Double or tandem mechanical seals are typically used in services involving toxic, flammable, or hazardous liquid to prevent the liquid, or its vapor, from leaking into the environment due to a leak or seal failure.
A double mechanical seal is a combination of two seals mounted back-to-back with a non-hazardous barrier liquid injected between the two seals at [ * ] to [ * ] PSIG above the stuffing box (or seal chamber) pressure. The barrier liquid comes from an external source such as a pressurized reservoir in a closed loop piping system connected into, and out of, the seal gland.
Double seal arrangement is intended to seal the barrier liquid, not the process liquid. The arrangement consists of an inner or primary seal installed in backward position, and an outer or secondary seal installed in normal position. The backward position of the inner seal prevents the barrier liquid from entering the pump at normal condition. In the event of an inner seal failure the barrier liquid enters the pump and prevents the process liquid from leaking into atmosphere. In the event of outer seal failure, the non-hazardous barrier liquid leaks into atmosphere while the inner seal contains the process liquid.
A tandem mechanical seal arrangement is also a combination of two seals but they are mounted in the normal position, as opposed to the back-to- back position of a double seal. A barrier liquid is also injected between the two seals from an external reservoir in a closed loop piping system but it is not pressurized.
The barrier fluid is circulated by the centrifugal force effect of a pumping ring in the seal. The pressure in the stuffing box is higher than in the barrier chamber so when the inner seal fails the process liquid enters the barrier chamber and forces the oil reservoir level to rise. A high oil level switch will trip an alarm and the pump is shut down. The process liquid is piped out from the of the reservoir into a safe disposal area.
Double and tandem seals are usually flushed using API Plan 52 or Plan 53 (or the equivalent piping plans in non-API pumps.)
The use of double or tandem seal arrangement increases the seal frictional horsepower (HP) draw on a pump; numerous tests indicate that it can be as high as [ * ] HP. This should be taken into consideration when sizing the driver of small pumps such as size 2x3x13, or smaller, and those requiring [ * ] HP, or less. The failure to account for the seal frictional HP can result in selection of drivers with insufficient torque.
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