6 replies to this topic

[arun261085]

During a hydrotest, equipment is initially filled with water and then water is pumped in using a +ve displacement pump to increase pressure. If water is incompressible how come water enters the equipment ?

[breizh]

http://hydrotestpump.com/

let you consider this resource .

Breizh

[ankur2061]

arun,

Who told you that water is an "incompressible" fluid? Water is compressible to a certain extent or it can be called as a partially compressibe fluid. The measure of compressibility of any fluid is its bulk modulus. Higher the bulk modulus of a fluid lower its compressibility and vice versa. To sum it up, all fluid are compressible, only the magnitude of compressibility differs, depending on the bulk modulus. Refer the link below for bulk modulus:

http://en.wikipedia....ki/Bulk_modulus

Regards,
Ankur.

[kkala]

In case of hydro testing a steel vessel (and generally not piping) water could be approximately considered as incompressible. As it fills the vessel, vessel volume increases, stresses increase, thus pressure of contained water increase. Water compressibility plays a minor role here.
Note: Suppose you throw water into a balloon, its volume will increase and the pressure of water in it will get a bit higher than atmospheric.
Of course water is not quite incompressible. Water compressibility has a significant contribution to pressure increase, when piping is hydro tested (due to small diameter, compared to a vessel).
Look at an arithmetic example in http://www.cheresources.com/invision/topic/13466-how-to-calculate-loss-of-water-from-pr-loss, (attached) file "elasticity.xls".

Edited by kkala, 02 July 2012 - 11:46 AM.

[ankur2061]

In case of hydro testing a steel vessel (and generally not piping) water could be approximately considered as incompressible. As it fills the vessel, vessel volume increases, stresses increase, thus pressure of contained water increase. Water compressibility plays a minor role here.
Note: Suppose you throw water into a balloon, its volume will increase and the pressure of water in it will get a bit higher than atmospheric.
Of course water is not quite incompressible. Water compressibility has a significant contribution to pressure increase, when piping is hydro tested (due to small diameter, compared to a vessel).
Look at an arithmetic example in http://www.cheresources.com/invision/topic/13466-how-to-calculate-loss-of-water-from-pr-loss, (attached) file "elasticity.xls".

Wow, increase in the vessel volume during hydrotest defies all principles of physics if we are talking of a steel tank or vessel and not a bladder made out of an elastomer. Could be a magic trick by "Criss Angel" the American illusionist.

Anyway who on earth would want to hydrotest a balloon, certainly not a chemical engineer.

[kkala]

Volume increase of a steel (pressure) vessel is much less compared to a balloon, yet this is the main reason why
more water can be injected into the vessel during its hydrostatic test after it has been completely full. Specific volume reduction of water (due to increasing pressure) is a secondary reason. Vessel wall is not rigid, it is expanded.
Careful reading of http://www.cheresources.com/invision/topic/13466-how-to-calculate-loss-of-water-from-pr-loss/, as mentioned in post No 4, is believed to be convincing concerning the above.
On the other hand derogating "flares" of post No 5 do not help in finding the truth, apart from lessening the forum. I personally believe that post No 5, apparently written in a hurry, had better be corrected.
Independent opinions on the subject from others are also welcomed.

Editing note 6 Jul 12: Second and third line changed for clarity.

Edited by kkala, 06 July 2012 - 03:12 PM.

[kkala]

Wow, increase in the vessel volume during hydrotest defies all principles of physics if we are talking of a steel tank or vessel and not a bladder made out of an elastomer. Could be a magic trick by "Criss Angel" the American illusionist.
Anyway who on earth would want to hydrotest a balloon, certainly not a chemical engineer.

Correction of above text and independent opinion from others was requested on Jul 2nd 2012, with no result (so far).
Repeating some parts of posts No 4 and 6, along with some new material, may be worth while. It is also pointed out that posts No 4 and 6 make reference to a Cheresources Forum link, whose post No 5 has already clarified the matter (arithmetic application in therebelow attached "elasticity.xls").
1. Reply to post No 1: during hydrostatic test of a pressure vessel, injected water can be considered as nearly incompressible, increasing vessel volume whose walls are elastically expanded. A balloon receiving water resembles to a sphere during hydrotest, of course sphere expansion is much less.
But during hydrostatic test of piping, pipe can be considered as nearly rigid and the water compressible.
Developed hoop stress of a hollow cylinder (vessel or pipe) is roughly σ = 0.5*P*D/s, where P=hydrostatic test (gauge) pressure, D=external diameter, s = wall thickness. At given P, σ is much lower on pipe than vessel (mainly due to much lower D), strain is much lower on pipe (Hook's law), and so is % "expansion" of pipe. Therefore injected water finds space mainly by compressing the rest water, pipe expands much less than vessel (percentage wise).
Above view for vessels or piping is simplifying for interpretation. Water contraction and vessel expansion actually occur in both cases, but the predominant phenomenon is different. This will be indicated in the arithmetic application later.
2. Some information from web about hydrotesting vessels:
2.1 Water is commonly used because it is nearly incompressible, therefore requiring relatively little work to develop a high pressure, and is therefore also only able to release a small amount of energy in case of a failure....The vessel is then depressurized, and the permanent volume increase due to plastic deformation while under pressure is measured .... http://en.wikipedia.org/wiki/Hydrostatic_test.
2.2 Incompressible fluids like oil and water are used for this test. ....Water is the most commonly used liquid in hydrostatic testing because it is more incompressible than any other liquid...(see attached "hydrostatic-testing-1.pdf).
2.3 Since water is almost incompressible, to increase the pressure of 600 m3 form atmospheric to 250 bar we require only an additional amount of one cubic meter of water. This is faster and requires much less effort. In case of the failure of any part the pressure also is reduced immediately preventing further damage. http://www.brighthub.com/engineering/mechanical/articles/26093.aspx.
3. Arithmetic application (based on mentioned "elasticity.xls").
Hydrostatic test on 5 m diameter vessel (20 mm wall thickness) and on 14" Sch40 pipe, each one of 410 m3 volume (water content 410 ton before compression).
Assume that pressure increases from 0 barg to 15 barg during hydrostatic test, no leaks.
3.1 Total injected water: Vessel 1.023 ton, pipe 0.386 ton (to increase pressure from 0 to 15 barg).
3.2 Metallic cylinder expansion: Vessel 0.744 ton, pipe 0.107 ton (0.18% versus 0.026%).
3.3 Water contraction: Vessel 0.279 ton, pipe 0.279 ton (same, since initial and final pressure is same)
For more explanations see "elasticity.xls" in the link mentioned in posts 4 and 6.

 

Editing note, 21 Feb 13: "of 600 m3" added from the source.

Attached Files

•  hydrostatic-testing-1.pdf   65.08KB   55 downloads

Edited by kkala, 21 February 2013 - 03:31 PM.