How high (no, not an oriental joke!) can you lift water?

That's for water. For petrol, the vapour pressure at normal temperatures is much higher, so the maximum lift is lower than for water, depending on the temperature.

Brilliant!

Agreed. You can't quite get a vacuum, you will only get down to the vapour pressure of the petroleum, which depends on the temperature in the tank. But petroleum is lighter than water, so that will give you a bit more height.

Of course if you keep on "sucking" at the top, you will suck out petrol vapour, so if you condense this you will then get some liquid petrol. The evaporation will cool down the fuel in the tank, reducing its vapour pressure. The rate at which heat diffuses in from the ground will then determine the rate you can get petrol out at.

Let me try to understand this...

So you are suggesting, that rather a near vacuum being created over petrol, that instead of the vacuum you get petrol vapour?

Aside from that, petrol is lighter than water and therefore ought to rise higher up the suction pipe, when pushed by atmospheric pressure.

They can definitely be traced back to the fifteenth century and may well have been in use earlier.

Colin Bignell

Out of interest, it is possible for a plane flying at the right height and in the right radius circle to deploy a rope (or hose) such that the end remains stationary (but rotating) at ground level. This has been proposed as a cheaper and more available means of rescue than a helicopter.

SteveW

That makes it a whole lot less interesting. (and I meant the truck would be driving in circles, not flying :-)

Only to the extent the petrol at the bottom of the pipe continues to boil as you suck. Due to latent heat of vaporisation, the petrol in the pipe will cool down, and the boiling rate will reduce. Also, you won't get petrol vapour off, but mainly some of the lighter fractions. That could lead to an interesting experience when you start (or explode) the engine.

, so if you condense this you will then get some liquid petrol.

Yes, and at room temperature that's at about a third of atmospheric pressure for petrol. That's assuming that petrol consists of a single compound, which it doesn't, so you initially get a higher pressure as the lighter fractions (Short chain hydrocarbons) boil off first, gradually decreasing as all the light stuff is sucked through the pump.

When pushed by the difference between atmospheric pressure and the vapour pressure of the petrol, it will rise less far than water when pushed by the much bigger difference between atmospheric pressure and the water's vapour pressure.

Course you also get water vapour above water and mercury vapour above mercury. The salient point here is that these are a lot less than that for petrol at ordinary temps and so you'll get nearer to a vacuum and so less "lift" is lost.

In message , Chris Holmes writes

Look up "Toricelli vacuum"

Except that water contamination in the fuel could write off the aircraft.

Er, what?

IIRC, maximum static lift for a conventional EMC (Electic Metering Column aka 'Petrol Pump' is around 15 feet. Severe cavitation takes place over that, which overwhelms the air separator in the pump, and leads to metering accuracy problems. There was a dalliance with submerged pumps some years ago, but were difficult to maintain. Pressurised systems also introduced further complication with respect to safety. ( Think 'pump' wiped out by errant motorist). Apart from vapour recovery systems, I don't think things have changed too much from my time as a petrol pump tech.

The downside is the need to prime them before they'll work (well, that and having two pipes rather than one), but they do get around the limit of how high you can suck water with a conventional pump.

cheers

Jules

You could get a bit more than 30' with fuel (SG less than 1, guessing between

From about the year dot!

Then again, it was a novel and novelists do make mistakes. They can't be an expert on everything they mention in their novels.

Colin Bignell