Every pump is different from another and that’s why you need to keep certain features in mind while choosing one. According to hydro test pump suppliers, there is a variety of pumps available in the market and you must be clear about your requirements while picking a pump. If you keep few pointers in mind, you can easily make the right selection. Let’s take a look-

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Pressure and volume

There are two important aspects to consider while making a selection. Those two factors are pressure (in pounds) per square inch and volume (in gallons) per minute. If you want to use the pump for life safety applications, then you can buy an electric pump. If you are looking for a powerful pump, you can go for a pump with a higher PSI, gallons per minute and horse power. You can easily add more pressure to the system by using a pump with higher gallons per minute.

Weight

Suppliers selling hydraulic testing pumps say that another consideration is weight. Pump which are light in weight, can be operated with just one hand, while pumps with large dimensions require two people to lift them. These pumps are mounted on wheels. Simple rule is, the more the power, the higher the weight. A pump made of aluminum is way lighter than of those made with components such as copper, steel and iron.

Pressure gauges

There must be a pressure gauge to indicate the pressure which is being supplied to the system. In order to stabilize the needle of gauge, glycerin, silicone and other substances are used in liquid filled gauges. You must know that hydraulic testing pumps have moving components which generate vibration and heat. It is obvious that these forces are enough to cause damage to pivots and links within the system. Gauge may also fail resulting in inaccurate readings and that’s why pressure gauges are a crucial part and major consideration while selecting a pump.

Pump hose and electrical cord

It is good to have cable of long lengths as they can be used for facilitating connections over a long distance. Pumps which have cords of short lengths can be coupled with standard extension cords. It is recommended to discuss this crucial aspect with the manufacturer.

The best way to choose a hydraulic testing pump is to discuss your requirements with a good hydro test pump supplier. They have an in-house team of experts to guide you through.

Hope you found the post useful. For more information pertaining to hydrostatic test pumps, feel free to get in touch with us.

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Hydrotest vs. Testing with Air

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Hydrotest vs. Testing with Air

Hydrotest vs. Testing with Air

clooney

(Mechanical)

(OP)

15 Jan 05 06:02

Dear gents,
I have not come across any work instructions or specs in our office establishing a guideline on "deciding when one should consider testing a piping system or pipe line by Hydrotest vs. Pneumatic Air Test?" or any other method for that matter.  Can anyone direct me in finding the answer?  I have been checking with the more experienced people in our office but, despite all their past achievements, none of the seniors were able to give me a straight answer or at least guide me in my search . Thanking you in advance.

Farzad

RE: Hydrotest vs. Testing with Air

zdas04

(Mechanical)

15 Jan 05 09:30

Some of the guys working on it felt that the accumulated energy in a pneumatic test created an unreasonable risk.  The ASME (in B31.8) disagreed.  If the hoop stresses allow, I will always do an air test preferentially a water test because:  (1) I live in the Rockies and a 1,000 ft elevation change results in a 431 psi difference in test pressure between the bottom and the top of the hill, for a 900 psi test that puts me in the position of either not really testing the top of the hill or getting really high hoop stresses in the bottom; and (2) disposal of test water is becoming a big issue (you have to treat the water for microbes, and the EPA is cranky about putting biocides into ditches-washes-rivers.

B31.8 has some pretty extensive information about when you can use air, nitrogen, or natural gas (as long as your hoop stress is less than 30% of SMYS, sometimes you can buy your test media from a nearby mainline pipeline and get the gas already at test pressure and you don't have to throw it away, you can sell it back).

I've got a document under "samples" on my web page where several of us tried to provide guidance on choice of test media and other topics related to static testing.Some of the guys working on it felt that the accumulated energy in a pneumatic test created an unreasonable risk. The ASME (in B31.8) disagreed. If the hoop stresses allow, I will always do an air test preferentially a water test because: (1) I live in the Rockies and a 1,000 ft elevation change results in a 431 psi difference in test pressure between the bottom and the top of the hill, for a 900 psi test that puts me in the position of either not really testing the top of the hill or getting really high hoop stresses in the bottom; and (2) disposal of test water is becoming a big issue (you have to treat the water for microbes, and the EPA is cranky about putting biocides into ditches-washes-rivers.B31.8 has some pretty extensive information about when you can use air, nitrogen, or natural gas (as long as your hoop stress is less than 30% of SMYS, sometimes you can buy your test media from a nearby mainline pipeline and get the gas already at test pressure and you don't have to throw it away, you can sell it back).

