Process Puzzler: Forego False Economies

A “penny wise, pound foolish” approach prompts plant problems

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This Month’s Puzzler

We needed to correct an unsafe practice in our blending area — using a forklift to raise and then hold a tote in place while we pump ingredients into our blending tanks (see drawing “a”). Corporate engineering proposed an automated design with six tanks with valves on the top of each tank (drawing “b”) that would cost about $49,000. I thought this was a dumb idea and one for which we’d never get funding anyway, so I had maintenance simplify the design (drawing “c”). We don’t need valves on top of the tank; we’ll control the weight added with a valve near the pump. Because we don’t need valves, we ran individual pipes to each blending tank. What does corporate engineering know anyway?

Unfortunately, we’re having trouble meeting the 1-lb accuracy needed. In addition, we can’t get all the ingredients out of the pipe. Our parking lot is filling up with product rejects. Many ingredients are in the 1–70-cP viscosity range. We tried pumping 990-cP liquid; even the air diaphragm pumps won’t handle it, so we’re hauling it up in buckets.

The corporate engineer says our system won’t work, but the CEO thought it showed initiative at solving the problem on the cheap. The corporate engineer also warns the PVC pipe won’t survive the 180°F water to be used next year for cleaning; his design has CPVC pipe. Is the corporate engineer correct? Did we waste our time?

Use A Frame

In the U.K., all totes (we call them IBCs — intermediate bulk containers) carrying liquids are designed specifically to be transported around using forklift trucks; they have slots underneath the framework for taking the forks of forklifts. Drivers must be appropriately trained in order to drive a forklift truck — this helps reduce the chance of human error. I can recall having seen liquid being drained from totes lifted by forklifts, but only very rarely. Most totes/IBCs are drained from a ground-level position or on top of one another, so they do not need to remain on the forks of a forklift truck.

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I would not advocate draining a tote while lifted high on a forklift truck — except perhaps for nonhazardous liquids. At a low height for hazardous liquids yes, but not from a [more-than-minimal] height — for safety reasons. Placing the tote at [the necessary] height on a frame regardless of the contents would be the safest approach.
Ron Davies, control engineer,
Thames Water, Reading, U.K.

Install End Valves

A pipe without a valve at the end will drain all its contents into the blending tank up to the first static height above the pump. Except for the inaccuracy of weighing, this isn’t a problem with the unsafe operation because the liquid forms a static column above the lip of the tank. This is an argument for making the line as small as possible. A 1-in. Sch-80 pipe will hold 0.037 gal/ft; a similar 2-in. pipe holds 0.153 gal/ft. Given the maximum extension of a forklift of about 14 ft and a run of perhaps 10 ft, that’s about 1.53 gallons draining into the blending tank. The vertical 14 ft won’t flow into the tank. Assuming the density of water, that’s 8.34 lb/gal or 12.76 lb/10-ft. That’s a problem.

Here’re some ideas for fixing this problem inexpensively: 1) install two valves per blending tank — a block valve and a vent/bleed; 2) add a centrally located return line with a hose connection — this will allow circulating solvent or hot water to clean the pipe in a loop back from a cleaning tote. It’s best to automate operation of the block valve rather than having an operator manipulate it. The bleed, upstream of the block valve, serves as an air vent to pump the ingredient back to the tote after ingredient addition. I recommend automating the whole process to improve accuracy. Based on my calculations, you’ll have to keep the temperature below 140°F because of manufacturer limits; CPVC would allow 200°F, hot enough to melt many waxes.

As for problems with high viscosity ingredients, I think you’re stuck. You will have to use two pumps in tandem — one on the floor near the tote and a booster pump on the mezzanine. Otherwise, you’ll have to consider something other than an air-operated diaphragm pump — perhaps a gear or lobe pump.

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