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Pumps & Systems, July 2007

Conclusion of our series covering applications in which they are used, why they are used, how they fail and things operators can check/do to maintain the performance of the seal flush plan.

Plan 52

Plan 52 uses an external reservoir to provide buffer fluid for the outer seal of an unpressurized dual seal arrangement. During normal operation, circulation is maintained by an internal pumping ring. The reservoir is usually continuously vented to a vapor recovery system and is maintained at a pressure that is usually at or near atmospheric pressure. The inner process seal of the dual unpressurized arrangement usually has its own flush plan.

For example, Plan 11 might be used on the inner seal along with Plan 52 for the outer seal. In such cases, the complete flush plan might be described as Plan 11/52. The reservoir size can range from two gallons to 5+ gallons of liquid capacity and has an internal coil of tubing which is used to remove heat.

Unlike heat exchangers, in the reservoir the cooling water flows through the coils while the buffer fluid flows over the exterior of the coils. In some cases the cooling coils are replaced by an external heat exchanger or even finned tubing if the heat load is very small.

The circulation rate within a Plan 52 depends on the performance of the pumping ring within the particular closed loop system. The reservoir size is selected based on the size of the seal and the pump shaft speed. Small ANSI pumps typically have the smaller 2-gal reservoir. Larger pumps may require larger reservoirs, especially at elevated pumping temperatures. Heat soak, seal generation heat along with turbulence energy within the outer seal chamber, must be considered in determining the desired circulation rate.

The general rule of thumb is that the reservoir should be located within a plot plan radius of about 3-ft, with the bottom of the reservoir 18-in to 30-in above the centerline of the pump. Many customers require that the liquid level provide at least 3-ft of static head to the outer seal.

Piping should be large in diameter and minimum length with long radius bends to minimize the pressure drop within the system to maximize flow. To prevent vapor locking, the piping should slope up from the gland plate to the reservoir.

In the event that the process, or inner, seal fails, there will be a pressure and/or level increase in the reservoir to set off an alarm. When this occurs, the reservoir should be blocked in by closing the valve near the top of the reservoir. As the process seal continues to leak, pressure and fluid level in the reservoir continues to increase. The reservoir can potentially reach the same pressure as the process seal chamber.

For all practical purposes, the outer seal takes over as the primary seal. For this reason, the recommended operating procedure is to begin planning an orderly shutdown for seal replacement as soon as possible after failure of the process seal.

In the past, most reservoirs for Plan 52 were vented to atmosphere, but due to environmental concern most systems are piped to a vapor recovery system. Plan 52 is used for both non-volatile and volatile process services. Although the leakage rate across the inboard seal is the roughly the same as for a single seal, the leakage is directed into the reservoir of the dual unpressurized seal system.

In non-volatile services the leakage from the process seal increases the liquid level in the reservoir. In volatile services, the leakage from the process seal vaporizes and rises to the top air space inside the reservoir. When the reservoir is connected to a vapor recovery system, the actual emission rate can be very low.

Seal Flush Plan 52

Advantages

•  Plan 52 is a necessity for dual unpressurized seals using a liquid buffer fluid. In comparison to single seals, the dual unpressurized seal can provide reduced net leakage rates as well as redundancy in the event of a main seal failure.
• The buffer fluid should not enter the process stream and contaminate the process fluid, because it is unpressurized.
• The buffer fluid can serve as a quench for the process seal.
• The pressure rating for a dual unpressurized seal is usually greater than that of a single seal.

Disadvantages

• Plan 52 is more complex and expensive than any single seal and the associated piping system.
• There will always be some leakage from the process seal into the buffer system where the buffer fluid is contaminated by the process fluid. Thus, buffer fluids must be selected with great care. It is possible over time that heavier process fluids will displace the buffer fluid resulting in the outer seal to be sealing the process fluid, thereby losing a buffer between the product and atmosphere.
• If the process fluid has a low vapor pressure margin, the heat from the outer seal can further reduce the margin causing the inboard seal to run with partial to full vapor between the sealing faces so an alternate system with dry running containment seals would be the recommendation.
• Venting is essential for Plan 52 to prevent vapor locking should vapor bubbles collect near the pumping ring.
• Selection, design and location of the pumping ring along with inlet and outlet ports is crucial to the successful operation of Plan 52.

  

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