There are many failures of mechanical seals . Common types of corrosion are as follows.
Corrosion failure
1. Surface corrosion
Due to the corrosive effect of the corrosive medium, the mechanical seals will have surface corrosion. In severe cases, corrosion perforation may also occur. The spring parts are more obvious. The use of stainless steel materials can reduce surface corrosion.
2. Pitting
Spring covers often have large-area pitting or regional pitting corrosion, and some lead to perforation. Such local corrosion will not cause very serious consequences for the use of seals, but they should be replaced during major repairs.
3. Intergranular corrosion
The tungsten carbide ring stainless steel ring seat is connected by brazing, and the stainless steel seat is prone to intergranular corrosion during use. In order to overcome the effect of sensitization, the stainless steel should be solution treated.
4. Stress corrosion cracking
Metal welded bellows, springs, etc., under the combined effect of stress and medium corrosion, tend to break. The sudden failure of the spring will cause the seal to fail. Generally, the diameter of the spring wire is increased to solve it.
5. Crevice corrosion
Between the inner hole of the moving ring and the surface of the bushing, between the screw and the screw hole, and between the O-ring and the bushing, the gap is corroded due to the difference in the concentration of the medium inside and outside the gap, in addition to the ceramic ring and the metal ring seat Crevice corrosion will occur. Generally, ceramic is sprayed on the surface of the bushing, and the surface of the ring is coated with adhesive to reduce the corrosion of the crevice.
6. Electrochemical corrosion
Dissimilar metals often cause electrochemical corrosion in the medium. It loosens the ring and affects the seal. Generally, it is also overcome by applying adhesive to the joint.
Heat loss effect
1. Hot crack
If the sealing surface is in dry friction, cooling is interrupted suddenly, impurities enter the sealing surface, and evacuation, etc., it will cause radial cracks on the ring surface, which will cause rapid wear of the dynamic and static rings and rapid increase in leakage of the sealing surface. Thermal cracking of tungsten carbide rings is more common.
2. Foaming, carbonization
During use, if the graphite ring exceeds the allowable temperature, the resin will precipitate on the surface, and the resin will carbonize near the friction surface. When there is a binder, it will foam and soften, which will increase the leakage of the sealing surface and seal failure.
3. Aging, cracking, swelling
If the rubber continues to be used beyond the allowable temperature, it will rapidly age, crack, harden and lose its elasticity. If it is an organic medium, it swells and loses its elasticity, which will cause the seal to fail.
The key to seal failure caused by heat loss is to reduce frictional heat as much as possible, improve heat dissipation, and prevent sudden temperature changes at the sealing surface.
Wear failure
If the friction pair material is poor in wear resistance, the friction factor is large, the specific pressure of the end surface (including the specific pressure of the spring) is too large, and solid particles enter the sealing surface, etc., the sealing surface will wear too quickly and cause seal failure. The use of a balanced mechanical seal to reduce the specific pressure on the end surface and the appropriate reduction of the spring pressure during installation are beneficial to overcome the failure caused by wear. In addition, the use of good friction pair materials can reduce wear.
The materials are arranged in the order of wear resistance: tungsten carbide-carbon graphite, cemented carbide-carbon graphite, ceramic (alumina)-carbon graphite, sprayed ceramic-carbon graphite, silicon oxide ceramic-carbon graphite, high speed steel-carbon graphite, surfacing Carbide-carbon graphite.
