×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Compressor start up from settle-out pressure using VFD

Compressor start up from settle-out pressure using VFD

Compressor start up from settle-out pressure using VFD

(OP)
Recently we had a big problem with a large compressor (27MW)start up using VFD. The issue is that the vendor sized VFD capacity is too small to start the centrifugal compressor from settle-out pressure. Several options were proposed including:
 1. Change to a larger VFD, which means a four-million dollars to be added and also a long time for delivery. Since the VFD is customized, the current VFD could not be used unless in the future there is a similar motor+compressor could match.
 2. Blowdown the process gas from settle-out to lower pressure so that the current undersized VFD could start up. It is estimated a two to four thousands of dollars to be lost on every start up since the vented the gas (refrigerant)is very expensive.
 3. Still use the current undersized VFD, the vendor will pay the cost of each start up for 10 years, which is roughly about 2-4 million dollars.
 4. Recovery the vented process gas from option 2 by using a recip compressor and a drum to the refrigeration loop. This is another two million dollar cost since of the recip compressor and drum and additional piping.
 5. Revise the current piping layout and adding drums etc to the compressor inner-connection piping volume so that the settle-out pressure could be reduced. This is not quite sure feasible, and still costly and time consuming since some part the piping has been done on site.
  
  Each option mentioned above looks ridiculous from the user's standpoint.

  I am thing a option 6. I don't know if it feasible.  

(Note the compressor is two-stage centrifugal compressor with inter-stage and post-stage air coolers )

My idea is:
  
  Add a control valve between the compressor 1st stage and 2nd stage. In this case, when the compressor is stopped, there will be two settle-out pressures before and after the control valve. The settle-out pressure in the first stage is lower and in the second stage is larger. I am guessing this may reduce the overall compressor start-up torque, and hence require lower VFD capacity. The problem is how I can estimate the start-up torque and power consumption in this scenario. Is there any problem if I add a control(block) valve between the 1st stage and 2nd stage?
 
  Does anyone have any ideas or insight? Thanks a lot!
 

RE: Compressor start up from settle-out pressure using VFD

Your option 6 : flow will cirrculate in the balance line for interstage  and end balance pistion that will reequilibrate pressures between both stages.

Are you able to throttle during start-up?
 

RE: Compressor start up from settle-out pressure using VFD

blow off to a storage tank, evacuate tamk after start up for the next start up???

RE: Compressor start up from settle-out pressure using VFD

Well, it seems that an improperly sized VFD relates to an electrical power matter; meaning too much power consumed by VFD during a start-up condition with "settled-out" pressure.  Electrical power consumed by a motor driven centrifugal compressor is largely dependent upon massflow rate, developed head, and moleweight of gas.

Lowering any of the these values will decrease power consumption during start-up.  However, decreasing massflow rate (thus volumetric flow rate) can be risky as sufficient flow must be maintained to avoid a centrifugal compressor surge condition.

Decreasing the gas moleweight means blending &/or purging of lower moleweight gas after start-up.  Without knowing system design, difficult to advise on.  Also, blending lighter moleweight gases with refrigerants does not bode well on the process.

Decreasing the head is difficult to do as this mainly related to # of impellers, impeller diameter, compressor geometry, and gas moleweight.

Hope this gives you some ideas to think and discuss among those involved.

Good luck!
-pmover

RE: Compressor start up from settle-out pressure using VFD

Instead of a CV maybe just a check valev? I know that they are not gas tight and that pressure over time will settle out - but that will take some time - and thus you wont have to blow down every time?

A CV will have to respond very quickly and will cause a permanent pressure drop (larger than for a check valve anyway.

Bets regards

Morten

RE: Compressor start up from settle-out pressure using VFD

Another idea...
why not try a start-up valve it includes a sophisticated control system that will allow a start up along the compressor Control Line?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources