Single seals do not always meet the shaft sealing requirements of today’s pumps, due to the small amount of required leakage when handling toxic or hazardous liquids; suspended abrasives or corrosives in the pumpage getting between the seal faces and causing premature wear; and/or the potential for dry operation of the seal faces. To address these situations, the seal industry has developed configurations which incorporate two sets of sealing faces, with a clean barrier fluid injected between these two sets of seal faces. The decision to choose between a double or single seal comes down to the initial cost to purchase the seal vs. the cost of operation, maintenance and downtime caused by the seal, plus the environmental and user plant emission standards for leakage from the seal.
The more common multiple seal configuration is called a Double (dual pressurized) seal, where the two seal face sets are oriented in opposite directions. The features of this seal arrangement are:
Potentially five times the life of a single seal in severe environments.
The metal inner seal parts are never exposed to the liquid product being pumped, which means no need for expensive metallurgy; especially good for viscous, abrasive, or thermosetting liquids.
The double seal life is virtually unaffected by process upset conditions during pump operation.
The other multiple seal configuration is called a Tandem (dual unpressurized) arrangement, where the two individual seals are positioned in the same direction. This seal arrangement is commonly used in Submersible wastewater pumps, between the pump and motor, with oil as the barrier liquid. The typical features of this seal arrangement are:
The pressure between seals is lower than the seal chamber pressure (typically atmospheric)
The external fluid only lubricates the most outside set of faces
Pumped fluid lubricates most inside faces
The outside seal serves as a safety seal or containment device
Leakage to the atmosphere is external fluid, possibly mixed with small amounts of pumped fluid.
