Centrifugal water pumps play a crucial role in fluid transfer systems. Their operation is based on the principle of centrifugal force, whereupon starting, the pre-filled water within the pump is set in rapid rotation by the impeller. This induces a centrifugal effect, causing the water to be ejected forcefully into the outlet pipe. Concomitantly, a low-pressure region is created near the shaft, facilitating the continuous influx of water from the inlet pipe under atmospheric pressure. The continuous high-speed rotation of the impeller, powered by an external source, ensures the uninterrupted pumping of water from a lower to a higher level. Understanding this fundamental principle is the cornerstone for effective maintenance. In this article, we will explore in-depth the maintenance strategies and failure analysis related to centrifugal water pumps.

Sealing Surface Leakage: In-depth Analysis. The leakage of the sealing surface between the moving and stationary rings is a critical issue. The flatness and roughness requirements of the end faces are of utmost importance. Deviations from the specified standards can lead to improper contact and subsequent leakage. For instance, if the surface finish is not within the tolerance range, it can cause irregularities in the sealing interface. Additionally, scratches on the surface can act as channels for fluid escape. Particulate matter trapped between the end faces can disrupt the synchronous operation of the two components. During installation, meticulous attention must be paid to ensure proper alignment and seating. Incorrect installation methods can introduce misalignment and stress concentrations, exacerbating the risk of leakage.

Compensation Ring Seal Ring Problems. The seal ring of the compensation ring is prone to leakage due to several factors. Deformation of the gland can cause uneven preload distribution, leading to localized areas of high stress and potential leakage paths. Incorrect installation practices can also compromise the integrity of the seal. The quality of the sealing ring itself is a crucial determinant. Substandard materials or manufacturing defects can result in poor sealing performance. Moreover, improper selection of the sealing ring based on the operating conditions can lead to premature failure. For example, using a sealing ring with insufficient chemical resistance in a corrosive medium will inevitably lead to degradation and leakage.

Common Failure Modes and Root Causes of Sealing Elements. The end faces of the dynamic and static rings are the most frequently failed parts. During installation, an oversized gap between the sealing surfaces can prevent the effective removal of heat generated by the friction pair. This can cause the temperature of the end face to rise rapidly, leading to overheating and damage. The vaporization and expansion of the liquid medium can exert significant forces on the sealing surfaces. When these forces exceed the adhesive strength of the lubricating film, the film is disrupted, resulting in direct contact between the surfaces and subsequent overheating. Poor lubricity of the liquid medium, combined with high operating pressures, can cause asynchronous rotation of the sealing surfaces. In high-speed pumps, the high linear velocities exacerbate this issue. For example, in a pump with a rotating speed of 20445r/min and a sealing surface center diameter of 7cm, the linear speed can reach up to 75 m/s. Any lag in the rotation of one of the sealing surfaces can generate instantaneous high temperatures and cause severe damage. Blockage of the sealing flushing fluid orifice plate or filter screen can lead to insufficient water supply for cooling and lubrication, ultimately resulting in mechanical seal failure.

Additional Factors Affecting Sealing Element Failure. The cleanliness of the liquid medium is essential. Tiny hard particles present in the medium can cause abrasive wear on the sealing surface. These particles can penetrate the interface and scratch the end face, compromising the sealing integrity. Poor coaxiality of the driving parts of the centrifugal water pump can induce vibrations and misalignment of the end face during operation. Each revolution can cause rubbing and shaking, leading to vaporization and overheating wear. Frequent changes in the hydraulic characteristics of the liquid medium can cause vibrations in the pump set, which can dislocate the sealing surface and lead to failure. Corrosion of the sealing elements by the liquid medium, stress concentrations, material incompatibility, erosion, and deformation of auxiliary sealing components such as O-rings, V-rings, and concave rings are all factors that can contribute to mechanical seal surface damage and failure. Therefore, a comprehensive analysis of the damage form is necessary to identify the root cause and implement appropriate corrective measures to ensure the long-term operation of the mechanical seal.

General Shutdown Procedures. Immediately after the centrifugal water pump ceases operation, it is imperative to close the inlet valve to prevent backflow and potential damage to the pump. Subsequently, once the pump has cooled down, the valves of the auxiliary system should be systematically closed in a sequential manner. This ensures the proper shutdown of the entire system and minimizes the risk of any residual pressure build-up or fluid leakage.

