January Process Puzzler: Sticky valve

Readers offer ideas for the cause of a sticky valve in Chemical Processing's monthly Process Puzzler feature.

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I also would check to make sure that the new valve is a like-for-like replacement of the old valve. In many instances, the group specifying and purchasing the valve aren’t the people that operate or maintain the valve. The new valve may be quite a bit different than the old valve and you should verify that the port size, trim, packing, and seals are all the same material as the old valve.

Brad Piggott, engineer
Air Liquide America, Houston

Review past history

I believe that one should look at the history of past failures which sometimes give enough clues to solve the problem if it isn’t altogether a new problem. In a well proven process, a new problem cropping up is rare. Start with the simple things. Tracing is provided to keep the line at certain minimum temperature to avoid solidification. There is a history of one tracing failure causing flow problem. It doesn’t seem logical that piping is a problem. It has worked without trouble for many years. I would pull the valve during the next scheduled maintenance and have it checked for internal troubles. The past history should be checked for similar failures at predetermined intervals.

Rashtriya- V.S.Nandekar, chief engineer
Chem. & Fertilizer Ltd., Mumbai, India

Could it be thermal expansion?

Perhaps the problem lies in different expansion rates of metal parts within the valve. The clue is the problem occurred before when the heat tracing failed. The external metal is cooling as the outside air gets colder. The internal fluid stays slightly warmer causing a differential. Increasing the insulation on the valve and checking the heat tracing effectiveness at the valve might solve their problem.

John Mattson, market manager
SACHEM, Austin, Texas

March's Puzzler

We are formulating a plan for collecting mercury from the blowdown in our scrubbers. Our on-site contractor has provided a basic design for consideration involving ion exchange (IX) and sulfide vacuum filtration (Figure 2). The proposal is for continuous removal of mercury using the maximum concentration range of 1.5-2.0 μg/l as a design basis (EPA studies). The proposal includes a reference to separation processes developed by the U.S. Department of Defense and Department of Energy to handle mercury waste. According to these studies, using sludges of heavy metals, a recovery of 99% can be assumed for the ion exchange. The regenerated waste produced by the IX unit will be treated with a sulfide and run through a vacuum filter: filtrate will then be reprocessed through the IX unit while sludge is collected for disposal. The vendor’s sales engineer claims that it may be possible to turn a profit by distilling mercury from the solid waste collected after dewatering. Are there any flaws in the proposal? How should we proceed?

Figure 2. Proposed process for treating Hg waste from power plant scrubber.
Figure 2. Proposed process for treating Hg waste from power plant scrubber. (Click to enlarge.)

Send us your comments, suggestions or solutions for this question by February 16, 2007. We’ll include as many of them as possible in the March 2007 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to ProcessPuzzler, Chemical Processing, 555 W. Pierce Rd., 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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