What is the impact on the dry gas seal after a centrifugal compressor shuts down suddenly?
Thread Content
This post was last edited by ajilisi520 on 2017-7-28 at 10:15. The compressor shuts down due to a fault-related interlock; the anti-surge valve opens automatically. At this point, the high-pressure gas at the compressor outlet flows back to the compressor inlet, and simultaneously the pressure of the primary seal gas in the dry gas seal decreases. It is possible that for a short period of time the pressure of the primary seal gas falls below the pressure in the balance line. Could this situation cause damage to the dry gas seal (two-way groove) from that moment until the compressor comes to a complete stop? Some systems are equipped with interlocked quick-opening valves under high-pressure nitrogen, or pressure boost pumps are installed on the main sealing gas pipeline; I wonder if anyone has encountered such a situation
My workshop’s compressor seal air system is equipped with a pressure booster pump. However, the slight abrasion between the moving and stationary rings over time is acceptable; this causes much less damage to the seal compared to turning the shaft for long periods of time.
Generally speaking, the pressure difference between the main seal gas and the balance pipe does not become negative, and it is also difficult for this to occur during an emergency shutdown, as the balance pipe is subjected to pressure reduction through a labyrinth seal as well
It is possible to monitor the flow rate of the main sealing gas; a certain flow rate indicates that the sealing performance is good. When this flow rate drops to a certain level, it is possible to activate the pressure booster pump to maintain the required flow rate of the main sealing gas
This post was last edited by Xibei Guyan on 2017-7-28 at 22:12. We are the olefin separation unit of an 1.8 million-ton MTO plant; it has two centrifugal compressors – one for the reaction gas and another for propylene cooling. The main seal gas supply for these two compressors comes from the process gas at the compressor outlet, high-pressure nitrogen, and ethylene gas (this source was discontinued after a technical upgrade as it was considered uneconomical). The pressure difference between the seal gas and the chamber, as well as that with the vent lines, is used as a reference value for adjusting the main seal gas flow. At startup, the main sealing gas is switched from high-pressure nitrogen to process gas ； During normal shutdown, the main sealing gas is switched from process gas to high-pressure nitrogen ； During an emergency shutdown, the main sealing gas can be switched to either high-pressure nitrogen or ethylene gas! The emergency situation mentioned by the poster will certainly cause some damage to the dry gas seal. The compressor shuts down urgently, the pressure of the main sealing air supply drops, and the thickness of the air film between the rotating and stationary rings becomes uneven, which can easily lead to frictional damage between these rings! This is inevitable! All we can do is prepare for accidents, reduce the time of friction, and improve the handling of emergency situations!
We have hydrocarbon compressors; any shutdown triggered by an interlock condition will result in the introduction of high-pressure nitrogen as a sealing gas to protect the unit. It is recommended to check the unit’s logic – the conditions under which the main sealing gas switches from process gas to high-pressure nitrogen vary from one unit to another
This type has a balance disc on the rotor, so the main shaft does not experience axial displacement. If the excessive axial displacement is indeed caused by a damaged thrust bearing, not only will the sealing strip be damaged, but the impeller will also be damaged. As for the damage to the sealed air groove (possibly called a comb-shaped seal strip), it is likely due to impurities in the air supply. For reference.
During an emergency shutdown, is it through interlock switching or manual activation of high-pressure nitrogen or ethylene gas?
In the event of an emergency shutdown here, the main sealing gas is manually switched to ethylene gas or high-pressure nitrogen!