Mechanical Seals – Why and How they fail

General Overview

A mechanical seal is expected to achieve its maximum life when it is operated within its design specifications. A seal is considered to have failed when leakage of the fluid starts between the engine and pump. This failure may occur before or after the seal has achieved its design life. A failure of a seal may be a major contributor to the pump’s failure and ultimately downtime of the unit. An understanding of why a seal can fail may lead to extending the life of your Pumps Australia product by improving seal maintenance, installation, and operating procedures.

Causes of Mechanical Seal Failure

The failure of a seal may be the result of:

– Incorrect selection of seal design of materials for the application temperature, pressure, speeds, and fluid properties.
– Incorrect handling of the seal components before installation including chipping, scratching, nicking, or allowing mechanical seal’s face to become dirty.
– Incorrect installation of the seal setting, or placing of components in the backplate or impeller.
– Incorrect start-up of the pump by dry running, or failure of environmental controls.
– Improper pump operation.
– Contamination of the sealing fluid with abrasive or corrosive materials.
– Equipment with excessive shaft runout, deflection, vibration, or worn bearings

Possible Seal Leakage Points

A single stage mechanical seal may leak along one of four paths, see image below:

1) Stationary Seal face leakage is visible at the point where the shaft exits the gland or at the drain connections
2) Rotary part of the seal leakage is also visible at the point where the shaft exits the gland or at the drain connections
3) Static secondary seal leakage is visible at the point where the shaft and impeller meet.
4) Glad gasket leakage is visible at the rubber boot seal at the backplate.

Analysing a Failed Seal

Much can be learned by how a seal failed by simply looking at the following:

– Evidence of pitting (cavitation) in pump bowl.
– Alignment measurements.
– The condition of the backplate.
– Condition of impeller
– The condition of the shaft (sleeve), fretting corrosion
– Condition of the seal, discolouring, corrosion
– The build-up of sediments, caking, etc.
– Wear pattern on the face of the seal.

Understanding Seal Wear Patterns

By inspecting a seal – we can determine what happened to a seal, much like CSI and those other fancy cop show.

Normal Wear; all wear looks even and consistent, contact is light. The seal is working within its design.

Narrow Wear; Wear points are narrow and concentrated in a point. This is a result of operation above the design pressure, Reduce the back pressure on the pump immediately!

Wide Wear; Wear points are wide and almost take up most of the seal’s face. This is a problem of misalignment, bad bearings (when application), shaft deflection, bent shaft or pump cavitation. Please return the unit to Pumps Australia immediately for a qualified fitter to inspect.

No Wear; Wear points are non-existent. This is a result of the rotary face not turning against the stationary face. Remove slipping of the rotary side of the mechanical seal.

Intermittent Wear; Wear points are not connected. This shows that uneven contact between the faces, Ensure the seal’s faces are flat. Check alignment of the pump and the tightness of the bolts to the backplate. Check the rotary part of the seal to see if it’s seated correctly.

Uneven Wear; The Wear points are narrow at some parts. Realign the equipment, check the seal and gland centering and inspect for build-up and accumulation around the seal

 

Deep Wear; This is a result of inadequate lubrication between the faces, essentially the pump has run dry. Replacement is the only option, check the pump to see it’s holding prime.

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