Pressurised LPG system

[Guy From Nowhere]

G'day all,

ENGINE:
Brocky black engine 308 bored/stroked to 355 with VN rebuilt heads running straight gas.

SITUATION:
On the weekend I encountered the strangest thing, I couldn't fill my gas tanks with LPG. I tried two different hoses. I tried three different service stations. Thus, it had to be something to do with the tank. I believe that like pouring water into a bottle the liquid coming in can only do so if it displaces the atmosphere/gas already occupying that space. This leads me to believe that this part of the system is not working. We had filled up earlier that morning and had driven about 250 km. The manual gauges on the tanks indicated about half full. I have two tanks connected by a hydrostatic valve for the outgoing gas (i.e. engine use); the incoming gas is seperate; i.e. a filler for each tank. Both tanks had the same problem. After waiting about 45 min and tapping the hydrostatic valve we were able to put gas into one tank but not the other. I also noticed that after driving another 150 km that the tank we could not put gas in had not had any gas come out of it.

QUESTIONS:
Q1) Does anyone have any idea what has happened, or gone through a similar situation?

Q2) How does a hydrostatic valve work in laymans terms?

Cheers,
GFN

[date]

Was it a hot day? I have had a similar problem once with my old Range Rover (twin tanks) and twice with my Pajero (single tank). Both vehicles had the tanks underneath. On each occasion, I was nearly empty. The ambient temperature was probably only about 30 Deg C, but I suspect that the blacktop was making things a bit hotter than that. A squirt of water over the valve block on the tanks got the system going. Once the gas started to flow into the tanks, it really went.

Other than possibly a weak transfer pump, I cannot offer a reason why this was so, but it did happen. The Rangie actually didn't stop filling and I got a really full load of gas before I stopped it myself. If you touch the inlet valve on the gas tank during the filling, it is COLD. With the Rangie, I think it actually froze up the inlet float valve. I haven't had this happen lately - hope it doesn't too. I know this doesn't answer your question, but it may help someone else to offer a reason.

[Guy From Nowhere]

Was it a hot day? I have had a similar problem once with my old Range Rover (twin tanks) and twice with my Pajero (single tank). Both vehicles had the tanks underneath. On each occasion, I was nearly empty. The ambient temperature was probably only about 30 Deg C, but I suspect that the blacktop was making things a bit hotter than that. A squirt of water over the valve block on the tanks got the system going. Once the gas started to flow into the tanks, it really went.

Other than possibly a weak transfer pump, I cannot offer a reason why this was so, but it did happen. The Rangie actually didn't stop filling and I got a really full load of gas before I stopped it myself. If you touch the inlet valve on the gas tank during the filling, it is COLD. With the Rangie, I think it actually froze up the inlet float valve. I haven't had this happen lately - hope it doesn't too. I know this doesn't answer your question, but it may help someone else to offer a reason.

Yeah it was a hot day and the worked engine I have throws out A LOT of heat. Put it this way, in a 1982 vehicle, with me wearing thongs, it gets too hot for me to leave my foot in one position for any longer than about 20 min (even with the rubber thong insulation)! The tank that would not fill at all sits in the tray right above the gear box. The other one is right at the back of the tray and underneath. The exhaust passes both of these.

My concern is that the gas was not coming out of both tanks. Isn't a hydrostatic valve supposed to ensure this?

What do you mean by "the valve block"? Is this just where the hoses come in and the on tank gauge is situated?

Cheers,
GFN

[date]

Yes - the "valve block" is the casing on the side of the tanks where the hoses go in and out of the tank. The hydrostatic valve is supposed to isolate one gas tank from the other so that you use up all of one tank before it switches over to the other. On my Rangie, they had to move the gas gauge from one tank to the other because the first tank showed empty well before I ran out of gas. After that, I would run for about 200 km before the gauge started to show any movement and when that showed empty, I was getting close.

The fact that your exhaust pipe runs close to both the tanks suggests that they are getting hot. What sort of heat shield do you have? I have put an extra shield on the end of my Pajero tank where the exhaust passes, but there is little you can do about shielding the tanks from the waste engine heat. I have also put a shield which covers the full length of the gas tank as an attempt, but I doubt that does anything.

Note that in filling the tank with gas, there is already a gutful of air in the tank and this is not vented. That air simply gets compressed to the pressure of the incoming gas, and also gets further compressed if the tank sits still all day on a hot day. Squirting water over the tank simply cools teh tank down a bit and the gas can start to flow. I have found that it uses less water if teh water can get to the valve block, but maybe this is imagination.

Hope this helps a bit.

