Clear Up a Compressor Conundrum

Readers decipher a compressor surge.

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Abhay Singh Gour, process control engineer
N.I.T., Trichy, India

LOOK AT CAUSE OF POWER DEMAND
A motor power surge is the result of power demand from the compressor. This
could be due to:

The packing could be “floating,” or expanding, because of gas upward flow. Packing demands a minimum upward flow of gas [i.e., saltation velocity], below this minimum gas flow it tends to “settle” and add resistance or pressure drop to the upward gas flow. This resistance could increase the discharge pressure and consequently delay (say, by 1 second) opening of the recycle valve. If real time trend analysis is available in the distributed control system, check the trends for current, absorber differential temperature and flow across the compressor. You will also want to identify which parameter moved first and how they are related to each other.
Another possible cause could be the unusual pressure drops in the system. Inspect the check valve at the compressor discharge or at the inlet of the absorber and verify the movement of its spindle. The movement may be restricted, sometimes by residual materials left in the piping during replacement. Test or measure the pressure drop across the absorber at 100% load before and after the packing change-over.
Investigating these two causes can serve as a starting point.
Anirudh Rajendra Kumar, chief process engineer
Viva Methanol LFTZ Enterprise, Eurochem Corp., Lagos, Nigeria

IMPLEMENT CASCADE CONTROL
Are the pressure and flow control tracking one another? You might try cascading the pressure and flow with pressure being the primary. The flow should false load during startup and upset condition to avoid flow changes and compressor surging.
Porter Yarbrough, engineer
Bayer MaterialScience, LLC, Pittsburgh

APRIL’S PUZZLER
We’ve just discovered what appears to be a design flaw during the first hazard and operability study involving solvent extraction evaporators. The evaporation system is a three-effect backward-feed design using long-tube vertical evaporators. The viscosities should be moderate, i.e., below 100 cP; there’s only a slight possibility of solid build-up unless the bulk temperature falls below 100°F. With backward feed, 150-psig steam enters the first unit as the feed while a dilute concentration of ferric chloride and hydrochloric acid enters the third unit. The product, a concentrated aqueous solution of ferric chloride, exits the first effect. It’s passed with the evaporated steam from the first unit to the second and then to the third, where it’s condensed and decanted after the final effect. The problem is the level control and pump cavitation. Operating at the design capacity of 50 gpm, the pump can empty the reservoir, i.e., the receiver, below the pump in less than a minute. This problem is most severe during startup. At this stage of the design what can we do to make this process operate more smoothly? Unfortunately, the evaporators, which require expensive materials, already have been ordered.
Send us your comments, suggestions or solutions for this question by May 8, 2009. We’ll include as many of them as possible in the June 2009 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.

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