Bulk Propane Technical Question
 

Hoping there is a tech guru out there that can clear up a question I have.
House uses LPG from a bulk tank. Coming to the end of my contract thought I would look around for a new supplier.

With that in mind I looked at my usage over the last 19 months. This is what I found:

See attached SS.

http://forum.a8parts.co.uk/attachmen...1&d=1513972935

My question is, how can there be such a discrepancy in the number of litres to a single % between the first and last?

CtL the Confused

That has to be ambient temperature, especially if you look at the dates - highest volume per 1% is in the winter where the density of the liquid propane is highest. There must be a difference in how the measure volume as it comes of the truck compares to how it behaves once its in the tank - perhaps it comes out as gas but the tank level is showing the liquid?

I can understand that for the small differences but the first delivery?

Was the tank empty before the first fill?

The subsequent re-fills were all from about the same point, maybe the gauge isn’t as accurate at 1% or maybe the gas is more compressible as the tank is empty?

I'm sure thats it ^ 1% level on the first fill so the vapour pressure will have been quite low and it'll have been almost all gas. It will have taken more volume to build up the pressure to get the gas to convert to liquid.

Is the volume measured at the tank?
Or off the truck?

The base measure of 1% will just be an inaccuracy of however they measure it (float or pressure measure).
It takes quite some pressure to liquify propane, which means quite a volume of gas, and a float measure (which is common in LPG tanks, so possibly in bulk propane tanks too?) will only start to measure once it can float on a liquid. That should explain the initial discrepancy in % full to volume.

You're dealing with a pressure vessel. Volume won't be accurate, as it varies with ambient temperature.
Off the truck is the most accurate measure, as it should be able to measure flow volume, much like an LPG pump does.

Does the tank sit on its side, or vertically?

Bulk propane levels/consumption is a 'black art'. I've had countless 'discussions' with customers over the years with users of LPG powered forklifts and tank volumes/consumption etc.
I've had Calor and Flo-Gas tech guys both explain it to me and it still doesn't make sense!

MJ is on the right track though - it all boils down to temperature on delivery, ambient temp and the likes.
I had a cross reference chart from Calor somewhere giving the volume on LPG as a vapour and as a liquid at different ambient temps.

Thanks for the replies. The tank was completely empty on that first fill. It sits on its side.
I have a record of daily levels from the tank telemetry. It will vary with temperature, but only by a percentage point or two.
My inept understanding of Boyels law indicates that while it is definitely in liquid form when it leaves the tanker and is counted in fluid litres. When it enters the empty tank it will initially phase change to a gas, but once the tank reaches the prescribed pressure the gas will liquify and all further LPG will remain in liquid form.
While the space for propane in its gaseous state decreases, the pressure will remain constant while it phase changes to liquid as the space is reduced by the incoming liquid. The capicty is then measured by a float.
While the volume of the liquid may change due to temperature it won't be by much; ergo the 1 or 2 % differences.
But 5 (approx) liters per % point? It makes me wonder.....

And with the tank on its side you will never get an accurate measure of actual volume using a float due to the cylindrical shape of the pressure vessel.
You could make a fancy circuit or use a computer to try to extrapolate a volume, but no one ever does. Floats just give a level.

Much like trucks with round tanks - those truckies need to watch that last 1/4 tank!

The only really accurate way to measure is in the same way the gas suppliers meter the delivery normally via a paddle wheel flow meter.

In theory the volume supplied should equal the volume drawn off in a liquid format.
Only downside to all this is getting hold of an EX rated flow meter - and fitting it to the pipework. A truck breaker might help if they have an old calor LPG tanker with the pumping equipment still on it