Why is the seal of screw pump often broken?How should it be sealed?

1.There are a wide variety of mechanical seals used for pumps with different models, but there are five main leakage points :(l) the seal between the shaft sleeve and the shaft;(2) Sealing between moving ring and shaft sleeve;(3) Sealing between moving and static rings;(4) Sealing between the stationary ring and the stationary ring seat;(5) Sealing between the end cover and the pump body.

2. Sealing: Analyze the cause of leakage and judge the static test of installation. After the installation and debugging of the leaking mechanical seal is completed, a static test is generally required to observe the leakage amount.When the leakage amount is large, it indicates that there is a problem between the friction pairs of the dynamic and static rings. On the basis of the preliminary observation of the leakage amount and the judgment of the leakage location, the sealing ring of the dynamic and static rings is in trouble when the leakage amount is not significantly changed.If there is an obvious change in the leakage amount during turning, it can be concluded that there is a problem with the friction pair of the dynamic and static ring.Such as leakage along the axial injection of medium, the ring sealing ring problem, leakage of medium around jet or leak from the water cooling hole, is more static seal failure and leakage channels can also exist, but generally speaking, there are difference between primary and secondary, as long as observe carefully, be familiar with the structure, must be able to correctly judge leak test run of pumps with mechanical seal after static test, high speed rotating centrifugal force, when working could suppress the medium leakage, therefore, when the mechanical seal leakage test run after exclusion between shaft and end cover seal failure, largely due to the dynamic static ring friction pair destroyed.

The main factors causing seal failure of friction pair are as follows: (l) During operation, due to the abnormal phenomenon of cavitation erosion and pressure holding, large axial force is caused, which makes the contact surface of dynamic and static ring separate;(When installing mechanical seal, the compression amount is too large, resulting in severe wear and abrasion on the end face of friction pair;(2) The moving ring sealing ring is too tight, the spring cannot adjust the axial floating amount of the moving ring;(3) The static ring seal ring is too loose. When the moving ring is floating in the axial direction, the static ring should be released from the static ring seat;(4) There are granular substances in the working medium, which enter the friction pair during operation and detect the end face of the dynamic static ring seal;(5) The above phenomena, such as wrong design and selection, low specific pressure of the sealing end face or large cold shrinkage of the sealing material, are often seen in the test run. Sometimes they can be eliminated by appropriate adjustment of the static ring seat, etc., but most of them need to be reassembled and replaced.
Failure due to loss of lubricating film on the end faces of the two seals: a) Dry friction occurs when the pump is started due to lack of liquid in the seal cavity due to the existence of the end face seal load;B) When the medium is lower than the saturated vapor pressure, the end surface liquid film will flash and lose lubrication;C) If the medium is a volatile product, when scaling or blocking occurs in the mechanical seal cooling system, the saturated vapor pressure of the medium will rise due to the end surface friction and the liquid stirred by rotating elements, and the pressure of the medium will also be lower than that of the saturated vapor pressure.
Failure of mechanical seal caused by corrosion: a) pitting corrosion of sealing surface, or even penetrating b) Intergranular corrosion of stainless steel seat in use due to welding of tungsten carbide ring and stainless steel seat;C) Welding metal bellows springs, etc. are prone to rupture under the joint action of stress and dielectric corrosion.
Mechanical seal failure due to high temperature effect: a) Hot crack is a high temperature oil pump. For example, the most common failure phenomena, such as oil slag pump, return refining pump, and pressure reducing tower bottom pump, are radial cracks in the torus when dry friction cooling water is suddenly interrupted at the sealing surface and impurities are evacuated into the sealing surface.B) when the graphite carbonization is the use of carbon graphite ring seal failure because one of the main reasons in use, if the graphite ring once more than the allowable temperature (generally - 105 ~ 250), its surface will precipitate resin, friction occurs near the surface resin carbonized, when a binder, foam will soften, increase the sealing surface leakage and seal failure;C) Auxiliary seals (such as fluoroelastomer, ethylene-propylene rubber, all rubber) will rapidly age after exceeding the allowable temperature, crack and harden. Elastic flexible graphite used now has better high temperature and corrosion resistance, but its resilience is poor and brittle, and it is easy to be damaged during installation.

Seal failure due to wear of seal end face:A) materials poor abrasion resistance of friction coefficient of the friction pair big face specific pressure (including spring pressure) is too large, etc., will shorten the service life of mechanical seal of the commonly used material, according to the wear resistance of the order as follows: silicon carbide carbon graphite carbide carbon graphite carbon graphite spraying ceramic carbon graphite silicon nitride ceramic carbon graphite carbide surfacing high speed steel carbon graphite carbon graphite b) containing solid particles of the medium, the sealing surface into the solid particles is a major cause of lead to make the seal failureSolid particles into the friction pair end surface grinding agent role, make the seal breaks due to severe wear and seal face reasonable interval, and the balance degree of mechanical seal, and the seals of liquid film flash is caused by open end and make the solid particles into the c) of the main causes of the balance of the mechanical seal degree beta also affects the wear of seal under normal circumstances, the equilibrium degree of beta = 75% optimum beta & lt;For mechanical seals with high load (high PV value), due to the greater heat of friction on the end face, generally take 65%~70% is appropriate, for low boiling point hydrocarbon media, etc., due to the temperature of the medium gasification is more sensitive, in order to reduce the impact of friction heat, take 80%~85% is good.
Mechanical seal leakage is caused by the error caused by installation operation or equipment itself, mainly in the following aspects:

1) The contact surface of the movable and static ring is uneven, and it is damaged by collision during installation;2) The sealing ring of the moving and static ring is damaged or not pressed;3) There are foreign bodies on the surface of the moving and static ring;4) The v-shaped sealing ring of the dynamic and static ring should be installed in the opposite direction, or the opposite side during installation;5) Axle sleeve leakage, sealing ring is not installed or the pressing force is insufficient;6) The spring force is not uniform, a single spring is not vertical, the length of the spring is different;7) The end face of the seal cavity is not perpendicular enough to the axis;8) There are corrosion points on the moving part of seal ring on axle sleeve.

In the operation of equipment, mechanical seal leakage of the main reasons:

1) The pump impeller's axial channeling momentum exceeds the standard, the rotating shaft experiences periodic vibration and process operation is unstable, and the pressure in the seal chamber often changes, which will lead to periodic leakage of the seal;2) The friction pair is damaged or deformed and cannot run and close, causing leakage;3) Improper selection of sealing ring materials, swelling and loss of ammunition;4) Big spring steering is wrong;5) The vibration of the equipment is too large during operation;6) Scale is formed between the dynamic and static ring and the shaft sleeve, which makes the spring lose its ammunition and fails to compensate for the wear of the sealing surface;7) The seal ring cracks and other pump in the stop for a period of time after the start of leakage, this is mainly because of the solidification of the media near the friction pair, friction pair has scale spring corrosion obstruction and the pump shaft disturbance is too large.