Hydrotest cannot be done after vessel alteration. Any alternatives? 1

[LanceM]

Hello,

Our company was planning to do a sectional replacement of the thinning tower shell due to CUI. The problems that arose were:

1. As of the latest UT data of the tower, bottom ellipsoidal head can no longer withstand the hydrotest pressure.
2. Even if we do leak test pressure only, the documents for the foundation analysis were no where to be found.


If it helps, the design pressure of the tower is 310 kPag with operating pressure at 139 kPag at 141 degrees Celcius.

So with those in mind, are there any other alternatives for hydrotest? I've read on API 510 that an appropriate NDE may be performed however it didn't specify any.

 

[metengr]

Why would you perform a hydrotest? Your concern are the attachment welds because the shell thickness should be above minimum thickness and you have new shell material. I would specify a volumetric examination of the shell to new shell attachment welds and use either API 510 or the NBIC for guidance.

 

[r6155]

Acoustic emission examination may be useful and you can avoid the replacement.

Regards
r6155

 

[keyen]

metengr: isn't a hydrotest standard for repairs and alterations? Volumetric examination of the welds wouldn't exempt you from a hydrotest of a new equipment, correct? Why would a volumetric examination be enough for a repair?

 

[TGS4]

It depends. Where is your facility?

 

[keyen]

While we're at it, what's the original code of construction?

 

[metengr]

keyen;
Review the NBIC 2017 Edition. It provides guidance for repairs and alterations. First, you need to determine if this is even an alteration.

 

[weldstan]

Of the hundreds of repairs and alterations made by my former company, only a small fraction there of was a hydrotest performed.

 

[LanceM]

TGS4 - it is a naphtha splitter tower

keyen - the code of construction is ASME VIII Div 1 1970s. again, i know hydrotest is a requirement for repairs and alterations but the issue is that based on the thickness data of the tower it can no longer sustain the hydrotest pressure.

metengr - it is an alteration since a sectional replacement of the shell course will be performed.

Anyways, i've suggested that our company do full radiograph testing of the welds with no indications and PAUT (phase array ultrasonic test) of the weld.

 

[david339933]

There are exceptions when pressure testing is not practicable...as others have stated. Open the book and read it. 4.4.2

 

[weldstan]

If the sectional replacement is the same material and thickness, it is a repair not an alteration under the NBIC. We have done this type of repair numerous times.

 

[keyen]

metengr: full disclosure, I don't deal with NBIC, but I am curious about your opinion. Based on my quick review of NBIC Part 3 Section 4, is it correct to say that a pressure test (hydrotest) of a VIII-1 vessel (that's been through a repair or alteration) is required unless the local pressure vessel authority agrees to an alternative form of NDE?

LanceM: is the bottom head no longer suitable for pressure testing because it has thinned? What's changed between the time it was pressure tested and now?

 

[mk3223]

IMO, if required hydratesting after repaired or alternated, it's difficult to be waived because the foundation condition was unknown or the bottom ellipsoidal heal was too thin. It's better to involve other peoples, in additional to Engineering, for the acceptable solution.

 

[metengr]

First, the pressure test and hydrostatic test are NOT the same. 1) A pressure test is a catch all to look for defects that can result in leaks either in new material and in repair welds. The pressure test can be done within a range from less than 1.5 X operating pressure to atmospheric pressure. 2) Hydrostatic testing is done based on the requirements in the original code of construction for purposes to validate design (because of an alteration) and weld quality.

As others here who are experts have noted, if you are simply replacing a section of shell regardless of size and you are using the same material and thickness for the replacement shell as original shell material, this is a repair and not an alteration. The NBIC provides a definition of an Alteration with examples. An Alteration relates to a change in pressure retaining capability and other distinct features. You are doing an in-kind replacement, this is a repair.

Now, use the NBIC Part 3 to perform NDT of the welds used to join the new shell material with existing, in lieu of pressure testing. The rules are in the NBIC to follow. Pressure testing is one of several options versus NDT to ensure repairs are pressure worthy. You do not need to do both.

 

[TGS4]

As of the latest UT data of the tower, bottom ellipsoidal head can no longer withstand the hydrotest pressure.

Define "withstand".

And my question was not what the process of the tower was, but where in the world you are located.

 

[r6155]

To avoid CUI see Hydrocarbon Processing magazine April 2018 "Optimize your insulation to reduce costs"

Regards
r6155