Is it still meaningful to use Seal Gas after the compressor unit is shut down?
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For large centrifugal compressor units, is it still meaningful to use seal gas after the compressor unit has been shut down? Since the separation of the rotating ring and the stationary ring in a dry gas seal relies mainly on the gas pressure generated by the rotation of the rotating ring, if the compressor stops rotating, do the rotating ring and the stationary ring then come into contact with each other? Is it still necessary to use a dry gas seal in this case? One explanation was provided: in a stationary dry gas seal assembly, the moving ring and the stationary ring are pressed together by spring force; it is difficult to maintain a sufficient degree of contact between these two rings in order to ensure static sealing (this is related to the spring force, as too high a force may affect dynamic sealing). If untreated natural gas enters the space between the moving ring and the stationary ring, it can damage the sealing surfaces, which is why it is necessary to supply externally treated sealing gas to the dry gas seal system. Additionally, the manufacturer’s control program also ensures that sealing air is supplied as long as there is pressure inside the housing. There is no problem with the manufacturer’s design procedure; all centrifugal compressor units I have come across operate in this manner. However, after the compressor unit stops, the stationary and rotating rings come into contact with each other; is it still meaningful to continue supplying dry gas at this point? Where did the supplied dry gas go? I hope everyone can discuss this together!
Of course it makes sense... Although the static and dynamic rings of the rotor are pressed together and thus no air film can be formed, the sealing gas still enters the cylinder body through the comb teeth; this prevents process gas from entering the dry gas seal chamber and damaging the seal surfaces.
After the compressor stops, I think it’s necessary to keep the sealing gas in use; once the oil circulation stops and pressure is built up inside the cylinder, then the sealing gas can be turned off.
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After the system is shut down, will the system pressure be released from the dry gas seal system?
Personally, I think it’s advisable to keep the system in operation even after it stops working; just as we do during the system protection phase, we should use the minimum amount of sealing gas if possible. However, this gas is pure nitrogen; since it prevents external gases from entering the unit during shutdown, it is best to use it.
Reply to 1# betoy: First of all, it is very meaningful to use seal gas after the compressor unit is shut down! 1. As long as your lubricating oil pump is running, your seal gas supply must not be stopped; otherwise, you’ll end up with lubricating oil entering your dry gas seal system. Our factory encountered such a situation due to an operational error, and in the end it was necessary to remove and clean the entire dry gas seal system, which was a costly mistake! By the same logic, remember to fill the seal air before starting the vehicle, and then start the fuel pump; otherwise, the same problem will occur. As for where the sealed gas goes, I can tell you that part of it goes to the exhaust manifold for venting, while another part goes into the cylinder block on the process gas side. 2. One more thing I’d like to tell you: if the unit is shut down for an extended period of time (more than 1 month), it is essential to supply dry nitrogen to the dry gas seal system during that downtime. This is done to prevent the stationary and rotating parts of the dry gas seal from coming into contact with each other over time, which could lead to sticking or jamming of the moving components. Additionally, dry nitrogen should be filled throughout the entire cylinder of the centrifuge unit as a protective gas. This prevents moist air from entering the cylinder and reaching the dry gas seal system; it also helps to avoid corrosion of the components inside the cylinder due to exposure to moisture over a long period of shutdown. Our factory once suffered a loss: at that time, the factory shut down for a month for a comprehensive maintenance campaign. Since our units were new, they were only shut down for one month. Due to the lack of the aforementioned protective measures, it was found upon restarting that both the pressure and flow rate of the dry gas seal system were not at their normal levels before startup. It was determined that the dry gas seal components had become damp and stuck as a result of the long period of inactivity, preventing an effective air barrier from being formed between the moving and stationary rings. Subsequent disassembly and inspection confirmed that this diagnosis was correct. After this incident, we decided to install an additional dry nitrogen sealing pipeline for use during long-term shutdowns; for normal operation, we use the existing sealing gas pipeline (we use the exhaust gas supplied by the downstream equipment as the sealing gas, whose main components are carbon monoxide and carbon dioxide). By the way, the process gas we compress is acetylene cracking gas, which is quite dirty and therefore cannot be used as sealing gas. I hope my experience can be of some help to fellow sailors!
Reply to 7# cqtangseng: Brother, what you said is great! Add me on QQ: 545160695. Our factory’s dry gas seals use three types of gas: hot high-pressure nitrogen, cold high-pressure nitrogen, and cold low-pressure nitrogen. Hot, high-pressure nitrogen is used as the process gas for sealing (syngas at 200 degrees), while cold, low-pressure nitrogen is used for the sealing oil to prevent the oil from entering the dry gas system. The role of cold, high-pressure nitrogen isn’t yet clear to me; I’m ashamed! One compressor has been shut down for 4 months; it hasn’t been operated at all during that time. It was working fine before it was stopped. It’s ready to start now, but the compressor vibrates too much and won’t start. Upon opening the inlet, it was found that the volute was filled with water, and the end cover of the dry gas seal had rusted completely; therefore, the dry gas seal needs to be replaced. I just found out today that nitrogen needs to be introduced during parking; thank you so much.
Reply to 7# cqtangseng: Was the engine not turned over during parking? If the shaft is turned, then there shouldn’t be these problems, right?
What is the relationship between turning the turbine and the dry gas seal? Could someone upstairs explain that?
The relationship between crankshaft rotation and seal air depends mainly on how the compressor is driven. After the compressor stops operating, the SEAL GAS should remain in operation; this is mainly because the oil pump is not necessarily stopped after shutdown, and if the oil pump continues to run it can cause damage to the sealing mechanism. Furthermore, when parking for an extended period of time and the sealing gas for that line is not in use, it fails to provide drying protection for the stationary and rotating rings of the dry gas seal.