Why we use higher pressure than the normal operating pressure in.. 5

[Engineerator]

Hello

Any body knows why we use a higher pressure than the normal in Hydrotests in water pipelines?

and is it just ok to make the test in the normal operation pressure?

thank you

 

[IRstuff]

Presumably, one would like to ensure that pipe survives all the variations that come with "normal" operating pressure. If not, you might be unpleasantly surprised if the system experiences a pressure surge, or something similar.

TTFN

FAQ731-376

 

[desertfox]

Hi qaengineerenator

In short no you can't just make the pressure test with normal operating pressure.
One needs to make sure that the pipe can cope with an adverse condition should things go wrong as IRstuff stated.

desertfox

 

[Engineerator]

So that's it! especially when the pipes and all joints and flanges are made of plastics, that higher pressure can shake the system and reveals any installation defects.

thank you all for inlighting me!

desertfox, what do you mean by adverse condition and how it can be happen?

 

[msquared48]

Waterhammer for one.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BigInch]

Sorry to disagree, but none of the above as far as what the hydro does test. Those above (pressure variations, including transients) can be determined and can be protected against by analysis and setting MAOP according to the code limitations.

The reasons I would say for hydrotest are to prove Integrity & close immediate leaks and would include,

Failure Pressure Versus Defect Size

Manufacturing Defects

Pipe manufacturing codes (for example API 5L [9]) give limits of acceptable defects, which set the standard for detection; many mills use internal standards that meet the criteria given in codes but can still miss some defects. Hydrotest ensures that remaining defects are small enough to still allow operation within MAOP. The hydrotest is designed to induce a stress that would cause failure of under-matched or substandard pipe properties (e.g. wall thinning, tensile properties below SMYS, or defects).

Transit by road, rail or boat

May damage the pipe or subject the linepipe to cyclic stresses that can result in fatigue cracks.

Deficient Workmanship

Since pipeline integrity is much more likely to be affected by longitudinally oriented defects than by circumferentially oriented defects, hydrostatic testing is one of the best ways to demonstrate the integrity of a pipeline.

To expose those and any other remaining defects that have survived previous test and inspection processes.

Hydrotest is considered the final test of pipeline integrity.


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[desertfox]

hi qaengineerenator

Adverse condition means an unexpected event like a pressure rise in the system due say to a partial blockage which is not expected.

desertfox

 

[desertfox]

hi qaengineerenator

Have a look at these links

http://en.wikipedia.org/wiki/Hydrostatic_test

http://www.ohiowater.org/oawwa/Trai...drostatic Pressure Testing of Water Mains.pdf

http://www.engineersedge.com/testing_analysis/hydrostatic_testing.htm

desertfox

 

[BigInch]

If liquid transient pressures can rise to 50% over operating pressures, how can a hydrotest to only 1.25 x operating pressure (this is a pipeline) prove 1.5 x pressure capacity?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[CRG]

qaengineerenator,

Typically, hydrotests use pressures greater than the normal operating pressure and a service test is where the line is put into service and inspected. Service tests are common for many water systems (for example, the copper water pipe in my house); however, you stated water pipeline. What is the code of construction you are using for your pipeline?

 

[ione]

There are often if not always different opinions as to what a hydrotest is meant for.
Some believes the major concern is for detecting leaks or construction flaws, others consider the test is performed to create stresses exceeding those at operating conditions. Some feels it is a method to detect errors made in design calculations, others consider it can produce a benefic stress relief effect. I personally consider hydrostatic test as the method to ensure completion, compliance and most of all safety of the equipment. It is the last test to be carried out (as BigInch noticed), and it can be repeated periodically to ensure that new factors, which affect the equipment during its working life, have not jeopardized the system integrity.

 

[BigInch]

Answer to above. In that case you have to raise design pressure until the maximum transient pressure is only 10% higher than Pd. Set the design pressure = 1.5 (the max transient pressure) / 1.1 transient allowance = 1.36 X original design pressure. Then the 1.5 transient pressure / 1.10 transient allowance * 1.25 test factor = 1.7 Original design pressure > 1.5 transient pressure.

Yes ione. Redistribution of localized stresses within the plastic zones can be an added benefit.



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[CRG]

Isn’t this really a code issue and we do not know what piping code is required by the owner or other regulatory agency?

Per B31.3, with the owner’s approval and category D fluid, service testing is permitted. A low pressure water pipeline, not exceeding 150 psig, does not necessary need to be tested above the operating pressure. So to answer qaengineerenator question, “is it just ok to make the test in the normal operation pressure?” It depends on the piping code and what you mean by “normal operating pressure”. If the owner specified designing to B31.3 then with the owner’s approval, and if it the operating pressure is less than 150 psig, and the water temperature does not exceed 366 degF then it would be ok to test at the operating pressure.

There are a lot of good reasons why you should/might test at a higher pressure than operating pressure given in many of the above comments; however, depending on the code, a service test may be sufficient. What are the consequences of a worst case failure? Are the costs associated with a hydrotest significantly greater than a worst case failure? If so, stay away from a service test.