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

RE: Hydrotest vs. Testing with Air

1969grad

(Mechanical)

15 Jan 05 10:41

I prefer to use a pneumatic test for equipment fuel gas lines and lube oil lines because of the difficulty of getting the line dry after testing.

RE: Hydrotest vs. Testing with Air

danthemeterman

(Petroleum)

15 Jan 05 23:20

I refuse to use pneumatic, same reason as "Z"'s coworkers gave. I have witnessed the destruction a vessel can cause when it belches.If you havent seen it, don't wish for it. Some automitive shops have used 125 gallon discarded propane(no longer refillable, working press. was 350#)for extra capacity. One in particular had approx. 145#'s of air, in the middle of the night(thank God) the tank let go at the seams. this shop "WAS about 75' x 100", the property taxes have declined considerably, the insurance company refused to pay since they were using a non spec tank for thier backup air.


A word to the wise is not wasted

RE: Hydrotest vs. Testing with Air

clooney

(Mechanical)

(OP)

16 Jan 05 04:04

Thank you guys.  At least now I have an idea of what the advantages and pitfalls are in using either of these methods. And thank you Dave Simpson for putting up your site.  I'll definitely look it up.
much obliged

RE: Hydrotest vs. Testing with Air

zdas04

(Mechanical)

16 Jan 05 09:54

Danthemeterman,
From what I can tell from your post, the propane bottle explosion was a failure in service of a vessel being used for an air receiver for a period of time. One has to assume that the failure mode was fatigue.

I wonder what the hoop stressesn the vessel were?  ASME has some very specific restrictions on hoop stress during a pneumatic test, and I'll bet your ad hoc reciever did not conform to those restrictions.

Auto shops are not the only people who follow this dangerous practice, I recently did some flow testing at the engineering department of a major US university and their air reciever was a 500 gallon propane tank.  I couldn't find a code stamp on it and asked when was the last time the tank had been certified as fit for service.  The tech gave me a blank stare and the department head said he would take care of it.  I haven't been back to see if he did.

I have never found the fact that air-receivers fail in prolonged or inappropriate service to be a reason not to use pneumatic tests.

David

RE: Hydrotest vs. Testing with Air

danthemeterman

(Petroleum)

16 Jan 05 10:37

Zdas
I fully agree there is no reason to assume that beacuse of a catastrophic failure in an inappropriate installation would be reason to cancel a proven procedure. however i maintain that due to the possilbility of even a new vessel (tank,pipe etc) failing it's integrity the hydrotest method is manyfold safer than pneumatic. When I test a flex coupling to 415 psi. I put it in a cage I built to somewhat contain the possibility of schrapnel.
The failure of the propane tank was most likely due to the moisture introduced from the compressor and the climate in the shop causing humidity?? Just a thought. Since air tanks are built with coating.

RE: Hydrotest vs. Testing with Air

danthemeterman

(Petroleum)

16 Jan 05 10:57

Zdas, What is hoop (stressing)? Are there any good sights that can get into detail? Thanks

RE: Hydrotest vs. Testing with Air

zdas04

(Mechanical)

16 Jan 05 20:23

That was supposed to be "hoop stress in ...".  ASME B31.8 has a good explaination of hoop stress.  It is also discussed in the document on my web page that I referenced above.

So if the propane tank failure was due to the introduction of water, was it a corrosion failure?  Did anyone get pieces of the shrapnel to verify that the failure was corrosion related?  Most corrosion failures I've seen have been more weeping than tearing failures.

