The principle of centrifugal water pump is to fill the pump with water before starting. After starting, the rotating impeller drives the water in the pump to rotate at high speed, and the water will be centrifugal, thrown out and pressed into the outlet pipe. After the water is thrown out, the pressure near the impeller decreases, and a low pressure zone is formed near the shaft. The pressure here is much lower than the atmospheric pressure. Under the action of the atmospheric pressure, the water outside will rush through the bottom valve and enter the pump from the inlet pipe. The rushing water is thrown out again during the high-speed rotation of the impeller and pressed into the outlet pipe. The impeller rotates continuously at high speed driven by the power machine, and the water is continuously pumped from low to high.

1. Failure analysis of mechanical seal of centrifugal water pump

The shutdown of centrifugal water pump is mainly caused by the failure of mechanical seal. The failure is mainly caused by leakage. The reasons for leakage are as follows:

① The main reasons for the leakage of the sealing surface of the moving and stationary rings are: the flatness and roughness of the end face do not meet the requirements, or the surface is scratched; There is particulate matter between the end faces, which causes the two ends to not operate at the same time; The installation is not in place and the way is incorrect.

② The main reasons for the leakage of the seal ring of the compensation ring are: the gland is deformed and the preload is uneven; Incorrect installation; The quality of sealing ring does not meet the standard; The selection of sealing ring is incorrect.

The actual application results show that the most failure parts of the sealing elements are the end faces of the dynamic and static rings, and the cracks on the end faces of the dynamic and static rings of the centrifugal water pump seals are common failures. The main reasons are as follows:

① During installation, the gap between the sealing surfaces is too large, and the flushing fluid has no time to take away the heat generated by the friction pair; The flushing fluid leaked from the gap of the sealing surface, resulting in overheating and damage of the end face.

② The vaporization and expansion of the liquid medium will cause the two ends to be separated by the vaporization and expansion force. When the two sealing surfaces are tightly connected, the lubricating film will be damaged, resulting in overheating of the end surface.

③ The lubricity of the liquid medium is poor, coupled with the overload of the operating pressure, the tracking rotation of the two sealing surfaces is not synchronized. For example, the rotating speed of the high speed pump is 20445r/min, the center diameter of the sealing surface is 7cm, and its linear speed is up to 75 m/s after the centrifugal water pump is running. When one of the sealing surfaces lags behind and cannot track the rotation, the instantaneous high temperature causes damage to the sealing surface.

④ The sealing flushing fluid orifice plate or filter screen is blocked, resulting in insufficient water volume and failure of the mechanical seal.

In addition, the sliding groove on the surface of the sealing surface and the gap on the end face caused the failure of the sealing element. The main reasons are as follows:

① The liquid medium is not clean, and there are tiny hard particles, which slide into the sealing surface at a high speed, scratching the end surface and causing failure.

② The coaxiality of the driving parts of the centrifugal water pump is poor. After the centrifugal water pump is started, the end face is shaken and rubbed once every revolution, and the moving ring is not concentric, resulting in the end face vaporization and overheating wear.

③ The frequent occurrence of hydraulic characteristics of liquid medium causes the vibration of centrifugal water pump set, resulting in the dislocation of sealing surface and failure.

Corrosion of sealing elements by liquid medium, stress concentration, matching of soft and hard materials, erosion, auxiliary sealing O-ring, V-ring, concave ring and liquid medium incompatibility, deformation, etc. will cause mechanical seal surface damage and failure. Therefore, the damage form shall be comprehensively analyzed to find out the root cause and ensure long-term operation of mechanical seal.

2. Requirements after the centrifugal water pump stops running

① After the centrifugal water pump stops running, close the population valve of the pump, and then close the valves of the auxiliary system in turn after the pump cools down.

② The shutdown of the high-temperature pump shall be carried out according to the provisions of the technical documents of the equipment. After the shutdown, the centrifugal water pump shall be turned for half a turn every 20-30min until the temperature of the pump body drops to 50 ℃.

③ When the cryogenic pump is stopped, the centrifugal water pump shall be filled with liquid frequently when there is no special requirement; The suction and discharge valves shall be kept normally open; For the cryogenic pump with double-end mechanical seal, the liquid level controller and the sealing fluid in the pump sealing cavity shall maintain the grouting pressure of the pump.

④ For the pump that is easy to crystallize, solidify, precipitate and other media, the centrifugal water pump should be prevented from blocking after stopping, and the pump and pipeline should be flushed with clean water or other media in time.

⑤ Drain the liquid accumulated in the centrifugal water pump to prevent corrosion and frost crack.

3. Storage of centrifugal water pump

① The unpainted surface of the centrifugal water pump that has not been installed shall be coated with a suitable layer of anti-rust agent, the bearing lubricated with oil shall be filled with appropriate oil, and the bearing lubricated with grease shall be filled with only one kind of grease, and mixed grease shall not be used.

② The centrifugal water pump personnel shall clean the liquid in a short time, wash the suction pipeline, discharge pipeline, pump casing and impeller, and drain the flushing liquid in the pump casing, suction pipeline and discharge pipeline.

③ Drain the oil in the bearing box, fill it with clean oil, thoroughly clean the grease and fill it with new grease.

④ Seal the suction and discharge ports, store the centrifugal water pump in a clean and dry place, protect the motor winding from moisture, and spray the inside of the centrifugal water pump housing with anti-rust and anti-corrosion liquid.

⑤ The centrifugal water pump shaft shall be rotated once a month to avoid freezing, and the bearing shall be lubricated.

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