Petrol vapour / fuel quality.

[Mirror Painter]

Hi,

The detachable tank for my outboard has the usual vent screw which I was told to open when the can is in the car boot on hot days. Of course there is a rush of petrol vapour with hissing noise.

But what is rushing out? Is that vapour pure petrol or is it just some of the constituents? In other words am I making the petrol less pure with each vapour release? I can vaguely remember something about fractional distillation in Chemistry lessons but that was 30 years ago and I was looking at Paula...

 

[rivonia]

I realy dont know, however all petrol stations that store fuel have vent pipes for the petrol fumes. If you did not vent there is a risk of the tank expanding and eventually an explosion - be aware

Peter

 

[dleroc]

Hi,

The detachable tank for my outboard has the usual vent screw which I was told to open when the can is in the car boot on hot days. Of course there is a rush of petrol vapour with hissing noise.

But what is rushing out? Is that vapour pure petrol or is it just some of the constituents? In other words am I making the petrol less pure with each vapour release? I can vaguely remember something about fractional distillation in Chemistry lessons but that was 30 years ago and I was looking at Paula...

Track down Paula - she might remember!

 

[Bru]

I realy dont know, however all petrol stations that store fuel have vent pipes for the petrol fumes. If you did not vent there is a risk of the tank expanding and eventually an explosion - be aware

Well yes and then again no

Yes, the main storage tanks are vented to cater for pressure changes within the tanks but under normal circumstances petrol vapour is not vented to atmosphere (there is a vapour recovery stage in the vent)

When the storage tanks are being filled by the tanker, there is a vapour recovery pipe attached to the tank pipework which recovers the head vapour from the storage tank (forced out under pressure by the incoming liquid fuel) back into the tanker (it is then dealt with back at the depot)

There is also, at all new stations and most larger existing stations, Petrol Vapour Recovery (PVR Stage 1 or Stage 2) on the delivery nozzles that you use to fill up with on the forecourt

The objective being to minimise or if at all possible eliminate the venting of petrol vapour to the atmosphere

Opening the vent on an outboard tank on a hot day to reduce the pressure build up in the tank is not, of itself, a bad idea. Doing it in the boot of a vehicle is though! Actually, it's a really really REALLY bad idea!

Petrol vapour is the thing that goes bang very very easily so if it is going to be vented it needs to be done in the open air well away from any potential source of ignition (including vehicle electrics, hot exhausts, and so on)

As for the long term effect on the fuel itself, I'm not a chemist but I think I'm right in saying that it is vapourisation of the light hydrocarbons in petrol that over time leads to it becoming "stale" and ultimately to it not wanting to go "bang" when it's sucked, squeezed and ignited!

 

[theoldsalt]

Quote : "Opening the vent on an outboard tank on a hot day to reduce the pressure build up in the tank is not, of itself, a bad idea"

But a waste of time as the pressure willl soon build up again. Just leave the vent closed until the fuel is required.

See the following (my) posting from a previous thread on a similar subject

"Petrol evaporates in a sealed container until the 'vapour pressure' is reached. This is the pressure at which no further evaporation can occur. Vapour pressure increases with temperature so the container will 'balloon' more on hot days. This is not just due to the expansion of the liquid petrol."

 

[Bru]

Quote : "Opening the vent on an outboard tank on a hot day to reduce the pressure build up in the tank is not, of itself, a bad idea"

But a waste of time as the pressure willl soon build up again. Just leave the vent closed until the fuel is required.

See the following (my) posting from a previous thread on a similar subject

"Petrol evaporates in a sealed container until the 'vapour pressure' is reached. This is the pressure at which no further evaporation can occur. Vapour pressure increases with temperature so the container will 'balloon' more on hot days. This is not just due to the expansion of the liquid petrol."

Yep, makes sense to me although I hadn't thought of it before (ain't a chemist, like wot I said before innit )

And I assume I'm right in assuming that properly designed containers including outboard tanks are designed to be structurally sound at pressures well above the vapour pressure

 

[theoldsalt]

It is essential that a petrol storage container is capable of withstanding the max vapour pressure it will be exposed to. So that is why an approved type must be used.

Also these containers should not be overfilled as liquid (whether petrol or diesel) is incompressible therefore fuel expansion due to temperature increases can cause a pressure sufficiently high to rupture even an approved container. It may then be reasanable to open a vent to release this pressure, but best not to over fill in the first instance.

 

[Mirror Painter]

It is essential that a petrol storage container is capable of withstanding the max vapour pressure it will be exposed to. So that is why an approved type must be used.

Also these containers should not be overfilled as liquid (whether petrol or diesel) is incompressible therefore fuel expansion due to temperature increases can cause a pressure sufficiently high to rupture even an approved container. It may then be reasanable to open a vent to release this pressure, but best not to over fill in the first instance.

Great advice - thank you.

Somebody I know ... over-filled his approved container recently and in warm weather petrol was pushed up in to the 'glass' of the inbuilt fuel gauge. Yesterday the same person put enough fuel in to the same tank to exceed the 'safe fill line'. He did it because it meant fully emptying a seperate 5 Ltr can but, having read your comment, he wont do it again.