The complexity and sensitivity of dry gas seals generally requires two maintenance provisions that are not typically needed with other compressor seal designs. These include:
• Sparing of the complete seal assemblies
• Inspection, overhaul and testing by a qualified repair facility (generally the OEM).
Once an assembly is tested, it should not be altered prior to installation. Testing should include the following measurements on both primary and secondary seals:
• Static leakage rate at various pressure levels, up to settle-out or discharge pressure
• Break-away torque at the same above pressures
• Dynamic (rated speed) leakage rates at various pressure levels, up to suction pressure plus 25% or 10 psi (whichever is greater)
• Leakage rates at rated speed and sealing pressure after 1 hour run
• Static leakage at settle-out or discharge pressure immediately after 1 hour run
• Leakage rates during 15 minute run following a “hot restart” (within 5 minutes of 1 hour run, at settle-out pressure).
Additional testing can be done to simulate emergency shutdown, depressurization and reverse pressure scenarios.
Seals that have been stored for long periods of time may need to have secondary sealing elements changed to counter relaxation and degradation that can occur during long storage durations or the upcoming time in service. The OEM should be consulted for specific recommendations for a particular seal design. The sealing element changes should also be done by a qualified repair facility, and the seals should again be tested to verify the integrity of the parts and the re-assembly work.
The suggested initial seal inspection interval is five years, until inspection results and performance demonstrate that longer intervals are possible. Immediate or earlier inspections should be considered if there is an event that compromises the environment of the seal faces. Some event examples, in approximate rank order of urgency (high to low), include:
• Filling of the compressor case with liquid
• Loss of filter integrity
• Loss of seal gas or purge gas
• Degradation of leakage performance
• Extreme compressor vibration levels
• Loss of separation gas
• Prior inspection results that indicate problems not yet resolved.
A qualified manufacturer’s representative should be present to perform or oversee, installation of seals, at least until plant personnel are very comfortable with procedures and methods. Use of the representative for seal removal is also advised, especially if it has never been performed by plant personnel.