In chemical production, it is often necessary to transport fluid from a low point to a high point, from a low pressure to a high pressure, or along a pipeline to a remote location.
Centrifugal pumps are the most common type of power machinery in fluid handling. The mechanical seal is the shaft seal device of the centrifugal pump, which has the advantages of small leakage and long life.
1.Mechanical seal failure phenomenon and analysis
Pump mechanical seals are available in a wide variety of models, there are five main leak points:
(1) Seal between bushing and shaft.
(2) Seal between moving ring and bushing.
(3) Seal between moving and stationary rings.
(4) static ring and static ring seat.
(5) The seal between the seal end cap and the pump body.
mechanical seal wearing photo
1.1 Leakage when installing static test
After the mechanical seal is installed and debugged, it is generally necessary to perform a static test to observe the amount of leakage.
(1) If the amount of leakage is small, it is more likely to be a problem with the moving ring or static ring seal.
(2) When the amount of leakage is large, it indicates that there is a problem between the friction pairs of the moving and stationary rings.
(3) On the basis of preliminary observation of the leakage amount and judging the leakage location, the vehicle is manually observed. If there is no significant change in the leakage amount, there is a problem with the static and dynamic ring seals.
(4) If there is a significant change in the amount of leakage when the vehicle is in a car, it can be concluded that there is a problem with the friction pair of the moving and stationary rings.
(5) If the leaking medium is sprayed in the axial direction, it is more likely that there is a problem with the moving ring seal.
(6) If the leaking medium is sprayed around or leaks out of the water cooling hole, it is more likely that the static ring seal will fail.
1.2 Leakage during test run
After the static test of the mechanical seal of the pump, the centrifugal force generated by the high-speed rotation during operation will suppress the leakage of the medium. Therefore, the leakage of the mechanical seal during the test run is basically caused by the failure of the friction pairs of the moving and stationary rings after the failure of the seal between the shafts and the end cap seal.
The main factors causing the failure of the friction pairs seal are:
(1) During operation, due to abnormal phenomena such as evacuation, cavitation, and rolling, a large axial force is caused to separate the moving and static ring contact surfaces.
(2) When the mechanical seal is installed, the amount of compression is too large, resulting in severe wear and abrasion of the end face of the friction pair.
(3) The moving ring seal ring is too tight, and the spring cannot adjust the axial floating amount of the moving ring.
1.3 Sudden leakage during normal operation
A small amount of sudden leakage of the centrifugal pump during operation is due to normal wear or has reached the service life, and most of them are caused by large changes in working conditions or improper operation and maintenance.
There are:
(1) Evacuation, cavitation or rolling for a long time, resulting in seal damage.
(2) The actual output of the pump is too small, a large number of medium pumps circulate, heat accumulates, causing gasification of the medium, resulting in seal failure.
(3) The return flow is too large, causing the sediment at the bottom of the suction side container (tower, kettle, tank, pool) to rise and damage the seal.
2. Causes and improvement measures for Failure of Mechanical Seal
2.1 Periodic leakage
(1) The axial displacement of the pump rotor is large, the interference between the auxiliary seal and the shaft is large, and the moving ring cannot move flexibly on the shaft. After the pump is turned over and the moving and stationary rings are worn, the compensation displacement is not obtained.
Improvement measures: When assembling the mechanical seal, the axial yaw of the shaft should be less than 0.1mm, and the interference between the auxiliary seal and the shaft should be moderate. While ensuring the radial seal, the moving ring can be flexibly moved on the shaft after assembly ( Pressing the moving ring against the spring can bounce back freely).
(2) Insufficient amount of lubricating oil on the sealing surface causes dry friction or brushed end faces.
Improvement measures: The height of the lubricating oil surface in the oil chamber cavity should be added to the upper and lower sealing surfaces.
(3) The rotor periodically vibrates. The reason is that the stator is not centered with the upper and lower end caps or the impeller and the main shaft are unbalanced, cavitation or bearing damage (wear), which shortens the seal life and creates leakage.
Improvement measures: The above problems can be corrected according to the maintenance standards.
2.2 Leakage due to pressure
The leakage of the mechanical seal caused by the high pressure and pressure wave is too large due to the design of the spring specific pressure and the total specific pressure, and the pressure in the sealed chamber exceeds 3 MPa, so that the sealing end face is overpressured, the liquid film is difficult to form, the sealing end face is seriously worn, and the heat is generated. Increased, causing thermal deformation of the sealing surface.
Improvement measures: When assembling the machine seal, the amount of spring compression must be carried out according to the regulations. It is not allowed to be too large or too small. The mechanical seal under high pressure should take measures. In order to make the end face force reasonable, minimize the deformation, use hard alloy, ceramic and other materials with high compressive strength, and strengthen the cooling lubrication measures, choose the available transmission methods, such as keys, pins and so on.
2.3 Leakage due to media
After most of the submersible sewage pump mechanical seals are disassembled, the auxiliary seals of the static ring and the moving ring are inelastic, and some have decayed, causing a large amount of leakage of the machine seal or even a phenomenon of grinding the shaft. Due to the high temperature, the weak acid and weak alkali in the sewage, the corrosion of the static ring and the moving ring auxiliary rubber seals caused the mechanical leakage to be too large. The dynamic and static ring rubber sealing ring material is nitrile-40, which is not resistant to high temperature and not resistant to acid and alkali, it is easy to corrode when the sewage is acidic and alkaline.
Improvement measures: For corrosive media, rubber parts should be made of fluororubber with high temperature resistance, weak acid resistance and weak alkali.
