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How to Extend the Life of Mechanical Seal

日期:2019-08-07


How to extend the life of mechanical seal is one of the very common questions of maintenance and design engineers.  The life improvement of mechanical seal will increase the availability of pumps and reduce the cost spend for seal maintenance.Before we seeing the Life Extend of Mechanical Seal, first we need to understand the causes of mechanical seal failure.

Causes of Mechanical Seal Failure

As per API 682, the average life of the mechanical seal is 3 years. Nowadays the seal manufactures has increased the mechanical seal life significantly and increase the availability of the pumps for production.
In many cases, the mechanical seal service life can fall short of expectations due to the following reasons
  1. Exposed to contamination
  2. Outside the design & operating conditions of mechanical seal
  3. Improper flushing plan selection
  4. Improper installation of flushing system
In this article, we will use a systems approach to evaluate the failure reason of mechanical seal and seal system. Before this evaluation, it is very important to know the root causes of mechanical seal failure.



Root Cause of Mechanical Seal Failure

The premature failure of mechanical seal will occur for several reasons. The most common root causes are
  • Outside the operating range of pump
  • Sudden change in pumping fluid properties
  • Improper Seal flushing plan
  • Pump with variable speed drive
  • Pump vibration
  • Improper Installation

Mechanical Seal Life Extension

The following points are to be taken care of mechanical seal design, selection and maintenance.

Mechanical Seal Selection

Selecting the best type of mechanical seal is not always easy and straightforward as there is usually a compromise between economic and technical factors. If the mechanical seal is properly selected, then 60% of premature seal failure will never occur.

Installation

Proper installation of mechanical seal will reduce the premature seal failure about 20%. Here the installation refers not only mechanical seal and also its seal system.

Nowadays cartridge type mechanical seals are highly used, due this proper operating length/working length of seal is easily maintained.

Air Pocket in Seal Chamber

The Air Pocket in Seal Chamber is due to the poor startup procedure of pump. Normally this problem can be corrected by bleeding the air from the seal chamber, before startup, is always a good procedure. Nowadays most of the OH pumps are the self-vented type. Hence this problem will never occur in Overhung (OH) pumps.

But on the other hand, this problem will occur in Between Bearing(BB) pumps. A procedure should be implemented to ensure that all the air is bled from the pump and the seal chamber. If this problem continues, a pressure switch may be installed on the discharge to prevent startup until the flow is indicated.

Bearing Failure

Bearing failure causes instant problems. When a bearing fails, it loses its ability to support the rotating shaft. Due to this seal will fail immediately.



Important Checkings

Following things to be check while doing maintenance in order to enhance the mechanical seal life
  1. The total indicated run out, it should not be more than 0.003” (0.08mm) at mechanical seal area.
  2. The end play (Axial Movement) – maximum of 0.005” (0.13mm).
  3. All keyways, threads, and shoulders for sharp edges, which could damage elastomers. Round edges or tape over before installation.
  4. Check for shaft concentricity to the housing/seal chamber, which should be within 0.005” (0.13mm) maximum.

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