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We have wet gás compressor (centrifuge compressor) in our DCU and we inject heavy coker naphta to wash the impellers. Is it usual to inject HCN to wash ? What is it the flowrate ? Any particle spec (< 25 micra) ? What happens if we do not inject HCN (compressor vibration ?) ? What is it the recommended stream to inject ?
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yes, wheel wash is normal. Flow is not continuous. It’s hard to say without knowing the details of the compressor design what the flow should be when running. Who’s the OEM for the WGC?
Yes, balance and vibration issues start accumulating without washing. Washing it typically done with the vibration starts coming up. You should look back at the baseline SOR trends. There should be a trip vibration level too. Just a guess but I would think washing should be triggered at approximately 30% of the difference between SOR and trip.
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The OEM for the WGC is from Germany (huge company). So, the naphta injection is not continuous? The OEM recommend a continuous injection. We have this experience with no injection starts the vibration. After we start the injection the vibration decreased. Do you know any paper that has more information about this injection or even in this site ?
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I did not think it was continuous but listen to the OEM not me, I don’t have any details…..
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I think it is very common to have a naphtha wash available to inject into the wet gas compressor. I have heard that some sites inject naphtha continuously, others inject intermittently while others do not inject naphtha during the course of the run.
I think the conventional wisdom is the naphtha injection helps to prevent polymerization of the dienes in the wet gas. If the temperature of the compressor discharge is higher than 300 F (150 C), there is a risk of polymerization. My understanding is that this is a common problem in ethylene plants that have a much higher olefin content in the gas stream. In those cases, they try to limit the compressor discharge temperature to less than 240 F (115 C) to prevent that polymerization and injection of a liquid into the compressors helps prevent that problem.
Each installation is different as the performance of the overhead accumulator to separate liquid and vapor is different at each unit and the compressor design is different for every site. The temperature of the overhead accumulator determines the amount of heavy hydrocarbons in the wet gas stream, which impacts the discharge temperature of the compressor. All of these could impact the need for naphtha injection into the wet gas machine.
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Which naphtha is better to inject in the compressor ? Heavy or light naphtha ? Why ?
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My guess is that using heavy naphtha is preferred. The combined liquid and gas should not go through a dry point inside the compressor meaning that there should be some liquid in the fluid as it leaves the compressor. A higher boiling fraction will result in a two phase flow on the compressor discharge. A lighter naphtha would require more liquid be added to prevent a dry point in the compressor.
A dry point would result in deposition of material on the compressor impellers and then no liquid to mitigate the increase in temperature due to compression. There will be more dienes due to the liquid injection and the probability of polymerization is higher than if no naphtha injection was done in the first place.
I think either could be used, but enough naphtha has to be injected to prevent a dry point in the compressor.
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