David

RE: Hydrotest vs. Testing with Air

danthemeterman

(Petroleum)

16 Jan 05 20:44

zdas
Sorry , so long ago I cnnot remember the stats, just remeber the occurrence.
  Side Bar: Did you hear about the 30,000 gallon propane tank that blew it's end off in Gwinner N. D. approx. 1988. the tank was full of propane. The end blew off, sent it approx. 100 yds. into a block wall and flew another 200'? hit another block wall and came to rest. No one hurt, happened about 1/2 hr. before maintance arrived. NO FIRE!!!
I witnesed a 100# l.p. tank some years after it blevied, some one put it in the basement full of gas, gass heated faster than the furnace used it. The tank was found splaid out flat with the ends missing.

RE: Hydrotest vs. Testing with Air

SteveK4

(Mechanical)

16 Jan 05 20:50

  The choise of Hydro versus Pneumatic really depends on the specific piping system being tested. If there is a big elevation change, the possibility of contaminating the system, disposal considerations, or if your are testing a low pressure system such as service air; you would most likely opt for pneumatic. However for most systems a hydro-test would be correct choice. Several of the piping codes give general guidlines on testing.  On a side note, "Hoop Stress" is the tensile stress applied to a pipe or vessel caused by the pressure contained by them. You calculate it by P*D/(2t) where P=pressure D=outside diameter and 2t is twice the wall thickness. the result is the stress applied to your pipe or vessel wall. it is important to keep this number below the allowable strees of the material used in construction.

RE: Hydrotest vs. Testing with Air

Contact us to discuss your requirements of hydrostatic pressure pump. Our experienced sales team can help you identify the options that best suit your needs.

bvi

(Mechanical)

17 Jan 05 18:52

Hydrostatic is almost always preferred for a number of reasons.  In piping per some codes, the hydrostatic test is conducted at a high pressure which results in "warm prestress" which increases the resistance of the tested equipment to brittle fracture.  This is from crack blunting and putting crack tips, post test, in compression.  The risk is also certainly less, because the consequences of a fracture in a pneumatic test can be much more severe than a hydrotest, since the stored energy of compressed gas is so much higher.  

Having said all that, there are a number of good reasons for performing a pneumatic test.  However, in the absence of these specific reasons, which have generally been cited by the other posts, a hydrostatic test should be performed.

RE: Hydrotest vs. Testing with Air

zdas04

(Mechanical)

17 Jan 05 22:25

"Almost always prefered" by whom?  The codes don't show a marked preference and at least 2 of us in this thread will always do a pneumatic test over a hydrostatic test if the hoop stresses are in the right place.

I don't mean to jump on you, but I keep running into folks that feel somehow superior for generating thousands of tons of waste material that is toxic to the environment and difficult to dispose of.  This is simply an engineering decision, and for systems that are compatible with pneumatic tests, I find them to be quite reasonable.

David

RE: Hydrotest vs. Testing with Air

DVNMech

(Mechanical)

19 Jan 05 03:33

In Europe, following the new European standards (EN13480-3 for piping or EN13445-3 for unfired press. vessels), the proof test shall be a hydrostatic pressure test (exceptions however are possible) The pressure equals the max of (1.43*design pressure) or {1.25*design pressure*allowstress Ttest/allowstressTdes}. In some cases, for example when a pipe or vessel operates in creep area, this leads to very high pressures.

RE: Hydrotest vs. Testing with Air

pmover

(Mechanical)

19 Jan 05 11:54

all,

the responsible engineer is to consider the varying conditions in which the piping system (pipeline or otherwise) is to be tested and then utilize the appropriate codes & test medium for implementing the test. certainly, ambient conditions (i.e. temperature) and elevation changes need to be considered.

i have witnessed many hydro and pneumatic tests on pipelines, piping systems, and pressure vessels with the test medium decision based on sound engineering judgement and by following the applicable code(s). in some cases, agreement between the contractor and client was obtained to conduct the pneumatic test, followed by a hydro test when ambient conditions allowed for hydro test.

hope this helps...
-pmover

RE: Hydrotest vs. Testing with Air

ARenko

(Mechanical)

27 Jan 05 15:34

zdas,

I've not read 31.8, nor do I have access to it at the moment, but I am curious - does the hoop stress you mention not apply to hydrostatic testing?  In other words, does the spec consider that some additional safety is required due to the risk of damage if a failure were to occure while testing with air as opposed to water?  Might this mean that if someone was intending to design a vessel for testing with air that it may be overdesigned for testing with water?  I would've thought that the 30% you reference would apply to any test media.

