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Cryogenic nitrogen (nitrogen that has been liquefied) can damage the carbon
stationary faces during slow-speed operation -- turning gear racheting 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
can 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 DGS 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
Optimizing the life of carbon used in seal faces and separation seals is
Using small air separation units that produce moist nitrogen (dew point > -
Using a nitrogen bubbling system to condition 'bone dry' nitrogen so that the
dew point > -30oC (-22oF)
Currently air separation units produce nitrogen with dew points below -50oC
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).