by Gordon Buck and Ralph Gabriel, John Crane Inc.
flow rates.jpgApproximate 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.

Purge Rates for Secondary Containment Seals

API Plan 72 is designed to have an inert gas purge through the containment seal area with the intent to reduce emission levels to the atmosphere. The purge gas mixes with leakage from the primary seal, thereby reducing the concentration of the hazardous fluid (liquid or gas). Leakage rates from the various types of containment devices will vary from high rates with bushings to low leakage rates with contacting face seals.

Leakage to atmosphere will also have a wide variation depending upon operating conditions, length of time in service and equipment conditions, as well as a myriad of other lesser considerations. When deciding on the purge rate, consideration should be given to the type of containment device, the flow rate past the orifice, the fact that excessive purge rates can dry out the sealing cavity and possibly decrease the life of contacting face seals, and that excessive containment seal cavity pressures can decrease the life of the containment sealing device with the possible exception of non-contacting containment seals.

A simple rule of thumb is to have a flow rate on the order of ½ SCFM to the containment seal cavity. This relates to the rough flow rate for a 5-psi differential pressure across a 1/16-in orifice. This rate can be adjusted upwards or downwards depending upon the specific application.

Influence of Static and Dynamic Dual Gas Seal Leakage on Pumps

Even though leakage from dual gas seals is normally very low, the following issues related to pump design and installation may require attention, depending upon the seal duty:

  1. Static gas leakage can displace the liquid in the pump and prevent start up. This is particularly relevant in the case of vertical standby pumps.
  2. Dynamic pump performance can also be affected by a loss of pump efficiency, differential head and increased NPSHr (Net Positive Suction Head Requirements).

Summarized below are the background and recommendations to eliminate these potential problems.

Static Gas Leakage

Inboard static gas barrier leakage may be at a minimal rate, but in a vertical pump in a standby condition, or stationary in a stop/start batch process, barrier gas can collect in the pump casing and disable the ability of the impeller to prime on start-up.

Some exceptional horizontal installations also suffer the same circumstance when suction pipework originates from below the shaft centerline. Not all vertical pumps are vulnerable, as the sensitivity is dependent on the relative positions of the impeller and the suction inlet. Some in-line units using a Plan 13 flush (in conjunction with a Plan 74 for the dual gas seals) have the ability to naturally vent through the suction valve, if the piping orientation permits.