1 reply to this topic

[znadzi]

Hi,
I need some opinion on a technical study on a gas compression system on a facilities that have been operating for 5years. (2 trains of Gas turbine driven multistage centrifugal compressors- LP-HP)

We want to increase the gas export pressure from 181.5 Bara to 194 Bara. The compressors will be operating closer to their surge control lines and the discharge piping, discharge coolers need to be checked against the new process requirement - all have to rerated to 210bar.

In process standpoint, based on future production gas profiles, it is possible to reduce gas flow by 10-20%. Hence, based on OEM preliminary assessment (based on the test performance curve), we could still meet the new op condition by reduce flow and recycling gas in order to operate within the compressor map. This is definitely not so efficient but at least no major compressor modification / restaging (cost, downtime etc) will be required.
The compressor will operate @ higher speed but still below 105%. So the existing turbine can still be used with some reduced life penalties operating above base load.

The problems that we foresee now is operating the HP compressor casing near to its design pressure.
The new discharge pressure of 194 Barg will result in the HP compressor casing design pressure requiring re-rating from 200 Barg to 210 Barg. The HP compressor casing was designed at 200 Barg pressure; therefore the HP compressor casing will not comply with API requirement when operated at 210 Barg. This means that the HP casing needs to be replaced or we will have to consider 'amending' from API requirements.

Prior to shipment, the compressor was hydrotested 1.5 times the maximum working pressure at 300 Barg (as per API 617). However, it is noted that, for Vessels, the ASME VIII Clause UG99 on Hydro test requirement has been revised from 1.5 to 1.3. Could this possibly be used to justify deviation from API requirements? The other option is to perform FEA study on the casing and check the casing MAWP.

It seems like all other process equipment (i.e. pressure vessel and heat exhangers) are also found not suitable for rerating based on ASME code calculation alone on vendor data. Although they were tested at 300barg before, it was at ambient temperature and surely cannot be used to justify the rerating to 210barg. I am pretty sure that they must be a lot of conservatism in the casing thickness and design. What else can I do to show that the pressure vessels can work at higher operating pressure?

I would really appreciate the forumers input on this study. (sorry the question is long - I need to put as much info for people to understand the background). Cheers

[JoeWong]

Don't really encourage to extend your working and design envelop especially those "used" and "deteriorated"...May see if you can adopt recently modification of test pressure (1.3 instead of 1.5). Apart, may check any chances of Short term overpressure excursion...there are many operation and management requirements binding to the application...