Repad on structural supports attached to SCH 10S pipe 1

[FlashPurp]

Hello, We have SCH10S piping and have various structural supports welded to it (dummy legs, base supports, etc.) Because it is thinner pipe, do we need to include repads? Concern would also be the weld burns through the thin wall.

Will CAESAR II be able to check this somehow? Or what recommendation would there be to validate results/findings?

 

[LittleInch]

I would say you would need FEA to determine this depending on what you know of the loads, moments etc on the piping.

But it seems a bad idea to simply weld bits of metal to a pipe as thin as Sch10S without reinforcement.

My recommendation is that piping shouldn't be used as a structural member and instead build a structure to support it without it being attached to said structure. Guides are better,

 

[FlashPurp]

I would say you would need FEA to determine this depending on what you know of the loads, moments etc on the piping.

But it seems a bad idea to simply weld bits of metal to a pipe as thin as Sch10S without reinforcement.

My recommendation is that piping shouldn't be used as a structural member and instead build a structure to support it without it being attached to said structure. Guides are better,

Hello LittleInch,

Thank you for your insight. Our piping is supported on rack, with dummy legs used on elbows throughout the layout.

With absence of licensing for FEA software, would hand calcs be suitable? I was told Kellogg method could be used for checking and that some excel file out there has been created with correct equations/validations.

 

[LittleInch]

Is that what you meant by dummy legs? I/e pipe welded to an an elbow almost like its a swept tee? - I was a bit confused

Don't know is my answer I'm afraid - I leave all that stuff to the stress wallahs.

I guess it depend son the loads, forces and moments, but I just think 10S is too thin for it to have any real bending capacity for something welded to it.

 

[GD2]

OP,
Why one needs to weld the pipe to its support? There are hundreds of pipe supports that can be clamped, strapped, U-bolted, supported on shoes.
There is no repad concept in pipe supports. Repads are considered only where pipe intersects (branching) or vessel intersections (nozzle, manway etc).
Caesar is a analysis tool. In piping, it is primarily used for flexibility analysis. It will essentially tell you where to put what supports to limit the stresses within Code allowable.
Caesar will not tell you whether to weld/repad or not weld a support.

Dummy legs are typically welded.

 

[Snickster]

When designing piping systems you should check for local stresses at pipe supports, especially when using thin wall pipe such as Sch 10s. I had a project in which we used all Sch. 10s stainless steel and the local stresses at pipe supports were way above limits. The piping in this case rested directly on the structural members without any attachments such as dummy legs or shoes so the contact point was very small. The solution was to either install repads at each support location or reduce span of the supports. We chose to reduce the span of the supports from 20 ft. to 10 ft. this allowed also for the installation of electrical conduit on the supports since they could not span much more than 10 ft.

If you have an attachment to the pipe at the support the contact area of the attachment itself may reduce the stresses to within acceptable limits. There are methods to determine the local stresses in piping at supports. Caesar does not check local stresses at supports. If you want to check manually without computer like FEA, you need to use methods such as Welding Research Council Bulletins WRC 107 and WRC 297 or other acceptable methods. I have a Kellogg common method used by most design firms to check local stresses at supports manually without computer that is easier to use than WRC 107. I will need to look for it on my computer and will post later.

 

[Snickster]

Here is the Kellogg method. It was the first method used and is still used today by design firms I worked for as the first check. If check passes the Kellogg method then no further analysis is required.

 

[Snickster]

Caesar does have a WRC 107 sub-program. It has been a while since I used Caesar and the WRC 107 sub-program but I believe it can be used independent of a stress analysis. For instance, if you model a dummy leg and node at the connection of the dummy leg to the pipe then Caesar piping stress analysis will give you the forces and moments at that node point. Then you can plug these forces and moments into the WRC 107 sub-program and it will give you the resulting local stresses.

 

[r6155]

Piping: design pressure / temperature?, diameter of sch10s?

 

[FlashPurp]

@LittleInch Yes pipe welded to elbows but in some cases to the side of the pipe like a trunnion. Thank you for your input much appreciated!

@GD2 Not welding the pipe to supports but welding steel to piping for structural support like dummy legs. Either pipe or in some cases even a beam. I attached a picture of the type of supports I am asking about.




@Snickster Thank you for the attached Kellogg method as this is most helpful. Are the Caesar stress results reflecting these localized stress results at points of restraint? I did see the WRC sub program in Caesar but would the geometry of the dummy leg welded to the elbows be reviewable with the program?

@r6155 For this specific project the conditions is as follows: Max Design Pressure is 170 PSIG and Max Design Temperature is 150 F. The pipe is 10" A312 TP304.
However I am just asking as a general question as this question has arise before for other projects and this one will have different size/design conditions at another area of the plant as well.

 

[r6155]

Never weld on pipes, elbows, ....fittings made of SS material and for any thickness.

 

[GBTorpenhow]

Never weld on pipes, elbows, ....fittings made of SS material and for any thickness.

Because...?

 

[GD2]

@LittleInch Yes pipe welded to elbows but in some cases to the side of the pipe like a trunnion. Thank you for your input much appreciated!

@GD2 Not welding the pipe to supports but welding steel to piping for structural support like dummy legs. Either pipe or in some cases even a beam. I attached a picture of the type of supports I am asking about.

