VicS
Well-Known Member
The difficulty will be ensuring that when in use the drain hose falls continuously from the locker to the outlet fitting. BSS 7.3.4/R
Gas Locker ( I feel a survey coming on!!)
- Thread starter pcatterall
- Start date
The difficulty will be ensuring that when in use the drain hose falls continuously from the locker to the outlet fitting. BSS 7.3.4/R
pvb
Well-Known Member
I think, if it has to comply with BSS, the side-opening door must be gas-tight to above the valve (or other high-pressure bits).
Kukri
Well-Known Member
Owning one of her twin sisters I look at the gas locker with a very jaundiced eye...
The explosion was caused by someone changing bottles over carelessly (solution - avoid the change over and just have one bottle hooked up at one time) and by poor attention to the gas tightness of the bottle locker (solution - less obvious) but having dealt with those two issues the locker is still let into the compartment with the Eberspacher and with the diesel genset and the piping passes through it. I am singularly underwhelmed by the Navy's approach of "take fifty strokes on the bilge pump at the change of watch!"
jdc
Well-Known Member
I am skeptical about the drains needing to be low down to allow gas to flow downhill under gravity. The density of gas isn't that great, Propane has the same density as CO2 for instance, and I think it will mix with air and diffuse out of the top of a locker rather faster than it will 'drain' out of a narrow pipe.
The real function of the drain which the BSS insist on is in case of a drastic and catastrophic gas leak, for instance from a faulty regulator, as it means that with the lid is closed the pressure will not build up and indeed most of the gas will go overboard safely. Hence it matters little imho if the bottom of the pipe is sometimes an inch below the WL (ours is when well heeled on stub tack) or not in a very favourable 'lead'.
The real function of the drain which the BSS insist on is in case of a drastic and catastrophic gas leak, for instance from a faulty regulator, as it means that with the lid is closed the pressure will not build up and indeed most of the gas will go overboard safely. Hence it matters little imho if the bottom of the pipe is sometimes an inch below the WL (ours is when well heeled on stub tack) or not in a very favourable 'lead'.
peteK
Well-Known Member
A lot of boats due to their design wont allow a draining gas locker without radical alterations if this is the case your survey will take account of this and so should your insurance company,if they don't see sense change insurance company.
pvb
Well-Known Member
The BSS guidance applies to open-topped lockers too, so I don't think it has anything to do with pressure build-up. Nor do I think it's aimed at drastic gas leaks; it's more probably designed to cope with gentle seepage.
As for the drain pipe itself, it is quite important that it has a favourable lead and a constant downwards gradient, in order to avoid puddles of water building up in it and blocking the vent.
JumbleDuck
Well-Known Member
Gentle seepage will diffuse away. The notion that propane sits in pools in the bilges of boats, waiting for a chance to go bang, is a myth. In fact it's the diffusion of propane into air which gives you an explosive mixture and makes it dangerous. Decent ventilation is needed more than anything else.
pcatterall
Well-Known Member
Ahh yes.. of course! I wondered why it had to fall all the way but had not considered blockage by water. That will make using a longer flexible pipe more problematic. Drawing board again!
jdc
Well-Known Member
Well I think that the 'drain' is useless even if sloping gently and without any water in it. Try the following thought experiment:
Assume that gravity will separate molecules with molecular weight 44 (CO2, or C3H8) from molecules with molecular weight 32 (O2). Then all one would need need to do to solve global warming would be to build a number of gas lockers and leave the lid up: the CO2 will drain out of the bottom and can be collected. Do you really think this will work?
pvb
Well-Known Member
Well, butane is about twice as dense as air, so it will settle towards the bottom of a gas locker in the absence of air movement. Whether it will drain out very quickly is an unknown. However, what you think really doesn't matter one way or the other, because the people who set the guidelines - and the surveyors who follow those guidelines - have decided that gas lockers need a drain.
jdc
Well-Known Member
Sorry, but this is nonsense; do the statistical mechanics.
dn/n = -mg / kT dh (see http://www.feynmanlectures.caltech.edu/I_40.html for instance if you don't want to take my word for it)
Thus, using Butane (C4H10 -> 58 g / mole), and a locker height of 50cm gives the variation of concentration of Butane over the locker of 0.01%: put another way, at the top of the locker it's 99.99% as concentrated as it is at the bottom of the locker. It simply doesn't flow out of the pipe because it's 'heavier than air'.
