Sciencemadness Discussion Board

DIY Flashback Arrestor

Panache - 18-7-2010 at 01:15

Has anyone made a flashback arrestor that can connect to ground glass, how was it tested for efficacy?
Cheers!

watson.fawkes - 18-7-2010 at 07:33

Quote: Originally posted by Panache  
Has anyone made a flashback arrestor that can connect to ground glass, how was it tested for efficacy?
As always, it's useful to know what the application is exactly, as solutions differ depending on the gas composition. Now I've not done this myself, but I've got a design in my head for an arrestor that can sense when it's active and output a signal to a solenoid shut-off valve, all of which is to say that I've done some research on the subject. Their main principle of operation is that they cool off the inflammable gas mixture below its ignition point so that the flame front goes out.

There are two ways I've seen this done. The first is to lead the gas down thin tubing so that a flame front can cool off by dumping its heat through a large relative surface area. This doesn't scale to arbitrary sizes, as it's a scale-dependent square-cube kind of effect. It does work, however, for low enough flow rates. This is used as an implicit mechanism in certain hand torches; one I saw for oxygen-hydrogen comes to mind.

The second one is more common, as it's better suited as an add-on commercial product, which is a tube packed with metal gauze. It's the thermal mass of the gauze that cools off the flame, on balance. It's only "on balance" because the gauze can burn if you're unlucky. Commercial ones seem to use brass or bronze wool, presumably because they're a little harder to oxidize than steel wool. The only issue with these is that the gauze should be treated as a consumable, so that after a flashback event they should be taken out of service and inspected. Inspection is going to be easier with glassware, clearly.

As for testing, what's the gas composition? I'd take greater precautions with oxygen-fuel than with fuel-only.

I think the easiest way to make one is to pack a Liebig condenser with bronze wool.

densest - 18-7-2010 at 13:08

Flashback arrestors used for welding are often sold combined with anti-backflow (one-way) valves. The intent is that a flashback will cause a pressure pulse so closing the gas or oxidizer feed should help break the conditions that caused the flashback. The biggest drawback is the amount of pressure and flow needed to open the valve in the forward direction.

Commercial welding flashback arrestors are not exorbitantly expensive for use with acetylene, methane, propane, oxygen, and air at flows up to 50 l/min or above. They don't seem to restrict gas flow too much. All the ones I've seen use brass cases - presumably for heat conductivity and strength. It might be safer and easier to make a set of standard taper to gas/air thread or welding quick-disconnect adapters. According to the manufacturer the arrestors I have do not require replacement or inspection after a flashback unless obviously damaged. If new ones are too expensive perhaps some used ones could be bought cheaply from a trusted source.

I'd suggest keeping the tube or vessel volume as low as possible wherever flashback might occur to limit the energy output and have the most favorable ratio of wall strength to volume. As w.f noted, this can be intrinsic flashback protection by itself. AFAIK, the longer the stretch of narrow tubing the more effective. Glass should be thick walled and well annealed borosilicate. Also, using plastic with less tendency to shatter (fluorocarbon, etc.) or metal where possible might be safer if flashback does occur.

High ambient and equipment temperatures make flashback more probable - again, it's an energy balance problem.

I, too, would be curious about what the gases are...

Panache - 20-7-2010 at 07:31

Hydrogen gas. i have a fireboy in the lab i use frequently and on occasion worry about a system full of hydrogen (from a dissolving metal or whatever) sparking. I worry because i once was the stupid reciepent of a face and arm full of tiny glass splinters when a 60L distillation full of hydrogen flashedback and pulverised the condenser head (the vessel was stainless). I was very lucky as i was very close to it (~10cm max) and below it and as such the bulk of the scrapnel went/over me.
I was so close to it because i was refitting the outlet hose that was to take the produced hydrogen well away from the system and dilute it, because it had slipped off, so to soften it to fit is over barb i lit one of those butane soldering torches, i knew it was all so stupid but it was late and i was very tired and wanted to go home and made a stupid decision. i'll never forget how quickly the flame licked back up the system to the boiler, and the bass of the boom was unlike any i had heard before. 10 seconds later i was covered in blood but without any serious injury, dumb luck.
Importantly though i would like to be able to burn outlet gases benignly over a bunsen and for this i need a reliable arrestor, otherwise benignly becomes well, unbenignly.

densest - 20-7-2010 at 10:43

Standard welding arrestors work fine with H2, AFAIK. Probably cheaper than a condenser & distillation head. Type "T" welding fuel hose is pretty resistant to misc organics, and barb-to-"B size" adapters are pretty cheap.

