Pressure Testing of Corroded Piping 3

[Sarahm554]

Deal All,

I had discussion with an Inspection Engineer of oil and gas plant and He has raised a query that pressure testing of corroded/old piping shall not be carried out at 150% of design pressure. My point of view is that, as far as, minimum required thickness is still available then pressure testing should be done at 150% of design pressure. This minimum required thickness also considers all other loading's apart from pressure design thickness.

can anyone guide on this matter.

Thanks in advance

 

[DSB123]

How do you know the entire piping system meets the minimum required thickness?

 

[Sarahm554]

The pressure testing is required after a repair activity and that portion of piping not meeting the minimum required thickness criteria has already been replaced. The remaining portion of piping is corroded with different value of remaining corrosion allowance.

 

[DekDee]

I am quite interested in reading any responses to this from experienced piping engineers.
As far as I am aware (I stress I am not an engineer) 1.5 times design pressure is for new piping.
Again, as far as I am aware there are codes/ standards that deal with piping that has been in service.

My only question would be if it was tested at 1.5 times design pressure when piping was brand new why would you test at the same pressure if it has corroded down to minimum required thickness ?

Regards,
Shane

 

[BigInch]

You need to test all piping at 1.5 x design pressure now, regardless of any corroded pipe you have left in the system. Apparently you have already replaced all known problem areas with new pipe. All remaining piping should then be within design pressure as long as your remaining wall thickness is above your original minus the corrosion allowance. You sound like you have little or no confidence in the work that was done.

 

[LittleInch]

I'm with BI on this. The Inspection Engineer needs to be asked why he thinks that and whether he will become personally responsible for any defects which emerge in the piping and cause damage or injury.

Lets go back to basics here:

Pressure testing to 1.5 times design pressure is included in design codes (B31.3 for example) to demonstrate that a particular piping system is able to perform at that pressure plus a suitable margin to allow for operational loads, expansion etc in service.

It matters not whether this is new piping or old piping.

If you are in essence re-certifying the piping after a repair. If the test results in loss of containment of an older corroded section, then it needs to be replaced or the design pressure lowered to meet the new conditions.

It is not unknown that systems become revised when after a couple of tests, you cant achieve more than a certain pressure without some part of the piping leaking, this pressure being less than 1.5 x design pressure. They then revise the Design pressure down so that it meets the 1.5x requirement.

E.g. You have a design pressure of 800 psi. Your test pressure is then 1200. However you can't get beyond 900 psi in your pressure test. The new design pressure is 600 psi. Only you can decide if you need 800 or whether 600 will do. Up to you and the owner.

Pressure testing is a bit of a blunt tool, but if your system passes then it makes everyone happy - management, lawyers, insurance, etc that you have undertaken what is required by the codes to certify a particular system.

Dekdee - the codes require a certain design thickness. you can add on corrosion allowance to that if required / necessary, but that minimum thickness is required to meet the design pressure. Therefore if you still have that thickness, why wouldn't you test to the same pressure?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DekDee]

LI,
Thanks for the response.
I was trying to get my head around the difference between design thickness and minimum required thickness.

I thought if piping was fabricated to design thickness and then it had corroded but was still above minimum required thickness that it would have a reduced test pressure ?

I think I will stick to questions about welding - a bit safer for me. LOL !
Cheers,
Shane

 

[TGS4]

If this is B31.3 piping, then the hydrostatic Leak test needs to follow 345.4.2, which states that the test pressure is 1.5*P*St/S. Don't forget about the St/S component - this is intended to add pressure for higher temperature situations.

 

[BigInch]

Technically B31 provisions only apply to the original design and does not include requirements for rebuilt systems, although logical is logical and it follows directly that, if the intent is to requalify to original criteria, that is what must be done. If a qualification is to be made to lower design pressures, other criteria could be applied.

Minimum design wall thickness results from applying the allowable stress criteria to design pressure and temperature, while superimposing all the other possible loading conditions, wind, seismic, hydrodynamic, etc. that might be concurrently applied to the design pressure and temperature condition and then adding the corrosion allowance to that. That result is in effect the minimum required wall thickness.

 

[MickMc]

IMHO as long as your existing piping meets the calculated wall thickness for DP which I assume is your minimum thickness + whatever allowance for corrosion you can safely test at 1.5 x DP.

Can you not isolate the replaced section of piping and test this only with maybe the final weld radiographed (golden weld) with client permission but maybe this is not possible if various sections of the system are replaced?

 

[LittleInch]

MickMc,

surely you mean just minimum thickness as calculated for the design pressure?? The CA is exactly that, an allowance for corrosion so that if it does corrode, the thickness doesn't reduce below the minimum you need for the design pressure.

