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February 1, 2001 Issue


February 15, 2001 Issue




February 1, 2001

Fire Extinguishers

By Dave Carnell

For at least twenty years I have carried a 5-B,C dry powder extinguisher in my car on the floor under the driver's seat. The same extinguisher has passed through a half dozen cars. A few weeks ago I opened the rear door on the driver's side and saw a pile of white powder on the carpet under the seat. Rolling back and forth under the seat the trigger handle was getting hit so that it began to unscrew to the point of discharging the gas and releasing the powder around the valve. The valve was screwed completely out, but the stuck pressure gauge still indicated a full charge. Time to review the basics of fire extinguishers.

I have put out a lot of fires with hand-held extinguishers, both real and practice, in my career in chemical research and development. Up until after World War II, we used carbon dioxide C02 which is effective against both Class B (solvent, gasoline, other flammable organic materials) and Class C (electrical) fires. Because C02 is a gas, you had to get right up to the fire and hold the nozzle practically into the base of the flame; too close, and you could blow the fire around, and the fires often restarted. When dry-chemical extinguishers came out, they made easy work of putting out fires.

You could stand off a comfortably safe distance and lay the powder right on the burning material, and the fires stayed out. One afternoon I walked into a semiworks just as a 55-gallon drum of benzene solution over-flowed and caught fire. This was just like a gasoline fire with flames two stories high immediately. I grabbed a 15-pound dry-chemical extinguisher from the wall and knocked the fire down in a few seconds. I put dry-powder extinguishers in my boat, car, shop, and kitchen. I had fires in the engine compartment of my VW Microbus and a backfired oil burner in a utility room that I handled easily.

Some years later, the A:B:C-rated dry-chemical extinguishers came out. These looked like a decided improvement, although their capacity for Class A fires (paper, wood, trash, etc.) is small. The numbers in the ratings refer to the size of a standard test fire that the extinguisher can control; a #1 fire is only a tenth the size of a #10 fire.

But then in 1985, 1 heard the owner of a safety equipment company say that he did not recommend the A:B:C dry-chemical extinguishers because after they were used on a fire, you could expect delayed failures of electrical and electronic equipment in the area, but which had not been involved in the fire. He cited radios, TVs, appliances, power outlets, but he did not give an explanation why the A:B:C extinguishers caused these problems and the B:C extinguishers did not.

I did some thinking about it. The B:C extinguishers use sodium bicarbonate, a powder which is quite inert. The A:B:C extinguishers use monoammonium phosphate. This powder is acidic (corrosive), contains the ammonium ion which is hard on copper alloys, especially, and when heated is apt to vaporize ammonium compounds, which can then condense in cooler areas. Even if it does not condense on a surface, the particles are so extremely small that they will drift and settle some distance away.

I wrote to Factory Mutual Research, Underwriters Laboratories, and the Coast Guard, agencies which test extinguishers, and to Walter Kidde, a prominent manufacturer of extinguishers, and asked for their comments on my reasoning and on possible problems with A:B:C extinguishers.

I never heard from the Coast Guard, but both Factory Mutual and Underwriters Laboratories confirmed that there are corrosion and cleanup problems with the A:B:C (monoammonium phosphate) extinguishers that do not exist with the B:C (sodium bicar-bonate) extinguishers. Walter Kidde agreed that the monoammonium phosphate can be more corrosive.

Factory Mutual's Loss Prevention Data, Section 4-5, "Portable Extinguishers", contains this paragraph on multipurpose-type dry chemical (A:B:C, monoammonium phosphate): "The multipurpose-type dry chemical forms a soft sticky mass when heated and clings to hot surfaces when they cool. Consequently it usualy cannot be brushed or blown from surfaces as sodium bicarbonate and potassium bicarbonate-base dry chemicals often can, particularly from metallic surfaces. Therefore, it is not recommended for areas such as textile card rooms or any other locations where many fine machine parts may require individual cleaning after a fire. Multipurpose-type dry chemical, in combination with moisture, can corrode copper and copper-alloy material."

My experience with cleaning up bicarbonate powder in laboratory and semiworks and at home is that blowing equipment clean and vacuuming up the powder is easily done.

I threw away the A:B:C extinguishers in my boats, cars, shop, and kitchen and bought iew B:C extinguishers. A pail of water, or a seltzer bottle will do a better job on a Class A fire than a puny I-A 5-BC extinguisher.