I've mostly tested high pressure equipment (10ksi to 15 ksi rated), and therefore have done hydrostatic testing.  However, I once did some testing of liquid tank containers at 20 psi.  We filled those nearly full with water before testing with air in order to allow a minimal amount of air in the vessel.

RE: Hydrotest vs. Testing with Air

zdas04

(Mechanical)

27 Jan 05 17:38

djv,
There are many hoop stress limitations throughout the testing section of B31.8.  The one I refered to is the most restrictive and doesn't allow testing with natural gas to pressures that result in hoop stress over 30% of SMYS.  There is another restriction that deals with personnel approaching a line when under a pneumatic test that results in hoop stress over 70% of SMYS.

All of the limitations in B31.8 are specific to a given set of conditions and location classes.  It is tough to generallize it.

David

RE: Hydrotest vs. Testing with Air

moltenmetal

(Chemical)

28 Jan 05 07:58

It's a simple matter of comparing risk versus risk.  If a hydrotest is feasible and won't produce secondary hazards, you should do a hydrotest because it's safer- the stored energy is far lower than in a pneumatic test and hence the hazard resulting from a sudden failure is FAR lower.

If introducing water into the system in question will introduce secondary hazards (i.e. corrosion, damage to equipment, or generation of an unmanageable amount of hydrotest fluid), you have to weigh that risk against the risk of the test itself.  You may choose to increase other nondestructive testing on the work in question to give yourself more confidence prior to the pneumatic test.

We frequently do hydrotests for basic integrity followed by a pneumatic test for leakage.  The first test is low hazard and gives confidence that the pneumatic test can be carried out safely.

RE: Hydrotest vs. Testing with Air

rmw

(Mechanical)

29 Jan 05 01:41

Danthemeterman,

Right after reading your post, within a day or so, I was at my shop, an old building of about 2500 ft^2 and I fired up the air compressor, only a 33 gallon receiver, but it is direct connected to an older receiver with a bum compressor, about 60 gallon for extra storage, and just about the time that it pressured up to the max and shut off, I heard a sudden hiss of air that indicated something had sprung a leak.  Suspecting a hose, I quickly checked out the hoses, and realized that it wasn't a hose, and that it could only be that the tank had sprung a leak (at the bottom, naturally, where the moisture collects) and all I could think of was what you described in your post, and I was torn between running out of the wooden building, and trying to open up something to reduce the air pressure before the tank burst.

It did not burst, and I got the pressure off, but what you described in your post sure played on my mind, and gave me a fright.

rmw

RE: Hydrotest vs. Testing with Air

quark

(Mechanical)

29 Jan 05 07:39

It was good that you took running for a precautionary measure. That could be as dangerous as you thought. There was one accident, I know, of a 0.5m3(132 gallon) receiver with a bottom leak jetting into air and landing 50feet away. This was three years back but I have yet to know whether the receiver was properly bolted to the foundation or not.

Regards,

rmw,It was good that you took running for a precautionary measure. That could be as dangerous as you thought. There was one accident, I know, of a 0.5m(132 gallon) receiver with a bottom leak jetting into air and landing 50feet away. This was three years back but I have yet to know whether the receiver was properly bolted to the foundation or not.Regards,

RE: Hydrotest vs. Testing with Air

mgp

(Mechanical)

31 Jan 05 12:34

In 1995 there was an incident in a british construction yard where three workers were killed during a nitrogen pressure test. Here's the story as I have heard it.

The project was an offshore topside gas facility.
The pipe to be pressure tested was an 18" gas line - in order avoid wetted piping, reasons were good to use nitrogen for pressure testing instead of performing a hydrotest. For bolted joints, Grayloc type clamp fittings were used.

