This Month’s Puzzler
At our plant, which manufactures a powder, we are suffering sudden problems with a pulsed-air cartridge dust collector. The unit, installed in 1978, is 20-ft tall and has an inside diameter of 6 ft and a 35° cone bottom. It has run flawlessly until the last month. The collector is downstream from a hammer mill in another unit. The mill is having a little trouble with wetting but otherwise runs like a Swiss watch. I’ve been asked to install an inspection platform in the collector and to resolve three reliability issues: 1) the external cartridge bags are wetted but only on one side; 2) like most dusts, ours is combustible, and collects up to 6-in. deep on some of the supports inside the collector; and 3) the wall thickness is only 10-gauge stainless steel.
What should I include in my inspection report?
For any piece of equipment that long has run well and then suddenly doesn’t, I suggest starting your troubleshooting by asking what has changed upstream. You may not need to alter the downstream equipment if the change is temporary, seasonal, or its effects are minor.
I wouldn’t assume just yet that all is well with the hammer mill. Hammer mills are fairly dynamic; when they do quit from build-up, it could be catastrophic. I would look at this upstream issue immediately.
As for the question of why the bags are wetting only on one side, you need to look at gas distribution. Something may have changed or gone unnoticed. There could be new blockages in the gas distribution plate, a reduction in the blower capacity restricting flow, or it could be the pulse jets.
Many years ago, I discovered choked flow in a plant’s compressed air system. It first showed up in blinded cartridges in a dust collector. As I recall, the blinding was worse on the side farthest away from the air line header. Look at this carefully.
As for the inspection platform, this sounds like a lingering item on somebody’s wish list. Ask maintenance how they grab the bags now. Chances are they don’t need a platform.
If do you have to install the platform, the thin-gauge tank shell poses a challenge. In addition, you face risks from welding and cutting inside a tank that works with a combustible dust. I suggest welding a ring or donut outside of the tank and bolting together the structure of the platform inside the tank. If not, the platform structure will punch through the tank wall like a screw driver through a soda can. I suggest using about 25 lb/ft2 with a 250-lb central load for a design basis. However, this is not a structural engineer’s opinion; it may be possible to get away with only drilled holes and a completely bolted-in platform.
The platform will provide more surface for dust to collect. Go with polished roundstock for supports; square tubing is a bad idea because it collects dust no matter how much it’s polished. There may be specialized grating that could be used that won’t collect dust. Instead, though, I would plan for sections that can be removed after the work is done inside the collector. Grating will collect a lot of dangerous dust; structure, especially round-stock tubing, won’t collect much dust. Besides, it will be much easier to clean the outside of a tank for welding than to clean the inside.
If operations can’t live without the grating, then make sure workers entering the dust collector wear electrostatic dissipating shoes, use non-sparking tools — usually bronze, and delicately remove as much collected dust as practical before entering the vessel. Also, if there’s a way to humidify the air in the tank without making work difficult, do so. Ideally, you want avoid working on tanks during the winter when air is especially dry.
Dirk Willard, consultant
We migrated our distributed control system (DCS) last year as part of a major expansion. Our old system, purchased for $221,000 in 1995, handled about 148 analog points and about 272 discrete points. The new system runs about 201 analog points and 346 discrete points; we originally planned for 192 analog points and 320 discrete points — and allowed for 20% extra capacity. We used a rule-of-thumb of $500/point, which our consultant said was conservative. We spent $478,000; our budget was $256,000. Management isn’t happy with the cost but they are thrilled with the performance of the DCS.
Accounting criticized the project team for eating the contingency for the expansion. Then, they asked us why the project was designed for 20% excess capacity and why we didn’t plan better since we used 7% more capacity than expected. Lastly, corporate grumbled that we destroyed the rate of return for the expansion.
Meanwhile, production complained to corporate management that we constrained them on points. We replied that we couldn’t fit more racks in the electrical room and this caused a lot of the extra cost because the old boxes couldn’t be re-used; we had to expedite getting replacements to complete the project. Our DCS is in the center of a hazardous area close to the operations.
What do you think we did wrong? Is there a better way to plan a DCS project?
Send us your comments, suggestions or solutions for this question by June 15, 2018. We’ll include as many of them as possible in the July 2018 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.