This Month’s Puzzler
We had a fire and resulting explosion in our gas storage facility. The fire turned out to be caused by relief valves popping off on individual liquefied-propane bottles stored in the sun on black top. The bottles are standard high pressure ones equipped with simple safety-relief valves vented to the atmosphere. They typically are stored in steel pallet racks side-by-side to maximize storage space. Our manager demands that we redesign the facility so that such an incident can never happen again. How can we change the layout and operating procedures of the area to reduce the risk to an absolute minimum?
TAKE SEVEN STEPS
Large fires and explosions in cylinder storage facilities occur because the flammable gas venting from relief valves can catch fire and rapidly overheat nearby cylinders.
It only takes a temperature of approximately 125°F to lift the typical relief valve set at 250 psig. In addition, the springs in older relief valves can fatigue, causing the relief valves to leak prematurely. Another concern is overfilling the cylinders can cause relief valves to open much sooner than expected because the liquid hydraulically overfills the cylinder as the temperature rises.
Measures to limit the temperature are critical. However, without an industry-wide change in cylinder design pressures there will always be the potential for fire caused by a small leak to cascade through a large group of cylinders. I propose the following countermeasures:
1. Ensure cylinders are painted white.
2. Compartmentalize storage.
3. Improve maintenance and inspection.
4. Provide covered storage.
5. Reduce inventory.
6. Install a deluge sprinkler system ― and inspect it.
7. Change the site culture.
The danger of overheating is not just from sitting on blacktop. It also extends to cylinders sitting in trucks waiting for delivery. The single most effective measure is to ensure the cylinders are painted white. There are low absorption coatings such as roof paints that keep a roof cool enough to touch in the middle of a summer day. The surface of the blacktop should also be changed by applying roof paint, dusting with concrete, etc.
The greatest risk from a small fire is numerous cylinders relieving with a high potential for causing an explosion. It is mandatory that bays be designed for pallets and separated with fire barriers like concrete walls to keep a fire from spreading. Two ends of the bays should be left off to allow for circulation and keep flammable vapors from collecting.
The manufacturers of small inexpensive relief valves often recommend outright replacing the valves on a periodic schedule such as 5 years instead of trying to test or overhaul the valves. New valves may leak if they are pop-tested. A method to ensure new valves are set properly and don’t leak is to test on air at 90% of set pressure before placing them in service. In addition overfilling can be prevented with internal float valves, weighing, etc.
Covering the storage area would shield the cylinders from the sun. There should be adequate overhead space to ensure good ventilation to keep the air temperature down and allow any vapors to dissipate.
Consider reducing the number of cylinders kept on hand and changing to bulk storage, if practical. This would significantly reduce the number of potential leak points and the potential for problems caused by a small fire to cascade.
A sprinkler system could significantly reduce this fire risk. The only downside is that if a cylinder “explodes” due to localized fire impingement the sprinkler system can be damaged. Periodic inspection assures that the deluge system improves as accidental fires are better understood.
The manager never wants this to happen again. However, there is the inevitable pressure to minimize costs. The company can weigh the costs and associated risk reduction. If it is serious then it should do the majority of these items ― not the minimum.
Bob Siml, process specialist,
S&B Engineers & Constructors, Houston
PAINT IT WHITE
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.
LOWER THE FILL DENSITY AND TEMPERATURE
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.
CONSIDER SPACE BLANKETS
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
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.