help with a Design Calculation. 2

[iam42]

Can anyone help me with a material thickness and weld size on the following problem.

I have to hydrotest a 12" Sch 160 P91 Pipe Spool. The spool has machined Buttweld ends. In the past the company has plugged the pipe internally with a 1 1/2" thick plate and welded it in place with a 2" fillet weld. I would like to confirm that this is ok mechanically, but I'm not sure how to calculate minimum plate thickness or minimum fillet size.

The Hydrotest pressure is 5055 psig.

Any help would be greatly appreciated.

IAM

 

[metengr]

iam42;
Did you know the special precautions required for handling P91 material and welding? How did you arrive at using a hydrotest pressure of 5055 psig? What is the intended application of this pipe spool - steam service or gas or water?? The wall thickness of pipe for service conditions should be based on using one of the formula's in ASME Piping Code Standards (ASME B31.1 or other).


The hydrostatic test pressure is normally 1.5X the maximum design pressure for service. So, your hydrostatic test pressure of 5055 psig would be about 3370 psig pressure in service.

Regarding fillet or butt welding, you need to understand the precautions necessary for welding P91 material. The ASME piping Codes have strict recommendations for preheat, filler metal and post weld heat treatment for ANY welding on P91 for fillet or butt welds. Do not do anything more until you obtain a copy of ASME B31.1, and review it.

 

[iam42]

meteng,

I was a little unclear of my intentions in my last post. I am aware of the special requirements when welding P91, and these have been taken. The welding is complete but we need to hydrotest it. 5055 psig is the customer (PED) requirement. As the pipe ends are machined BW ends and not flanged, it is difficult to Hydrotest. To remedy this problem, 10.132" Dia. plugs are welded in each end of the Pipe spool for hydrotest. My question was what thickness should these plates be and what fillet weld is required.

Hopefully I have explained it a litle better.

Thanks

 

[metengr]

I would go to ASME Boilre and Pressure Vessel Code, Section VIII, Div 1, and use the thickness formula's in PG-34 "Unstayed Flat Heads and Covers". Regarding the fillet weld size, this would be based on strength formula's referenced in ASME Section VIII, Div 1, Part UG.

 

[iam42]

Thanks,


I will give it a try.

 

[stanweld]

Have you thought of using matching pipe end caps to the spool, butt welded and post weld heat treated.

 

[iam42]

Stanweld,

That is an option I would rather not use due to the fact that the BW ends are machined ends and the spool is difficult to machine.

Thanks

IAM

 

[DeltaIV]

Does anyone has photos or sketch of TIG weld Heated Affected Zone and Fusion area? Thanks

 

[SnTMan]

iam42, what I have done in the past for a vessel nozzle with buttweld end is to fillet weld a ring outside the pipe, back of the weld prep and to the ring weld a standard (larger size) cap. In the field they cut off the cap and leave the ring. The weld prep and any PWHT are unaffected. Just have to watch your fillet weld sizes versus test pressure.

Regards,

Mike

 

[unclesyd]

iam42 How are you going to remove the plug and take care of the weld area?
What little experience I've had with P91 I am awful leery of this approach.
Have you thought about mechanical plugs. The only problem I see is whether they rent them your size.

On some heavy wall piping spools we have left a tag on each leg in the spool and welded on a cap then cut the offending ends off and prepped the spool to dimension.

I can't see the problems you mention in cutting and prepping your pipe.

 

[iam42]

Thank SnTMan,

That sounds like it may be a better option than what i'm using.


Ian

 

[iam42]

unclesyd,

Our customer has specified a combined weld prep. which can only be done by machining....as far as I know.

We remove the plugs by flame cutting, leaving 1/2" to grind to pipe ID.

Ian

 

[unclesyd]

This scares me a lot more. Hopefully metengr will comeback with his thoughts on this approach based on his experience and knowledge of the P91 material.

What you are proposing, a single filet weld on a flat head at your pressures wasn't allowed. This type head requires a full cross section weld. I think it was prohibited even for lower pressures by code. The only time I've seen with an outside filet weld was when the ends of the pipe had been swaged in and the filet weld approached a grove weld.

 

[iam42]

unclesyd,

Any advice is appreciated. This is a practise that I have inherited. That is why I am questioning it.

 

[metengr]

IAM;
I am going to ask the obvious questions, why are you hydrotesting this pipe spool? Wasn't the spool or original pipe stick hydrotested by the mill during fabrication? What is the material specification for this pipe spool?

Regardless of your past practice, P91 material is sensitive to welding and post weld heat treatment. As a practice, after fabrication is completed, I would not permit any additional welding with a subcritical post weld heat treatment (this is not a normalize and temper) because of the inherent risk of exposure to improper heat treatment and introducing another weld heat affected zone into this material.


Have you explored using mechanical plugs for hydrotesting as mentioned by unclesyd above? I would prevent any post fabrication welding on P91 material, period. You need to discuss this with your customer.

Any type of welding on Grade 91 by Code requires PWHT (either a subcritical or re-heat treatment). Subjecting this spool to any subsequent subcritical PWHT by welding cover plates for hydrotesting (ID or OD makes no difference)is increasing your risk of having Type IV cracking in service.

As a customer, I would prohibit your current practice for welding plates on the ID surface.

 

[SnTMan]

I would agree with unclesyd that closing out a vessel section with a single fillet as both iam42 and I proposed is not generally a Code allowed detail, however these are temporary test closures and it can perhaps be justified on that basis.

While I have not done so in the past the ring-and-cap detail would lend itself to full penetration welding if desired.

Regards,

Mike

 

[iam42]

Meteng,

The plugs were welded during the fabrication stage prior to PWHT, so there will be no welding after PWHT.


IAM

 

[unclesyd]

I still have some concern, somewhat alleviated by the installation prior to heat treatment, but you still have the possibility of having a metallurgical notch from the welding and a good possibility of a mechanical notch from the grinding.
I'm basing this one the some of the parameters/conditions set forth when a local fab shop fabricated a considerable number of P91 spools. Things like all Id's had to match, machined no grinding. No grinding on OD, heaven forbid if a welder took a three corner file to the toe of weld.

I liked the comment of one welder on all the conditions he had to meet when he said It's a wonder they don' t put in a requirement that we have no sex during the work week. The next day this requirement was included