Parameters for mechanical seal safety and reliability

July 16 2014 - William E. Forsthoffer

Experience shows that pump MTBFs (mean time between failures) are directly related to mechanical seal reliability. It is well known that pump mechanical seal MTBF is the lowest of all machinery components. To optimize mechanical seal MTBFs, it is essential to confirm the actual seal operating conditions on the data sheet (PT, vapor pressure, S.G., P1 and P2).

Of all machinery components, mechanical seals are the most affected by the process conditions. Accurate definition of the following parameters on the pump and seal data sheet will go a long way towards assuring optimum pump mechanical seal MTBFs:

  • Fluid vapor pressure
  • Fluid specific gravity and viscosity
  • Fluid pumping temperature (PT)
  • Suction pressure
  • Cooling medium temperatures (if a flush cooler will be used)
  • External flush fluid pressures, temperature, vapor pressure, viscosity and specific gravity

Failure to properly specify the correct process conditions on the pump and/or mechanical seal data sheet will result in lower than optimum seal MTBFs. Most ‘bad actor’ seals (seals with more than one failure per year) result from improper specification of process conditions on the data sheets.

Always check all operating seal process conditions against the data sheet values when investigating a ‘bad actor’ seal. If the proper instrumentation (pressure, gauge, etc.) is not installed, have the appropriate instruments installed at the time of seal replacement. This best practice has been used since 1990 to troubleshoot mechanical seal problems and to ensure maximum mechanical seal MTBFs (greater than 100 months).

Optimal seal plans for various applications

Pre-select mechanical seal and flush system during the pre-FEED project phase, based on plant, company, and/or industry experience, to optimize mechanical seal MTBF.

Use plant, company and industry lessons learned to properly select a flush system that will result in optimal seal life in your plant. Frequently, the flush system is selected by the process licensor or the EP&C (contractor) and does not reflect the actual plant conditions.

Be proactive early in the project design (pre-FEED phase) to convince project management of the proper flush systems to apply for all pump services.

When would certain flush systems be better than others? Mechanical seal flush must possess the following qualities for optimal seal life. They must be cool, clean, be approximately 345kPa (50 psi) above vapor pressure (psia), and most importantly, it must be cost effective. Therefore, if a flush system for an application can provide all the above qualities, then you have selected the optimal flush system.

Different seal flush scenarios have considerations that are specific to each. Ultimately the seal vendor should be consulted when selecting the seal and flush plan; the more input they have the higher seal reliability your plant should see as a result.

The use of the following flush system components where they are not warranted has resulted in low mechanical seal MTBFs and has exposed plants to safety issues:

  • Flush line strainers
  • Cyclone separators
  • Flush line coolers

Flush line strainers can expose plants to seal failure and safety issues, since they are not monitored in the control room and can result in flush line blockage, which will fail the mechanical seal and can expose the plant to significant safety issues in hydrocarbon applications.

This best practice has been used since the mid-1970s when I became involved with pump selection. Since that time, prohibiting the use of flush line strainers and cyclone separators has resulted in optimum mechanical seal safety and reliability during field mechanical seal reliability optimization audits for all projects.