The analysis of mechanical seal failure

The analysis of Mechanical seal failure

A wide range of mechanical seals for pumps, models vary, but there are five leak:
 
(1) between the sleeve and the shaft seal;
(2) ring and the seal between the sleeve;
(3) Dynamic and static inter-ring seal;
(4) of the stationary ring and the seal between the stationary ring seat;
(5) sealing the seal between the cover and pump body.
1, the installation of static when the leak test
After mechanical seal installation, generally the static test, observed leakage. If leakage was smaller, and more for the ring seal ring or static problems; leakage is large, then the dynamic and static friction between the ring problem. Leakage in the initial observation to determine the leaking part of the basis, and then manually turning observed, if the leakage were no significant changes in static and dynamic ring seal in question; as Barring significant changes in leakage can be concluded that there is a move problems of static friction ring; such as leakage of the media along the axial jet, the ring seal problems mostly to the surrounding medium leakage from the water spray or leakage of cooling holes, are much more static ring seal failure. In addition, the leak paths may exist, but generally there are differences between primary and secondary, as long as careful observation, familiar with the structure, will be able to judge correctly.
2, when commissioning of the leak. After the pump mechanical seal static tests, running high-speed rotation of the centrifugal force, will inhibit the media leak. Therefore, the commissioning of mechanical shaft seal leakage between the exclusion and cover seal failure, basically are due to dynamic and static damage caused by the friction ring. Seal failure caused friction factors are:
(L) operations, because of the time to cavitation, choke pressure and abnor mal, causing large axial force, the dynamic and static ring contact surface separation;
(2) the installation of mechanical seal compression excessive, leading to friction face serious wear and tear, scratches;
(3) The tight ring seals, spring can not adjust the offset of the axial ring;
(4) stationary ring seal is too loose, when the ring axial float, the stationary ring from the stationary ring seat;
(5) working media are granular materials, running into friction, testing static and dynamic ring seal face;
(6) The design and selection of an error, lower than the pressure face seal or sealing material shrinkage is greater and so on. This phenomenon occurs frequently in the test run, and sometimes can be adjusted to eliminate static and other Central Block, but most need to re-entry, replace the seal.
As the two seal faces caused by loss of lubricant film failure:
A) due to the presence of seal load, lack of liquid in a sealed chamber occurred when starting the pump dry friction;
B) lower than the saturation vapor pressure of media, making the face film flash occurs, the loss of lubrication;
C) If the medium is volatile products, mechanical seal or block the cooling system in the scale, due to end friction and rotating components produce heat leaving the liquid mixing saturated vapor pressure of the medium increased, but also caused the saturation vapor pressure is lower than the media pressure conditions.
Due to corrosion caused by the mechanical seal failure:
A) sealing surface pitting, and even penetrate;
B) the tungsten carbide and stainless steel seat rings, such as welding, the use of the seat is easy to produce stainless steel intergranular corrosion;
C) welded metal bellows, springs and other media in the stress and the common effect of corrosion prone to rupture.
Effect due to high temperature mechanical seal failure resulting from:
A) high temperature thermal cracking is a pump, such as the diesel pump, back to the refinery pumps, atmospheric and vacuum tower bottom pump failure as the most common phenomenon. In the sealing surface at the dry friction, sudden interruption of cooling water, impurities into the sealing surface, taking the time and other circumstances, can lead to radial cracks appeared torus;
B) is the use of carbonized graphite carbon – graphite ring one of the main seal failure. As the use, if graphite ring once more than the allowable temperature (usually in the -105 ~ 250 ??), the surface will precipitate resin, friction will occur near the surface carbonized resin, when binder, it will soften the foam, increase the leakage of the sealing surface, seal failure;
C) secondary seals (such as fluorine rubber, ethylene propylene rubber, all rubber) over allowable temperature, will rapidly aging, cracking, loss of playing hard. Flexible graphite is used in high temperature, good corrosion resistance, but its elasticity poor. And easy to rattle, easily damaged during installation.
Seal face wear and tear as a result of seal failure:
A) friction materials used in poor wear resistance, friction coefficient, end pressure (including the spring pressure) is too large, etc., will shorten the life of mechanical seals. Commonly used materials, arranged by order of resistance: silicon carbide – carbon graphite, carbide – carbon graphite, ceramic – carbon graphite, painting ceramic – carbon graphite, silicon nitride ceramic – carbon graphite, high-speed steel – carbon graphite, surfacing alloy – carbon graphite.
B) For medium containing solid particles, the sealing surface into the solid particles is the main reason leading to seal failure. Face of solid particles into the friction from the abrasive effect in that the seal failure occurrence of severe wear and tear. Sealing surface of a reasonable gap, and the extent of the balance of mechanical seals, as well as film of the seal face are caused by flash and other solid particles into the face to open the main reason for leaving.
C) the balance of mechanical seals |? also affects the degree of seal wear. Under normal circumstances, a balance of about |? = 75% level of the most appropriate. |? “75%, although the reduced wear, but the leak increased, increasing the possibility to open the sealing surface. For the high load (high PV value) of the mechanical seal face friction heat as the larger, |? generally take 65% to 70% is appropriate for low-boiling hydrocarbon medium, etc., the temperature of the gasification medium is more sensitive, in order to reduce the impact of friction heat, |? take 80% to 85% as well.