Fire Boats & Relief Valves
Vol.20 No. 5
IFW has focused for 21 years on industrial fire and emergency response management challenges to see if we area missing any messages. In this issue two key messages kept coming through. One is to adapt existing technology for better performance in today's fire world. The other is to know when to mount an engine response and when to protect the perimeter risks instead. I'll set your mind to thinking and hope you'll find the jewels in the stories we've learned from our search for solutions in this issue.
In the United States, more than half of our ports do not have fire boats. This creates an enormous challenge for fire and emergency response personnel to reach a fire aboard a ship at the dock or, worse, away from the dock. In 1970, New York City had 10 operational fire boats. Today, the same port functions with only two boats on duty and another two in reserve.
When you have two ships collide in the middle of the harbor or a tanker burning from end to end, there is no substitute for the classic fire boat able to pump 20,000 gallons a minute. But big is not the only way to go. Fire boats come in all shapes and sizes starting at 20-foot models with small pumps. These boats can be very effective for small spills and small ship fires. They can even be used effectively to pump water for fire operations ashore along the harbor.
These smaller functional boats fulfill the broader needs of emergency responders today. Being a fire boat alone just isn't enough. A emergency vessel must also be able to handle security, EMS and water rescue. These smaller boats may not be the vessel you need when that 800-foot tanker starts burning but for a wide range of emergencies it may be just what the doctor ordered. An innovative boat manufacturer has captured the changing boat needs in their product line. You can read about them on page 19.
On the scarier side, several stories focus on recent fires involving pressure vessels and atmopheric storage containers. In the early 1970s the acronym BLEVE (boiling liquid expanding vapor explosion) did not exist. That doesn't mean they didn't happen. Fatal tankcar explosions occurred in Houston, TX, in 1971 and Kingman, AZ, in 1973. The Kingman explosion alone took the lives of 12 firefighters. Added to the list were several pressurized propane tank failures.
With a little research, we soon had a name to go along with this destructive phenomenon. A BLEVE is basically a catastrophic rupture of a pressure vessel. Say the magic word anywhere in the world today and everyone instantly knows what you're talking about. But just because we have a word for it doesn't mean people understand what leads up to it.
Take a 55-gallon drum, fill it half full of water and build a fire around it. If the drum is sealed it will BLEVE with substantial danger from the flying pieces. If you do the same thing using a drum half filled with gasoline the result will be breathtaking, to say the least. So what makes a massive storage tank with its contents at atmospheric pressure any different from that drum?
Some people will tell you that storage tanks have atmospheric vents. If you build up pressure, the vent will relieve it and keep the tank from rupturing. That is profoundly wrong. The vent is not designed to contain the pressure or relieve it in the event of a major impinging fire on the vessel. If you are dealing with a product such as diesel an impingement in the vapor space can cause the vessel to catastrophically fail from an internal vapor/air explosion. If you have flammable vapor, oxygen and heat from impinging flames you have everything you need to go ka-boom. Red hot metal that is 700 to 1,000 degrees F in temperature is far hotter than the 400-to-500 degrees F needed to ignite gasoline or diesel fumes. That results in the kind of spectacular explosions that enveloped a Fort Worth chemical plant in July. (See page 6)
Another aspect of BLEVEs we need to understand comes to us by way of the recent fire and explosion at a St. Louis facility for repackaging pressurized commercial gas. (See page 10) Those who saw the live coverage on television will remember the sight of pressured vessels rocketing through the sky. Any vessel has a top and a bottom. What people don't understand is the bottom may blow out long before the top does. These tanks do not have a weak roof with a shell flangible seam as many people believe.
Those types of tops work by having the top 'dome up.' The problem is that any tank under 60 feet in diameter does not have enough top surface to dome up before blowing the top off. Instead, the bottom may blow out. The result is 10- and 12-foot diameter tanks being launched like missiles.
True, pressurized vessels such as propane bullets and acetylene cylinders do come with relief valves of a sort. Acetylene cylinders have a disk in the bottom that blows out at a specific pressure. Like bigger pressure vessels, these cylinders will fail if impinging fire heats the metal and causes a red hot point of impingment. Any cylinder or tank can and will often fail without the relief valve ever opening.
Technicially, the vessel does not fail from an internal over pressure. Take a propane container pressurized to 200 psi. The relief valve is set to release at 250 psi. The problem is that if impinging flames heat the metal above the liquid level the hot spot will bulge like a blister. When the metal is stretched thin enough it tears, causing a BLEVE. The pressure was never sufficient to trigger the relief valve. Also, once a relief valve opens there is no guarantee that internal pressure is not building up faster than the valve's ability to relieve it.
That very scenario killed two Iowa firefighters only a few years ago. They had been taught that if the relief valve is closed the vessel is safe. It is not shamefull to walk away from one when you can not change the outcome positively. That means taking home everyone you took to the fire. Vapor space dictates when a cylinder will BLEVE, not the relief valve. o