Somewhere on that line a valve or something had been temporarily removed during the test and the line was blinded off with a blind hub and a Grayloc clamp. A couple of meters upstream of that line there was a check valve and further upstream was the pressure indicator used for the hydrotest. The check valve was not removed as nitrogen was being filled from the upstream side, however with the line being blinded off downstream there was a considerable volume trapped between the check valve and the blind hub.

The pressure test was performed, the system was depressurized using the PI, but because the PI was located  upstream the check valve the trapped volume remained pressurized to some 180 barg (2610psig) without anybody noticing it.

The workers went back to reassemble the removed valve. First unbolting the clamp - it stuck so nothing happened.
The poor guy then hit the clamp with a hammer to it loosen it and that was the end for him and to of his colleagues.
The nearest guy virtually dissapeared as he was blown to pieces.

It must be said that my information is second hand as I have it from several colleagues or ex colleagues who at the time worked for the company that designed the module. All I could find during a google search was this:
http://www.wwne.demon.co.uk/ww2000/d00-4.htm#letter

One can argue that this incident was due to human error, but it surely demonstrates that pneumatic pressure testing should be done only with extremely high safety precautions.
Safety cost money, so if there are not very good reasons for using air or nitrogen - use liquid.

Whenever we do pneumatic testing - e.g. for instrument air, nitrogen (much smaller pipes and lower pressure) - we have to evacuate the whole area etc etc. during the test. This is a legal requirement.

Just my comment

Regards
Mogens






 


.

I agree with moltenmetal and others that a hydrotest should always be preferred unless there are good reasons not to do so - there may well be.In 1995 there was an incident in a british construction yard where three workers were killed during a nitrogen pressure test. Here's the story as I have heard it.The project was an offshore topside gas facility.The pipe to be pressure tested was an 18" gas line - in order avoid wetted piping, reasons were good to use nitrogen for pressure testing instead of performing a hydrotest. For bolted joints, Grayloc type clamp fittings were used.Somewhere on that line a valve or something had been temporarily removed during the test and the line was blinded off with a blind hub and a Grayloc clamp. A couple of meters upstream of that line there was a check valve and further upstream was the pressure indicator used for the hydrotest. The check valve was not removed as nitrogen was being filled from the upstream side, however with the line being blinded off downstream there was a considerable volume trapped between the check valve and the blind hub.The pressure test was performed, the system was depressurized using the PI, but because the PI was located upstream the check valve the trapped volume remained pressurized to some 180 barg (2610psig) without anybody noticing it.The workers went back to reassemble the removed valve. First unbolting the clamp - it stuck so nothing happened.The poor guy then hit the clamp with a hammer to it loosen it and that was the end for him and to of his colleagues.The nearest guy virtually dissapeared as he was blown to pieces.It must be said that my information is second hand as I have it from several colleagues or ex colleagues who at the time worked for the company that designed the module. All I could find during a google search was this:One can argue that this incident was due to human error, but it surely demonstrates that pneumatic pressure testing should be done only with extremely high safety precautions.Safety cost money, so if there are not very good reasons for using air or nitrogen - use liquid.Whenever we do pneumatic testing - e.g. for instrument air, nitrogen (much smaller pipes and lower pressure) - we have to evacuate the whole area etc etc. during the test. This is a legal requirement.Just my commentRegardsMogens

RE: Hydrotest vs. Testing with Air

SMF1964

(Materials)

31 Jan 05 12:39

Hydrotest vs. pneumatic test:  Let's start on the reasons for performing the pressure test to begin with - to assure that there are no leaks.  

If you do this via pneumatic test and you hear 'hissing', that tells you only that the system as a whole has a leak somewhere.  You must then go look for the leak.  

If you do this via hydrotest, then your leak will be readily visible because it's got a wet spot.

