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Pressurized Dual Seal Plans

In the past there was only one Plan 53, but with the 2^nd Edition of API 682 and the 1^st Edition of ISO 21049 other variations of Plan 53's were created.

Plan 53A is the former Plan 53. Plan 53B is what had been in the past denoted as Plan 53 Modified; this is especially popular in European and other countries in the Middle East. Plan 53C is a variation of this that has also been used in the past and is now formally recognized.

The major difference in the plans is that Plan 53A uses an external reservoir, while Plans 53B and 53C run within a closed loop system with a make-up system piped to it for replenishment of the barrier fluid.

In dual pressurized sealing arrangements the inner process seal can have its own flush plan; in such applications the complete flush plan system designation should include both plans. For example, Plan 11/53A means that the inner seal has its own flush plan, Plan 11. The API/ISO default is for no separate flush plan when using any of the Plan 53's, but this can vary with the application conditions.

With the older traditional back-to-back seal arrangement the inboard seal usually does not require a separate flush. In applications such a hydrofluoric acid, where it is both extremely hazardous and corrosive, a Plan 32 can be used in conjunction with a Plan 53. The dual pressurized face-to-back seal arrangement eliminates some of the potential problems associated with the back-to-back design. This face-to-back seal arrangement sometimes incorporates a reverse pressure capability that is not a default with the back-to-back design.

Also, face-to-back arrangements do not have a dead zone underneath the inboard seal that can become clogged by dirty process fluid and lead to seal hang-up. However, the face-to-back arrangement is not a cure-all. With the product on the seal O.D. and with it being used on API pumps that still incorporate throat bushings, it is advantageous to provide a flush for the inboard seal on a number of applications.

Abrasives can accumulate in the more closed API type seal chambers compared to the newer generation chemical duty pumps with large cylindrical bore or tapered bore chambers. The use of a Plan 11 or similar bypass type flush for the inner seal has advantages. It can help keep the seal chamber clean. It also has an improved overall heat transfer setup versus just using a Plan 53 system alone.

In comparison to a Plan 54, Plans 53A/B/C are usually less complex and less expensive. With Plans 53A/B/C, both the inner and the outer seals are lubricated by the barrier fluid, which can be selected for optimum seal performance. Plans 53A/B/C are usually selected for dirty, abrasive, or polymerizing process services which might be difficult to seal directly with single seals or with dual unpressurized seals using a Plan 52. There will always be some leakage of the barrier fluid into the process with any pressurized system.

With some of the Plan 53 systems the volume of barrier fluid is limited, especially compared to a Plan 54 system. Venting of the seal chamber is essential for all Plan 53's where vapor locking can if vapor bubbles collect near the pumping ring or in the piping.

Plan 53A

Plan 53A uses an external reservoir to provide barrier fluid for a pressurized dual seal arrangement. Reservoir pressure is produced by a gas, usually nitrogen, at a pressure greater than the maximum process pressure being sealed. The gas pressure is regulated by a system that is outside the schematic of the piping plan. Circulation of the barrier fluid is maintained by an internal pumping ring.

Like Plan 52 reservoirs, cooling is accomplished internal coil of tubing to remove the heat. Also like Plan 52 reservoirs, the volume of barrier liquid can vary from two gallons to 5+ gallons, where API and ISO standards specify 3-gal and 5-gal, depending upon the shaft diameter.

For non-API specifications, smaller reservoirs - typically 2-gal - are often used, especially at ambient pumping temperatures. Pressure alarms, pressure gages and level switches are typically standard equipment and are required by API 682/ISO 21049.

The circulation rate in a Plan 53A system is like a Plan 52 system described earlier. The piping to and from the seal chamber and location of the reservoir is also the same as a Plan 52 system. Unlike Plan 52, the elevation of the reservoir does not contribute to the pressure in the sealing cavity.

The usual guideline for Plan 53 barrier pressures is that they be a minimum of 20-psi to 50-psi above the maximum process pressure seen by the seal. Barrier pressure is normally supplied by a plant wide distribution system. Nitrogen bottles should not be used as they require a lot of attention and maintenance.

API 682/ISO 21049 recommends that the system be limited to 150-psig due to gas entrainment into the barrier fluid. Field experience has shown that with the proper barrier fluid, Plan 53A systems can be used up to 300-psig if the temperature is controlled to less than 250-deg F. A variation to this would be to use an accumulator to eliminate gas entrainment.

Seal Flush Plan 53A

Advantages (vs. other Plan 53 systems)

•  Least expensive of the various Plan 53 systems.
• Should the loop be contaminated for any reason, the contamination is isolated to a single installation.
• Wear particles that are heavier than the barrier fluid will settle to the bottom of the reservoir away from the reservoir outlet to the seal chamber.
• The volume of barrier fluid is dependent upon the size of the reservoir. Larger flow rates should use larger reservoir sizes so that retention time in the reservoir is maximized for longer fluid life.

Disadvantages (vs. other Plan 53 systems)

• The barrier fluid in Plan 53A is subject to gas entrainment due to direct exposure to the pressurizing gas. Different barrier fluids have varying levels of gas entrainment.
• Heat dissipation capacity is limited to the coiling coils unlike Plan 53B/C, which have separate and potentially larger capacity.
• Installation should be limited to a single seal installation even on between bearing pumps. Therefore for a large number of installations, Plan 53A can be more expensive than Plan 53B or 53C.

  

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