Using air to increase pressure in Hydrostatic test 3

[CoolDude003]

hello all,

I would like to know if filling a hose with water and then adding air on top of the water to increase pressure would work. I believe its the same theory as a air over hydraulic pump which can get 100 psi of air pressure to create 10,000 psi of oil pressure.

I attached a very professional photo of the basic idea.

1. open water supply and opposite end of hose until it is completely full with no air in the hose.
2. close opposite end and then water supply.
3. open air supply until water pressure increases to desired test pressure.
4. stop air supply. release pressure at opposite end of hose.

Would this work? Any links, standards, procedures, examples, special equipment, anything at all would be fantastic.

Thanks for reading!!

 

[CoolDude003]

Left out important detail:

The water pressure I want to get to is only 525 psi.

 

[rutherford703]

If you do pressure test that way, you have to treat your pressure test as a pneumatic pressure test. Built energy is a big concern for pneumatic pressure test.

 

[zdas04]

A hydraulic (or pneumatic) pressure multiplier is only a reasonable device if the applied fluids is completely separated from the in situ fluid. A dual-ended piston with the area on one send twice as big as the area on the other end will exert a force equal to the area on the large end of the piston times the pressure applied. The piston will move away from the applied force until it either bottoms out or raises the pressure in the other side by enough where the small piston area times the pressure acting on it equals the big end of the piston times the applied pressure (i.e., twice as much pressure for a piston with a 1:2 size ratio).

The tricks are: (1) the piping on the small side of the piston must be completely full of liquid and mostly not compressible; (2) the seal between the two ends of the piston must be nearly perfect; and (3)there must be enough travel in the piston to handle the small compressibility that you do have.

Just pumping air into a water line will just turn the test into a pneumatic test and you will have to add enough air to get to your test pressure without any multiplier effect at all.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[btrueblood]

Google "hydrostatic test pump" or "air powered hydrostatic test pump", or go to

http://www.mcmaster.com

and enter either search term there. Air in a hydrstatic test is a pain in many ways.

 

[CoolDude003]

Thanks for the responses. I felt like it was too convenient to work.

So, the air would push it's way through the water into the hose?

 

[zdas04]

in milliseconds.

 

[btrueblood]

The air will dissolve into the water over time, faster with increasing pressure. This makes your hydro-test hard to control, as the pressure keeps decaying.

The 2nd problem is that the pressurized gas contains a lot more explosive potential than water, and so is more dangerous. Even if the air pocket is small, the gas dissolved into the water can un-dissolve very rapidly, giving a lot more "boom" when a pipe or fitting ruptures.

 

[Latexman]

Compressed air (gas) contains much more energy than pressurized water (liquid). It's the work done on the environment (∫PdV) when the vessel spontaneously deconstructs that's the problem.

Good luck,
Latexman

 

[bimr]

No, it will not work.

Air will dissolve in the water which will tend to lower the pressure reading during the testing time.
It is too dangerous to hydrotest with air pressure. Most air pressure tests limit the air pressure to approximately 20 psi or less.

Unlike water, which is incompressible, air is very compressible, making it hydraulically equivalent to a large mechanical spring. If something were to break or come free during a high pressure air test, the released air could propel an object a great distance with much force. This is why the necessary elimination of trapped air when initially filling pipelines can be so hazardous; air tries to move things to relieve built up pressure; water, because it is incompressible, does not.

http://www.ejprescott.com/media/reference/PresTestwAirvsWaterR-25.pdf

 

[moltenmetal]

If you have a SMALL vessel or other device which is completely full of water, and you use a small diameter tube or hose to introduce compressed air into the vessel from the top, it is possible to do a test which is of acceptable safety to modest pressures (I'd say 525 psig would qualify as modest pressure). A pressure regulator AND relief device on the gas side of the testing system is essential. An excess flow valve on the gas feed is also a good idea.

The test will be greatly less sensitive to leakage than a test where water is used as the pressurizing medium IF a pressure gauge is used as the means of detecting leakage. The gas pad will dampen pressure losses due to small liquid leaks, and as mentioned will also tend to dissolve (slowly) into the water.

This sort of test is nowhere nearly as dangerous as a full pneumatic test, contrary to what some people have stated above. The hazard of such a test is proportional to the volume of stored gas at a given pressure, which should be small unless the vessel under test is quite large, the test pressure very high and/or the item under test is very elastic- I don't know the nature of your hose, but that might be a problem in your case.

As to pressure multiplication, you have the answer above: you need two diaphragms or pistons of different areas, coupled by a rigid shaft. You only need a pump if one stroke of the cylinder/diaphragm doesn't give you enough water, which is likely even if your hose is quite rigid.

 

[bimr]

In this edition of The Cold Front, technical aspects of pressure testing piping systems are discussed.

http://www.irc.wisc.edu/pda.php?/file&id=328

 

[1gibson]

Safety concerns aside, I would not label that a "hydrotest" because the compressed air will mask a small amount of liquid leakage, as already mentioned.

Losing 0.5 psi from a hydrotest could just be due to temperature. Losing 0.5 psi from your pneumatic/hydro combo could represent enough liquid loss to be of concern. Visual inspection should catch it but you never know.

 

[moltenmetal]

From a safety of execution perspective, properly designed it IS a hydrotest.

From a sensitivity perspective using pressure loss as the means of detecting leakage, I agree that a test performed this way is less sensitive than one performed with the unit certified liquid full by evacuating and then filling with water. However, most people who perform hydrotests are far less scrupulous than that, tending to leave trapped gas in nozzles etc. Those gas pads behave in exactly the same way as far as test sensitivity is concerned.

 

[bimr]

With the limited information available, one would not think that it is not prudent to encourage someone like CoolDude003 that comes to an internet forum for information for the procedures to conduct such a test.

CoolDude003 posts that he is pressure testing a hose, not a vessel. Pressure testing of flexible hoses is different than pressure testing of rigid wall tanks. Unlike a vessel, a hose will stretch and release energy in a springlike fashion.

http://www.jgbhose.com/data_returns/testmethods.asp

CoolDude003 doesn't post whether this hose is 1-Inch or 500-Feet in length, nor does he post the diameter, materials, sevice life, pressure rating, or material of construction.

With the limited information posted, the best course of action is for CoolDude003 to take his hose to someone that does pressure tests for hire, rather than attempt this by himself.

Moltenmetal:

"Always use water for pressure testing."

See page 11.

http://www.gates.com/downloads/files/catalogs/IndHosePrevMainMan.pdf