Dear stress engineers

I would like to know how RF pad cuts from pipe.

how it is fabricated & erected at site.

can we provide RF pad to the elbow?
if yes, then how it is made?

Are these questions for a Caesar II forum ?

1. You cut RF pads with a gas axe (flame cutter), then trim grind.

2. Ask the Pipe Fitter foreman.

3. Yes you can reinforce an elbow, often with a pad cut from another elbow, but may be rolled from plate.

Dear stress engineers

I would like to know how RF pad cuts from pipe.

how it is fabricated & erected at site.

can we provide RF pad to the elbow?
if yes, then how it is made?

Go away.

It is not cut/fabricated from pipe. It is made form metal plate of same material as parent pipe.
On elbow you can not provide RF pad. You can use higher sch elbow if essentially required.

Good grief. Where do you guys come from ?

Nodal, you are quite wrong on both points. Re-pads for straight pipe are commonly cut from parent pipe. Re-pads for elbows ... actually usually rings are commonly cut from another elbow.

Not necessary Moverz.. pad can be fabricated from a metal plate .. can be fabricated in two shelves too.
It should be more thick than the pipe.

We specify repads to be cut from the run pipe, or suitable plate - up to the client depending on what they have available at the time, although as Moverz says it is usually cut from the run pipe.

We try and keep the repad the same thickness as the run pipe, or if we really have to we may go up a pipe schedule.

If you make it thicker than the run pipe where do you stop? Do you have a 30mm repad on a 12.7mm thick pipe? It looks wrong and the thicker the repad the less flexible the piping around that area - making it thicker can have a negative effect.

Typical rule of thumb is to limit the repad thickness to 1.5xT. Thicker pads tend to have a "hoop" effect where high stresses can be developed in the "thin" run pipe at the edge of the pad.

I have used pads at 2xT (very rarely), but only after rigorous evaluation of the external loads and importanly the pressure loads.

I do not recall using pads on ELLs. Best to relocate support or use thicker ELL. Always been able to work around.

I've used pads on elbows infrequently, and ALWAYS in preference to a thicker elbow. The reasons for this are more practical rather than technical:

1 - The heavier elbow is an out of spec item. Especially with modern 3D CAD based modeling, the systems correctly make is difficult to put in an out of spec component. When it drops on the ISO, there's a good chance that the fabricator is going to miss the fact that it is different.

2 - You cannot walk up to that elbow in the field and see that it is the heavier wall that you asked for. You will have to get a specific UT reading taken to verify.

3 - If you call for a repad, that becomes a separate line item in the Bill of Material (BOM) that stands out on it's own. It's much more likely to actually get noticed by the fabricator and it will be obvious to anyone who walks down the line during construction whether or not the pad was installed.

As my mentor has told me - you can't just specified something so that it can be understood, you have to specify it so that it cannot be misunderstood.

thank you very much for your good advice.

A repad on a piping system will have an effect on the stiffness and local stress of the piping. The B31 Code has given us rules when we use repads on branch connection. There is more info on this published by the WRC.

The Code committee has not given specific advice as to local reinforcement of elbows. The numerical stress value of reinforcing pads on elbows can be determined using finite element analysis. The attachment welds for external repads will probably be the limiting factor.

Engineering sensibility requires us to think of the effect of an external pad on piping components. At what size does the repad affect the flexibility of the elbow and the loading on other piping components?

Is the plate needed for internal erosion of the base elbow? I shall assume yes, since I can't think of another reason unless it would be for an attachment?? After the erosion of the base elbow, the external pad and weld has to maintain the pressure and bending stress boundary. That means that the weld has to sufficient for the loading on the elbow at the service temperature conditions. High temperature will require creep analysis and thermal fatigue study. Low temperature service would require evaluation of notch toughness and nonductile fracture.

The stresses at the weld locations are critical. The load path through a perimeter weld should consider eccentricity. The metallurgy of the attachment should be the same as the base pipe component. This is to ensure that consideration is given (by a qualified person) to weldability, mechanical properties, physical properties, and process fluid.

Using formed plate that has bending above a 5% extreme fiber stress would require post forming stress relief before welding. Either method might require post welding stress relief as required by the code. The weld will also have to be examined as per the Code.

At this point some of us will realize that we can't write the procedure for this in the expected time for acceptable isometric turnaround. We are increasing cost significantly by adding an external pad.

My suggestion is that you discuss this with your process engineer and if this is an actual erosion problem, you suggest localized internal refractory fastened using hex mesh.

If refractory isn't acceptable, then the significance of a thicker elbow should be calculated.

Rather than give a very short reply, I attempted this short reply. Much of this is contained in ASME PCC-2008. I suggest that if you want to code, only use the components that CAESAR II includes. That would be a thicker elbow.

If you want to know more about limitations I really do suggest getting a copy of the ASME Post Construction Code mentioned above.

Bill Edasi

I have never come across a re-pad intended to assist with erosion. That sounds like a recipe for disaster to me. The prime purpose of an elbow re-pad is to assist with local stresses where a trunnion or similar is attached.

As regards flexibility, the combination of pad and attachment such as a trunnion at an elbow will effectively destroy any flexibility and strictly speaking, its s.i.f's. However the prudent way to model this would be to ignore the bend element and its flex factor, but manually add the normal s.i.f's to the bend node.

I agree with Ed Klein, a re-pad is far more practical than a thicker bend.

errata: PCC is Post Construction Committee not Code.

Plant mechanical integrity managers are (should be) familiar with this. It is meant to be used in conjunction wth API 510, API 570 and NB-23.

ASME PCC-2 2008 Article 2.12 Fillet Welded Patches pg. 63. (I can't give you the pdf) The earlier 2006 edition does not have article 2.12.

Highlights of PCC-2 Repair of Pressure Euipment and Piping
http://www.ipeia.com/Misc_Docs/2009%20Reynolds.pdf

OP indicated "Erected at site". Didn't say new or old construction.

Repad. patch, scab have similar effects. They exist in the catalyst world or where there is slurry, coke fines, or decoking. Also where shutdown is highly avoided.

FEBend (Paulin Research Group) will give SIF's and flexibilities based upon actual geometry. Use a tiny attachment if only concerned about the pad effect. Review a model with and without torsion with various pad dimensions.