October Process Puzzler: Foil a Future Fire

Readers suggest ways to improve safety at a cylinder storage site. Also, December's Process Puzzler: Improving coagulant consistency.

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Bob Siml, process specialist,
S&B Engineers & Constructors, Houston

I once had a similar situation involving a liquid chlorine cylinder stored on the east side of a building in sunny Iowa.

We didn’t want to build a shed, as that would have made replacing empties hard because of the large size, 4 ft. dia. by 6 ft. We built a wooden cover that rested on top of the cylinder and allowed for air circulation; we painted the cover white. Over-heating was never a problem! A forklift was used to remove the cover when a replacement cylinder was installed. The facility needs shade for its cylinders and air circulation. Also, venting to a flare system would help.
Donald Phillips, engineer
Phillips Engineering, Melbourne, Fla.

The problem has two dimensions, temperature and density. As the temperature and density of the propane, a liquefied compressed gas, increase so does the resulting pressure. The normal propane DOT 4BW-240 cylinder, a cylinder with a test pressure of 480 psig and a rated pressure of 240 psig, is usually equipped with a pressure relief device set at 375 psig.
The permitted fill density for propane is found in 49 CFR 173.304a (d) and is 42% of the cylinder’s water capacity.

Current hazardous materials transportation shipping regulations permit the cylinder to be liquid full at 149°F (65°C) — the problem is you are storing the containers above that temperature.

So, reduce the package fill density [kg propane/L] by: a. understanding the variance associated with the internal volume of your package; b. understanding the variance associated with your filling process (scale, operator, etc.); and c. utilizing a 40% maximum filling density vs. the 42% maximum filling density.

Reduce the radiant heat gain by: a. storing the units on a lighter surface; b. covering the units from the radiant solar rays; c. ensuring that the cylinders are painted a light/reflective color (white or silver); and d. cooling the units for 5 min. every hour (between 10 a.m. and 6 p.m.) with a water spray.

Increasing the start to discharge pressure on the pressure relief device from 375 psig to 390 psig will do very little to help the problem.
William R. Fink, packaging engineer
Air Products and Chemicals, Trexlertown, Pa.

The problem statement did not indicate the number of cylinders that are
currently stored in the sun. The first step is to minimize the solar exposure, so install a roof over the storage area. If the area is so large that crane access is needed, build the roof with removable panels. The angle of the sun needs to be considered, so adequate overhang is provided to protect the bottles at the edge of the storage area. A less expensive solution would be to cover the cylinders with space blankets to deflect the solar radiation.
Darryl E. Laxo, consultant
Windsor, Calif.

When I worked with explosive materials in the military I noticed that everything was done to minimize temperature effects such as storage in bunkers and shading. Berms were built to minimize contagion and everything was grounded. A common mistake is to assume that placing a cylinder on concrete or asphalt is enough if the pad is grounded. This is not always safe. Cylinders should be grounded to racks. Racks should be grounded individually to a plate or cable connected to grid grounded to earth; soil conductivity should be confirmed during installation of the grid ― sand is less conductive than clay. Take care to avoid grounding to something that’s painted.

Nonconductive fluids such as hydrocarbons generate electricity as they flow. Grounding at the destination of the flow is critical to avoid a spark. Unfortunately, relief valves are a special case. Nothing is more dangerous than a hydrocarbon liquid atomized by a relief valve. Numerous sources suggest setting a maximum velocity of 15 ft./sec. for hydrocarbons. This is probably impractical for relief valves; however grounding of relief headers may help. These sources also suggest that the energy exchanged between atoms of escaping hydrocarbons may produce sufficient static electricity to cause an explosion in some cases. There may be little that can be done to prevent all fires with such gases as propane and propylene. Maybe locating the relief vents so that they don’t become blow torches when the relief valve lifts is the only sure option besides berms, shading and dispersing clusters of bottles.

Dirk Willard, process engineer
Citgo, Lemont, Ill.

We add a polymer agent as a coagulant to a cosmetic tank. During development, the ingredient was added by hand. In a large bulk tank, this is too inconvenient. So, we enlisted an open-impeller centrifugal pump to batch the agent to the bulk mixing tank, i.e., reactor. Now, however, the product, which should have a consistency of tapioca pudding, resembles runny soup. How should we investigate this problem and what could be the cause? How do you suggest we can improve the consistency?

Send us your comments, suggestions or solutions for this question by November 10, 2008. We’ll include as many of them as possible in the December 2008 issue and all on www.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 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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