slacken compression fitting to bleed air ... BANG!

[RobertL]

I thought I'd share by exerience of making a foolish and potentially dangerous mistake. I am replumbing the house at the moment and there are some 'dead end' pipes that lead up to the bathroom but are presently capped off with compression stop-end fittings like this:

these pipes were pressurised at mains pressure but had a lot of air in them. I thought it would be a good idea to bleed the air out. So I thought: "I'll just slacken the stop end compression nut a tiny bit and let the air hiss out, then tighten it up again."

Well, as soon as I slackened the nut a little there was a loud BANG and the whole fitting shot off at high speed. Luckily it did no damage. This was followed by a lot of water!

I now realise of course that once the fitting was slackened it did not take much force to slide the olive off the end of the pipe. The loud bang was because of all the compressed air in the pipe -about 1 litre of air at 70 Bar.

I felt really foolish. Learn from my mistake!

Robert

[Andy Burns]

1000psi mains?

[Baz]

If the fitting had been made off Correctly, the Olive should had deformed the Tube and that would not of happened.

And where did you get 70Bar (that is over 1000 psi)

Baz

> Anyway, these pipes were pressurised at mains pressure but had a lot

[Dave Plowman (News)]

A properly fitted compression fitting doesn't allow the olive to 'slide off'

The entire system would explode with pressure anywhere near 70 bar. Mains is rarely above 3 bar.

[Ian French]

This has got to be wrong, 70 PSI (pounds per square inch) maybe ?

Even this (70PSI) seems high.

(Many water boards only guarantee a minimum of 1 Bar (about 14.7PSI))

[RobertL]

oops yes sorry I meant 70 metres of head - although maybe it's more likely to be 30.

R

[Mathew Newton]

We often get 80PSI here in Bristol (certainly always above 70PSI).

Mathew

[David Hansen]

On Thu, 08 Jan 2009 11:04:49 +0000 (GMT) someone who may be "Dave Plowman (News)" wrote this:-

The static pressure at my house is 4.5 to 5.5 bar depending on the time of day.

[Tim S]

Dave Plowman (News) coughed up some electrons that declared:

Worst case I've seen consistently is 7.5 bar.

[Tim S]

RobertL coughed up some electrons that declared:

That's 7bar and 3bar respectively.

[Doctor Drivel]

> Anyway, these pipes were pressurised at mains pressure but had a lot

The olive is not meant to dig into the pipe. Depending on the water system you have, you should have left the air in the pipe. IT acts as a shock arrestor, until it dissolves.

[RobertL]

Are you sure? Here's a tool designed to slide off olives:

much force does this gadget use to slide off the olive? At a guess its something like 5 Newtons (force from thumb) x 20 (guess at ratio of distance moved by thumb/distance moved by pusher on olive). =3D

100 Newtons.

What force does the water and compressed air exert? 500,000 Pascals (5 Bar) * 2/10000 sq m (2 sq cm) =3D 100 Newtons.

So the force of the water in a slackened compression fitting is about the same as the force of an olive extractor. So it's quite posisble for a properly fitted fitting to blow off when slackened in this way.

Robert

[Baz]

Oh Yes it is!!

Baz

[Bob Eager]

Agreed. Drivel keeps the olive in place by relying on the hacksaw burrs.

[The Natural Philosopher]

To be fair, on steel pipes, it doesn't dig in that far...

[Doctor Drivel]

You need to get to know more about pipe fittings.

[Baz]

I doubt very much, that anyone on this NG knows any more than me about "Pipe Fittings".

For one you don't. Do you even know the difference between "Pipe" and "Tube"??

Are you the fool who had that silly thread going about "Push on Fittings" a few years ago?

Baz

[Baz]

Ferrules for Steel "Pipe" will have an "Edge" to bite into the "Pipe".

Baz

[Unbeliever]

Please educate the uneducated of the group then Baz as to what is the main difference "between "Pipe" and "Tube"" as defined for plumbing and heatings applications (and if you wish, engineering as well [such as gas and oil conveyance]).

Unbeliever

[The Medway Handyman]

Errm no. I have one and it takes a sh*t load of force to remove most olives. Usually a boc spanner on the tommy bar to get enough leverage.