by Gordon Buck and Ralph Gabriel, John Crane Inc.

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.

To prevent the likelihood of dual gas seal leakage in dynamic operation affecting the design pump performance, screening by consultants is advised on pumps operating between 40-gpm and 90-gpm, when operating at less than 50 percent of its BEP.

At high vacuum suction conditions the effect of dual gas seal leakage into the process fluid is exaggerated because the gas expands at the low pressure. This is not a normal pump operating condition, but on pump NPSHr proof testing it may occur. The normal measurement criteria of a loss of 3 percent in the head generated can be created by gas entrainment. In an NPSHr proof test with a low capacity pump design and dual gas seals, a conservative and inaccurate value may be indicated.

It is advised that if NPSHr proof tests are applied to pumps with a BEP capacity less than 40-gpm, the influence of gas seal leakage must be evaluated and if necessary use an alternate seal design.

Plan 01

Plan 01 is an integral (internal) recirculation from the pump discharge to the seal chamber, which is typically at a pressure slightly above pump suction pressure. It is similar to Plan 11 in that it uses the pressure differential between pump discharge and pump suction to develop flow, but is different in that there are no external lines (piping or tubing) on the pump. It is recommended for clean pumpage only and is typically limited to pumps with a Total Discharge Head of less than 125-ft.