February Process Puzzler: Isolate Electrical Incidents

Readers unravel a wiring riddle.

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Update the drawings, especially the single line diagram. Section 205.2 of the National Fire Prevention Association’s “Standard for Electrical Safety in the Workplace,” NFPA 70E, is a general maintenance requirement that “a single line diagram, where provided for the electrical system, shall be maintained.” If you can’t justify producing the drawings based on plant uptime, your emergency health and safety (EHS) people might require, and help pay for, producing the drawings to satisfy the refinery’s electrical safety program.

In the future, specify a control circuit transformer in each of the MCC buckets for a motor starter or contactor. Consider also specifying an interposing relay for remote control. The event could have been caused by disruption of common control circuit power for several motors. While the transformers and relays cost a little more, they isolate the control circuit for each bucket.
Jeff M. Goldsmith, engineer
GE Water & Process Technologies, Watertown, Mass.

Even though an MCC bucket may be limited to energizing one motor, auxiliary
contacts wired through the disconnect or starter circuit may be used in an
interlock scheme with other motors or control logic. MCC wiring diagrams usually show only 480-VAC wiring schemes, not 120-VAC or 24-VDC control circuits, which are typically shown on loop diagrams.
To avoid this problem in the future, before de-energizing a motor circuit: 1) visually inspect the motor control circuits to identify any 120-VAC or 24-VDC control wiring that exits the bucket which may be used in an interlock circuit; 2) scan the process control system (DCS or programmable logic controller) inputs and outputs to identify any motor I/O; identify any logic schemes where the motor I/O may be used, especially where it may be used in the run logic for other motors; and 3) whenever maintenance or other shutdown activities are executed, take a little extra time to verify the MCC wiring diagrams and to sketch motor control loop diagrams.
Scott Sommer, automation technology manager
Jacobs Engineering Group, Conshohocken, Pa.


This is the kind of incident OSHA gets curious about. It definitely counts as a near miss. I suppose no loop sheets are better than faulty ones! There’s no magical solution here, just tedium. During the next refinery turnaround, plan for at least two to three days of tracing 120-VAC wiring and DCS cables. One thing for sure, any turnaround work involving the crude unit pumps, and perhaps the whole unit, has probably been put on hold.

This may not be a problem that can be taken care of all at once. The best you can do is to trace the wiring and prepare a contingency plan for each solution. You may have to install new boxes and relays, which in today’s world means getting approvals for emergency management of change (MOC) orders. You’ll need to find a contractor that understands your DCS and can call in extra help if needed. This probably means a union shop.

Once you’ve solved the problem for the crude unit, it may be a solution required for other units in the refinery. Then, you have more tedium ahead as you inspect MCC throughout the refinery. Before a repeat incident, send out a notice to all units to watch for the same situation — this solution you can apply immediately.

I think, though I am uncertain, that you must notify OSHA that you have a potential safety issue. Obviously, the corporate lawyers and public relations people will want to talk about it first. Your headaches have just begun!
Dirk Willard, process engineer
CITGO Petroleum Corp., Lemont, Ill.

Our unit superintendent is concerned about a multistage centrifugal gas compressor. It’s fed by a separator and discharges to an absorber; there’s a pressure control valve at the compressor suction and a flow control valve in a recycle line from the compressor discharge. Ever since the first startup after the last turnaround, the motor current unexpectedly surges during startups and other upset conditions before settling into generally steady service. The packing in the absorber was replaced during that turnaround. What could be causing the problem? Should the superintendent be concerned? Can we do anything so we can safely limp through until next year’s turnaround?

Send us your comments, suggestions or solutions by March 13, 2009. We’ll include as many of them as possible in the April 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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