Pressure drop

[...CooLMan...]

I've just had a 20 psi pressure drop overnight. 1500-0800. It's a cold room with a 50m run. 3/4 and 1/2 inch pipe. 30m is outside. Liquid line is not lagged.
Solenoid valve is open and OFN was added into high and low side.
I couldn't find a leak and the pressure hadn't dropped in the 4 hours I was there. Could there have been any pressure equalisation (?) or is it possible the 7 degree temperature drop would have an effect?



Merry Christmas

[Mark]

http://www.refrigeration-engineer.com/forums/showthread.php?29090-N2-Pressure-Test-Formulae-Anomalies-Explained&highlight=pressure+test

Try the search box at the top right hand side of the page...DTlarca or MOB wrote this article...

[Mark]

Indeed, Happy Christmas to you and all on the forum:)

[Brian_UK]

20psi seems somewhat high and unfortunately you haven't given any figures to calculate the effect from.

If we assume that you started at 200psi at 12°C at 1500H and your temperature drop being 7° then it was 5°C at 0800H.

The following uses your Centigrade converted into Kelvin..

The third gas law states: P1/T1 = P2/T2

So P2 = (P1xT2)/T1

(200 x 278) / 285 = 195 psi as the end pressure. A drop of 5psi.

Hope this helps.. ;)

[Mark]

....yes

I would probably separate the installation into sections until the leaking part is determined...C/U, Pipework, Evaporator

Did you leave your gauges on or take them off?..

[...CooLMan...]

400psi down to 380psi. 12C down to 5C. I calculate this to be 390PSI. Gauges were left on.
So I've lost about 10 psi.
Gonna have to find the leak.

[jonjon]

try getting the system into a vacuum and attach a decent torr gauge

[...CooLMan...]

I have used a vg64. It took about 40 mins to pull 2 torr. As soon as I stopped the pump the vac went up. I've been told that these digital gauges read early and I need to vac for longer. Just purchased an analogue torr gauge and will try that to see if it will hold 1 torr for about an hour.

[hookster]

The digital vacuum gauges from your local wholesaler unfortunately are best chucked into a bin.
On a recent external calibration exercise none of the models were able to be certified. They failed at the below 5 Torr range where you really want them to be accurate and the readings received. The analogue torr gauges readings were repeatable and within acceptable tolerances.

[GHAZ]

I had a problem once with hose. It was a new hose took it out of the packet and put it on on the vac pump. The vac pump was pulling 1 torr and as soon i stop it the torr gauge would rise after about 20minutes. I was confident t.here was no leak.so i started to charge in liquid form and after a couple of minutes i saw white marks on the hose and it was forming ice. The hose was porous and i binned it.

[Tayters]

If you removed your gauge overnight, is there a chance the access port was leaking?

[Grizzly]

Hi Coolman.
Given the high pressure you are testing to. Is there really the need to test at such a high pressure. Strength test at those pressures by all means!
But for leak testing I would not normally push a system so high, particularly the low side for such a long length of time.
Your pressure drop is bordering upon a problem, but given the high pressures you are using I am not so sure as if you actually have a leak.
I have in the past seen a 5psi pressure diff on marine systems where the systems contain lots of steel and are basically in a huge steel box.
The difference temperature wise between mid afternoon and the following morning can be 5psi or more.

I have spent many hours scouring for leaks that were not there and the pressure difference was as you may suspect due to temp differential.

So in short feel free to leak check further.
But at the high pressures you have stated you would normally of found the leak earlier on.

I will be interested to hear what others say?
I would drop the nitrogen and try pulling a vac.
You may be pleasantly surprised.
Grizzly

[...CooLMan...]

Gauges were left on. :(

[...CooLMan...]

