Seal system issues are a common reason for alarms and shutdowns in turbocompressors. Upgrades to dry gas seal systems typically consist of two options:
• Seal gas filter system — the original filters are typically small in capacity and large in the mesh size. In most cases, new filters should be used which are 4-to-6 times bigger while having a mesh that is 5-to-8 times smaller
• Operation range of seal system instruments — seal panels are complex systems with various kinds of instrumentation.
Some instruments may not have been selected properly and the instrument range may not be sufficient. For example, seal system flow meters are often improperly sized. Take the case of a natural gas turbocompressor where the original seal filter meshing was 3-to-5 microns. An upgraded seal filter provided a mesh of 1 micron, and was six times bigger than the original.
Oil carryover can also be a problem.
Upstream compressors, whether oil-flooded screw compressors, oil-lubricated reciprocating compressors or old-fashion, oil-sealed turbocompressors can create problems. Turbocompressor manufacturers often preferred to use generic seal panels. Emergency and abnormal operational cases, which can make the seal system more expensive, are sometimes neglected. Additionally, all of the following should be considered during the bidding
stage:
• An intermediate labyrinth purged with N2 (to eliminate fugitive emissions) is nearly always recommended
• A tandem dry gas seal is the most common seal today for turbocompressors
• The primary vent should be situated between the two seals, usually connected to the flare system
• Back pressure is important and the seal should not be subjected to a reverse pressure condition as this can cause premature failure
• High pressure in the primary vent line usually indicates failure of the primary seal stage while low pressure indicates problems with the secondary seal stage
• For the secondary seal to function satisfactorily a minimum positive differential pressure is required; therefore, the primary vent pressure should carefully be controlled.
(The complete version of this story appears in the Nov/Dec issue of Turbomachinery International)
