Control the Skids

Essential information is needed to keep a project from going off track.

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In addition to venting, you may need N2 blanketing to reduce fragrance losses and odors. An activated carbon bed likely will be required for the conservation vent. Typically, these vents are sized at a 1–2.5 psig set pressure with a flow of 8.021 times the tank pump-out rate in gpm: for 2 gpm withdrawal, that’s 16.04 std ft3/h of air (60°F, 1 atm) that must be replaced by N2. The lower the pressure setting, the lower the loss of N2 to the conservation vent. Here’s a useful white paper: http://goo.gl/eqPPgR.

Although the fragrances typically will be Class III rather than Class II under NFPA-30, there is a risk if they are heated, which could occur after the static mixer. OSHA 1910.106(a)(19)(iii) states that Class II or Class III liquids — these normally are ignored in Electrical Area Classification (EAC) studies — must be treated as Class I liquids, i.e., they may not be ignored, if heated within 30°F of their flash points.

Another problem is that fragrances are known to swell many commonly used gasket materials such as EPDM and some fluoroelastomers but not PTFE. It provides a good static seal against this type of material but performs poorly as a dynamic seal, like a valve stem seal. Limited materials are available because the gasket must meet stringent FDA requirements to ensure that gasket components don’t leach into foods and drugs. Some of the newer soft perfluoro-polymers perform well, and are FDA-approved, but may incur additional cost because valve and equipment manufacturers don’t have dies cut for them.

Lastly, remember the fragrance is a non-conducting organic. Everything must be bonded (connected together) and grounded after the fragrance is added to the product. Otherwise, you risk operators being shocked and a fire.

There’s another risk we haven’t considered: the powder. It poses an obvious hazard in handling and an insidious risk in mixing in the reactor. Organic powders generally can be classified under NEC-500 as Group G: classification depends on enclosure and housekeeping. Refer to NFPA-499, “Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.” The insidious risk comes from the potential vapor pressure produced by dissolving the powder in the reactor. Thanks to “N/A” on the MSDS, you’ll have a real challenge with that one. My suggestion is to keep the reactor and other tanks under vacuum — but take care where you vent them and how you handle activated carbon beds during disposal.
Dirk Willard, consultant
Wooster, Ohio

June’s Puzzler

Our makeup-air heat exchangers seem to have suffered ruptured tubes. On the shell side, we use 45-psig steam reduced from 200-psi boilers. We send 40% propylene glycol/water through the tubes; at the steam control valve inlet with a 50% load it’s 42 psig. The exchangers are horizontal U-tubes with ¾-in. × 0.049-in. (16 BWG) copper tubes. The tubes were rolled into a 316L stainless steel tube sheet and sealed with fluorocarbon gaskets. Both exchangers are 24-in. diameter and 6-ft long. The exchangers are about 50% oversized sometimes while running about 105% during the winter months. We run one at a time. Another concern is that the tube-side relief valve was sized using the old 77% rule — is that okay? I think a classic case of water hammer causes the crushing of a number of tubes at the top of the tube bundle that we see. When we operate at only 50%, there’s a thermal reservoir in the shell that pulls a vacuum; another engineer believes that’s the culprit. What do you think?

Send us your comments, suggestions or solutions for this question by May 15, 2015. We’ll include as many of them as possible in the June 2015 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, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. 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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