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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.

It is customary to leave suction valves on standby pumps open. Static barrier gas leakage will eventually vent through this opening. On vertical pump installations, this venting is liable to occur in sudden and significant volumes when meniscus forces are broken. In these instances, there is a possibility that the volume of gas entering the main pump suction can be sufficient to affect the operation of the main pump, if it is of a low enough flow rating.

To accommodate these issues in vertical pump installations or horizontal pumps with non-venting suction lines, a provision for manual or continuous automatic venting of seal chambers must be incorporated within the total pump installation.

If for operational or hazard reduction reasons it is required to shut both the suction and the discharge valves and isolate a standby pump, it can be expected (as with any dual pressurized seal) that the pump casing stands the risk of becoming pressurized to the same pressure as the gas barrier source. Depending upon the effectiveness of the valve seats, the casing pressure could also rise to that of the pump discharge manifold, which might be in excess of the barrier gas pressure.

Even though the dual gas seal may have a reverse pressure design feature on horizontal units, it is possible that a small quantity of process fluid may contaminate the gas barrier chamber. This is not detrimental to the seal (unless the process crystallizes or hardens), but when restarting the pump there is a risk that this small volume of process fluid will be pumped through the outer seal to the atmosphere.

On horizontal installations requiring zero atmospheric emissions, which may be required to operate in a standby mode with the pump suction and discharge valves closed, it is necessary to connect the casing to a low pressure environment.

Dynamic Gas Leakage

Barrier gas leakage across the inner seal face during dynamic operation will eventually mix with the process flow. Depending on the seal size, operating conditions, pump size, pump design, and operation this leakage can affect the seal's performance. This may be an increase in the NPSHR, a reduction in differential head, and in extreme cases a loss of prime.

At normal leakage levels this may not be an issue, but when leakage levels approach a condition when failure is deemed imminent, the effect on pump operation should be minimized. The seal size, shaft speed, barrier gas pressure, pump flow capacity, impeller design, and level of operational flow compared to the pump's design BEP (best efficiency point) are all factors that determine the affect on normal pump operation.