As long as we're talking horror stories:  had a station air compressor fail at a flexible hose (2" diameter).  The hose burst like a balloon, sending at least three pieces of the steel flying, along with some wire filaments from the braiding.  One of the steel pieces caught the mechanic on the jawline, missing his jugular/carotid artery by less than 1/2" (1 cm, to those of you metric) but severing the major nerves in his face so that he cannot smile on one side of his face.

If you want to see the consequences of pneumatic vs. hydrostatic related failures first hand, pop a balloon with a pin and pop a water ballon with a pin.  The difference will be readily apparent.

RE: Hydrotest vs. Testing with Air

zdas04

(Mechanical)

31 Jan 05 12:47

I've got a whole book full of piping failures in gas service.  I don't think that using gas as a test medium is a significant portion of the list of failures.  That is because in a pneumatic test there are a list of things that must be checked prior-to and during the test.  Had the workers in the example above followed those procedures (and if the engineer designing the test had called out the proper procedures) they wouldn't have been killed that day.

Gas is more concentrated source of energy than liquid.  Any time you use it under pressure you need to use appropriate care.

David

RE: Hydrotest vs. Testing with Air

dwedel

(Mechanical)

1 Feb 05 00:06

zdas04,

I agree with you on the safety involved with pressure testing.  Hydrotests can fail spectacularly.

Can you share any of your stories?  I'm affraid I can't share my hydrotest story.

dwedel

RE: Hydrotest vs. Testing with Air

2

Scipio

(Mechanical)

1 Feb 05 12:03

Now, there are a number of reasons to justify pneumatic testing, like zdas04's elevation change (I'd do a pneumatic there, myself), but my default is a hydrotest.  In fact, in my region (Alberta, Canada) the local regulators require hydrotests be performed unless there's adequate justification for alternate testing, and then they want to see detailed testing procedures to cover the checklists zdas04 mentions, which has to be prepared and stamped by an engineer prior to testing.  There's an interesting article on the danger of pneumatic testing on page two of this pdf link:

http://www.absa.ca/newsletter/v07-2002/v7-iss2.pdf

The really interesting part is the equivalent energy table, which equates a 2 ft x 6 ft vessel of air at 500 psi to a pound of nitroglycerin.

One of the ways I look at it, if every pipe spool or pressure vessel was designed properly, fabricated per design properly, the original materials were cast or forged properly, and the joints made up properly, so there was no chance of a failure, there really wouldn't be a need of a pressure test. Since that's not the case, pressure tests are performed because occasionally, something's screwed up enough that the spool or vessel will fail pressure testing. If I have the option, I'd rather it fail during a hydrotest than pneumatic test.Now, there are a number of reasons to justify pneumatic testing, like zdas04's elevation change (I'd do a pneumatic there, myself), but my default is a hydrotest. In fact, in my region (Alberta, Canada) the local regulators require hydrotests be performed unless there's adequate justification for alternate testing, and then they want to see detailed testing procedures to cover the checklists zdas04 mentions, which has to be prepared and stamped by an engineer prior to testing. There's an interesting article on the danger of pneumatic testing on page two of this pdf link:The really interesting part is the equivalent energy table, which equates a 2 ft x 6 ft vessel of air at 500 psi to a pound of nitroglycerin.

RE: Hydrotest vs. Testing with Air

zdas04

(Mechanical)

1 Feb 05 13:47

Sicipio,
That case study has been kicking around a while and it is pretty scary.  While the arithmetic of energy storage is accurate, a vessel does not have the ability to deliver the energy as effeciently as a pound of nitro.  

I once calculated the stored energy in a 20 mile, 12-inch static test to 700 psig with air.  The numbers were in the kiloton range.  The problem with the calculation is that by the time the gas even 100 ft from a failure sees the hole, it is a flow calculation instead of an explosive decompression calculation.  I was unable to establish exactly what volume of gas would participate in the decompression, but I was able to determine that it is a fairly small subset of the total gas in the test.

David

RE: Hydrotest vs. Testing with Air

dwedel

(Mechanical)

1 Feb 05 15:13

zdas,

I deal mainly with maintenance on the compressors, so I am not exposed to the pipe testing as much, however, I do need to have knowledge of the testing.

