5 replies to this topic

[Lyne]

Hello, I'm reviewing some BOG centrifugal compressors P&ID, and it catched my attention a line of gas used as purge gas ( Hc not nitrogen) at the compressor disrcharge ? It says that it’s used for startup .... any idea about this requirement ? When purge gas is needed ? And does it apply for all centrifugal compressors ? Thank you

Edited by Lyne, 19 October 2025 - 12:55 PM.

[Pilesar]

You need to get rid of the oxygen in the line some way after maintenance. What is your concern with the current design? I like the idea of 'N2 first with CH4 purge to remove the N2' to avoid flammable mix at all times. At least this is a question to bring up in a HAZOP. I have had BOG compressors in LNG storage designs, but never looked closely at the purge setup.

[fallah]

Hello, I'm reviewing some BOG centrifugal compressors P&ID, and it catched my attention a line of gas used as purge gas ( Hc not nitrogen) at the compressor disrcharge ? It says that it’s used for startup .... any idea about this requirement ? When purge gas is needed ? And does it apply for all centrifugal compressors ? Thank you

 

All equipment handling Hydrocarbons need purging facilities using a HC or an inert gas. A compatible HC is normally used for purging to replace the existing air inside the equipment and associated piping by purge gas for ready to start condition. An inert gas like N2, conversely, is to be used to replace the existing HC inside the equipment and associated piping in order to prepare the out of service facilities for long shutdown or maintenance.

[Lyne]

Hello, I'm reviewing some BOG centrifugal compressors P&ID, and it catched my attention a line of gas used as purge gas ( Hc not nitrogen) at the compressor disrcharge ? It says that it’s used for startup .... any idea about this requirement ? When purge gas is needed ? And does it apply for all centrifugal compressors ? Thank you

All equipment handling Hydrocarbons need purging facilities using a HC or an inert gas. A compatible HC is normally used for purging to replace the existing air inside the equipment and associated piping by purge gas for ready to start condition. An inert gas like N2, conversely, is to be used to replace the existing HC inside the equipment and associated piping in order to prepare the out of service facilities for long shutdown or maintenance.

Thank you for your reply.

But is purging air with HC safe, we will be creating an explosive atmosphere?

[Lyne]

You need to get rid of the oxygen in the line some way after maintenance. What is your concern with the current design? I like the idea of 'N2 first with CH4 purge to remove the N2' to avoid flammable mix at all times. At least this is a question to bring up in a HAZOP. I have had BOG compressors in LNG storage designs, but never looked closely at the purge setup.

Thank you for your reply. N2 connection utility is provided, and another connection for HC gas which is included in the compressor startup sequence (Automatic), I find it not safe if it's about getting rid of O2 in the line.

[fallah]

 

Thank you for your reply.

But is purging air with HC safe, we will be creating an explosive atmosphere?

 

 

Yes, it can be safe, provided that the gaseous mixture being conducted to the flare system or a safe location...