High - temperature Pump Shutdown Precautions. For high - temperature pumps, strict adherence to the equipment's technical documentation is essential during shutdown. After shutdown, the pump must be manually rotated by half a turn at regular intervals of 20 - 30 minutes. This practice helps to prevent the shaft from warping due to uneven cooling and ensures that the temperature of the pump body gradually decreases to 50 ℃. By maintaining this procedure, the integrity of the pump's components is preserved, and the risk of premature failure due to thermal stress is minimized.

Cryogenic Pump Shutdown Requirements. In the case of cryogenic pumps, when no specific requirements are stipulated, it is necessary to frequently refill the pump with liquid. This helps to maintain the low temperature environment within the pump and prevents the formation of ice or frost. The suction and discharge valves should be kept in the open position to allow for proper venting and drainage. For cryogenic pumps equipped with double - end mechanical seals, the liquid level controller and the sealing fluid within the pump sealing cavity must be carefully monitored to ensure that the grouting pressure remains within the specified range. This is crucial for maintaining the integrity of the seal and preventing any leakage.

Pump Handling for Special Media. For pumps handling media that are prone to crystallization, solidification, or precipitation, proactive measures must be taken to prevent blockage after shutdown. Timely flushing of the pump and pipeline with clean water or an appropriate alternative medium is essential. This removes any residual media that could potentially cause blockages and ensures the smooth operation of the pump during subsequent start-ups. Additionally, draining any accumulated liquid from the pump is vital to prevent corrosion and frost cracking, which can severely damage the pump's components.

Surface Protection and Lubrication. For uninstalled centrifugal water pumps, the unpainted surfaces must be carefully coated with a high-quality anti-rust agent. This forms a protective barrier against corrosion and extends the lifespan of the pump. Bearings lubricated with oil should be filled with the appropriate grade of oil to ensure smooth operation. In the case of grease-lubricated bearings, only a single type of grease should be used to avoid potential compatibility issues. Mixing different greases can lead to reduced lubrication efficiency and premature bearing failure.

Cleaning and Flushing Procedures. Promptly after use or prior to storage, the centrifugal water pump should be thoroughly cleaned. This involves removing all residual liquid from the suction pipeline, discharge pipeline, pump casing, and impeller. The flushing liquid used for cleaning should also be completely drained to prevent any potential contamination or corrosion. Thorough cleaning ensures that no debris or contaminants are left behind, which could affect the performance of the pump during subsequent use.

Bearing Maintenance and Oil/Grease Replacement. The oil in the bearing box should be drained and replaced with clean oil. This helps to remove any contaminants or degraded oil that may have accumulated during operation. Similarly, the grease in the bearings should be thoroughly cleaned and replaced with new grease. Proper lubrication of the bearings is crucial for the smooth operation and long-term reliability of the pump.

Storage Environment and Protection. The centrifugal water pump should be stored in a clean, dry, and well-ventilated area. The suction and discharge ports should be sealed to prevent the ingress of dust and moisture. Protecting the motor winding from moisture is essential to prevent electrical malfunctions. Spraying the inside of the pump housing with anti-rust and anti-corrosion liquid provides an additional layer of protection against corrosion. This helps to maintain the integrity of the pump's components during storage.

Periodic Checks and Rotation. To prevent freezing and ensure the smooth operation of the bearings, the centrifugal water pump shaft should be rotated once a month. This simple yet crucial step helps to distribute the lubricant evenly and maintain the flexibility of the bearings. Additionally, during the rotation process, any signs of abnormal resistance or noise should be carefully noted and investigated promptly.

In conclusion, maintaining centrifugal water pumps requires a comprehensive understanding of their working principles, potential failure modes, shutdown procedures, and proper storage techniques. By adhering to the recommended maintenance practices and addressing issues promptly, operators can ensure the reliable and efficient operation of centrifugal water pumps, prolong their service life, and minimize downtime. Continuous monitoring of the pump's performance and staying updated on the latest maintenance technologies are also essential for optimizing the performance of centrifugal water pumps in the long run.