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Recommended Quench Rate

After all the above considerations, the recommended quench rate is the largest of the values. For most pump seals the recommendation can be simplified in the following table.

Quench rates for typical high temperature pumps. The rate is volume per inch of seal size. 

Controlling the Quench

The recommended quench rates are low enough that the flow rate may be somewhat difficult to control with any accuracy. There is little need for precision, especially with steam. Table 3 shows the approximate flow rate through a simple orifice, for steam and nitrogen.

Approximate flow rates through orifices. (ACFM = Actual Cubic Feet per Minute)

Water is typically used as a quench medium when the fluid being sealed has solids in solution or will crystallize upon exposure to atmosphere.  The flow rate for water does not have to be very large.  In some cases it can just be enough to keep a volume of fluid on the atmospheric side of the seal, while in other cases a slight flow rate of 1/8-gpm to ¼-gpm is sufficient to prevent build up of product underneath the seal faces.  This is one case where the containment device may be a lip seal.

Secondary Containment Seals

Plans 71, 72, 75, and 76 are new plans for dry running secondary containment seals used in conjunction with a liquid lubricated primary seal. The process, or inner seal, of the dual unpressurized arrangement usually has its own flush plan. For example, the flush plans for a dual unpressurized seal arrangement with a dry running secondary containment seal might be written as Plan 11/71, 11/71/75, 11/71/76, or as noted below 11/72/75 or 11/72/76. The Plan 11 for the inboard seal can be any of the plans normally associated with a single mechanical seal.

A secondary containment device is a means of containing and controlling the primary seal leakage from a mechanical seal. In contrast to a dual liquid lubricated mechanical seal, which operates in a buffer or barrier fluid, a secondary containment device operates primarily in the leakage from the process seal, although purges may be added.

There are many different types of secondary containment devices from simple bushings to mechanical seals. Leakage rates for the various secondary sealing devices can vary by several orders of magnitude. Selection of the secondary containment device and system will depend on the level of leakage to atmosphere that is considered acceptable as well as performance requirements for normal operation, upsets, and in the event of process seal failure.

By definition, the secondary containment device does not necessarily have the performance or rating of the primary seal; however, it may be able to temporarily tolerate seal cavity pressure and fluid in the event of a failure of the primary seal.

Large clearance devices like fixed bushings have the highest leakage rates; floating bushings with reduced clearance are much better. Floating segmented bushings have still lower leakage rates. Dry running mechanical seals, both contacting and non-contacting, may also be used as secondary containment devices and can approach the level of performance of a dual unpressurized liquid lubricated seal arrangement.