In many case, depending on the amount of corrosion, its depth, length etc, the pipe is still good for the design pressure, but that's a fitness for purpose assessment which is something different.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MickMc]

LI,

You are correct, I should have phrased better. My intent was that the thickness should be minimum for the design pressure but you would still have an additional CA if required to suit operational life otherwise if down to minimum and corrosion expected it should also be replaced.

 

[DekDee]

LI,
I am a little confused.
Your comment
Pressure testing to 1.5 times design pressure is included in design codes (B31.3 for example) to demonstrate that a particular piping system is able to perform at that pressure plus a suitable margin to allow for operational loads, expansion etc in service.

It matters not whether this is new piping or old piping.


This is from B31.3 and what I based my earlier comments on.

Intent of the Code
(1) It is the intent of this Code to set forth engineering
requirements deemed necessary for safe design
and construction of piping installations.
(2) This Code is not intended to apply to the operation,
examination, inspection, testing, maintenance, or
repair of piping that has been placed in service. The
provisions of this Code may optionally be applied for
those purposes, although other considerations may also
be necessary.

There are API standards that are used for inspection & testing of tanks and vessels that have been placed in service - I was unsure if there was something similar for piping that had been placed in service.

Regards,
Shane

 

[BigInch]

As I said before, "Technically B31 provisions only apply to the original design and does not include requirements for rebuilt systems, although logical is logical and it follows directly" that you should use the same test pressure for the new pipe.

In the pipeline code of federal regulations, CFRs Title 49 Parts 192 and 195, repairs to pipelines are addressed and, if the original maximum allowed operating pressure limits are to be maintained, the pipe used in the repair must be tested to the same test pressures used on the original pipe.

 

[LittleInch]

True, the code is not intended for repairs, however in practice, if you wish to "re-certify" a repaired pipe or a corroded pipe to the same design conditions as it was before then use of the same design code is applicable.

Section 345.2.6 could be used in this respect
"If repairs or additions are made following the leak test, the
affected piping shall be retested, except that for minor
repairs or additions the owner may waive retest requirements
when precautionary measures are taken to assure
sound construction.

There's no limit given of how long after the leak test......

There are standards for assessing fitness for purpose, for pipelines there is B 31G, piping appears to be API 579

Ultimately pressure testing is a simple, established method for providing a level of assurance about a piping system.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DekDee]

LI,
I don't think cutting out and putting in a new section of pipe into your piping system falls under "minor repairs or additions".
I do agree though with your comments that a successful pressure test makes everyone feel warm and fuzzy.

BI,
Agree 100% with your comment
"......although logical is logical and it follows directly" that you should use the same test pressure for the new pipe."

The OP stated the Inspection Engineer queried the testing of old, corroded in service piping at 150% of design pressure.
That is what I have been trying (unsuccessfully) to discuss.
Would it not be more logical to cap the new pipe, pressure test to 150% then install with two golden/closure welds ?
If it is an O&G plant there is a strong possibility the product doesn't like water so to fully remove the product, hydrotest with water, remove the water and reinstate ready to accept product could be quite time consuming.
Mind you, if it is water piping I have just wasted 15 minutes of one finger typing.
Cheers,
Shane

 

[BigInch]

DekDee yes you are correct. Repairs made with pretested pipe, welded into the old pipe, radiographing the golden welds, is usually faster and far easier than hydrotesting the entire section.

Test pressures should be specified according to the original requirements, if the pipeline is still to operate at the same pressure. That would be 150% for B31.3, but could be less than 150% for B31.4 or B31.8 designs.

 

[LittleInch]

Well that is exactly what B 31.3 says. The first half of the sentence doesn't say "minor" that's in the second half - "the affected piping shall be retested" .

The OP is just talking originally about pressure testing per se of "old corroded piping". If you're doing an inspection and part of the inspection is to certify that the current piping is good enough for the process conditions, one of the ways to demonstrate that is to do a pressure test. IMO that pressure test should be the same as the original unless the owner has decided to de-rate part of the plant and adjust all the safety devices accordingly ( trips, relief valves etc).

All depends on what you're trying to achieve - close your eyes to the rest of the plant and just test the bit you've had to replace or a full re-test and re-certification of the entire plant.

If you have confidence in the piping then you can do this with process fluid or gas.

A pipeline company I used to work for did a pressure test to design pressure every year in product as a means of demonstrating integrity. Only failed once or twice....



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DekDee]

LI,
My apologies.
I misread the B31.3 clause.
Cheers,
Shane

 

[SJones]

API 570 and ASME PCC-2 offer a number of paths to take dependent upon the situation. Might be worth having a squizz at those two documents and see if it helps the OP's thinking.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.