5 replies to this topic

[vnpetroleum]

Dear All,

There are some concerns relating to set the low pressure trip of compressor discharge needs your help.

As I know, PZLL for discharge pressure is set to,
- Prevent HC release in case of downstream rupture
- Detect BDVs/ Maintenance valve left open
- Keep compressor away from stonewall region

We operate the gas plant with two centrifugal compressors in parallel. For initial years, when the reservoir is healthy, the suction pressure is about 100 barg, discharge pressure 160 barg. PZLL was set 115 barg. Settle out pressure in case of two XDVs closed is 140 barg.

However, the current status shows that the suction pressure decreases to 70 barg due to reservoir pressure decline, some times when we want to run a compressor in UNLOAD mode, sette-out pressure is lower than 115 barg (existing setting) --> PZLL signal activated --> trip the compressor.

=>>> There is a request to revise the PZLL setting for discharge.

Please help to define what are the principles of setting PZLL. Is there any spreadsheet to do that?

Thanks - vnpetroleum

Edited by vnpetroleum, 08 May 2012 - 02:33 AM.

[jrtailor09]

Vnpetroleum,

As per data provided above i an see that there is lot change in suction pressure almost 30 %.

Do u you have original design compressor curve? I will advise you to consult with the original equipment supplier for the revised condiion. You may required to change the rotor of the compressor to take care of revised suction pressure.

Revised set pressure means you to look in other auxiliarries system of compressor also like lube oil sytem, seal gas etc.

Regards,

Jatin

[Art Montemayor]

I believe Jatin is exact and correct in pin-pointing that it is the SUCTION pressure condition that is of interest and concern - and not the discharge pressure.

One of the outstanding negative features of a centrifugal compressor - and one that should always be considered in the original design and selection stage - is the limiting, narrow range of suction conditions (and capacity) that this type of compressor can tolerate. Unlike a positive displacement device, it is very sensitive to the suction conditions and cannot tolerate much deviation. Jatin is exactly correct: the service conditions have changed - drastically - and the original designer / manufacturer should be consulted on the new, existing conditions being imposed on this machine. That is the best and smartest way to approach a reasonable and safe solution to a pending production problem.

[Zauberberg]

You've got good advices on how to proceed further. We need to take into account that all settings around the compressor have been configured for the original set of process conditions which were changed due to depletion of the reservoir by time.

You can look at Control Narratives and Compressor Databooks/Manuals to see what is the basis for setting PZLL value at 115 barg for original process conditions. Similar logic can probably be applied for evaluating a new PZLL value. As Art has correctly pointed out, you definitely want to consult the OEM and seek for their advice.

[vnpetroleum]

Sorry guys for my late response. I have a mistake in typing the current pressure of 70 barg. Now it is about 80-85 barg. 70 barg is the minimum suction pressure to need re-wheel the compressor.
Dear Jatin, compressor curve for your reference
Dear Art and Zauc, do you mean I need to contact with vendor to revise the PZLL for discharge pressure. Do you know the basic to calculate PZLL for that value?

Attached Files

•  Compressor Curve.doc   759.5KB   26 downloads

Edited by vnpetroleum, 16 May 2012 - 02:02 AM.

[Zauberberg]

From what I understood in your post, your original process conditions have been changed. This is, just by itself, a sufficient reason to contact the vendor and obtain their approval for the new PZLL settings.

If there is no feedback from the vendor, you can contact the engineering company who have done the EPCC work and ask them how did they arrive to the previous PZLL value, and how to obtain the new one. This will probably cost you some money.