October Process Puzzler: Foil a Future Fire

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.