Reliability (Mechanical Seals Failure Modes 2) - Details

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Clogging: In extreme cases, the sedimentation on metal bellows shaft seals may prevent the axial spring action of the bellows.

When the pumped medium has a high content of suspended particles and fibres, the seal can fail due to precipitation or consolidation of the particles and fibres on or at springs, bellows, seal-drivers or O-rings.

Particles and Deposits: Small amounts of hard particles on the seal faces result in increased wear, especially when using hard/soft seal face material pairings. In such cases, small, hard particles might be squeezed into the soft seal ring from where they act as a grinding tool on the hard seal ring.

Impurities between seal faces result in a high leakage rate, permanently or until the impurity has been grinded and flushed away. As an example, a human hair is 50 to 100 μm thick, as compared to the 0.3 μm (height) of a sealing gap operating under normal operating conditions.

Sticking ans Seizure: Sticking (British) and seizure (American) are two different terms with the same meaning. In the following, we shall use the term “sticking”.

Sticking occurs when the two seal rings are locked or partially welded together. The locked state results in a failure if the interconnection is higher than the starting torque of the motor. It may also result in mechanical damage of seal parts. The main causes of sticking are precipitation of sticky materials from the pumped medium on the seal faces or corrosion of the seal faces.

Chemical, physical degrading and wear:

All parts of the mechanical shaft seal must have adequate resistance to the chemical and physical environment to operate properly during the expected working life

Swelling of rubber parts: Swelling of rubber is an increase in volume and a decrease in hardness due to absorption of a solvent.

The volume increase depends on the type and grade of rubber, the type and concentration of the solvent as well as the temperature and time of exposure. In extreme situations, the linear dimensions of a swollen rubber part can be doubled.

Ageing of Rubber parts: Ageing often results in a change in physical parameters such as tensile strength and hardness. Ageing is divided into these two categories: • shelf-ageing • atmospheric ageing.

Shelf-ageing is basically oxidative degradation. Apart from the obvious influence of oxygen, the catalytic effects of heat, light, internal and external stresses or strains and pro-oxidant metals should be considered.As opposed to shelf-ageing, atmospheric ageing is characterized by the attack of ozone on the rubber.

Corrosion: Most seal rings materials are made of composite materials. In order to prevent selective corrosion, all phases of the seal ring material must be resistant to the pumped medium.

Corrosion in cemented tungsten carbide is typically seen as an attack on the metallic binder phase.

Erosion: On other surfaces of the seal rings, heavy erosion can occur where the binder phase is corroded.

When the glass phase has disappeared from the surface of the seal ring, the porosity increases.

Wear: Because the thickness of the lubricating film is of the same order of magnitude as the surface roughness, the seal faces will wear to some extent.

This normal wear on well-performing seals will be so small that the seal will be able to survive for many years.

Installation Failures: Some mechanical seal failures come from wrong mounting and handling. Examples can be shaft misalignment, seats not mounted perpendicular to the shaft, axially moving shaft and wrong assembly length, etc.

The position and width of wear tracks on the seat indicate various problem areas. If the width of a wear track on the seat is the mirror of the sliding face of the opposite seal ring, the shaft seal seems to be well aligned with no run-out of the shaft.

System Failures: When operating pump systems, the operating parameters may differ slightly from what the system was designed for. A change in operating conditions may affect the seal performance.

These parameters affect the performance of a mechanical shaft seal: • the pressure in the seal chamber • the temperature around the shaft seal in the seal chamber • the pumped medium • the speed • the shaft seal dimensions.

Reliability (Mechanical Seals Failure Modes 2)

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