gas locker drainage (sorry if this has been answered before)

[forrestmichael]

Hi, i've made my gas locker from plywood coated with grp inside and out and secured it to the underside of the aft deck, (i'm quite proud of my first fiberglass project) i'm ready to drill the hole near the centre of the transom as high as i can get it with a fall from the base of the gas box. I've got a 3/4" tube with a skin fitting sealed into the grp box and a skin fitting to go on the transom, is there anything i'm missing? I'd hate to sink my boat with this, it would definately be above the painted waterline. I understand non-return valves are not allowed from talking to the gas installer.

Any advice much apreciated as always

Mike

 

[paulburton44]

Can air get in the top to let any leaked gas out the bottom ?

 

[Twister_Ken]

If it's not too late, can you make a small 'sump' at the bottom of the box to plumb in the drain line? Mine (supplied by Calor) doesn't have this and consequently there's always a mm or two of water (rain or spray) left in the bottom which can't drain away because the exit pipe has a slightly proud flange around it. Consequently the gas bottles get rusty, which in turn leaves a rust stain on the transom from the drain pipe.

Double jubilee clips both ends, of course.

 

[VicS]

Chapter 7 of the Boat Safety scheme Does give a recommended minimum height for the drain outlet above the water line but is probably worth remembering that it is not aimed at sea going vessels. It also explains that the drain need not be from the very bottom of the locker nor, provided dead space below it is filled , from within the normal maximum distance from the bottom either.

There is also a table of minimum drain diameters vs bottle size

If you have a tight fitting lid to the locker there must also be a vent.


If your drain is sealed into the locker, as it must be, and to the transom you will not be in danger of sinking all the while the top of the locker is above water.

 

[mikemonty]

I don't think so.
There are recommendations that you should have a top vent to allow air in to replace gas exiting the drain.
But if you have an understanding of Daltons law you will know that the partial pressure of the gas will ensure that it will flow out the drain irrespective of any air vent. in fact it would flow out the vent even if it was above the level of the gas locker without needing any excess pressure above atmospheric inside the locker.
The only thing an air vent is going to do is allow a following sea to displace the air in the locker, pushing into the drain and possibly up into the locker, whereas without the air vent at the top, the effect is like an inverted bottle - water cannot fill it up.
Any air vent would have exit overboard, as a major rupture would probably force gas up the air vent also.

 

[VicS]

[ QUOTE ]
if you have an understanding of Daltons law

[/ QUOTE ] Dalton's law of partial pressures states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture.

Right that's cleared that up then!<span style="color:white"> ......................................................... </span> /forums/images/graemlins/wink.gif

 

[mikemonty]

It also implies that a mass of gas in an unbounded space will expand until its OWN pressure (ie partial pressure) is equalised.
So if you have butane in an open container (i.e. a gas locker with a drain) it will expand and dilute in the atmosphere until its partial pressure equalises with the partial pressure existing in the surrounding atmosphere - in other words it will dissipate until it is a negligible percentage of the total.
Not only will the butane FORCE its way OUT of the container, but the air constituents will FORCE their way IN.
Even if the there is no difference in total pressure between the gas locker and the outside atmosphere.

Being an Old Chemist I'm sure you understood the implications. /forums/images/graemlins/wink.gif

 

[discovery2]

A couple of wooden battens in the bottom of the locker to raise the bottle sufficiently to keep it out of the residual water.

 

[sarabande]

I think that ordinary GRP does not easily classify as "fire-resistant".

http://www.grpconsultants.net/fire_resistant_phenolic_mouldings.htm

and for DIY use

http://www.boater.com/repinfo/westsys/west4.html

 

[Eygthene]

Thanks for that David - easy when you know how! I've often been ashamed of the rusty bottle that I have taken back to exchange for a full one.

 

[mikemonty]

Yah - but the point of the gas locker is mainly to prevent a hazard arising.
If the flames were licking round the outside of my gas locker I doubt I'd be too worried about how fast it charred through.

Bear in mind his box is made of sheathed ply - the ply doesn't burn too quick anyway.

If I couldn't put the fire out in seconds, in other words, by the time all my extinguishers were discharged and I'd tried seawater if appropriate, then you are out of options and there no reason to be in there any more.

I'd be in the tender and rowing away.

What is the point of an especially fire resistant gas locker when it is surrounded by ordinary resins (the hull)?

(unless the locker is big enough to hide in)

And it is not going to delay a conflagration "until the Fire Brigade turn up": -

If you were alongside and the fire brigade turned up - their first question would be "Any flammables on board?" - and the answer "yes" would result in your being treated to the sight of a bunch of firemen watching your boat burn down to the waterline rather than risk injury.
I had this happen to me when I has my gas bottle stored in my timber garage (edit - garage burnt down, not boat) - You cannot imagine how frustrating it is to watch the FB watching your property burn!

