Is Hydrotesting Really Necessary on Low Energy Piping Systems? 3

[meyerske]

Is Hydrotesting Really Necessary on Low Energy Piping Systems?

For low temperature (but moderate pressure) piping systems where high quality piping material is used, what does a hydro test buy you that a pressure test at the max operating pressure wouldn't?

 

[metengr]

Is the intent of your question for new piping systems or for existing piping systems?

This question may seem odd but hydrostatic testing of new material can be quite beneficial, regardless of fabrication quality. Material that has been fabricated to form pressure containing components will see benefit during a hydrostatic test because any local areas of stress concentration in the component caused by discontinuities, inherent subsurface flaws or geometry changes will locally yield the material and it will become strengthened. This is a benefit that few people understand, in addition to finding gross leaks or defects in the material.

So, yes, I strongly believe in hydrostatic tests for new construction.

Topic #2 - hydrostatic tests on in-service equipment. Totally different matter because the material has already been squeezed (hydrostatically tested) after construction so a second and subsequent squeeze provides no benefit for the material. In fact, on aged components a hydrostatic test can possibly do more harm than good.

 

[vpl]

Code compliance?

Perhaps a bit tongue-in-cheek, but if the Code requires it, you need to do it.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[stanweld]

Hydrotests are essentially required to find leaks. I have found during my 35 years of experience that the greatest number of leaks occur in low pressure socket welded, small bore pipe and butt welds in thin wall, small bore pipe/tube. They are usually caused by improper tie-in, porosity and lack of "due diligence" with regard to inspection and assignment of appropriately skilled craftsmen.

 

[meyerske]

Yes, this is in reference to new piping systems.

I understand that hydros are required by Code, I'm just trying to under WHY they are required by code and what the benefits truly are.

Anyone have the history of how hydros came to be in the code?

I recognize that these questions are somewhat off-the-wall. My hope is to develop a better understanding of the purpose/benefit of hydros.

Apart from possible crack blunting, it seems like any system leaks detected from a high pressure hydro would be detected at operating pressure.

 

[davefitz]

" it seems like any system leaks detected from a high pressure hydro would be detected at operating pressure"

In the case where the fluid to be handled is more dangerous than water, why would one want to wait and see if there is a leak after the system is placed in service? Would you actually want to work near a system that is handling ammonia or naptha or other toxic fluid if the system integrity was not proven?

That being said, there are often cases where an "in service" test of systems that carry only air or water are accepted by the client for some low pressure systems ( such as circulating water systems ), provided the testing and subsequent repair of the system will not hold up commissioning.

As meteng stated, the extraordinary stresses imposed by the hydro test may work-harden some stress raisers, as well as place into a highly stressed state pipe hangars, supports, gasket surfaces, threaded or welded connections, etc. These "shakedown " stresses not only condition the pipig system and vessels for the lower continuous service stresses, but the hydrotest period serves as a logical transfer of responsibility from fabricator to operator. Without such a proof test, any subsequent component failure cannot easily be proven to be the responsibility of teh designer or fabricator, and the authority of the local inspector would have been mooted.

 

[Ashereng]

This was explained to me on why we hydro.

1) Water is fairly save, as davefitz mentioned. Water does not combust, is non-corrosive in most test intances, and is fairly available.

2) If testing with a gas (even inert), there is the possibility of explosion due to the overpressure. With water, pretty much a non-compressible fluid, it is more common to have a leak instead of explosion. Once the leak happens, the over pressure is relieved almost immediately.

I didn't know about the "any local areas of stress concentration in the component caused by discontinuities, inherent subsurface flaws or geometry changes will locally yield the material and it will become strengthened. This is a benefit that few people understand, in addition to finding gross leaks or defects in the material." I guess that's why I am a member of this site.

 

[vpl]

The ASME B&PV Code came to be in the 1880's because of problems with boiler explosions. Over the years, it has expanded to address other problems occurring in the mechanical engineering field. Even though the Code has become a huge and complex document, it is there to protect both the public and the engineers doing the design. When you've done the hydro, you, the engineer, know that your product is going to work in its intended application (or at least not explode and kill somebody.) That lessens any chance that you will be sued for improper design.

As to why a hydor's done above normal operating pressures: to help protect everyone from the unknown. There have been a number of times where I've come across or read about systems being unexpectedly overpressurized. Very seldom does that result in even damaging the piping, much less fatalities. The hydro is a controlled situation to make sure the system will be safe for normal operation.


Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[meyerske]

The systems I'm referring to contain only low temp water.

Drafting and approving the hydro procedures takes time and manpower (lots of trades manpower) and is very expensive.

Is the notion of eliminating a hydro plain old foolish or may it have merit if, based on many years of hydro test history, it can be shown that a hydro (above and beyond max operating pressure) is of little or no benefit.

Thanks for your opinions...

 

[metengr]

"Yes, this is in reference to new piping systems".

"I understand that hydros are required by Code, I'm just trying to under WHY they are required by code and what the benefits truly are."

Your question above has been answered. If you trying to search for information to work around Codes and Standards that is your business. Sounds to me like someone did not consider all aspects of the job.

 

[jte]

meyerske-

First, welcome to this forum! Please take some time to give us some of your background in the "Personal Profile" section on the top left of this page. This helps us understand your perspective on things. You can see mine by clicking on the "jte" above this post. My perspective may be different than someone from the power generation industry, for example.

Now... Metenger has provided his usual good service for providing some reasoning why you would hydrotest a system. One way that testing was explained to me was that you are expecting a failure. If everything was perfect, there would be no reason to test. You're driving at the consequence of a failure being low in your case so the risk of an online failure is low. That may be. Presumably you are working to B31.3 or related, and you have to stay code legal. That's simple also. Write a letter to the secretary of the Committee requesting a Code Case to exempt your situation from the generic for which the Code is written.

Also, if you plan to service test, be sure to do the test in the fully corroded condition! Having said that, I don't always call for hydro's on my work. I have one job now which involves a few spools which will be shop built and hydro'd and one field welded flange. Contents are waste water at around 50 psi. Working with my inspector we agreed to not hydro the field weld, but substitute two forms of NDE instead.

jt

 

[StressGuy]

Meyerske,

In B31.3 2004, refer to paragraph 345.1a. If you are talking about low pressure ambient water systems, you can probably come to an agreement with the facility owner to classify these lines at Category D and do a service test instead of a hydrotest.

Section 300.2 under Fluid Service defines category D as
1. fluids nonflammable, notoxic, and not damaging to human tissue
2. Guage pressure not to exceed 150psig
3. Temperature not to exceed 366°F.

So, Meyerske, the answer is that the code does recognize that some systems to not require the rigourousness of a full hydrotest - but is is always the owner's call to allow this.

If you are working with a different code, you should see if they make similar allowances. B31.3 is what I am most familiar with.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[UtilityLouie]

I'll agree with Stress Guy on this.

It seems that because you never stated what this pipe was carrying that maybe you were looking for a way to skirt the code - which no one in this forum would ever endorse.

My rule is - if it can hurt you real bad, whether the code requires it or not, do everything possible to make sure that system is safe. Combine NDE and certified welders with the proper leak testing and pressure testing.

 

[meyerske]

Thanks for the input.

The media is low pressure, low temperature water.

In no way am I looking to skirt the Code. I am looking to understand the Code and the benefits it provides.

 

[mechvhstech]

Does the hydro test have to performed with Water ONLY? Can the same pressure be generated with a water based coolant?

I guess i know the answer to this one ...but looking for comments anyways...
Thanks

vhs

=========================================
Mech Rulz

 

[JoeTank]

Not sure about ASME Piping Code, but ASME VIII would permit hydrotest with combustile liquids with flash point above 110F.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[stanweld]

Glycol or other coolants are often added to hydrotest water, especially when there is potential for freezing.

 

[davefitz]

If you are hydrotesting with a risk of freezing, then you are not meeting the assumed minimum hydrotest temperature ASME posts. You had better be extra sure that all components and weld interfaces have adequate ductility at the hydrotest temperature.

 

[stanweld]

davefitz,
Not necessarily. Filling for hydrotesting may take considerable time and may occur during Winter when atmospheric temperature may be well below freezing. Antifreeze compounds are added to prevent freezing during this period to protect the system/equipment from potential damage. ASME B31.3 does not prohibit hydrotesting at low temperatures but does require due consideration of brittle fracture. When testing boilers, heating the test fluid is done to meet the minimum required surface metal temperature throughout the test.

 

[davefitz]

Stanweld,

You are correct in the general case- in the case of boilers, ASME has a set minimum.