I was at a welding class, and they were talking about the hydrogen crack in a pipe.  If the longtitude crack in a pipe does occur, they said that the crack would propogate through the steel faster than the speed of sound in air.  Thus the pressure in the pipe would not relieve itself and the crack would propogate through a hole length of pipe.  Is this crack propogation faster than the speed of sound in water as well?

to everyone,

I guess, that I agree, that pressure testing with water is preferable to air.  However, many of the posts here are emphasizing the explosive power of air, and it makes me feel that people are getting the idea that testing with water is "safe".  I would emphasize that this is not the case, that a pressure test is dangerous no matter what the medium.  All pressure tests should be treated with caution and careful adherance to protocol.

RE: Hydrotest vs. Testing with Air

moltenmetal

(Chemical)

2 Feb 05 22:06

You're right that a hydrotest is not "safe", it is "safer".  That said, a properly-performed hydrotest is not just safer, it is ENORMOUSLY safer than a pneumatic test at equivalent pressure.

Crack propagation speed in metal is irrelevant to the safety difference between the tests.  The volumetric expansivity of the test medium IS.  A liquid's pressure decreases enormously with decrease in volume (i.e. it is nearly incompressible), and hence the amount of energy stored in a cylinder of compressed (subcooled) liquid is minor compared to the amount stored in a similar volume of gas at the same pressure.  

Hydrotests are safer than pneumatic tests because even the sudden generation of a crack in a vessel or pipe will merely generate a leak or spillage rather than schrapnel and a devastating shock wave.  Even the small deformations necessary to develop the crack will reduce the pressure in the vessel enormously if it is entirely filled with water.

A well-designed and executed hydrotest takes advantage of these factors.  By carefully eliminating the trapped air in all components prior to carrying out the test, a hydrotest can become a very sensitive leakage test as well as a very safe test.

RE: Hydrotest vs. Testing with Air

bigskyguy

(Mechanical)

3 Feb 05 01:18

I run pressure tests weekly (some time daily) on a variety of differant systems, I'll tell ya from experiance, I run with pnumatic tests at a lower pressure before I move on to full pressure hydrostatic tests. For a typical 150-300 PSI system that requires a hydrostatic test of 1.5 working pressure, throw a 50 PSI air test on it first.  Main reason being, if their is a leak, it sure is alot easier to fix if you only have to release air pressure instead of dumping out water.  I've argured this with a number of inspectors, my solution, stick to the specification but take precautions and use other means of testing that is more user friendly to repairs. And always put the gauge at the lowest part of the system....  

RE: Hydrotest vs. Testing with Air

mgp

(Mechanical)

3 Feb 05 02:29
Thread794-26767

Regards
Mogens

There was a thread about equivalent energy from a gas contained vessel which might be of interest:RegardsMogens

RE: Hydrotest vs. Testing with Air

joeswoes

(Materials)

3 Feb 05 08:29

My 2 cents as a chemical process equipment designer :

Looking for leaks ? low air pressure under water or with snoop detergent.

looking for safety/structural integrity ? 1-1/2X rated working pressure hydrostatic.

Rated working pressure pneumatic testing is extremely dangerous due to the compressability of gas vs. liquid.

RE: Hydrotest vs. Testing with Air

dwedel

(Mechanical)

3 Feb 05 08:49

We just had an accident with a hydrotest.  Maybe there were some problems and some procedures weren't followed, I don't know as I was not there.  But what information we are getting back, it sounded like it was pretty explosive.  I cannot assume that it is enormously safer to perform a hydrotest verses air test.

Have a good day,

Moltenmetal,We just had an accident with a hydrotest. Maybe there were some problems and some procedures weren't followed, I don't know as I was not there. But what information we are getting back, it sounded like it was pretty explosive. I cannot assume that it is enormously safer to perform a hydrotest verses air test.Have a good day,

dwedel
Hotrod Big Engines!

