TABLE OF CONTENTS In a tandem gas seal, seal gas is injected between the inner labyrinth seal and the primary gas seal (refer to chapter 3). The vast majority of this injected gas flows across the inner labyrinth seal and into the process side of the compressor. A very small amount of the seal gas passes through the primary seal and out the primary vent, which is normally connected to the plant flare system. Note that there is no primary vent per se in a double opposed gas seal system.
The primary vent (fig. 5-8) should be fabricated from stainless steel piping. The vent should be equipped with a low point drain and valve to allow removal of any accumulated liquids in the primary vent area that could cause damage to the primary seal. If the primary vent is connected to a flare system, a check valve must be included to prevent any potential back pressure from the flare system in the primary vent area, which could cause damage to the gas seal.
The best means of assessment of the condition of a tandem dry gas seal is by monitoring the gas seal leakage through the primary vent. This is usually accomplished by measuring the flow or pressure across a restriction orifice in the primary vent piping. An increasing flow or pressure trend indicates increasing gas seal leakage and probable deterioration of the primary seal. The flow restriction...
