lpg feed pipe size on 500 bhp engine.

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LPGC
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Re: lpg feed pipe size on 500 bhp engine.

#261 Post by LPGC » Tue Jan 08, 2019 1:50 pm

robertXX wrote:
Sun Jan 06, 2019 9:55 pm
From what Simon was saying above. Is it true to say that with a totally empty tank filled up , and then I used up about half the gas and filled again , that it would take more gas to fill the second time ?
Not quite what I said/meant Rob..

There's obviously air in the tank on the first fill, air above the liquid gas might compress to higher pressure (due to rising liquid gas level when you fill) than *would-be* gas vapour pressure in the tank if the tank didn't contain air. E.g. if remaining tank headroom after filling with gas is only 1/10th total tank volume then there will be 9bar of air pressure in the tank because we compressed the air above the liquid gas. But if the tank didn't contain any air the pressure in the tank would be solely and directly related to the gas pressure, which would only depend on it's temperature.

Obviously gas wouldn't flow from a pump that is only pushing 7 bar into a tank that has 9 bar pressure and this is irrespective of if the tank's pressure is due to compressed air or gas pressure. It seems some pumps output (pumping) pressure is related to temperature because where pumps have a pressure gauge they seem to push lower pressure when it's cold (weather or perhaps liquid gas temp in bulk storage tanks at forecourts) conditions than in warm conditions. Perhaps these pumps push at pressure that is relative to their input pressure, the input pressure would just be pressure of the bulk storage tank at the forecourt which is (again) solely dependent on temperature of the gas in the bulk storage tanks.

In cold weather propane pressure in a vehicle tank can be low enough for compressed air to be the limiting factor preventing further filling when pumping pressure is low.

To get rid of air in the tank, filling to half full and running til low then refilling wouldn't work. There's only one way for air to come out of the tank and that is by being pushed through the tank outlet toward the reducer but since the fuel pickup pipe is usually submerged in liquid gas that won't happen until the tank is completely empty of liquid gas. Getting rid of air out of the tank this way (pushed toward reducer) can happen much quicker on a mixer system, which is more likely to allow the engine to run at least at low load while there is just vapour in the tank, than on a sequential system where at least in part due to the higher reducer output pressure the reducer and rest of system won't allow the engine to run when there is just vapour in the tank.

Even with a sequential system some vapour containing air will be forced toward the reducer each time the tank is run to empty but it could take running the tank to empty many times for enough air to be pushed toward the reducer with a sequential system to make much difference to the amount of air in the tank.

To speed up the process of getting air out of the tank it should be possible to: Before fitting the tank put a few litres of gas in it, turn it upside down to get the pickup tube into vapour rather than liquid, open the outlet, air will come out with the gas vapour. On some installs this isn't possible (might need to fit the valve after fitting the tank) and (in any case) an easy way of getting at least some air out of the tank would be to fit the valve and tank, use as normal for the first tank full, when it is empty disconnect the pipe from tank outlet, open the tank solenoid to allow remaining gas in the tank push some air out... there will always be some pressure in the tank even when it's run empty. If you did this multiple times you'd get most of the air out but once or twice might be enough to prevent air pressure having an effect on fill level and speed even in winter

In winter weather tank pressure might only be 4 bar, in hot summer periods it might be 10 bar... This is Irrespective of how full the tank is as long as the tank contains some liquid and isn't filled to 100%. But 'irrespective' isn't true if the tank also contains air which is compressed (above the liquid gas in the tank) to the point that the compressed air is at higher pressure than tank pressure would be without the air in the tank.

One proviso in all of this - It all depends on the pump pressure, so is the pressure on pump gauges (where fitted) absolute pump pressure or do the pump pressure gauges reflect the pumping pressure above tank pressure. All of the above depends on the gauges showing absolute pressure. Since fill speed seems to slow as a tank fills (and since tank pressure is only effected by temperature unless it contains air) I do expect the pump gauges to show absolute pressure (well not strictly absolute but you know what I mean, relative to atmospheric pressure).
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robertXX
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Re: lpg feed pipe size on 500 bhp engine.

#262 Post by robertXX » Tue Jan 08, 2019 2:12 pm

Thank you simon , so that's a "no ,i wont get more in the tank on successive fills after it being totally empty" is it :).


incidentally the pump gauge at my local shell reads 165 psi pumping when the tank is empty ,then rises to 185 psi when it shuts off ,on the astra that is .which has a definite shut off.

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Re: lpg feed pipe size on 500 bhp engine.

#263 Post by LPGC » Tue Jan 08, 2019 5:08 pm

You might get more gas in the tank on successive fills if you run the tank to completely empty.

165psi is 11bar, would expect that pressure to be able to fill the tank to around 11/12ths full (91.7%) before air pressure prevents further filling regardless of what you do with the tank valve. Perhaps less because the one way fill valve on the tank (discrete from fill shut off valve) needs X pressure to open before it will allow filling.