I reckon his glass and ply box will serve well enough.

 

[VicS]

post deleted Total bollocks obviously.

 

[VicS]

[ QUOTE ]
I think that ordinary GRP does not easily classify as "fire-resistant

[/ QUOTE ] You are right of course but unless I have missed it there is no stipulation in the BSS that a fire resitant grade should be used. They merely refer to FRP which normally means fibre reinforced plastic. If they intend it to mean fire resistant, or retarded, plastic they do not say so as far as I can see.

 

[mikemonty]

This doesn't happen as two separate streams, one in each direction - it happens at the molecular level.
In a drain pipe there will be a FULL BORE flow in each direction at the same time - air UP the drain

(The air is moving to fill the space in the box where there is less nitrogen, oxygen etc than the atmosphere and therefore an air "vacuum") -

and butane DOWN (to the butane "vacuum" that is the atmosphere).

Analogy: a suspension of sand falling through a rising water column. or iron filings being drawn out of sand by a magnet.

Gas can move as freely against (or rather, through) the flow of air in the opposite direction - under the propulsive force of its partial pressure - as under the particularly weak force of a few inches head of gravity pressure.
In fact I'd not be surprised if the gas vented out the top too - purely down to - you guessed it - partial pressure.
The gravitational pull on the butane will not be as strong as the force of its partial pressure until it is at a very low concentrations and probably well under its Lower Explosive Limit.

This ain't rocket science - but neither is it intuitive.

 

[VicS]

OK then a vent is not necessary. It would also seem from your explanation that there is no reson for the drain to be at the bottom of the gas locker.

BUT please expalin how Daltons law, which states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture, has any bearing on the matter.

 

[mikemonty]

I don't know where you got the idea that I said the drain is not necessary!
Of course it is necessary. or else the gas partial pressures cannot equalise.

My argument is that the drain is perfectly effective without the need for a vent.
In fact a vent would have the disadvantage that it would allow water to enter the drain.
I think I explained the implications of Dalton above, but I'll try again:
Yes, the bare "law" is as you state - but the point is that each constituent of the gas <u>acts independently</u>
So you might start with a box of pure butane at atmospheric pressure, but because the pressure of the butane wants to equalise with the pressure of all the other butane in the atmosphere surrounding it, it expands.
Out of the box - through the drain - and into the atmosphere. Until it is at the same pressure as the butane in the air around the boat. Which is to say, virtually zero.
Now that is not to say that the box is now a vacuum, because the reverse also happens simultaneously, the air outside the box sees the box as a vacuum to it and is forced up the drain to equalise the pressures of all the constituent gases, nitrogen oxygen, carbon dioxide etc. until the partial pressures of each of these have equalised.
And the total pressure of all these constituents in the box remains at atmospheric throughout this process.
It is partial pressure which drives that process, not total or atmospheric pressure, and it is Dalton that expresses that principle.

 

[VicS]

[ QUOTE ]
I don't know where you got the idea that I said the drain is not necessary

[/ QUOTE ] I did not say that you did. Read what I actually wrote!

Anyway you now appear to be describing what is usually referred to as the phenomenon of Diffusion which is the tendency for any substance to spread uniformly throughout the space available to it. Although exhibited by solids, liquids and gases it is most commonly recognised with gases.

Grahams law of diffusion applies and tells us that the rate of diffusion of a gas is inversely proportional to the square root of its density. The interesting consequence of that is that in our gas locker air diffuses in more quickly than butane diffuses out. It is a principle that can be demonstrated with a fun experiment using hydrogen and air which results in the pupil in the middle of the front row of a science class getting wet! /forums/images/graemlins/grin.gif

Dalton's law which merely tells us that the pressure of a gas mixture is equal to the sum of the partial pressures of the constituent gases.
Because we can show that the partial pressures are directly proportional to the mole fractions of the individual gases it can be used to calculate gas mixture compositions from a knowledge of the partial pressures.

it is also of practical use in making gas mixtures of approximately known composition for subsequent standardisation and use in calibrating gas analysers. i imagine it can be directly applied to making gas mixtures for uses such as diving.

 

[mikemonty]

Diffusion describes the mixing of gases, not the principles that drive them to mix - partial pressure.

I was discussing the IMPLICATIONS of Dalton, a bit further into it than simply reciting a definition - which directly bears on the WHY of the drain/no vent question, not the HOW -(i.e. diffusion) which you have labelled.

I described diffusion - though not by name - because you seemed to be proposing that the gas was going to flow out like a liquid.

The WHY is dalton, the HOW is diffusion - The WHY is more important and explains my point, the HOW is merely a desription of a mechanism.

This is now well off topic - what IS your point?

 

[VicS]

[ QUOTE ]
you seemed to be proposing that the gas was going to flow out like a liquid.

[/ QUOTE ] To a certain extent it will. You can easily demonstrate the way in which a heavy gas will flow and can be poured in several ways.