Cheers grizzly.
I was under the impression that I had to pressure test at MAP for minimum 1 hour ideally 24h. I new and still at college so maybe a bit green. I will give the vacuum a good test. I'm not going back till the new year as the place has shut down.
I've worked with some engineers that only pressure test for 30 mins and then let the gas go. I didn't see a drop for the first 3 hours and then came back in the morning to see 20 psi gone. My heart dropped. Gutted.
Back there fresh in the new year I suppose.
Thanks

[Grizzly]

Cheers grizzly.
I was under the impression that I had to pressure test at MAP for minimum 1 hour ideally 24h. I new and still at college so maybe a bit green. I will give the vacuum a good test. I'm not going back till the new year as the place has shut down.
I've worked with some engineers that only pressure test for 30 mins and then let the gas go. I didn't see a drop for the first 3 hours and then came back in the morning to see 20 psi gone. My heart dropped. Gutted.
Back there fresh in the new year I suppose.
Thanks

No sweat CoolMan.
I am sure some of the guys will quote you some official times.
Tayters question is very valid.
I would suspect your gauges and lines may be the issue here.

I try and isolate the gauges or at least minimise their influence upon the test system pressures.
Because most gauges or their lines will leak after time.
So try using a prov-en, dedicated gauge and line. Particularly if you are testing to those sort of pressures.
If you have to use the gauges and lines, try to isolate them from the system. Leaving them pressurised.
Particularly if the system is to be left for such a long test.
Simple things like keeping the line as short as possible.
Measure as far as possible from the charge/ pressurise point.

Is this a new install or a repair?

You may be new, but you are asking the correct type of question.
So good on you!
Grizzly

[...CooLMan...]

Replacement of outdoor/indoor unit and most of the pipe work. Some of the existing pipework was inaccessible.

[Grizzly]

Where part of the system is not accessable, can be a problem.
As suggested by both Mark and Jonjon.
All you can do if there is still a problem after the vacuum test.
Is test zones isolating sections where possible and prove sections one at a time.
Although I still think you may be ok?
You will have to let us all know how you get on.
Grizzly

[...CooLMan...]

Will do..
Thanks again for the advice. I'm back there in the new year so I'll let you know.

Cheers

[r.bartlett]

If you removed your gauge overnight, is there a chance the access port was leaking?

9869

Been there done that...:-/

[...CooLMan...]

Happy 2013 everybody :)
@ r.bartlett we left the gauges on overnight.

OK, so we arrived back onsite today with fresh heads. My colleague did not want to try and pull a vac. so we pressure tested again. This time just feeding only from the high side. As soon as we reached 28bar it just kept dropping. Top it up it would drop again and again. We then decided to split the system. closed the valves to the rest of the system on the condenser and added pressure. Same thing, it just kept dropping. Thought we had a leak on the outdoor unit. It then levelled off at 24 Bar. We added more OFN and it held. we then tried the same thing on the whole system. It took about half an hour of just topping the system up until we got the pressure to hold.
It's been left under pressure over night.
It's a Danfoss Optyma Plus. We have the indoor solenoid open. Could this be a closed valve in the outdoor unit? Will it cause a problem when we come to vac it tomorrow? If the pressure has held..
Thanks

[...CooLMan...]

We are hoping that this would have bee the reason for initial pressure drop. We looked again this morning and couldn't find a leak.

[...CooLMan...]

Just letting you know that there was no leak. System held pressure and we were able to hold the vac.
Thanks for all the advice. :)

[install monkey]

stick a spectroline glostick in-for peace of mind- and yellowy/green hands!

[stufus]

Always connect Gauges on Hi and Low sides to ensure pressures have equalized .Use quick couplers and disconnect if leaving overnight ,always spray service connections after disconnecting.
But at least you got it sorted.
Cheers
Stu

[Mark]

Glad its sorted...

always worth checking before pressure testing if any Pressure Relief Valve is fitted (Liquid Receiver or Suction Line) as they will lift if the test pressure exceeds the PRV rating and then reseal as the pressure reduces under the lifting pressure (seen these fitted on these Danfoss units) check the MWP of the Condensing unit LP/HP as these have a maximum working pressure subject to damage...

Kind regards
Mark