What are the methods to prolong the life of mechanical seals

When the mechanical seal works in liquid medium, it generally relies on the liquid film formed by the liquid medium between the dynamic and static ring friction surfaces for lubrication. Therefore, maintaining the liquid film between the friction surfaces is to ensure the stable operation of the seal, so it is essential to effectively extend the service life of the seal.

Several structures for enhanced lubrication

1. Grooving on the end face:

It is difficult for high-pressure and high-speed machinery to maintain the liquid film between the friction surfaces. The liquid film is often destroyed by the friction heat generated by high pressure and high speed. In this case, it is very effective to take the form of slotting to enhance lubrication. Both the moving ring and the static ring can be slotted, usually on more wear-resistant materials. The moving and stationary rings should not be grooved at the same time, as this will reduce the lubrication effect. In order to prevent dirt or wear debris from entering the friction surface as much as possible, and to seal the liquid flowing in the direction of centrifugal force (outflow type), the groove should be opened on the static ring to avoid centrifugal force from introducing dirt into the friction surface. Conversely, when the liquid flows against the direction of centrifugal force (inward flow), the groove should be opened on the moving ring, and the centrifugal force helps to throw the dirt out of the groove. The small grooves on the friction surface have rectangles, wedges, or other shapes. The grooves should not be too many or too deep, otherwise the leakage will increase.

2. Hydrostatic lubrication:

The so-called static pressure lubrication is to introduce the lubricating liquid with pressure directly into the friction surface to play the role of lubrication. The lubricating liquid introduced is supplied by a separate liquid source, such as a hydraulic pump, etc. Use this pressurized lubricating fluid to oppose the fluid pressure in the machine. This form is also commonly called a hydrostatic pressure seal.

For the mechanical seal of gas medium, measures should be taken to establish gas film lubrication, such as the use of gas static pressure controlled film seals or solid lubrication, that is, self-lubricating materials should be selected as the moving ring or static ring. As long as conditions permit, the gas medium condition should be converted to the liquid medium condition as much as possible, which is convenient for lubrication and sealing.

3. End face eccentricity:

For general seals, the middle center of the moving ring, the middle center of the static ring and the middle center of the shaft are all on a straight line. When the line is offset by a certain distance, the ring can continuously bring in lubricating fluid to the sliding surface to lubricate when it rotates.

It should be pointed out that the size of the eccentricity should not be too large, especially for high pressure. The eccentricity will cause uneven pressure on the end face and cause uneven wear. For high-speed sealing, it is not suitable to use a moving ring as an eccentric ring, otherwise the machine will vibrate due to the balance of centrifugal force.

According to different conditions, the friction between the dynamic and static rings of the seal will appear as follows:

(1) Semi-liquid lubrication:

There is liquid in the pits of the sliding surface, and a thin liquid film is maintained between the contact surfaces, so the heat generation and wear are good. Because the liquid film between the dynamic and static rings has surface tension at its exit, Therefore, leakage of liquid is limited.

(2) Complete liquid lubrication:

When the pressure between the dynamic and static rings is insufficient and the gap increases, the liquid film will thicken. At this time, there is no solid contact, so there is no friction. However, since the gap between the dynamic and static rings is large in this case, the sealing effect cannot be achieved, and the leakage is serious. This situation is generally not allowed in practical applications (except for controlled membrane seals).

Most of the working conditions between the dynamic and static rings of the mechanical seal are in the state of boundary lubrication and semi-liquid lubrication, and semi-liquid lubrication can achieve better results when the friction coefficient is small, that is, the wear and heat generation are satisfactory. sealing effect.

To make the seal work under good lubricating conditions, factors such as medium characteristics, pressure, temperature, and sliding speed need to be considered comprehensively, but choose an appropriate pressure between the dynamic and static rings, a reasonable lubrication structure and improve the dynamic and The friction surface quality of the static ring is also an important factor to ensure the effective operation of the seal.

(3) Dry friction:

No liquid enters the sliding friction surface, so there is no liquid film, only dust and oxide layer, and adsorbed gas molecules. When the dynamic and static rings are in operation, the result will inevitably be that the friction surfaces will heat up and wear will intensify, resulting in leakage.

(4) Boundary lubrication:

When the pressure between the dynamic and static rings increases greatly or the ability of the liquid to form a liquid film on the friction surface is poor, the liquid will be squeezed out from the gap. Because the surface is not flat, but uneven, there is contact wear in the convex, and the lubricating performance of the liquid is maintained in the concave, resulting in boundary lubrication. The degree of wear and heat generation of boundary lubrication is moderate.

In short, strictly following the above operations to extend the life of the mechanical seal can greatly reduce the user's operating misunderstandings, avoid detours, and achieve the effect of extending the life as soon as possible.