Air vs Water - plumbing physics.

[k13wjd]

Right, maybe someone can answer this one.

After finishing the plumbing for my central heating system, i decided as i had the right kit, to pressurise the loop, in order to check for any major leaks. Cut a long story short, after some major issues with some rubbish brass olives(the ones with 2 groves in them), i finally got the system pressurised to 2 bar.

Went round with a bottle of soapy water, and started checking comp. fittings/o-rings etc...found quite a few very small bubbles appearing, so nipped up fittings etc.....Now, this leads me to a question.

When the system is at 2 bar with AIR, and there is a small leak ( im talking 10 tiny bubbles in 30 seconds)....does that mean that it will still leak with 2bar of water ?????


Be interested to hear some answers.


before anyone asks, a 15mm comp fitting onto the filling loop, a 5bar gauge, and a 230bar microregulated dive bottle. very very controlable down to 1psi !!!!

 

[Toddyplumb]

I would re joint it now while you can. 10 small bubbles is difficult to quantify , it may take up when hot or it may not . It might just be wet and evaporate everytime the heating is on.
I would not be happy leaving a joint knowing what you do , its so easy to sort out

 

[k13wjd]

yeah, quite glad i used air, as it appeared that one of my fittings had a nick in it, and no amount of tightening up would have sealed it.....


the brass olives issue was a right pain, but hopefully fixed. the system is back up to 2 bar, and i'll leave it like that for 3 hours, and if it is still at 2 bar, i'll fill it with water.


air can squeeze through very small holes compared to water.......but how small is the question...

 

[WaterTight]

So you're saying that the olives with two grooves along them caused leaks? Interesting. They do come with some fittings I've noticed. Never noticed whether it was those ones that ending dribbling or not. Come to think of it, what are those grooves for?

As for air to water escapage ratios, no idea but very interesting. If nobody answers I might phone up Dr Karl's radio show.

 

[turnpin]

are you using air pressure kit or hydraulic test bucket for your system and is your system copper or plastic pipe/fittingsregards turnpin

 

[k13wjd]

air pressure kit - as i mentioned, starting from a regulated dive bottle, through a flexi, into the filling loop, with a decent gauge. seems to be holding 2 bar pretty well. I'm pretty certain that just because air leaks out, that doesn't mean that water will. What i mean is that water won't pass, but air will. Goretex is a prime example of this, so is breathable roofing material, Lets air through, but is completely water tight, Due to a water molecule being 0.278 nm, and the smallest in air being 0.10nm.


system is 1mish of copper in 22mm from the boiler, then reduced to 15mm HEP push fittings throughout.

The olives with the 2 groves - i have NO idea why the grooves are there, but after inspecting one that has been tightened up ( alot ), i see that the centre peice of the olive ( between the 2 grooves) ends up signnificantly bigger than it started off. so big that it makes the fitting very tight to do up....

I have no experience with these brass olives. Think i'll go out in the morning and buy a load of proper copper olives, and replace them. I did consider anealing them instead, but i think i'd prefer copper. No leak sealer, no ptfe.

 

[turnpin]

all pressure testing of rigid [copper/iron pipework] and plastic pipework should be tested to British standard 6700 i.e 1.1/2 times max working pressure for the relevant timingregards turnpin

 

[justlead1]

Hi. I think you may find the viscosity of air and water are much different. As a result air will show up where water would not. However testing with high pressure air can be highly dangerous, as the air compresses (unlike water) in the event, as example of say a stop end being pushed off by the pressure of air, the cap would respond, as does an air gun pellet from an air gun. Only with a lot greater potential danger.

 

[k13wjd]

thanks for the word of warning.....though its really not all that dangerous - at 2 bar, the force applied to a stop end really isn't a concern....the only thing that could actaully go anywhere, as you mention is a stop end....Of which there is none in my system, and even then, it would'nt leave with sufficent force to do any damage...Though if i get a spare 1/2 hour, i might just set a rig up to test that theory.

As it happens, The bottle i used to fill the system, is the same one i used to fill my PCP Airgun to 200 bar...now THAT would do some damage !!!

 

[Dannypipe]

200 Bar??? That's a crazy amount of pressure. Whats a PCP airgun?

We air test our c/h installs. Just use a compressor and fill to 1.5 of working pressure. Any leaks we sort em and test again. Go off for a cup of tea, if it's still holding up, we fill her up.

Can save on silly mistakes, like a drain off still open etc.

Not worth going crazy on pressure as air compresses easily and water doesn't.

Oh and on the subject of brass (AKA Crap) olives. The two rings are to aid compression, and they could not be the cause of your leaks. The olives seal on the face/edge of the olive in the cupped part of the fitting.

The brass ones just need a bit more welly to get them to compress down, hence the grooves. I generally bin them though and use a copper one. I don't know why people still produce them. It's like fibre washers in tap connectors. Most of the time fine, but sometimes a weep. Rubber ones never leak of you don't over tighten. So just produce rubber ones (please manufacturers).

 

[k13wjd]

PCP = Pre-Charged Pnumatic. in basic, a multi shot air rifle, that has a small chamber, like 150cc. Its pre-charged from a dive bottle, to around the 200 bar mark. Then, a simple valve releases a small burst of air, in order to propel the pellet. Makes for a very VERY accurate rifle, with zero recoil.

Anyhow, its dropped 3psi over night, and has held there all day. Yesterday was pretty warm though, so i'll wait untill tonight, and if its still the same, i'll fill her up.

 

[Toddyplumb]

Come on ! universal gas law P1 V1 /T1 = P2 V2 /T2 . Volume constant so dead easy

 

[k13wjd]

yes, i did consider that, at around 18 gallons in my system, 3 % change ( 3 psi from 2bar ( nearly 30 psi ) is fairly insignificant - as temprature change would be more than that.


I'll leave it for another 24 hours, and check for a further drop.

 

[k13wjd]

well, i released the air tonight, and filled it to 2 bar with water - after a good session of bleeding and filling, i have 3 joints that weep slightly, all compression joints, all the ones with brass olives.


Nipped them up, will check in the morning.