View attachment 1145


@Snickster Thank you for the attached Kellogg method as this is most helpful. Are the Caesar stress results reflecting these localized stress results at points of restraint? I did see the WRC sub program in Caesar but would the geometry of the dummy leg welded to the elbows be reviewable with the program?

@r6155 For this specific project the conditions is as follows: Max Design Pressure is 170 PSIG and Max Design Temperature is 150 F. The pipe is 10" A312 TP304.
However I am just asking as a general question as this question has arise before for other projects and this one will have different size/design conditions at another area of the plant as well.

Hi FlashPurp,

Load carrying capacity for trunnion/dummy supports must be checked for local piping loads/stresses (longitudinal, circumferential and axial loads and the corresponding moments) to be below the allowable stresses.

A repad will increase the load carrying capacity (reduce the bending stress).

Snickster had provided you guidance to use M.W.Kellogg design.

 

[Snickster]

@Snickster Thank you for the attached Kellogg method as this is most helpful. Are the Caesar stress results reflecting these localized stress results at points of restraint? I did see the WRC sub program in Caesar but would the geometry of the dummy leg welded to the elbows be reviewable with the program?

No. Caesar does not check local stresses at pipe supports directly in the stress analysis program at least at the time I was doing stress analysis. Even though they included the WRC 107 sub-program you still has to do the local stress analysis independent of the piping stress analysis program by taking the forces and moments at the support point and plugging into the WRC 107 program. However it has been a few years since I have used Caesar so I don't know about the latest version of Caesar.

 

[r6155]

A support welded to the pipe can cause excessive heat input to the pipe.
Lack of preheat for welding can cause serious problems.
Welds cause residual stress that is impossible to measure and may require PWHT.
Some welds during installation are not inspected.
The pad requires an air test.
Kelow does not mention this

 

[GBTorpenhow]

A support welded to the pipe can cause excessive heat input to the pipe.
Lack of preheat for welding can cause serious problems.
Welds cause residual stress that is impossible to measure and may require PWHT.
Some welds during installation are not inspected.

Never is a long time, and these four statements hardly add up to "never weld attachments to any SS pressure pipe of any thickness" IMO. All four are valid concerns, and may be more or less important for certain loads, thicknesses, pressures, and materials, but are also all valid concerns when welding pressure pipe to other pressure pipe and somehow we manage that.

Certainly if a welder can't make a pipe weld don't let them go near the attachment welds.

 

[FlashPurp]

@Snickster Ahh I see, I'll have to do the check outside of the stress analysis, thank you very much.

@r6155 and @GBTorpenhow I also appreciate the discussion and concern about welding to the pipe itself. Unfortunately we do need these types of dummy legs due to configuration of piping in certain areas.

 

[Snickster]

A support welded to the pipe can cause excessive heat input to the pipe.
Lack of preheat for welding can cause serious problems.
Welds cause residual stress that is impossible to measure and may require PWHT.
Some welds during installation are not inspected.

Never is a long time, and these four statements hardly add up to "never weld attachments to any SS pressure pipe of any thickness" IMO. All four are valid concerns, and may be more or less important for certain loads, thicknesses, pressures, and materials, but are also all valid concerns when welding pressure pipe to other pressure pipe and somehow we manage that.

Certainly if a welder can't make a pipe weld don't let them go near the attachment welds.

As the design engineer I would provide the contractor with a support attachment detail with welding symbol showing weld type and size. Never had to get into fine details of welding as this is left up to the contractor in his welding procedures and qualifications. This has been the case whether welding attachments to pipes, pressure vessels or tanks and of any kind of attachment you can dream of.

 

[GD2]

A support welded to the pipe can cause excessive heat input to the pipe.
Lack of preheat for welding can cause serious problems.
Welds cause residual stress that is impossible to measure and may require PWHT.
Some welds during installation are not inspected.
The pad requires an air test.
Kelow does not mention this

Attachment welds are almost always done in a piping system. All you need is to develop and qualify a WPS complying to the Code requirement.

 

[StressGuy]

As others have noted, Caesar won't check the local stresses that you need to consider for a support attachment - this is true for dummy legs, lugs, saddles, whatever. That's just not within the capacity of a Caesar beam element model.

The Paulin suite of tools is really something every stress engineer should have alongside Caesar or whatever general pipe stress program they work with. Nozzle Pro and FE-Bend making checking local stresses a relatively easy process. The Kellogg method is ok for a very quick first check, but it is so grossly conservative that I find it hardly worth bothering with anymore. There are plenty of connections that fail Kellogg that easily pass under FEA.

As for the general principle of welding a dummy on 10S pipe, again, I've done it routinely and found that it's not the boogeyman we were lead to believe. It's not going to collapse like a beer can, and the lack of corrosion allowance to worry about means that the wall thickness doesn't carry the penalty that carbon steel does.

I generally recommend upsizing the pipe diameter of the dummy leg before considering a repad on the elbow.

As for the idea that welded attachments shouldn't be used for pipe supports - that's just not the way things work in B31.3 world with our clients. Welded dummy legs and base supports are the industry standard like the attached pictures. These are typically shop welded by the pipe fabricator and shipped ready to assemble.