The Revised API 682 Mechanical Seal Standard

The 4th Edition includes details on the revised product coding system, the seal system selection process and seal supply systems.

Written by:
Thomas Böhm and Markus Fries, EagleBurgmann
Published:
August 29, 2013
  • Arrangement 1—single seals are differentiated
  • Arrangements 2 and 3—double seals with and without pressurization
When sealing aggressive and abrasive crude oil in pipelines, reliability and extended service intervals are required. Challenging conditions place high demands on the design limits of sealing and supply systems, which can handle frequent stops/starts and occasional pressure reversals or reverse pump rotation.

Details regarding the supply system—specified as “Plan”—are in the old and new code. The addition of precise information regarding material selection and shaft diameter is new. This gives more meaning to the code and guarantees a clear specification of the mechanical seal and its operation—from selection to documentation. Industry experts agreed that the expanded coding system will prove itself in practice and endure permanently.

More Precision During Selection

The selection process of an API seal system is complicated. Several flow charts and tables on more than 10 pages are dedicated to this topic in the new edition. To provide more precision in the technical selection process when determining the arrangement, an alternative selection tool (Annex A.4) has been included in the 4th Edition for the first time. This method is based on the established “Risk & Hazard Code” and has been tested in practice.

The starting point is the pumped medium. Its real hazard potential is accurately recorded and described by the “Hazard & Risk Code” in the “Material Safety Data Sheets.” Decisions can be made quickly and securely, for example, about whether a single seal (Arrangement 1) will suffice, or if a double seal with barrier pressure system is required.

Practical Experience

The experience-based, “lived” standard of the API 682 edition is demonstrated by the two silicon carbide (SiC) variants, reaction-bonded silicon carbide and self-sintered silicon carbide, which are treated equally as default materials for sliding surfaces in chemical (Category 1) as well as in refinery/oil and gas applications (Category 2 or 3). Until now, sintered SiC was set for chemical applications because of its superior chemical stability, whereas the reaction-bonded variant established itself in the refinery sector. This restrictive allocation was canceled because of practical application examples (best practices) that were brought to the attention of the task force, which called for a course correction.

Chapters 8 and 9, dealing with the hardware for the supply systems and instrumentation, were subjected to intense revision. They were reorganized so that the topic is now handled in three stages, which makes it more systematic. The first block introduces the supply systems in total. The piping and the components are addressed next.

Seal Supply Systems

Plan 53 with a pressurized barrier fluid belongs to the more complicated supply systems. In detail, three types are possible:

  • Plan 53A is the solution with the constructively least amount of effort. The pressure on the barrier medium is generated directly via gas pressurization—normally with nitrogen—in the tank. However, the application has limits, since higher barrier pressures could cause a dissolution of the nitrogen in the barrier medium. The consequence would be the risk of inadequate lubrication in the sealing gap of the mechanical seal. That is why Plans 53B and 53C are used for higher barrier pressure.
  • Plan 53B uses a clever solution, which makes it popular. Pressurization occurs via an elastomer bladder in the reservoir that separates the nitrogen from the barrier fluid. Pressure monitoring with consideration of the temperature in the bladder accumulator records the values and transfers them to the control room. The fill level with consideration of any temperature impacts is calculated there, and the correct time for refilling the barrier fluid is determined.
  • Plan 53C works with a piston accumulator, which makes it among the more sophisticated seal supply systems.
Refinery in which multiple mechanical seals are at work

A new prescribed refilling interval of at least 28 days has also been included in the 4th Edition of API 682. The fluid reservoir must be large enough to supply the seal with barrier fluid for this entire period—without refilling. To obtain the most compact reservoirs, the seal manufacturers are required to find optimized system solutions with minimal leakage values for the barrier medium.

Also, Plans 03, 55, 65A, 65B, 66A, 66B and 99 have been newly included in the regulations and, along with the already existing plans, are described in detail in Annex G.

Transmitters Replace Switches

Regardless of pressures, temperature, flow rates or fill levels, the 4th Edition heralds a change to modern transmitters for the supply systems. Switches had previously been the default, but transmitters have now taken the position. Although they can be more expensive, they transmit continuously measured values. The control room is now aware of the actual system status at any time and can immediately sound the alarm if problems arise.

The transition to transmitters as default is illustrative: the API specifications primarily concern operating and process reliability and only then consider economic viability. This universal application is also verified by the decision of the task force to permit only seamless pipes in the future for “Piping” for the supply systems. The use of welded pipes, which would be less expensive, was considered unacceptable.

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See also:

Upstream Pumping Solutions

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