Challenges with Dry Gas Seals for Turbocompressors

Modern dry gas seals are typically gas-lubricated, non-contacting end-face seals. The mating ring is rotating and usually contains grooves measured at a few microns. This generates a proper running gas gap, ~4 - 12 microns, due to the rotation. In other words, these grooves compress the seal gas, creating a pressure-gas dam between seal ring faces. This removes and separates the seal faces, which the seal gas flows over. There’s always a small amount of seal gas leakage that flows through the primary seal vent. Seal faces are separated by a thin film of gas based on a non-contacting seal concept.

CHALLENGES OF SHUTDOWN AND TRANSIENT CASES

At the compressor trip (shutdown), the primary seal ring (stationary) is usually held against the mating ring (rotating) with a series of springs. The low-speed operating—during shutdown, start-up, or other transient situations—could be problematic since the rings touch each other while rotating. The compressor rotor assembly may experience small axial movement during the operation; therefore, the gas film should be stiff with an optimal running gap to avoid seal-face contact.

The uniform gas pressure distributions on the seal-ring faces are also important. The gas seal support system (seal skid) should include sufficient instrumentation to continuously monitor operation and key parameters such as pressures, flows, temperatures, etc. This is necessary to ensure high performance, proper operation, and reliable seals.

NEVER USE WET SEALS

Wet seals—oil-lubricated seals—should not be used in modern turbocompressors as they cause operational and reliability problems, including high seal-oil consumption, issues related to used-seal-oil drainage, high-power waste, hazards, short life, high rates of wear/maintenance, complex skid needed for the seal oil, and contaminated oil, which can lead to serious malfunctions downstream. A seal oil skid is very expensive and challenging, and may contain different seal oil pumps, filters, various instruments, etc.

A RELIABLE DRY GAS SEAL

Compressed gas properties and conditions should be considered when selecting dry seal gas and the seal system/skid configuration. The phase change of the seal gas (the gas that flushes the dry gas seal) should be investigated and identified. Liquid or dirt should not be allowed in the seal gas. The separation gas system function prevents the lubrication oil from entering the dry gas seal. Compressor pressurization and depressurization details and sequences should be reviewed to ensure the seal system’s proper operation.

All cases of shutdown and start-up, particularly a black start-up (start-up from total plant shutdown) need attention. Every detail of the seal system requires careful attention; for example, small diameter orifices in some seal systems/skids could be plugged and shut down the seal system, consequently tripping the turbocompressor. The dry gas seal failure rate, if designed and operated properly, could be approximately 0.15 - 0.25 failure/year. In other words, a failure might happen every four to six years. Often, a complete dry gas seal inspection or overhaul is required every five to seven years.

COMMISSIONING OF DRY GAS SEALS

Liquid can cause problems for dry gas seals. Inspect all parts in a seal system/skid, including low-point drains in the primary and secondary seal vents and seal gas filters for the liquid. Low-point drains should be closed and plugged before commissioning. The sources of all utilities, including the separation gas (usually N₂) and seal gas should be operational and available for commissioning.

Step one is to static test the dry gas seal. During static testing, the gas seal is slowly pressurized by ~10% incremental increases up to the maximum operating pressure. The actual static leakage is compared to the seal manufacturer’s guaranteed static leakage rate. If the static leakage rate is acceptable, perform the seal dynamic test. During the seal dynamic test, accelerate the turbocompressor from rest to maximum operating speed. Monitor and record gas seal leakage during acceleration and compare it to the guaranteed leakage rate. Operate the turbocompressor for four hours and monitor/record leakage rates and other important seal parameters every 10 - 15 minutes. The dynamic leakage rates should be within acceptable limits as specified by the manufacturer.

OPERATION AND KEY PARAMETERS

The following operational parameters are critical for the dry gas seal and seal system/ skid:

• seal gas supply flow
• seal gas differential pressure
• primary and secondary vent gas leakage rates

Monitor these parameters for indications of the dry gas seal’s operational condition and health. A rise in seal gas consumption can indicate an increasing internal clearance of the seal due to wear. An increase in the primary-vent gas leakage could show an increased running gap of the primary seal.

The most common reasons for a turbocompressor unscheduled shutdown are related to the seal system. Reviewing key seal system parameters such as seal gas flows, differential pressures, and gas leakages is important to turbocompressor reliability and performance.

ABOUT THE AUTHOR

Amin Almasi is a Chartered Professional Engineer in Australia and the U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.