Circulation Systems for Single and Multiple Seal Arrangements (Part Three)

Written by Gordon Buck and Ralph Gabriel, John Crane, Inc.

Page 7 of 7

Plan 76

Plan 76 is designed for use with a dual unpressurized seal utilizing a dry running containment seal, where primary seal leakage is piped to a flare or vapor recovery system. It is intended to be used when the process sealed by the primary seal will not condense to a liquid at lower temperatures or pressures.

In this arrangement, the vent connection is located at the top of the outer seal gland for routing the vapors through an orifice that would create a back pressure to exist in the event of high inboard seal leakage. A pressure gauge and a high pressure alarm indicates this condition. API requires a minimum orifice diameter of .125-in, but smaller sizes may be necessary to provide a realistic leakage alarm point. It is recommended that the high pressure alarm switch be set at 7-psi above the mean operating condition in the flare or vapor recovery system.

The piping should continuously rise from the vent to the piping/instrument harness and should be properly supported so as not to impart strain to the gland. A drain connection in the piping is advisable in order to safely dispose of process fractions that may have condensed. A block valve is standard on this arrangement, to isolate the containment seal in the event of a primary seal failure.

While the secondary containment seal is less subject to clogging in this arrangement, the leakage from the primary seal may be a combination of a condensing and non-condensing fluid. When this is the case, the addition of a Plan 72/76 is highly recommended.

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Seal Flush Plan 76

Advantages of Plan 75 and 76

Lower initial cost alternative to liquid dual unpressurized seals using a Plan 52.
Lower maintenance requirements and associated costs compared to liquid dual unpressurized seals that utilize reservoirs (Plan 52).
Heat generated by secondary containment seals is small compared to contacting wet seals so minimal heat is added to the inboard seal. This is important in applications where the vapor pressure margin for the inboard seal is critical.

Disadvantages of Plan 75 and 76

The secondary containment seal may not be capable of running for extended periods of time in the event of a primary seal failure.
The secondary containment seal can become clogged with debris if the primary seal leakage contains a heavy fluid that can coke or crystallize upon exposure to air. This can be improved upon through the use of Plan 72 and a bushing that directs the fluid away from the seal and seal faces.
Should some of the primary seal leakage condense and accumulate in the seal chamber, the containment seal will generate more heat that can potentially cause coking of the product and shorten seal life.
All of the primary seal leakage will not go to the disposal system and can leak past the secondary containment seal faces to atmosphere.

Gordon Buck is chief engineer of field operations and Ralph Gabriel is chief engineer at John Crane Inc., 6400 West Oakton St., Morton Grove, IL 60053.

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