What is Mechanical Seal(III)

V: Mechanical Seal Structure and Maintenance

  1. Installation

(1) Preparation

① Check whether the radial runout, surface roughness, outer diameter tolerance, and axial movement of the shaft and shaft sleeve meet the accuracy requirements.

② Check if the model and specifications of the mechanical seal match the requirements. Check the integrity of each part, whether the seal ring size is appropriate, and if the surfaces of the rotating and stationary rings are smooth. If there are defects, they must be replaced or repaired.

③ Clean the parts of the mechanical seal with clean gasoline, then wipe them dry, paying attention to protecting the sealing surfaces.

④ When installing the mechanical seal, first check the working length of the spring from the manual. Then, measure the free length of the spring with a caliper to obtain the compression amount of the spring. During installation, ensure that the deviation of the spring compression amount is not greater than 1mm.

(2) Inspection and Measurement

① Check the floatability of the rotating ring, which requires a certain gap between the rotating ring and the shaft, ensuring a gap of 0.3~0.7mm.

② Check if the fixed ring is eccentric. In pump mechanical seals, the fixed ring (spring seat) and the shaft use sliding fit, and the clearance is very small. If the clearance is too large, the fixed ring may become eccentric, causing uneven spring force on the sealing surface and resulting in leakage or sealing failure.

③ Inspection of the fit surface between the rotating ring and the stationary ring: During inspection, a 90° square ruler can be used to measure the deviation of the fitting surface from the centerline of the shaft.

(3) Installation

First, assemble the components in the rotating assembly and the stationary ring assembly separately, and pre-tighten the spring. Then, complete the installation of the rotating ring assembly on the shaft and the stationary ring assembly in the pressure cover. Preliminary measurements of the distance from the rotating ring sealing end face to the sealing chamber end face and the distance from the stationary ring sealing face to the end cover end face. The difference between the two is the spring pre-compression amount of the mechanical seal. Assemble the bearings, adjust the compression amount according to technical requirements, refer to the measured compression amount, and tighten the pressure cover.

During installation, the cleanliness and integrity of the seal must be maintained, and it is not allowed to use tools to hit the sealing components to prevent damage to the seal. After the mechanical seal is installed on the shaft, push the rotating ring by hand to feel elasticity and smoothness. Then, add some oil to the sealing surface, uniformly tighten the end cover, and do not deviate from the pressure.

(4) Inspection and Pressure Test

After installation, turn the handwheel by hand to ensure flexible rotation and a certain degree of flexibility. Mechanical seals for important equipment must undergo static and dynamic pressure tests. Only after the test is qualified can it be put into formal use.

  1. Operation

(1) Precautions before starting: Ensure that auxiliary devices and cooling systems are installed correctly; clean the pipelines to prevent rust and impurities from entering the sealing chamber; manually turn the coupling to check if the shaft rotates smoothly. If the turning is heavy, check whether the relevant mating dimensions are correct, find the cause, and troubleshoot.

(2) Trial run and normal operation: Start the liquid sealing system and the cooling water system first to fill the sealing chamber with the medium. Then, start the main seal for trial operation. If a small leakage is found at the beginning but gradually decreases after 1~3 hours, it is a normal running-in process. If the leakage does not decrease, stop and check. If the mechanical seal becomes hot or smoky, it is generally due to excessive spring compression. Reduce the spring pressure appropriately. After the trial run is qualified, enter normal operation under operating conditions. The warming-up and pressurization process should be slow, and closely observe for any abnormal phenomena.

  1. Shutdown

Stop the main engine first, and then stop the auxiliary systems and cooling water system. When the shutdown time is long, drain the medium from the main engine.

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