Leakage Analysis and Improvement Plan of Mechanical Seal on Alkali Pump

Leakage analysis and improvement plan of mechanical seal on alkali pump

21/09/25 10:04:09

By analyzing the reasons for the frequent failure of the mechanical seal of the lye circulating pump of the ethylene plant, the sealing structure and friction pair materials have been improved, and good sealing effect and economic benefits have been achieved.

1. Basic situation: The pump is a single-suction cantilever horizontal centrifugal pump. The main technical parameters are as follows. Medium composition: 20% NaOH liquid containing colloidal and granular impurities.

Flow: 30m3/h

Head: 18m

Suction pressure: 1.078MPa

Discharge pressure: 1.272MPa

Shaft power: 2.7KW continuous use.

Motor speed: 2950r/m

Disassemble the failed mechanical seal, and its damage is as follows:

(1) The gap between the moving ring, the moving ring seat and the shaft sleeve is blocked

(2) There are annular wear marks on the moving ring and the static ring

(3) There are coke-like polymers, particles and other impurities and crystals in the sealing cavity. (4) The spring is seriously corroded, stuck by impurities and crystals, and has insufficient elasticity.

2. Failure analysis

Generally, there are four main forms of mechanical seal failure, namely corrosion, heat loss, wear and failure caused by factors such as installation and operation. After the pump is disassembled, the sealing surface has no scratches and wear, so the cause of physical damage to the sealing surface can be ruled out. Considering that there are many crystals and scales in the sealing cavity, and combined with many manifestations of mechanical seal failure, it is preliminarily determined that the easy crystallization and corrosiveness of the transported medium are the main reasons for the failure of the seal.

3. Improvement measures

(1) Change the structure of the elastic element: Change the elastic element from a spring to a bellows. The bellows is both an elastic element and an auxiliary sealing ring for the moving ring. The O-ring between the original moving ring and the shaft sleeve is omitted. A gasket is added at the tail of the seat to seal the material there, which not only solves the contradiction between the O-ring seal and compensation, but also changes the original quasi-static seal to a complete static seal, which improves the reliability of the seal. The use of bellows also saves the transmission pin of the spring mechanical seal and the mating surface between the moving ring and the moving ring seat, avoiding the failure of the moving ring action caused by the deposition of solid particles, and further improving the overall performance of the mechanical seal . The bellows provides both elastic compensation and torque, with better following and compensation capabilities.

(2) The static ring compensation replaces the dynamic ring compensation: in the mechanical seal assembly, the mass of the moving ring part is much larger than that of the static ring part, so the rotational momentum is also large during operation. After changing to static ring compensation, the rotational momentum is reduced, and the stability and reliability of the entire mechanical seal assembly are improved.

(3) Increase the clearance between the moving ring and the shaft sleeve: Enlarging the clearance can avoid the deposition of crystals and scales. On the premise of ensuring the specific pressure of the end face, the inner diameter of the moving ring can be increased or the inner diameter of the shaft sleeve can be reduced. According to the recommendation of relevant information, the gap should be increased from the original 0.38mm to 0.5mm.

(4) Appropriately increase the aperture of the flushing line: for the actual situation where there are many crystals and scales, increasing the flushing amount can effectively solve the deposition of solid particles, so the original flushing line aperture must be increased from 12.7mm to 19.05mm. It should be noted, however, that increasing the pore size too much will reduce the efficiency of the pump.

(5) Increase the mesh number of the inlet filter: Increasing the mesh number of the filter mesh can effectively prevent solid particles from entering the sealing cavity and provide an ideal working environment for the mechanical seal.

The concentrated lye circulating pump of the compression system of the ethylene plant is the bottom circulating pump of the caustic washing tower. During the actual operation of the pump, leakage frequently occurs, and the mechanical seal begins to leak slightly after a week of installation and operation, and the leakage increases within 1 to 2 months, making it impossible to continue to use. It seriously affects the environmental protection standard and long-term stable operation of the device.