Weld P91 to P22 2

[PMCap]

Does anybody have any experience welding P91 material to P22. I have heavy wall P22 nozzles that are to be welded to P91 pipe. Should an interface buttering material be used? Any other recommendations? thanks

 

[stanweld]

Assuming that your proposed encapsulation approach uses a clam shell design involving 2 longitudinal weld seams on the P91 reinforcement, I cannot see any benefit over similar P22 pipe reinforcement as per metengr. The Type IV creep properties of the the P91 reinforcement weldment are near equivalent to the creep properties of P22 in hoop and the difficulty of volumetrically examining the P91 longitudinal welds make the repair all the more dubious. Like metengr, I would recommend removal of the damaged weld or a pipe segment containing the creep damaged weld and replacing in kind.

 

[PMCap]

stanweld

A typical fossil plant main steam pipe run has quite a number of weld joints. I was considering constructing one piece components that were attached to existing pipe using full penetration circumferential corner joints with appropriate tapered welds.

Installation approach would be to remove a small section of existing pipe at as few location as necessary and the slip encapsulation components (quantity as necessary) over existing pipe (noting that encapsulation component has larger ID (root gap) than existing pipe OD).

Restoration where existing pipe was removed or installation of the encapsulation components would be repaired by installing a new spool piece as suggested by metengr.

Of course cost is always an issue - However, it is thought that the encapsulation approach could be less costly than wholesale removal of all weld joint areas as each area would require installation of restraints to maintain alignment - not an easy task.

 

[stanweld]

While I have used a similar technique to repair leaks in live gas transmission lines (still a clam shell design), I cannot see any real benefit to your approach over replacement of damaged pipe sections. You do have to cut the existing line and remove a section thereof sufficient in size to place the seamless pipe reinforcement piece.

 

[PMCap]

stanweld

You fellows sure make me think - good for the brain cells.

I would not classify my component as a clam shell design - that is if one interprets a clam shell design as a 2-piece wrapper plate that has the longitudinal seams full pen welded together and the ends attached by fillet welds. The component I am considering would be a 1-piece component with an inside diameter larger than the pipe OD, long enough to encapsulate the weld and HAZs, with its ends configured with a hub so that it can be full penetration welded (corner weld) to the pipe. This end hub / full penetration weld would be appropriately tapered and the weld ultrasonically testing for soundness. All code stress criteria would be satisfied.

Yes, I agree that cost issues would need to be considered. IMHO, if one were faced with having to restore a large diameter heavy wall piping system (think main steam line where pipe OD = 20” and pipe wall = >3”), then the economics of my method could govern. Consider the cost of cutting out and replacing all the bad girth weld areas which would require installing a large number of temporary restraints, perhaps on both sides of each cut area. For my proposed method, only a few sections of pipe would have to be cut and restrained as several of my components could be slipped onto the pipe at each cut location.

 

[stanweld]

I assume that you intend to swage the ends of the SA-335 P91 encasement pipe to permit the repair as stated. You do need to determine the costs for making these items and note that the componenets will require N&T afte swaging. If I were the Owner, I would not contemplate such a repair without finite element analyses of the corner groove weld and at the reinforcement at the swaged ends.

 

[PMCap]

stanweld

I agree with you.

Components would be fabricated from SA335GRP91 (Pipe) or ASME SA336 Gr. F91 (Forging) with appropriate axial and through thickness directions examination and material testing - both before and after component construction.

I have obtained preliminary hardware costing information that appears reasonable. Preliminary estimates for replacement of an entire piping system have been developed. Cost estimates for installation of encapsulation components and costs for installation of spools - including additional work for design and installation of the required restraints will be developed.

Affects on schedule to perform each of the pipe restoration approaches will be looked at as time is money.

Piping restorations using the encapsulation method would include detailed finite element analysis to document ASME B31.1 Code compliance of the encapsulation component itself and the effect of installing the new hardware on the piping system Code qualification of record.

 

[stanweld]

FYI.
We have made a number of small replacement spools in P91 pipe to repair locations where the original pipe was mis-heat treated. Spools were between about 3.5 feet long to 10 feet long and from 10" to 24" diameter. Average cost to cut out the existing pipe, make the replacement spools in a shop, and weld them into the existing lines was approximately $115,000 each, including all PWHT and NDE.

After you make your localized encapsulation repairs, what guarantee do you have that the remaining creep life of the "nondamaged" locations will be realized?

 

[PMCap]

Stanweld

Thanks for the cost information – this will be really helpful to me.

Based on my experience and hardware and field costs numbers I seen to date, I’m reasonably confident that my encapsulation repair method could be implemented for less than what your costs have been.

Again, I’m talking here about not a single spool replacement but restoration of a complete system including main steam lines and turbine bypass lines. My restoration method for this type system restoration, where I’d have to cut out a spool to get access to install 1-piece component, works best through an economy of scale. Also, for a large number of repairs on a single system, I believe my method could be implemented in significantly less time.

Regarding your question “After you make your localized encapsulation repairs, what guarantee do you have that the remaining creep life of the "nondamaged" locations will be realized?” Very good question with no direct answer or guarantee. Remaining creep life for the non-damaged pipe outside the HAZ areas would depend on the length and type of service the system has seen at the time the damages was uncovered in the HAZs requiring repair. What we know is that creep life in HAZs for P22 material is significantly less than for the non-HAZ areas. I would employ a team of experienced and qualified engineers and metallurgical “experts” familiar with the creep behavior of P22 material to assess what the remaining creep life of the system would be following restoration. The economics would have to be there to support a restoration of the damages areas vs. wholesale pipe replacement. For many systems, I believe that significant life of the non-damaged portions in the pipe would remain and thus warrant restoration of the weld/HAZ areas.

 

[davefitz]

As long as you are developing a detailed stress model of the asembly, you might as well check the thermal stresses developed during startup and shutdown for differnt assumed ramp rates- this series of calcs can be used to determine the allowable piping system temperature ramp rate that will not over-stress the fillet welds due to the DT between the inner P22 pipe and outer P91 sleeve- and confirm this is a faster ramp rate than other inherent limits ( STG, headers, etc).

 

[davefitz]

Also, there may be 2 corrosion related issues with the sleeved design that need to reviewed. First, the annular space acts as a crevice, and corrosion products may tend to open up the pre-existing crack that is being developed at the inner root of the fillet weld.

Second, the corrosion products may actually develop a high shear force between the sleeve and the inner pipe- similar issues have plagued some gas turbine bolts.

The steam conditions you have defined indicate a supercritical unit using "holy water", so most corrosion products are excluded, but even oxidation in steam environment yields corrosion products.