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Tandem seal (Courtesy of Flowserve Corp.)[/caption]
Gas from the compressor discharge enters the port closest to the compressor (labyrinth end) and the majority of the gas enters the compressor thru this labyrinth. To assure that process gas, which is not treated by the dry gas system, does not enter the seal chamber, (velocities across the labyrinth should be maintained between 6-15 m/sec (20–50 ft/sec). It is the writer’s experience that considering labyrinth wear, the design should be closer to 15m/sec (50 ft/sec).
Approximately 1.7-3.4 Nm3 /hr (1-2 SCFM) flow (standard cubic feet per minute) leak across the first tandem seal faces (primary seal) and exit through the primary vent. Based on the backpressure of the primary vent system, 1.7 Nm3 /hr (1 SCFM) or less will pass through the second tandem seal faces (secondary seal) and exit through the secondary vent.
To assure that oil mist from the bearing housing does not enter the dry gas seal chamber and that seal gas does not escape to atmosphere, an additional barrier seal is used and provided with pressurized nitrogen at approximately 35 kPa (5 psi).
Dry gas seal reliability depends on the condition of the gas entering the seal faces. The function of the seal gas supply system for any dry gas seal option is to continuously supply clean, dry gas to the seal faces. During start-up, when the compressor is not operating with sufficient pressure to supply the seals, an alternate source of gas or a gas pressure booster system should be provided.
Note that the following options exist regarding the primary, secondary vent and barrier seal instrumentation and components:
■ Primary seal vent triple redundant (2 of 3 voting) flow or differential pressure alarm and shutdown
■ Primary seal vent rupture discs in parallel with vent line to rupture at a set pressure and prevent excessive pressure to the secondary seal on primary seal failure
■ Spring loaded exercise valves in the primary vent line to exert a backpressure on the primary seal to close the faces in the event of dynamic ‘O’ ring hang-up
■ Secondary vent line flow or differential pressure alarms and trips
■ Barrier seal supply pressure alarm and permissive not to start the lube oil system until barrier seal minimum pressure is established.
Tandem seal and barrier seal typical housing arrangement[/caption]
Tandem seals for saturated gas applicationsThe changes required for a saturated gas are solely in the seal system. A typical system incorporates a cooler, separator and heater in addition to the normal components used for a dry gas application to assure that saturated gas does not enter the seal chamber. Typical values for the cooler are to reduce the gas temperature to 30°F below the saturation temperature of the gas. The typical dimensions for the separator vessel, complete with a demister, are 460mm (18 inches) diameter and 1.8 meters (6 feet) high.
The typical requirements for the heater are to reheat the gas to 15°C (30°F) above the saturation temperature. Temperature transmitters are provided upstream and downstream of the cooler and downstream of the heater. As a precaution, in the event of cooler or heater malfunction, a dual filter/coalescer, complete with a drain back to the suction is provided.
Tandem seals with interstage labyrinthThis arrangement features a labyrinth between the primary and secondary seals. This action assures that gas vented from the secondary seal will always be nitrogen since the nitrogen supplied between the primary and secondary seals is differential pressure controlled to always be at a higher pressure than the primary seal vent thus assuring that only nitrogen will be in the chamber between the primary and secondary seals.