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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 oilflooded screw compressors, oil-lubricated reciprocating compressors or old-fashion, oil-sealed turbocompressors can create problems.
Turbocompressor manufacturers often prefer 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.
About 90% of primary seal failures in difficult gas services occur because the seal cannot be protected from the gas inside the compressor.
To ensure high reliability of dry gas seals, clean and dry gas at the seal is required. Seal gas supply systems usually draw gas from a higher-pressure stage of the turbocompressor, filter the gas and then use it to flush the seal. About 90% of primary seal failures in difficult gas services (such as contaminated, corrosive or sour gases) occur because the seal cannot be protected from the gas inside the compressor. It is important to provide clean seal gas at the proper pressure and conditions during transient periods such as start-up or shut-down.
When a reliable source of seal gas is unavailable, therefore, the required seal gas should be supplied from an alternative source. One solution is a gas seal booster compressor. In general, seal gas booster compressors are a requirement for turbomachinery. These small compressors should be compact, reliable, oil-free, robust, rugged, capable of providing a relatively small amount of seal gas with relatively high pressures, reasonably priced and delivered quickly.
Traditionally, oil-free positive displacement compressors (such as small oil-free reciprocating compressors) have been used. Pneumatically driven positive-displacement compressor systems have also been employed for years. However, these positive-displacement compressor boosters are often not available when needed. Reciprocating compressors, in particular, have a relatively low service life and tend to wear down quickly.
Modern seal gas boosters based on high-speed multistage centrifugal compressor technology, then, have a crucial role to play in our industry. Only a reliable multi-stage centrifugal compressor booster has the capability of ensuring continuous operation with zero leakage and no oil contamination in a seal gas setting. They generally offer a small footprint, low power requirements and a simple connection to the seal system.
It is also recommended that seal gas centrifugal compressor boosters be electric motor direct-driven, high-speed, fully stainless steel, and without internal seals or oil lubrication systems. These machines are robust, maintenance-free and have no wear-sensitive components. Vertical directdriven designs (the electric motor driver on top) have become popular options because of their compact footprint.
Amin Almasi is a Chartered Professional Engineer in Australia, Queensland and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.