[Guy From Nowhere]

This is definitely food for thought. I will park my ute in the shade and near petrol station tomorrow and see if this does indeed change the situation. Thanks for the info.

I don't have heat shields under there as the mechanic who built the engine used to have the same model truck as mine with the same engine and he said he had never had a problem. I might call him back and let him know what you have said. He was stumped when I called him on the weekend.

Cheers again,
GFN

[MrGrim]

i used to have the same sort of problem in the summer with twin tanks ...i used to just go to the car wash and $1rinse the tanks before i filled up to cool them down

[V8Patrol]

QUESTIONS:
Q1) Does anyone have any idea what has happened, or gone through a similar situation?

Q2) How does a hydrostatic valve work in laymans terms?

Cheers,
GFN

ANSWERS:

1/
Common issue in summer. Atmosphereic pressure in the tank prevents filling by being at a higher pressure than the 'pump' can push at.... try pumping up an aircompressor with a tyre that has 40psi in it, its basically what your doing.
Without getting to tech, its related to the days ambient temperature V's the pressure and LPG's boiling point in the tank

easy fix ...... fill up in the early morning when its cool, preferably with a low amount of LPG in the tanks.
On one occasion I had an issue that was over-come simply by parking the rig over a sprinkler for 15mins...it was 45degrees in the shade



2/
The hydrostatic valve is very misunderstood.....
ALL it does it prevent LPG from 'moving' from one tank to the other, basically its 2 oneway valves that are a single unit.

The LPG can ONLY go from the tank to the converter ....... remove the hydrostatic valve and LPG will "back fill" the tank with the lower pressure.
The key word there is pressure, note I didnt use the word volume.


In the case where twin tanks are used, they can be 'wired' to run in two differant ways

A/
both tanks supply lpg at the same rate and empty at the same time, usually on a common filler setup.

B/
electrically controled so that one tank supplys whilst the other remains unused untill 'switched' on to allow LPG to flow.... in this case only one of the tanks is in use at any given time and that tank can be emptied fully whilst the other remains completely full.

Option "B" is the perfered choice as one tanks is considered "the main tank" whilst the other is utilised as the "reserve tank"

Kingy

[date]

V8Patrol - thanks for the reply. My problems have come when I was on a long trip, so the option of filling early in the morning was not really on. Also, not every fuel station has a hose or is willing to let you pour water over the tank (and their driveway). Hence I tried to devise a heat shield covering the whole front of the whole tank. So far it seems to have worked. Incidentally, I have found that I can get more gas in when it is colder, but most times it is a fairly constant fill.

[Guy From Nowhere]

V8 PAtrol:
Thanks a heap for that helpul list of information. I thought that was what a hydrostatic valve did, but my understanding was elementary and as it was the only thing related to both tanks, deduction seemed to point to it as the issue. I didn't even consider ambient temperature as a common factor. This explains why the tank in the back would not let me fill it up; I have a hard top, so the temp inside the tub is going to be more than under the vehicle (i.e. no air coolinf in the tub and a heated up metal tub keeping the tank warm). This water solution is all well and good, but my tank in the tub is usually stored with my swag, clothes camping gear etc. and I do not want to wet these. I will keep thinking and see if I can devise a cooling system (there is no room for a heat shield for the one in the tub having looked last night)

Cheers again,
GFN

[date]

ManfromNowhere: My "hot" experiences have been when travelling on blacktop with ambient air temps of about 32 - 35 Deg C. I cannot explain why, but it is only when I was nearly empty that thew problem occurred. I suspect that teh main part of teh problem was lack of pressure in the filling system because I have had plenty of times when the temps have been higher and the gas tank has been empty or nearly so, and it has filled quite well. Only once have I been forced to return the next morning to fill up.

I would be inclined to try another pump or another station if you cannot get gas (assuming that this is an option).

[V8Patrol]

...... This water solution is all well and good, but my tank in the tub is usually stored with my swag, clothes camping gear etc. and I do not want to wet these .....

Old mate had a similar problem ( HZ Holden 1tonner ute) ...... We made up an alloy box so it fully enclosed the tank, drilled some 90mm holes in the tray so air could circulate around the tank, then fitted the box and sealed it to the tray floor.
This box has a lockable lid that allows easy access to the tank, it also is a good place to store gear that could normaly be stolen. The lid is reasonably durable, so even stacking stuff on it has had little effect to its looks / structral ridgity.

non filling has never been a problem since we fitted the box .... plus he scored a lockable area thats largely weather resistant too for storage.

win win ..... & win situation

[Guy From Nowhere]

Cheers all, I have a project for the weekend.

Man from nowhere - ha ha ha, I htink I might change my online name.

cheers again,
GFN