in the pump body, it is necessary to change the installation angle of the water pump to ensure correct installation, in order to eliminate the problem through exhaust or pump filling.
5. Motor reversal
For three-phase motor water pumps, motor rotation is a prone area to errors. When the motor rotation is not verified during debugging, the water pump may reverse, which can cause a sharp decline in pump performance and fail to provide effective flow and head.
It is possible to confirm whether the water pump is reversing by observing the direction of rotation of the motor. The correct direction can be seen from the external markings of the pump body or identified based on the appearance of the pump head and impeller.
For the problem of motor reversal, any two phase line sequences can be swapped to achieve it. If the water pump is driven by a frequency converter, changing the direction requires adjusting the wiring sequence between the motor and the frequency converter, or adjusting the parameters of the frequency converter.
6. The impeller falls off
When the system frequently experiences water hammer, the impeller may reverse and loosen, eventually leading to a drop phenomenon. After the impeller drops, the operation of the water pump will not be able to drive the impeller to work on the water, and there will naturally be no flow or head performance. At this time, the current of the motor is roughly the no-load current, which can be used to assist in judging this problem.
The repair of impeller falling is relatively simple, just disassemble the pump body and reinstall it, but the key is how to determine the cause of the falling and avoid falling again.
7. Inconsistent system resistance
In some systems, the performance of the water pump itself meets the design parameters, but the system cannot reach the design operating point during operation. This problem may be related to the system rather than the water pump, and may be caused by the system's resistance deviating too much from the design operating point.
For example, in a circulatory system design, the pipeline is too thin and there are many elbow valves, resulting in a steep resistance curve. Even if the valves are fully opened, the pipeline resistance cannot be reduced, leading to a lower water flow rate than the design value.
In this situation, by adjusting the valve, it was found that the operating point of the water pump can only work on the left section of the curve, and the system needs to be modified to reduce system resistance in order to release the water pump flow.
8. Performance testing point error
In rare cases, the abnormal performance of the water pump we see is not actually abnormal, but may be a "misjudgment" caused by errors in the collection points of flow and head. This type of error mostly comes from data feedback from pressure gauges or pressure sensors. When we use a pressure gauge/sensor at the wrong point, the water pump head read may be consumed by resistance elements such as valves or check valves, and may be lower than the true head of the water pump.
It is necessary to determine whether there is an issue of inaccurate head calculation based on the location of the pressure point in the system, and to measure the pressure value near the inlet and outlet of the water pump.
9. Controller setting error
Some water pumps with variable frequency control usually allow the setting of pressure or frequency to achieve the energy-saving effect of frequency reduction. However, if the pressure or frequency is set too low, it may lead to insufficient water output performance of the pump. In this case, only the correct setting of the frequency converter is needed to solve the problem.
10. Low speed
Unlike the issue of frequency setting errors in frequency converters, when replacing the motor, a low-speed motor was mistakenly used, resulting in a decrease in water pump speed and affecting water discharge performance.
The actual speed of the motor can be found on the motor nameplate, and the correct speed can be found based on the water pump nameplate or water pump information. When the speed difference is too large, it is necessary to replace the motor with a suitable speed.
11. Impeller assembly error
Errors in impeller assembly are often seen after on-site disassembly and maintenance of water pumps. The order of impeller reinstallation is incorrect, and the positioning shaft sleeve is installed in the wrong position, resulting in axial movement of the impeller, damage to the structure of the mouth ring, a large amount of backflow at the impeller suction port, loss of flow and head, and a decrease in pump efficiency.
For this issue, it is necessary to disassemble the pump head and measure the installation dimensions of the impeller to check. If it is indeed an installation error, it needs to be reinstalled.
12. Impeller damage
Due to long-term cavitation or foreign objects entering the pump body, the impeller wears out, and the blades and cover plate suffer from damage such as missing flesh and penetration, which can affect the hydraulic performance of the impeller and cause a decrease in flow and head. This type of damage is difficult to determine from the outside and requires disassembling the pump head to inspect the impeller.
For severely damaged impellers, replacement is necessary. Replacing the impeller is not difficult, but it is still necessary to check the cause of the impeller damage to avoid further damage in the future.
Regular inspections allow us to detect pump abnormalities as early as possible, identify the cause and handle them promptly to reduce costs. However, most people are unable to accurately identify the cause of pump abnormalities, resulting in low efficiency and even damage to the pump.
