[PSUBS-MAILIST] Pressure Test Chamber

Thu Jun 22 18:05:07 EDT 2017

Hi Steve,

Not sure what you are aiming at with reconciling the diesel engine example. A diesel engine has a trapped air volume that is compressed at approx. 18:1 and higher (21.5:1) (theoretical temperature of 725 deg F) and then the diesel is injected in as a fine spray to ensure combustion.   

I have left out a few details in what I described and also the formula was incorrect as I did not bring back the temperature in the formula from Rankine however I did in my calculations. 

Anyway the 1600 deg F was only if all the air is compressed without adding more air as the air added lowers the temperature as does the dissipation of heat into the walls of the tubing, valves etc.

The point of the whole message is that high temperatures can be reached at only 2000 psi or less.  Rather than say it is OK to use air I wanted to emphasize the possibility of  dangerous practices and the reasons behind it.

I started off with B.A. Fire service and Scuba compressor maintenance and later went on to gas compressors with CNG/NGV.  The industry has seen a bunch of cowboys, explosions, irresponsible engineering etc.  at this stage of life I have forgotten more than I have learned but I know who to ask and when.  Normally it is Cliff & Sean who pick up on these things.  We try to idiot proof our valves but that is hard to do.  We have just been working on a breakaway coupling and found that if the O’rings are damaged on assembly then it can create a fast decoupling situation.  We have now fixed that.

There is a good mix of guys like Hank who just get on and do things and there are some ballsy things going on so that is just great to see.

Good to catch up even though by way of forum.

Regards,

Hugh

From: Personal_Submersibles [mailto:personal_submersibles-bounces at psubs.org] On Behalf Of Stephen Fordyce via Personal_Submersibles
Sent: Thursday, 22 June 2017 10:24 PM
To: Personal Submersibles General Discussion
Subject: Re: [PSUBS-MAILIST] Pressure Test Chamber

Hi Hugh,

Thanks a bunch for the detailed explanation - that makes good sense for the SCUBA compressor example and I hadn't considered adiabatic heating.  

I should perhaps have qualified what I was getting at - that I always like to reconcile my empirical experience with the theory behind it, because the "real world" interactions that are often counterintuitive to the theory.  I'm also an engineer and I find it's the combination of background knowledge and empirical experience that makes an expert.  My background is in cryogenic and industrial gas systems which are strictly oil free, but I'm still a little surprised I've never even come across this concept.  I also do things with SCUBA compressors, which is where those examples come from.

Really interesting to hear about your HP seats spontaneously combusting!  Do you think that's from reaching heat of ignition on the seats?

On a big compressor the final stage sections may get up to 200bar+ in a matter of 10 seconds or so as a matter of normal operation.  I guess this still counts as "slow compression".

What are your thoughts on reconciling the diesel engine example?  I would think they spontaneously combust the fuel/air mix by pressure at similar pressure, and the fuel will have a much lower flashpoint than the oil.  Surely that would be fast compression?

Cheers,

Steve

On Thu, Jun 22, 2017 at 2:10 PM, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

Thanks Hugh,

great example. I can imagine the sonic boom travelling up the valley &

your wife running out of the house!

I  was at Global dive recently getting them to look at the sloppy thread on

my pressure transmitters that fit on to my first stage regulator. I might run 

them past you for a second opinion if that is ok.

Thanks for the offer of the 40,000 psi tester. I thought my lights might

implode around 2000psi but for my use they only need to stand 500psi.

Cheers Alan

Sent from my iPad

On 22/06/2017, at 11:15 AM, Hugh Fulton via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

Alan,

We have a pressure tester that goes up to 40,000 psi.  If you need to do a high pressure test we can assist.

We had an explosion with a 1” line that was pressurised with 3,500 psi air but there was oil left in the line after hydrotesting with oil.

The explosion blew out the side of the shed  and reached an estimated 25,000 psi in the pipes which ended up looking like Spaghetti.

I had explained in detail to the guy who did the fitting of the pipes that everything had to be taken apart after the hydrotest and cleaned using solvent, then soapy water etc. as well as telling him stories of explosions.

The guy, an aircraft engineer, did not do it properly and left oil in a branch tube, and I did not do a check on it.  Very lucky my son did not get killed as he opened the valve  doing tests.  These things do not just go pop when they fail and this forum needs to hear of these dangers.  The explosion was heard all round Clevedon. That was our first accident of that nature in 40 years of high pressure involvement. We only use water now for testing and make sure there is no air present.  We did have another incident when we hired out our tester to another company who tested a tank to 1500 psi and the sight glass was held in with a very fine thread.  The sight glass went through the roof iron along with the fittings and we never found the parts.  The inspector had looked in the sight glass about 20 seconds before it let go.  If you are using tube fittings always disassemble after swaging and check the ferrules are in the right way and that a proper joint has been achieved.  There are some fittings which are copies and not up to the job.  We had issues with “DK-lok” (from Korea) with NPT threads where the taper was ½ the required taper but the thread gauge said it was OK. They were also not full form threads.   Also had some fittings called “Tubefit”  from I don’t know where and they were poor copies of “Swagelok”.  

We use SSP fittings out of USA which are fully interchangeable with Swagelok but in my opinion they are better quality.  

Air from a scuba tank over water is OK but make sure you have a concrete wall between you and the thing to be tested.  Also you should have an excess flow valve in the line.  You can get those from  Swagelok etc.

Hugh

From: Personal_Submersibles [mailto:personal_submersibles-bounces at psubs.org] On Behalf Of Alan via Personal_Submersibles
Sent: Wednesday, 21 June 2017 5:58 PM
To: Personal Submersibles General Discussion
Subject: Re: [PSUBS-MAILIST] Pressure Test Chamber

Thanks Greg & Hugh,

there was a bit of doubt in my mind about it so mentioned my

intended process. I have just googled accumulators in hydraulic systems

& the danger of high pressure air! Yikes just as well I've got you guys

looking out for me.

What about air from the scuba tank pressurising water?

I can get all the air out of the test chamber apart from the air in

the line pressurising it!

Other than that I have done some previous tests with the water blaster

& may have to suffice with it's pressure!

Cheers Alan

Sent from my iPad

On 21/06/2017, at 3:33 PM, james cottrell via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

Alan,

Pressure testing that high needs to be done with liquid not gas. High pressure air is VERY dangerous. A pressure washer pump using water would be a safer choice.

Greg

  _____  

From: Alan via Personal_Submersibles <personal_submersibles at psubs.org>
To: personal_submersibles at psubs.org 
Sent: Tuesday, June 20, 2017 9:01 PM
Subject: [PSUBS-MAILIST] Pressure Test Chamber

I am making a plastic container to fill with water & stand inside my

pressure chamber. There is a gap between the wall of the container

& the chamber which I intend to fill with oil. There will be about an

inch of air at the top of the chamber & I'm using a scuba tank to

pressurise the unit up to 3000psi.

Will car engine oil do to fill the gap between my plastic container &

the chamber wall, or do I need to go with a hydraulic fluid.

Am using the plastic container to keep water from rusting my test

chamber. (Aussie Steve's idea)

Cheers Alan

Sent from my iPad

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