The main common failure causes of mechanical seals

Mechanical seal is a device to prevent fluid leakage. It is composed of at least a pair of end faces perpendicular to the rotating shaft under the cooperation of fluid pressure, the elastic force (or magnetic force) of the compensation mechanism and the auxiliary seal to maintain adhesion and relative sliding.

The mechanical seal structure is mainly composed of the following components:

1) Static ring

2) Moving ring

3) Elastic element

4) Spring seat

5) Fixing screws

6) Auxiliary sealing ring of moving ring

7) Static ring auxiliary seal

An anti-rotation pin is attached to the gland to prevent the stationary ring from rotating.

The mechanical seal has the advantages of reliable sealing performance, small leakage, long service life, small power loss and wide application range. It is widely used in various technical fields, especially under the working conditions of high speed, high pressure difference and expensive, toxic and corrosive process media. At the same time, the mechanical seal is also one of the weakest links in the equipment.

In order to prolong its service life, in addition to selecting the correct friction pair material and the correct end face specific pressure, correct installation and maintenance can play an important role.

Failure and leakage cause analysis

There are many types and models of mechanical seals used in chemical equipment, but there are five main leak points:

1) Seal between shaft sleeve and shaft

2) Seal between the moving ring and the shaft sleeve

3) Seal between dynamic and static rings

4) Seal between static ring and static ring seat

5) Seal between gland and pump body

The wear of the end faces of the dynamic and static rings leads to the leakage of the mechanical seal

No matter what type of mechanical seal, its main feature is that the sealing surface is perpendicular to the rotating axis, that is, the axial seal that is easy to leak is changed to the end seal that is not easy to leak. Therefore, the main form of mechanical seal failure is the wear failure between the static ring and the moving ring. The friction pairs at the ends of the moving ring and the static ring are mainly fitted together by spring thrust to prevent leakage. The tighter the dynamic and static rings, the less likely they will leak, but the friction between them will also increase. The contact surfaces of the dynamic and static rings will wear quickly under the action of large friction, and eventually fail and leak.

Mechanical seal leakage due to unstable operating conditions and poor installation

Vibration caused by unstable working conditions and poor installation, as well as equipment evacuation and vaporization, will lead to damage to the liquid film between the dynamic and static rings of the mechanical seal, so that the mechanical seal runs "dry" without lubrication. The temperature of the sealing ring will rise rapidly, some will burn directly, and some will cool sharply when the pump returns to normal operation, resulting in thermal shock and fragmentation. Poor flushing fluids and operating conditions can also cause thermal shock, which will cause radial cracks in the seals and increase the likelihood of wear failure of the moving and stationary rings. At the same time, when the temperature of the graphite ring exceeds the operating temperature, crystals will precipitate on the surface of the graphite ring, carbonization will occur near the friction pair with a higher temperature, and the particles will enter the friction pair, causing the dynamic and static rings to wear quickly.

The failure of the sealing ring of the mechanical seal is also the main reason for the leakage of the seal

The dynamic and static ring seals are poorly assembled; the surface finish of the shaft or bushing matched with the seal is not enough or the matching size is too small; the physical or chemical reaction, corrosion, deformation, aging, etc. between the seal and the sealing medium will lead to leakage.

Mechanical seal leakage due to improper assembly

The mechanical seal parts are not cleaned, the parts are scratched or scratched; the installation is not in place; the spring assembly (installation) is skewed, and the fastening screws are not tightened; the mechanical seal is damaged during disassembly, etc., all of which are early failure of the mechanical seal reason.

Improper selection and design of mechanical seals lead to leakage of mechanical seals

Due to the diversity of process medium characteristics in chemical plants, improper design and selection of mechanical seals, excessive or small specific pressure of the sealing end face, and large shrinkage rate of sealing materials, it is easy to cause mechanical seal failure and leakage.

Responses

According to the above analysis, in order to prolong the service life of the mechanical seal, the following measures and maintenance methods should be taken.

spring compression adjustment

Adjusting the spring compression is to adjust the specific pressure of the end face of the mechanical seal, which is related to the important parameters of the seal performance and service life, as well as the seal structure type, spring size and medium pressure. If the specific pressure of the end face is too large, the friction pair will be damaged; if the specific pressure is too small, it is easy to leak, so the manufacturer usually gives a suitable range, generally 3 kg/cm2 to 6 kg/cm2. The free length a of the spring, the spring stiffness k (the load to be endured when generating unit compression), and the specified specific pressure p are all parameters given by the manufacturer. When the compression dimension is b, then p/(a-b) = k, b = a-p/k, which is the spring installation compression dimension. If the size of the spring is too large after installation, the thickness of the adjusting pad can be increased between the spring seat and the spring. If the size is too small, the thickness of the adjustment pad can be reduced accordingly. The thickness of the adjustment pad is measured with a micrometer.

The tightness of the moving ring seal

Over-tightening of the moving ring seal is harmful and unhelpful. First, it will aggravate the wear between the sealing ring and the shaft sleeve, resulting in premature leakage; second, it increases the resistance of the axial adjustment and movement of the moving ring. When the working conditions change frequently, the moving ring cannot be adjusted in time; third, the spring It is easy to be damaged due to excessive fatigue; fourth, it will deform the sealing ring of the moving ring and affect the sealing effect. In order to ensure the floating performance of the moving ring, its inner diameter is 0.5 mm to 1 mm larger than the shaft diameter, which is used to compensate the vibration and deflection of the shaft, but the gap should not be too large, otherwise it will be stuck in the sealing ring of the moving ring and damage the mechanical seal of the mechanical seal. performance. After applying lubricant, the tightness of the sealing ring should be relatively easy to press in by hand.

The tightness of the static ring seal

The static ring seal is basically in a static state, and a relatively tight sealing effect is better, but too tight will lead to: 1. The deformation of the static ring seal is too large, which affects the sealing effect; Excessive stress can easily lead to fragmentation; 3. It is difficult to install and disassemble, and it is easy to damage the static ring. Therefore, the inner diameter of the static ring is generally 1 mm to 2 mm larger than the shaft diameter, and the tightness of the sealing ring (after applying lubricant) can be felt by the pressure of the installation with both hands. If the hand can be pressed in lightly, it is too loose; if the hand cannot be pressed in hard, it is too tight.

Replacement of old and new seals

Relatively speaking, the effect of using the new mechanical seal is better than that of the old mechanical seal, but when the quality or material of the new mechanical seal is not selected properly, the fitting size error will affect the sealing effect. In polymerizable and deeply permeable media, if the static ring is not excessively worn (cracks, peelings, scratches, pits, flashes and eccentric wear on the sealing surface, and the scratches and pits penetrate the entire sealing surface, it is called For excessive wear), it is best not to replace. Since the static ring is in a static state for a long time in the static ring seat, the polymer and impurities are deposited as a whole, which plays a better sealing effect.

Overhaul

Once the mechanical seal leaks, do not rush to disassemble and repair it. Sometimes the seal may not be damaged. Leakage can be eliminated by adjusting the operating conditions or properly adjusting the seal. For example, adjust the compression of the spring, eliminate the vibration of the running equipment, remove the oxide scale that makes the spring lose its elasticity, and the oxide scale on the dynamic and static friction pairs. In this way, waste can be avoided and practical problems can be solved. At the same time, it can verify the maintenance personnel's ability to judge faults, accumulate maintenance experience, and improve maintenance quality.