In fact, the whole concept of 'heavier than air' is entirely irrelevant when it's allowed to mix with the air and one has to consider the paths taken by the individual molecules moving around under thermal motion. Gasses simply don't separate as liquids do.
The reason for the drain, which I entirely agree is both mandatory and extremely sensible, is to allow an escape to go out without bursting the locker, and to make sure that as much as possible of a catastrophic leak does go overboard rather than into the cabin. But it's nothing to do with gravity. Hence a minute quantity of water in a locker drain pipe won't actually make any appreciable difference to the safety of the installation.
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pvb
Well-Known Member
I'm sorry you think it's nonsense. If it has nothing to do with gravity, why do you think the recommendation is to have the drain at the bottom of the locker? And if gravity isn't involved, why are people encouraged to fit gas detectors in the bilge?
lw395
Well-Known Member
1) Because a lot of people will regurgitate 'advice' when they know sweet FA.
2) Initially as gas escapes, it's generally cold due to adiabatic expansion and/or latent heat of evaporation.
So it convects downwards until/while it diffuses into the air.
My take on it is that the drain should be at or near the bottom, but you should not rely on the gas falling downwards. Gas lockers where the top is open to the cabin are very very bad IMHO.
Gas detectors should be near and below the potential leak e.g. cooker. Not separated from the cooker by the cabin sole or whatever.
JumbleDuck
Well-Known Member
To let water out? Or because the people who wrote the specs didn't understand gas dynamics very well?
Again, could be ignorance. Or perhaps it's because bilges, being poorly ventilated, are where explosive concentrations might arise first. Or - and this is my favourite theory - it's because leaking gas has expanded and therefore tends to be colder and therefore tends to head downwards, regardless of density.
However, it's a very good question. I wonder if any research has been done into the placement of gas sensors on a boat. Could be one for PBO ...
jdc
Well-Known Member
Very good questions, to which I can only guess the answers! Here goes (but I freely admit they are only guesses)
Drain in the bottom of the locker.
1. To make it self draining if full of water - where else would you put it?
2. To make it useful if liquid gas or other fuel is spilled, eg petrol.
3. Because gas when spilled can be very cold. That does indeed make a difference to its density, at least until it returns to thermal equilibrium.
4. It's just as good at the bottom anyway so no merit in changing the time-honoured advice
5. Out of the very common misunderstanding of the kinetic motion of gasses
Gas detector in the bilge:
1. Because if there's been a serious, liquid, leak of LPG it will have gone down hill to the bilge
2. After a short time (a minute or two), with no ventilation, it's as good a place as any as it will read the same as the cabin, so why not put it in the bilge?
3. Supposing the leak has good gas-exchange with the bilge space, and the cabin is well ventilated (and both conditions apply in my boat when we're aboard) there may be higher concentration in the bilge: ie, paradoxically, the best place for the detector could be in the least ventilated place!
4. Because it's not only LPG one might try to detect: how about spilt petrol where the concentration is derived from diffusion from a localised source of high concentration (where it's leaking from).
5. Out of the very common misunderstanding of the kinetic motion of gasses.
By the way, I'm not really arguing with you or anyone else other than myself! I entirely agree with you that it's always said and largely accepted that gas accumulates in low places, I just don't get the physics and so challenge the conventional wisdom. My hang-up I admit, but it really bugs me when simple physics and rules of thumb disagree. Hopefully others will help the debate along and/or improve my understanding.
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pvb
Well-Known Member
Tammany
Well-Known Member
All this hassle reminds me why I removed the gas system off mine within a month of buying the boat lol. Mine had a plywood sealed box in a cockpit locker with a plastic pipe to a thru-hull underneath that exited the boat through another thru hull. Didn't look that gas safe but assume it was. Think it had been their since the 1970's judging by the cookers I removed.
Poignard
Well-Known Member
I was thinking it was time to renew my gas installation a few years ago when I was invited on another boat for coffee which had an Origo alcohol cooker. I became an instant convert, bought an Origo from a German chandlery at a good price and enjoyed ripping out and dumping all the pipework, cocks etc.
I have never regretted it. True there is no grill but we can make toast in a Diablo gadget and in marinas we use an electric toaster. Also there's no need to go out in the rain to turn a gas bottle on/off!
I have never regretted it. True there is no grill but we can make toast in a Diablo gadget and in marinas we use an electric toaster. Also there's no need to go out in the rain to turn a gas bottle on/off!
pvb
Well-Known Member
Didn't you have an on/off valve near the cooker?
Tammany
Well-Known Member
Another origo convert. Much safer too. A little slower but who's in a hurry.
