On or around Fri, 13 Oct 2006 13:19:31 +0100, "Greg"
<
[email protected]> enlightened us thusly:
>"hugh" <hugh@[127.0.0.1]> wrote in message
>news:[email protected]...
>
>> Overfilling in itself won't cause any running problems. The only danger
>> is that you reach 100% fill and the pressure increases due to increase
>> in temperature e.g. you park in the sun, then the solenoids lock.
>
>In his case overfilling did cause problems, it started venting on the
>motorway and he had to evacuate the vehicle, as I explained in my post.
>Greg
>
indeed:
"Then he managed to find a pump that apparently wasn't set right and over
filled his tank, at least that's what the dealer claims, it was ok until
next day when it was warmer and he got a flashing message on the dash
telling him to evacuate the car immediately, which he did even though he was
on a motorway, only to see clouds of gas escaping from somewhere under the
car."
see, I don't understand that bit, unless there was something wrong with the
system. To get from the garage to wherever he was sleeping and then onto
the motorway he must have driven at least 10 miles, which, even if the tank
was full to capacity with liquid (fairly unlikely but possible) would have
used some gas and created a vapour space. OK, not as big a vapour space as
required by the regulations (20% of tank capacity) but it should have been
enough - notwithstanding the comments about temperature, you have to expand
the liquid in the tank by enough to use all the vapour space to make it
over-pressurise and vent.
Unless, of course, the people fitting it didn't purge the tank properly,
which I suppose is quite possible. In that case, the tank would have air in
it as well, which will compress, but will occupy part of the vapour space
and that part will be unavailable for the LPG vapour. (assuming I remember
my partial pressures thing correctly) [1]
either way, the system *has* to have malfunctioned for this to occur. The
most likely is a fault in the 80% fill device,
[1] Consider a 60l tank. If the tank has air in it at atmospheric pressure,
which is not purged, then once liquid LPG is present (in significant amount)
the tank pressure will go up to 7-8 bar, or about 100psi or so. let's use 8
bar cos the maths is easier. The 60l of air, compressed, will now occupy
60/8 or 7.5l. The vapour space, provided by the 80% fill device, will be
20% of 60l, or 12l, when the tank is nominally "full" of LPG, so there's now
only 4.5l available for the LPG vapour, which is still plenty. The 48l of
liquid gas in the tank has to expand by almost 10% to cause any notable
increase in pressure - all the time there's some vapour space available the
tank pressure should stay more or less constant. I've just been googling
and for no obviously good reason it's not easy to find actual figures for
the CoE of liquid propane, other than one site which describes it as
"relatively high".
Even if the liquid gas does expand by more than 4.5l, thereby using up all
the available space, you still have the air in the tank. To double the tank
pressure, for example, it's got to expand a further 3.75l to halve the
remaining space full of air. I'm not sure what pressure the PRV is supposed
to operate at, but it's liable to be significantly higher than normal
working pressure or they'd be going off all the time. I found one mention
of 375 psi, which is more like 3 times normal working pressure.
So although a tank full of air which is not purged is an installation fault,
it's not a major safety issue. The problems come with tanks which have air
at higher pressure still present following, for example, a safety test. If
the air was at 2 bar, say, then the compressed air at 8 bar will now occupy
1/4 of its original volume, or 15l. If the tank is filled to 80% with
liquid gas, then the tank pressure will be above 8 bar, since the
aforementioned air will now have been compressed to 1/5 it's original
volume: it'll now be at 10 bar. This will preclude vapour-phase propane (I
think). It won't necessarily be dangerous - if the PRV operates at 25 bar,
there's still plenty of margin.
BTW, for would-be installers... you purge the tank by putting a small amount
of LPG in it, inverting it, and opening the outlet valve. In a
well-ventilated area... This is not needed for vapour delivery tanks, as
any air will be at the top of the tank, and the outlet is also at the top.
Vehicle tanks have a dip tube and deliver liquid LPG from (near) the bottom
of the tank.
--
Austin Shackles.
www.ddol-las.net my opinions are just that
"Carpe diem, quam minimum credula postero" (sieze today, and put
as little trust as you can in tomorrow) Horace (65 - 8 BC) Odes, I.xi.8