How do you know if a dry-chemical extinguisher is ready for action? The gauge on the handle of my extinguisher completely detached from the cylinder still showed that the pressure was right where it should be because the gauge mechanism was rusted tight. The new extinguishers now say "watertight, stainless steel guages". But how long will a gauge that sits there always showing the same pressure stay operative. I don't know. We weighed the old carbon dioxide extinguishers to check that they were full, but the amount of gas in a dry chemical extinguisher is not practicably weighable. For a while the extinguishers were made with a small button you could press and get a reassuring burst of gas, but each time it was tested that way part of the gas was lost; obviously not a good situation.

We have wasps who build solid mud nests in small openings such as fire extinguisher nozzles. I found three of the four extinguishers in my shop and boat plugged solid with wasp nests. After I drilled them clear I put loose-fitting caps (lipstick tops) over the extinguisher nozzles. Don't tape them on and be sure they fit loosely enough to fall off or blow off without any resistance.



February 15, 2001

A $10 - 10 Minute Steam Box

By Jack Dice

I needed to bend some wood the other day. I was installing new gunwhales and inwales on a fiberglass skiff (an old "Plastic Pete's") that was molded with a rounded bow as many of those fiberglass skiffs are. The original wood had rotted away after one of those nice canvas covered town rub rails had been installed, collected water and never dried out. I was loath to build a real steam box just for the time it would have taken so I came up with this quick steam box, and it only took a few minutes and cost only about $10.

Buy a 10' section of 4" diameter Schedule 20 PVC pipe at your building supply yard. This pipe is used for foundation perimeter drains in the building industry. The pipe I got is distributed by Genova, is made by ADS, and is labeled "30001b Triple Wall". The wall construction of this pipe is corrugated just like a cardboard box but is made from PVC (polyvinyl chloride). It will easily withstand the temperature of boiling water and steam at ambient pressure. Not only is it light but it has the great advantage of having a high insulating value. This is desirable because the steam will not condense as readily on the walls of the pipe and lose its effectiveness at steaming the wood.

Avoid buying Schedule 20 or 40 PVC pipe with a solid wall as it will not give you as effective an insulating effect. Buy a couple ofend caps that fit over the ends of the pipe to close it off. The pipe that I purchased had a coupling molded onto one end that took a special cap on that end. As a cautionary note, you should never bum PVC because it gives off toxic gasses when ignited.

Now all that is needed is a boiling vessel, a piece of old garden hose (or suitable substitute) and a camp stove. The boiling vessel can be anything that will hold and boil a couple gallons of water. I used an old five gallon galvanized can that I found at the junk yard that might have been used for kerosene or other solvent. An old metal gas can will work but be sure to rinse out all the gasoline before putting it over a fire! A large sauce pan or dutch oven would work as well. If you have none of these you can buy a cheep aluminum pot with cover for only a few bucks.

Drill a hole in the cover that is slightly smaller than the garden hose outside diameter so that the hose can be stuffed into it and seal itself in the hole. Drill the same diameter hole in one of the end caps near the edge of the cap so that the condensate will not form a puddle in the pipe and will run back into the boiling vessel. The cap should fit tightly over the PVC pipe. If a tight fit cannot be had, use duct tape to seal it. Tie the PVC pipe on a stiffening board and put it on some saw horses or anything to elevate it higher than the boiling pot with the far end of the pipe slightly higher than the inlet end so that the condensate will run back toward the supply hose and drain back into the boiling vessel.

Install the hose (make it only long enough to go from the boiling pot to the steaming pipe and no longer) from the boiling vessel into the PVC pipe end cap, put a couple gallons of water in the pot, and put the pot on your camp stove. In the opposite end of the PVC pipe put in the wood to be steamed and then install the end cap. If that cap is tight fitting, drill a couple of small holes in it (.25 inch) so that the steam will migrate down to the far end of the pipe and vent out.

I steamed two pieces of white oak 1/2"x 1"x 8' about an hour and they bent around the bow of that skiff not quite like limp spaghetti but close to it. Amazing!

Jack Dice, Oceanville Boat Shop, RRI, Box 864, Stonington, ME 04681, <oceanvilleboat@gwi.net>

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