Causes and analysis of leakage in constant pressure water supply pump mechanical seals

Main forms of leakage in constant pressure water supply pump mechanical seals: leakage due to pressure, leakage due to medium, periodic leakage of mechanical seals, dry friction or roughening of sealing end faces due to insufficient lubricating oil, periodic rotor vibration, and leakage of mechanical seals due to other problems. There are also some unreasonable aspects in the design, selection, and installation of mechanical seals.

Definition of mechanical seal of water pump: Also known as end face seal, it has a pair of end faces perpendicular to the rotating axis. Under the action of fluid pressure and compensating mechanical external force, the end face relies on the cooperation of auxiliary seals to maintain contact with the other end and slide relatively, thus preventing fluid leakage.

I. Leakage due to the medium
(1) After most submersible pump mechanical seals are disassembled, the auxiliary sealing elements of the stationary ring and the dynamic ring are inelastic, and some have rotted, causing a large amount of leakage of the mechanical seal, and even shaft wear. Due to the corrosion of high temperature and weak acids and alkalis in wastewater on the auxiliary rubber sealing elements of the stationary ring and dynamic ring, the mechanical leakage is too large. The material of the dynamic and static ring rubber sealing rings is nitrile-40, which is not resistant to high temperature and acid and alkali. When the wastewater is acidic or alkaline, it is easily corroded.
Countermeasures: For corrosive media, rubber parts should use fluororubber that is resistant to high temperature, weak acids, and weak alkalis.
(2) Mechanical seal leakage caused by solid particle impurities If solid particles enter the sealing end face, it will scratch or accelerate the wear of the sealing end face. The accumulation rate of scale and oil stains on the surface of the shaft (sleeve) of the high-temperature gear pump exceeds the wear rate of the friction pair, causing the dynamic ring to be unable to compensate for the wear displacement. The service life of the hard-to-hard friction pair is longer than that of the hard-to-graphite friction pair, because solid particles will be embedded in the sealing surface of the graphite sealing ring.
Countermeasures: In positions where solid particles are likely to enter, mechanical seals with tungsten carbide to tungsten carbide friction pairs should be selected.

II. Periodic rotor vibration. The reason is that the stator is not aligned with the upper and lower end covers, or the impeller and main shaft are unbalanced, cavitation or bearing damage (wear). This situation will shorten the seal life and cause leakage.
Countermeasures: The above problems can be corrected according to the maintenance standards.


III. Leakage due to pressure
(1) Mechanical seal leakage caused by high pressure and pressure waves When the spring pressure and total pressure are designed to be too large and the pressure in the sealing chamber exceeds 3 MPa, the end face pressure will be too large, the liquid film will be difficult to form, the sealing end face will wear seriously, the heat generation will increase, and the sealing surface will be thermally deformed.

Countermeasures: When assembling the mechanical seal, the spring compression must be carried out according to the regulations, and no excessive or insufficient phenomenon is allowed. Measures should be taken for mechanical seals under high-pressure conditions of gear pumps. In order to make the end face stress reasonable and minimize deformation, hard

(2) Mechanical seal leakage caused by vacuum operation During the start-up and shutdown process of the heat transfer oil pump, due to the blockage of the pump inlet and the presence of gas in the pumped medium, the sealing chamber may be under negative pressure. If the sealing chamber is under negative pressure, it will cause dry friction on the sealing end face, and the internal mechanical seal will leak gas (water). The difference between vacuum sealing and positive pressure sealing lies in the directionality of the sealing object, and the mechanical seal also has its adaptability in a certain direction.
Countermeasures : Use a double-end face mechanical seal, which helps to improve lubrication conditions and sealing performance.

IV. Dry friction or roughening of sealing end faces due to insufficient lubricating oil.
Countermeasures: The lubricating oil level in the oil chamber should be higher than the sealing surfaces of the dynamic and static rings.

V. Periodic leakage of mechanical seals
The axial movement of the rotor of the explosion-proof self-priming pump is large, and the interference fit between the auxiliary seal and the shaft is large, and the dynamic ring cannot move flexibly on the shaft. After the pump is reversed and the dynamic and static rings are worn, they cannot be compensated for displacement.
Countermeasures: When assembling the mechanical seal, the axial movement of the shaft should be less than 0.1 mm, and the interference fit between the auxiliary seal and the shaft should be moderate. While ensuring radial sealing, the dynamic ring should be able to move flexibly on the shaft after assembly (the dynamic ring can be freely bounced back when pressed against the spring).

VI. Mechanical seal leakage due to other problems There are also some unreasonable aspects in the design, selection, and installation of mechanical seals.

(1) The spring compression must be carried out according to the regulations, and no excessive or insufficient phenomenon is allowed. The error is ±2 mm. Excessive compression increases the end face pressure, generates excessive frictional heat, and causes thermal deformation of the sealing surface and accelerated wear of the end face. Insufficient compression results in insufficient end face pressure between the dynamic and static rings, which cannot be sealed.

(2) The end face of the shaft (or shaft sleeve) where the dynamic ring seal is installed and the end face of the sealing cover (or shell) where the static ring seal is installed should be chamfered and polished to avoid damaging the dynamic and static ring seals during assembly.