Take a bottle of bromine and you will find that you can pour the heavy vapour into another vessel almost as quickly as you can pour a liquid. However put a drop of liquid bromine into a tall vessel and cover or seal it and allow the bromine to vapourise. You will have a layer of bromine vapour in the bottom which will over time diffuse until it is a uniform concentration throughout. It will take several hours to do that though.

Fill a gas jar with carbon dioxide, place small piece of an ordinary candle in the bottom of a beaker and light it. The you will find that you can pour the carbon dioxide into the beaker and extinguish the candle.

You suggested that the partial pressure of a gas in a mixture will force it selectively out of the vessel containing the mixture. this produces a picture of butane coming out of a gas bottle locker drain in the same way that it is forced out through a gas burner for example.

The rate of diffusion as well as being related to density and hence RMM is also proportional to the mole fraction of the gas in the mixture and on that score since the partial pressure is proportional to the mole fraction the rate of diffusion is proportional to the to the partial pressure.

However the "driving force" of diffusion is the random movement of the molecules as described by the kinetic theory of gases q.v., the partial pressure ( and the density) merely affect the rate.

 

[mikemonty]

“To a certain extent it will. You can easily demonstrate the way in which a heavy gas will flow and can be poured in several ways. “

Yes, it will, but that mechanism does not assist a vent-less gas locker to empty significantly, and if it did, you have still added weight to my assertion that the box does not need a vent!

”Take a bottle of ….. It will take several hours to do that though.”

It doesn’t matter if it takes several hours! If your gas bottle in the locker fails catastrophically then you will get a good flow of gas out the drain, until the locker pressure equalises with the outside atmosphere, then the diffusion, driven by partial pressure, driven by the energy of the gas takes over. If it fails slowly then the only difference is no appreciable flow - and diffusion - driven by partial pressure et c. operates from the start.
And if we stick with your “gas heavier than air” theme then you can open the gas locker with impunity as long as the gas pressure has equalised with the atmosphere, because it is going to just gently slide out the drain.

”Fill a gas jar with carbon dioxide, place small piece of an ordinary candle in the bottom of a beaker and light it. The you will find that you can pour the carbon dioxide into the beaker and extinguish the candle.”

I don’t see the point of that, I did not suggest that the dispersal was instantaneous, that seems to be YOUR inference from my point.

”You suggested that the partial pressure of a gas in a mixture will force it selectively out of the vessel containing the mixture. this produces a picture of butane coming out of a gas bottle locker drain in the same way that it is forced out through a gas burner for example. “

Errr No! Again- YOUR inference.
(Edit - actually as I re-read this, I took the bunsen burner analogy to be quantitative which would, after TOTAL pressure equalisation, be nonsense, but if it is purely a qualitative statement, i.e. the gas is FORCED out then yes the bunsen burner analogy works to an extent. it breaks down where a bunsen burner gas supply flows too fast for the air pressure outside to force air constituents back up the pipe, analogous to the catastrophic failure scenario.
So is it a qualitative analogy or a quantitative one Vic? - Actually... no - don't answer that, lifes too short.)

Let’s look at what could happen:
1. catastrophic failure: gas rushes out of bottle – turbulent flow/mixing – all air entrained and pulled out of locker – locker total pressure equalises with atmosphere – partial pressure forces equalisation of pressures of constituent gases.
2. slow failure: gas weeps out of bottle – density of gas pulls to bottom of locker and out drain – air floats above gas in locker like an “airlock” and prevents locker filling with gas in short term. Within a few seconds gas/air interface blurs as gas diffuses into the “airlock” – ending with an equilibrium balanced on several factors. – until the bottle contents have completely vapourised.

And we are back at the same end point.

”The rate of diffusion as well as being related to density and hence RMM is also proportional to the mole fraction of the gas in the mixture and on that score since the partial pressure is proportional to the mole fraction the rate of diffusion is proportional to the to the partial pressure.

However the "driving force" of diffusion is the random movement of the molecules as described by the kinetic theory of gases q.v., the partial pressure ( and the density) merely affect the rate.”

I am well aware of the relationships, I have, after all, been designing hydrocarbon gas screw compressor sets for fifteen years and working in the screw compressor industry for the fifteen years before that.

There would have been no sense in my stating that the kinetic theory of gases was responsible for the mechanism which will empty the gas locker of gas. It is one step too far up the causal chain.
That is like me stating that the kinetic theory of gases is responsible for your car engine turning when all that is required by way of explanation is “petrol, air, spark –bang”

I’m still not sure what your point is! You seem to be engaged in a rambling ill-considered gainsaying of what I write, based on inferences and irrelevancies and the only reason I persist in responding is because I detect some kind of challenge in amongst the pedantry.

My original point (remember that?) is that a vent is at best unnecessary and at worst, a way to deliver flammable gas nearer to the topside of your boat.