Don't know if the pump could actually push gas at 185psi (12,3bar) during actual tank filling but if it could that would change above figures to 12.3/13.3 full (92.5%)... not a great deal of difference to 165psi. The fuller the tank gets the more any air in the tank has an effect of preventing gas flowing into the tank, the pump pressure would have to rise at an increasing rate - If we wanted to fill the tank to 99% full and the tank started empty but containing air the pump would need 99bar (1485psi) pressure just to overcome the air pressure in the tank, the air in the tank would have been compressed to 1/99th of it's original volume which was the same volume as the empty tank. OK atmospheric pressure is 14.7psi not 15psi lol, but you get the concept. Or look at it from the other angle, if you wanted to fill the tank to half full air pressure in the tank would only be 1 bar because the air would be compressed into half it's original volume, so between half full and 99% full a pump would need to increase pumping pressure by 1470 psi lol... Except it wouldn't really be only 1 bar in the tank with the tank half full because the pressure of LPG in the tank would be maybe 5 bar this weather. Doesn't only apply with LPG though, it would be the same for any liquid that air could not dissolve in, if we tried pumping diesel into an un-vented tank the air pressure would still work against pumping. In the case of LPG which will find it's own pressure (temperature dependant) in a sealed container the pressure preventing pumping more fuel into the tank might be LPG pressure or might be air pressure but whichever is highest will be what prevents further pumping unless pump pressure is increased.

All the above due to just air pressure above the rising liquid gas. Now because gas vapour pressure at these sorts of winter temperatures is only around 60 / 70psi, suppose if someone did adjust their valve to fill to 95% or even disable the fill shut off valve completely.... it would be air pressure preventing the tank from filling further, not gas pressure in the tank and not the fill shut off valve. Now suppose someone modded the valve to what they think is around a 90% fill, they might go to the garage and indeed the fill might stop at 92%, they might then think they successfully modded the valve for around a 90% fill, but then come a hot day in summer they might go to fill again when the pump is pushing more pressure and some air has come out the tank and fill to near as dammit 100% full - combination of less air in the tank and higher pump pressure.

I fill a lot of different tanks with LPG, every time I convert one it gets filled to the top these days (unless the customer prefers me just to put a few litres in the tank so they can fill maybe cheaper local to them). I've done a lot of filling at Morrisons over the last few years and the pressure gauge this weather is down to 7 or 8 bar from maybe 11 bar or higher in warmer weather. At 7 bar pump pressure with a tank that started out containing air I can only get 7/8ths (87.5%) of a tank's absolute capacity into a tank regardless of whether I adjust/disable the valve etc because air pressure prevents filling further but if I got all the air out of a tank first I could fill to near 100% if I disabled the fill shut off valve.

Air pressure in the tank also slows the speed of filling, to greater extent with a low pressure pump. But even if I filled at your 11 bar pump and have modded the valve to fill to 90% I could still only fill to 90% (instead of 87.5% so only another 2 litres for a 90L tank) but the fill would be a bit quicker especially toward the end. If I could get all the air out of the tank before filling the fill speed would be the same all the while it was filling.
Last edited by LPGC on Wed Jan 23, 2019 8:07 pm, edited 2 times in total.
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robertXX
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Re: lpg feed pipe size on 500 bhp engine.

#264 Post by robertXX » Wed Jan 09, 2019 9:38 am

gotcha , thank you .

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Re: lpg feed pipe size on 500 bhp engine.

#265 Post by LPGC » Wed Jan 23, 2019 7:37 pm

Further to my posts above regards air pressure in tanks slowing tank filling and sometimes being the limiting factor in the percentage of gross capacity a tank can be filled to...

Since I last posted on this thread I've converted 5 vehicles to LPG, on all of them I put a bit of gas into the vehicle's tank from another tank I have in the yard (not much, just enough to get some pressure in and maybe a very small amount of liquid gas), then I released all the gas pressure from the vehicle tank (by removing the solenoid post), refitted the solenoid post and repeated a couple of times. This made a great difference to the speed of fill when I went to the forecourt to fill them with gas, particularly as the tanks neared the usual 80% fill capacity. But I bent the valves on all them to allow them to fill to 90%... Now usually when I bend the valve to allow filling to 90% and fill at the forecourt where pump pressure is only around 8 bar in this weather I still struggle to get much more than 80% in the tank (especially on the first fill, which is usually the only time I see the vehicle) but on these 5 conversions having pre-purged the tanks of air (which will have come out with the gas when I removed the solenoid posts) the fill rate was still relatively quick all the way up to around 90% which was when the float mechanism will have closed the incoming gas valve. Result! I will now be purging tanks of air in the same way on all of my conversions.

I'll do a thread on this at some point.
Full time LPG installer
Servicing / Diagnostics / Repairs to all systems / DIY conversion kits supplied with thorough tech support
Mid Yorkshire
2 miles A1, 8 miles M62,
http://www.Lpgc.co.uk
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robertXX
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Joined: Sat Dec 18, 2010 9:45 am

Re: lpg feed pipe size on 500 bhp engine.

#266 Post by robertXX » Wed Jan 23, 2019 8:29 pm

very interesting ..thank you Simon.

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