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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.

Seal Flush Plan 01

Advantages

• No product contamination. The flush source is coming from the pump and going back to the pump.
• No reprocessing of product. Unlike an external flush, the product does not have to be reprocessed.
• Simplified piping. There is no external piping on the pump.
• Useful arrangement on fluids that are highly viscous at normal ambient pumping temperatures so as to minimize the risk of freezing if exposed to low temperatures in external piping plans, such as a Plan 11.

Disadvantages

• Must be used for clean pumpage only as dirty pumpage could easily clog the passageway and it would require pump disassembly to repair.
• Flush has to be repumped. The circulation from pump discharge back to pump suction will decrease pump efficiency and increase power required for the application. The volume of the flush is based upon the pump OEM's design and the pressure differential.
• There is no external way to control flow. Unlike Plan 11, which can have an externally replaceable orifice to control flow, the internal design of a Plan 01 eliminates this possibility.
• The flush is not usually directed right at the faces, but may come in over the seal head.

Sizing and Controlling

The flow rate is dependent upon the pressure differential in the pump and the design of the line running internal to the pump casing. Changing the impeller design can affect the pressure differential and thus the flush rate. The pump OEM should be contacted to ensure that the flow rate is adequate to maintain a stable condition at the seal faces.

General

This flush system can perform its function well when used properly. Changes in pump impellers, or changing seal designs that can move the seal faces away from the flush hole can cause problems that result in seal failures. This system is not recommended on vertical pumps.