Unless you're planning on testing a DIY version 100 times or so, commercial safety equipment is usually a better bet IMnsHO.

Panache - 20-7-2010 at 15:57

Quote: Originally posted by densest  
Standard welding arrestors work fine with H2, AFAIK. Probably cheaper than a condenser & distillation head. Type "T" welding fuel hose is pretty resistant to misc organics, and barb-to-"B size" adapters are pretty cheap.

Unless you're planning on testing a DIY version 100 times or so, commercial safety equipment is usually a better bet IMnsHO.


true that, sound reasoning it is, just have to ensure that the back pressure is negligible.

watson.fawkes - 20-7-2010 at 18:08

I'm not sure that a flashback arrestor is the right component here, because I'm not sure that flaring off a hydrogen-atmosphere mixture is the right way to go. A flashback arrestor can only work if it can cool off the gas, which means that it has to stay cool. Now the speed of flame propagation for hydrogen is very fast. Oxy-hydrogen torches, accordingly, are (1) under pressure and (2) use tiny orifices, both of which allow the flame front to stay forward of the torch tip. In the case of a simple vent tube exhausting into a flame, it will constantly flash back, and so it's going to act as a pulse jet engine. In such an operation mode, I wouldn't rely upon the arrestor to stay cool and I wouldn't rely on the lifetime of the metal gauze inside.

A flare might be feasible, but it would take some engineering. You'll need a flashback arrestor as discussed, but also some effort in managing the output velocity of the gas mixture to ensure that it's high enough. You'll definitely need a pressure difference across the orifice to drive an adequately high velocity. You can get that either by positive pressure with respect to ambient or negative. Positive pressure means a tank or blower, but more importantly it means that if you're using tapered glass joints, that they'll spontaneously unseat and always leak a little, all of which means that you'll need to dilute and vent the whole apparatus anyway. Negative pressure means that you're sucking on the exhaust with some kind of blower, and it has to be of a type that can't ignite an inflammable gas mixture. This might work, although it's not obvious to me right now how to make it reliable and cheap.

You might get enough ΔP (I'm estimating all you need is about 5 psi) with an ejector pump, using atmosphere as the motive fluid. In that case, though, I'm guessing you'll also be diluting the exhaust gas below its combustion concentration limit. While this doesn't quite work as a flare, it does work within, say, a fume hood or even just a duct inlet.

It's possible that a better solution for disposing of hydrogen is catalytic oxidation. The whole point here is that you don't raise the temperature of the gas stream enough to ignite. But it need to be adequately hot to work. Closed loop control with an electrical heater and a thermocouple would work fine. There's an upper limit of the amount of combustion gas you could push through it so it doesn't overheat, but that's straightforward enough to manage.

densest - 20-7-2010 at 21:32

Back in the glory .05 days of cold fusion, catalytic recombination modules intended for sealed lead-acid storage batteries were the bee's knees.

Dunno how well they'd work in the presence of organic vapors. I -think- that as long as there are no S or P components floating in, the platinum-group catalysts work for a very long time.

I don't have any manufacturer or other information. Googling cold fusion recombination or recombiner may help.

Panache - 26-7-2010 at 07:32

does hydrogen react readily with ozone? i'm assuming yes, i have some 80W hg vapour lamps, quite small, running the exhaust gases from the system through/over one of these would do it, thnx wtson-fwks (i abbreviated your name because i am sooo time poor) that was basically your idea.

densest - 26-7-2010 at 12:14

How much H2 and total gas are you thinking about? I have a 150 mL/min H2 generator. Connecting it to a hypodermic needle gives a 2-3 mm long flame. It's very hot - the tip of the needle glows brightly for 1/4mm or so. I haven't tried testing flashback. My guess is that 10-20cm or so of capillary copper tubing would be quite sufficient to prevent flame traveling backwards. A small coil of nichrome wire energized by a low voltage transformer enough to glow red would ensure ignition.

I'm asking because that H2 generator would be sufficient to fill a quite large piece of apparatus and allow for some small leaks or a purging flow. At that rate of flow a water bubbler would act as a flashback arrestor and a one way valve. Just a thought.

aonomus - 26-7-2010 at 13:01

A water bubbler would act as a flashback arrestor but you want a weak point in the system as a blow-out plug, like a burst disc or a plastic film. Without it the pressure surge will be transferred through the water and back into your generator. Any glassware may get damaged or destroyed.

peach - 18-8-2010 at 04:51

This is a bad idea, and I'm the kind of none fume hood using, none respirator wearing, no goggles, no gloves person who does his washing up with 35% piranha.

