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The MTBF of the secondary (outside) seal in a tandem seal arrangement can be increased if the
differential pressure across the seal faces is greater than 50 kPa (7.25 psi).
Installing a backpressure device (spring loaded check valve or backpressure control valve) will increase the pressure in the seal chamber to above 50 kPa (7.25 psi) and allow the condition of the secondary (outside) seal to be monitored easily. Reduced primary vent pressure, which is the seal chamber pressure, immediately indicates secondary seal increased leakage.
A primary vent backpressure device is required to sufficiently pressurize the seal chamber and allow monitoring of secondary seal condition by a low primary pressure alarm transmitter.
A spring loaded check valve is recommended in the piping at the flare header connection. This prevents the back-flow of flare gas into the primary vent chamber, and maintains a positive pressure differential across the secondary seal.
The check valve is normally designed to exert a minimum back pressure of 35 kPag (5psig) in the primary vent cavity, ensuring a minimum 35 kPad (5 psid) positive differential over the secondary seal. However, the required secondary seal pressure differential should be determined by the seal vendor, based on anticipated turning gear operation and seal lift-off speed.
Recently, (since 2008) some vendors have been installing a back pressure device in the primary vent which is set at 140 kPa (20 psi) or greater. This approach increases secondary seal face differential pressure and enables monitoring of secondary seal condition (low primary vent pressure indication if secondary seal experiences excessive leakage).
Tandem seals that do not use a means of pressurizing the seal chamber above 50 kPa (7.25 psi) have lower MTBFs (less than 70+ months) than tandem seals using a means to pressurize the seal chamber.
Seal vendors concur that increasing the seal chamber pressure in tandem seal applications will prolong the life of the secondary seal since a greater positive pressure will reduce seal wear and minimize the possibility of seal face groove contamination.
This best practice has been recommended for all new projects and field seal system modifications since the early 2000s. This best practice has resulted in increased seal MTBFs.