Cryogenic nitrogen (nitrogen that has been liquified) can damage carbon stationary faces during slow-speed operation — turning gear ratcheting or slow roll — when the faces are in contact. Cryogenic nitrogen is typically very dry, with a dew point as low as -90ºC (-130ºF). But the self-lubricating quality of carbon is based on the ability of its crystalline structure to adsorb and hold certain gases, including water vapor, which significantly reduce rubbing friction.
In the absence of water vapor carbon has poor lubricating properties, and wear rapidly. Therefore dew point conditioning is required whenever carbon stationary elements are used in either face or circumferential seals, when rubbing contact is anticipated for extended periods.
For large steam or gas turbine driven compressors that require slow roll for extended periods below the BGS lift-off speed, the best practice is to condition the nitrogen upstream of the coalescing filter system, raising its dew point to -30ºC (-22ºF), or higher.
Methods to increase nitrogen dew point include mixing saturated nitrogen — from a bubbler chamber — with cryogenic nitrogen in an appropriate ratio, or mixing moist air with cryogenic nitrogen, keeping the oxygen content below 5%. A dew-point monitor and low-dew-point alarm are required for safe operation.
The best practice is to ensure that nitrogen dew points are above -30ºC (-22ºF) to optimize the life of carbon used in seal faces and seperation seals.
This is achieved by:
- Using small seperation units that produce moist nitrogen (dew point-30ºC (-22ºF)
- Using a nitrogen bubbling system to condition ‘bone dry’ nitrogen so that the dew point > -30ºC (-22ºF)
Currently, air separtion units produce nitrogen with dew points below -50ºC (-58ºF). The life of carbon seals (radial and face seal) is significantly reduced in dry gas applications where the nitrogen dew points are below -30ºC (-22ºF). The use of ‘bone dry’ nitrogen (dew points below -30ºC) for intermediate and separation sealing duties has resulted in low seal MTBFs (below 12 months).
In some cases, floating carbon seal wear was observed during the factory acceptance tests (FATs).
This best practice was first used in 2008. Since that time, specifications that require the dew point of supplied nitrogen to be above -30ºC (-22ºF) have been produced. It should be noted that small, dedicated nitrogen generators can produce nitrogen above -30ºC (-22ºF).
