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Boil off gas compressor

Boil off gas compressor

(OP)
Can someone please list out some specific things that we need to take care of when doing the engineering for screw type Ethylene Boil off Gas Compressors? How would it be different from any other process gas screw compressor?

RE: Boil off gas compressor

Experience with polymerization would be a question for the OEM.

RE: Boil off gas compressor

Oil compatibility is a big concern. An oil that mixes with gaseous ethylene would be a crappy choice for the screw-oil. Temperature control is always an issue with oil-flooded screw compressors, but by the time the product gets to an Ethylene Boil-Off step it has to have been deydrated to nearly zero humidity so the need to cook water out of the screw oil is unlikely to be a design-driver.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Boil off gas compressor

zdas04 is absolutely correct. I was thinking that you would need a dry screw. Not sure who you are considering, but http://kobelcocompressors.com//index.php/screw_com... may be an option. Apparently they have experience with my initial concern of polymerization.

Interested in reading the results of your review Divya11

RE: Boil off gas compressor

Use VFD for capacity control for the entire required speed range and avoid using the internal slide valve.

RE: Boil off gas compressor

(OP)
Our requirement is dry screw compressors, and yes nickelkid, kobelco is also under consideration. Georgeverghese, we would be using a VFD for capacity control. Is there any specific reason for avoiding internal slide valve?
How else would this compressor be different from any other process gas dry screw compressor?

RE: Boil off gas compressor

I've messed with a bunch of capacity control schemes over the years and the one that is absolutely the most reliable and moves the most gas for the least energy input is to keep the slide valve shut while the compressor is at other than max or min rpm. In other words you can use the unloader valve when you get to max rpm to prevent running out of hp and you can use it at minimum rpm to keep from running out of gas. Other than that use the VFD to control capacity. One of the instructors at an Ariel school used to say "if you don't compress it your efficiency is 100%", meaning that you should use the speed change to only compress the gas that needs compressing. The slid valve takes gas that you have compressed some and dumps it back to the suction.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Boil off gas compressor

Issues with temp rise within compressor casing during gas recirc. when slide valve is in operation as David explains - it gets worse when you have an oil flooded machine.

With a dry screw, would imagine you would have limited machine life as is typically seen in dry screw air compressors. A spare installed offline machine will help to maintain uptime.

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