Oil vent cap if that is what it is?

anotherquestion

Hi folks,

I have an old oil tank for central heating in the back garden probably about 36 years old.
On the top at front there is an opening with conical cover to put in oil, while more than half way towards the back of tank at top, there is simply a red lid type cover about 3 inches  in diameter that simple screws into threads in tank. 

This red cover was never tightly screwed in place, there is no washer in the lid cover, just screws into place on top of the tank.

Is this some sort of vent cap?

Anyway a few days ago, our autistic teenager manged to screw it off, luckily I think I found the lid in time before any rain down pour and tightly screwed it back on so he doesn't screw it off again.

As stated there is no washer on this cover to prevent water getting in. The tank does tip slightly off the horizonal towards the back, and the thought has crossed my mind that maybe water has got in down through the years. though heating is working ok.

A google has given me the name of a paste that can be added to a dip stick to check for water.

If this is a normal setup does the curvature of the top of the tank keep water from seeping in through the red lid or what..

Many thanks for any clarification.

ThisIsWeird

Hi. I don't know, but what you say makes sense.
This red cap, is it 'female', like the cap of a plastic coke bottle? Ie, the threaded tank part sticks up from the tank's top? If so, that should be pretty rain proof, tho' it's possible for some splashback to perhaps get up past the thread, if the cap is quite loose.
A slightly comical roof shape should certainly prevent pooling water, so that would theoretically only leave 'splash' as a means for rain to get very slowly through, but that will depend on things like the height of the threaded section, how far down the cap screws on, and just how loose it was.
Photo?
Is it a vent? Very possibly. But I don't know. If it is, then I guess it's needed.
I presume this is a plastic tank, albeit quite likely single skin? 
If it's a plastic tank (let's hope), does it have any details embossed on it - manufacturer, etc?
(If it's a steel tank, I'd suggest you may have a ticking environmental time bomb in your garden )
Check the low back side of the tank for a water drain !!!!!! - there ideally should be one there. 

FreeBear

Is it a cap similar to this -

The threads often used on these type of fittings are slightly tapered. As a result, no washers are needed as they are self sealing when done up tight.

Rodders53

It's always best to post a picture of such a thing...

but is it one of these?

The description given made me think of this item immediately...

ThisIsWeird

FreeBear said:

Is it a cap similar to this -

The threads often used on these type of fittings are slightly tapered. As a result, no washers are needed as they are self sealing when done up tight.

That has a built-in vent, doesn't it? So if it's like that, OP, then do it up tightly.

FreeBear

ThisIsWeird said:

FreeBear said:

Is it a cap similar to this -

The threads often used on these type of fittings are slightly tapered. As a result, no washers are needed as they are self sealing when done up tight.

That has a built-in vent, doesn't it? So if it's like that, OP, then do it up tightly.

But if it isn't a vent that the OP has, and there is no other fitting on top of the tank, best not do it up tight. Fuel oil expands & contracts, and the air inside the tank needs to go somewhere or it could spring a leak.

Photos of the tank and cap would be useful.

anotherquestion

The cap is like the black one shown on pic,  just red  in colour, so that explains why no washer needed, thanks so much to everyone for their very helpful replies.