I have used flashback arrestors for oxy/fuel welding a fair bit.

When there's a flashback (which can happen quite often if molten metal sprays back at the tip), there'll be a significant bang and thump as the gas in the torch and hose ignites and pressurizes.

A tube packed with material is what's called a flame catcher. It's used in things like turbine engines and big natural gas torches for furnaces and is there to hold the flame where it's supposed to be by providing it with a stable surface to burn on (like scratches for bubbles and crystallization in a flask).

If the mix in the stream becomes combustible and the gas genuinely tries to flashback, there's a good chance it'll go straight through a packed tube and pop the glass as it does.

It's perhaps worth bearing in mind that the arrestors on my welding equipment are being fed from a cylinder pure stream (and so are very unlikely to be contaminated with anything that'll help them flash coming in that direction). They're also heavy lumps of stainless.

I entirely agree with the idea of trying to be as safe as possible and putting everything we can in the way of a risk. But, I am also aware of a dangerous potential for that to encourage a false sense of security, which can be worse than not using the thing in the first place and simply being concerned about what may be about to happen. For the price of a genuine arrestor, buy one and add some hose barbs to couple it to plastic tubing, which can go on the glass. There should then be next to zero chance of it ever getting through, and tubing is more likely to come off before any huge amount of pressure develops (None barbed spikes would obviously work better for that, but I still wouldn't trust it's ability to open against the cost of buying new glass.). One possible option for pressure visualization and regulation may be to attach a balloon at some point around the pressure sensitive areas. It will fill up, to indicate the gas is pressurizing. I believe a full party balloon contains around 2psi of pressure when full, not sure what they burst at. There are, of coarse, numerous other balloons with varying burst pressures (e.g. medical catheters, which are also available in more inert, known materials, with 95% confidence and 99% guarantee numbers on the burst pressure).

Possible, indeed. Wise, arguable.

[Edited on 18-8-2010 by peach]

peach - 18-8-2010 at 06:16

Rinse.

Panache - 29-8-2010 at 02:38

As a follow up, i'd like to share what i have done with the forum, it has worked effectively during testing and when i eventually need it i'll also follow up with results from those circumstances.

Basically i have just mixed the outgoing non-condensed gas stream into the outgoing condenser water stream. Ordinarily i use a recirculating chilled system but for these circumstances i am going to run the tap.
The system wasn't idiot proof to setup up and also does not run as simply as an open distillation minus the flame threat.
i used two good quality (Schott), large capacity, glass, non return thingy's with inline course frit's (glass not pomme) to minimise the chance of condenser water entering the distillation system. In my experience these devices must be used in conjunction with frits otherwise a single hair can render them ineffective. I ordinarily do not like using them on distillations because of the backpressure however if you over spec them sizewise it poses no probl, the two are simply if one gums up or fails.
The system obviously can however fall into below atmospheric pressure and this needs to be understood, when generating non condensable gases it is an unlikely situation.

Thats it. Oh i also have a teflon manifold connected with three outputs, one goes to the condenser water as described, one goes to a large ballooon and the third to atmosphere. Sometimes i use these to double check the system is doing what i believe it is doing.

peach - 29-8-2010 at 03:06

PTFE manifold, nice...

Balloons, aw yeah! The cheapest and most easily popped of safety appliances.

I spent way too long thinking about the following, but condoms are made out of very specific latex compositions (or polyurethane for the latex intolerants) and they have to abide by very strict regulations on the burst pressures, as a measure of their tearability during angry animal sex.

There is actually an ISO standard from the regulators in Switzerland, 1kPa... or 0.145 psi.

Obviously, it'd only need rolling out part of the way

I wonder if the psi could be set to a few whole units by simply cutting one up into bits, then adding layers over the opening of a compression fitting (like a screw thread thermometer adaptor) and screwing them down. Then the adaptors could be dropped into any open ports along the paths in case anything blocks.

Contrabasso - 29-8-2010 at 03:49

The "lab" flashback arrester is the Beehive shelf in a tray of water. Bubbles of gas go one way only, flame cannot go through the water.

Flashback in ground glass is not nice because the pressure developed in the enclosed volume may well part the joints or smash the glass.