RE: Hydrotest vs. Testing with Air

PAN

(Mechanical)

3 Feb 05 09:54

Refer to ASME B31.3, a pressure relief device is necessary for pneumatic test. This should imply that pneumatic test is more dangerous than hydrostatic test.

In my company, the safety operation team will not allow us to perform pneumatic test if we can perform hydrostatic test.

RE: Hydrotest vs. Testing with Air

zdas04

(Mechanical)

3 Feb 05 10:06

The best pictures I've seen of static test failures have all been vessels tested at unreasonable temperatures with water.  Brittle failure of a 2-inch thick piece of steel is a really impressive energy release.

Maybe one of the reasons I'm such a vocal advocate for pneumatic tests is that I've seen too many people enter a static test with a binary mindset (i.e., "hydro good", "gas bad") and not specify minimum water or ambient temperature for a test, not specify liquid disposal technique, not specify an air-elimination period, etc.  I once reviewed a hydrostatic test "procedure" where the "engineer" hadn't calculated hoop stress that the test would impose - when I calculated it at 175% of SMYS (at the bottom of a hill) he was a bit shaken and we re-wrote his document to make it an air test at under 30% of SMYS (same bulk pressure, no hydrostatic head).  This guy was actually a pretty good engineer, but was suffering under a silly set of pre-conceived ideas that had been shoved down his throat by people who think "hydro good" and he thought the test was the easy part of his design.

Everyone is at least a bit scared of gas tests and they tend to be better specified.  Fear is a powerful motivator.

David

RE: Hydrotest vs. Testing with Air

moltenmetal

(Chemical)

4 Feb 05 08:40

dwedel:  

I don't know the test's conditions so I can't comment.  But I can tell you this:  there's a reason B31.3, ASME VIII etc. carry out a hydrotest at 1.1x rather than 1.3x or 1.5x design, and that reason is the energy content of the compressed gas in a pneumatic test.  You can test small devices to failure quite safely if you are scrupulous about eliminating all the trapped gas and other means of energy storage- in fact this is routinely done to establish pressure ratings without carrying out finite element analysis etc.

We will test systems of limited volume pneumatically at low pressure (15 psig) pneumatically to get the gross leaks out prior to a hydrotest.  If it's of really limited volume, we may extend that to full shop air pressure (100 psig), but by no means would I recommend this as a general rule.

David:  we agree that thinking, NOT rules of thumb, are a prerequisite to safe testing.  The assumption that hydrotests are intrinsically safe, regardless of design, can be FATAL.  It's also possible to design and carry out a pneumatic test safely- it's done every day.  Any test which ignores sources of pressure other than that supplied by the test pump or gas cylinder, or ignores secondary effects of the testing program, OR ignores the stored energy in the test medium, may be safe, but only by accident- regardless whether it's a pneumatic or hydraulic test.

But again, stored energy is the principal hazard in MOST tested systems- and most people are not testing long, thin pipelines where the energy release on failure is limited by pressure drop in the line.  The energy release from the failure of a 5-gallon air receiver during a pneumatic test, or by accident due to corrosion, can be spectacular and can lead to injury or death and serious property damage.  The same receiver tested to failure by a properly-designed hydrostatic test WON'T result in injury or death- in fact it may not even get anyone wet.

RE: Hydrotest vs. Testing with Air

dwedel

(Mechanical)

5 Feb 05 12:45

Your points make more sense in light of the relative size of the test.  I had not considered that.

Thanks for the reply Moltenmetal,Your points make more sense in light of the relative size of the test. I had not considered that.

dwedel
Hotrod Big Engines!

RE: Hydrotest vs. Testing with Air

acoilfld

(Mechanical)

6 Feb 05 21:41

  Generly we hydro test when ever possible - for safety reasons and also because in B.C. an above ground hydro test is 1 hour, 4 hours for below ground, 24 hours for a air test.
  I only air test fuel gas lines and engine oil lines.  On a small air test we fill the line with compressed air and "bump" it up to test pressure with nitrogen.
  Working with a pipe full of compressed gas is like handeking a pipe bomb - only larger.  

  A.C.

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