Assistance with Structural Tmin Calc 2

[daily123214]

Hi all,

I'm typically use the beam stress theory to calculation the structural minimum required thickness of a pipe. Here is the code that I'm using:

https://gist.github.com/flare9x/94c29240a1be02f987872ef2f4e5f1be

For reference, this method was derived from this article (paywall):

https://inspectioneering.com/journa...uired-structural-thickness-of-pipe-using--bea

There is one elbow i'm trying to calculate a structural Tmin on but i have trouble determining the pipe span.

I typically get luck and I usually do structural tmin calcs on piping that look like the first page below.

This is straightforward since the supports that form the span is in the same axis.

However, the elbow i'm working with looks like the elbow in the second image below. What distance do i use as the pipe span? Is it simply 10' upstream and 19' downstream?

For reference I'm not a mechanical engineer so i'm not well versed in some of these concepts.

 

[Snickster]

Your approach to designng piping is not correct. Piping wall thickness is only determined by circumferential hoop stress per your program calculation Step 1. Allowable stress is based on the Code allowable stress such as ASME B31.3 for plant piping. The thickness per Step 1 is the thickness to be used throughout the piping system and supports, etc. are placed so that the longitudinal stress also meets the allowable stress requirements. You never increase the pipe wall to accomodate for longitudinal stresses. Only in very rare cases do you increase the wall but only at very local places due to very special forces. For instance at a road crossing to protect against wheel loading you may increase wall of pipe under road. Also if you have a very heavy concentrated load that is causing very high local stresses in the pipe resting on a pipe support you will add reinforcing pads to the pipe to account for the high local wall loads.

Supports are placed such that the piping is not overstressed in the longitudinal direction considering the combmination of all loads that cause stress in the longitudinal direction. This includes the longitudinal pressure stress, weight of the pipe, contents, insulation, etc; wind loads, earthquake loads, possible loads from slug forces, etc are also included. It is customary to consider all loads acting in same direction although wind loads for instance may be acting horizontally while weight may be acting vertically. Also to protect against wind loads guide stops need to be placed on the supports to restrict horizontal movement due to the wind loads. For large diameter thin wall pipe, local stresses at pipe supports need to be considered so this will also go into setting the spacing of supports, or otherwise reinforcing pads may be needed at supports.

An analysis is done with pipe supports placed so that the longitudinal loads do not cause the allowable stress value to be exceeded. The basic allowable stress value for longitudinal loading is the same allowable for the hoop stress, except B31.3 allows 33% increase for occasional loads such as wind.

A piping flexibility analysis is performed which will determine that the longitudinal loading does not cause the allowable stress to be exceeded and to determine stresses due to thermal expansion with the code providing the allowable for thermal expansion using the basic allowable stress value for hoop stress and increasing it to a higher value as indicated in the Code. If there is no significant thermal expansion in the piping then a manual hand calculation can be performed for the longitudinal stress determination.

The piping layout is not good in any case. The piping should not have that much overhang of unsupported piping beyond the pipe supports like you have. Piping is flexible and will vibrate and cause cracking of welds or fatigue failure. There should be a support at all elbows withing a couple of feet. Just visualalize how much overhang you have with a pipe that is out in the air 10' on way and 19' the other way. Also for straight runs of piping supports should be placed to limit deflection to about 1/2" to 1" depending on pipe diameter. This is because if a pipe is sagging between supports too much it is also not stiff enough and will oscillate easy with wind loads. Standard tables for pipe support spacing are available which give maximum recommended spans of pipe supports.

 

[daily123214]

@Snickster

Thank you for the informative post! I learned a lot more on the importance of longitudinal stress.

To clarify, this is an existing piping system.

I'm a bit confused on the need to calculate structural thickness of a pipe according to that inspectioneering website.

Also, API 570 mentions structural thickness and the need to potentially calculate it instead of using the provided table in the code.

 

[KevinNZ]

Believe Snickster over inspectioneering. All the stress calculations required are built in to the code requirements. To work out the bending moments in the piping the system is treaded as joined beams. Usually this is handled by the pipe stress program but can be manual for simple systems. The moments then go back in the code formulars. If the pipe is too thin these calculations will tell you.

 

[Snickster]

I see what you your issue is. there are cases where the calculated wall thickness by B31 equations do come up with a ridiculously thin wall for low pressures and small diameter pipe per hoop stress calculation. In such cases you do have to select a higher wall thickness base on mechanical and structural strength considerations. In process piping design clients usually use standard wall - sch 40 minimum anyway which controls minimum wall use. Also standard tabes for pipe support spacing considers completely filled with water and maximum span deflection of about a half inch. This also sets a reasonable minimum thickness versus support spacing.

 

[Snickster]

Also to correct my original post B31 codes combines horizontal and vertical or inplane and out plane stress using a Mohrs circle type of combination not by direct addition.

 

[daily123214]

@KevinNZ Thank you

@Snickster

"Also standard tabes for pipe support spacing considers completely filled with water and maximum span deflection of about a half inch."

That makes sense and I agree but why does API 574 11.1.3 say engineering calculations may be needed? In what circumstances?

 

[1503-44]

As proof of design adequacy according to the facility's pipe design code.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Daily,
Snickster is 100% correct.

I think from what I could read of the article, the author was making the point that often people just either ignore other loads on the pipe other than internal pressure or use some rules / tables/ spans for what is "normal", i.e. pipe between two supports.

But each pipe is different and more loads could apply from thermal stress, wind, snow, ice, insulation, someone using it as a climbing frame / access to a valve etc

Basically all those loads normally go into a stress analysis, either a program like Caeasr or similar or an experienced pipe stress engineer looks at it and decides to add a bit more wall thickness.

your OP diagram is a really oddball bit of cantilevered piping and cannot just be compared to a standard pipe support span calcualtion.

You need to get this stressed in a pipe stress program really.

also you need to decide if there is some other limit like deflection that will actually govern the thickness / strength of the pipe. Usually people don't like pipe sagging visibly, event though it is within the stress limit. Also of starts vibrating then you're into all sorts of fatigue calcs.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Noting that you have not mentioned diameter, I would at least support near each ell on the 15ft span.

Spans over 20ft are usually not needed. Pipe racks are almost always constructed for 20ft max span.

Usually you can make a 20ft span work, if pipe is over 3"diameter pipe. 3" maybe yes, maybe no.
1" to 2" about 10ft and can usually be tied or suspended from an angle bar tied to larger pipes in the rack.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[daily123214]

@1503-44: Thanks for the advice

@LittleInch: Thanks as well for the feedback. i was really hoping I can do this calculation manually but looks like i have to plug it into Caesar now.

Halfway through that 4' span line (4" diameter) there is a 1" line that branches off from there. Can that be considered a "support"?

 

[LittleInch]

No.

More likely it is a point of high stress concentration that could easily crack and fail in service. You really do need to look at how that 1" pipe is allowed to move to avoid stressing the connection to the bigger pipe to AVOID it acting as support.

Small bore taps and connections are notorious for failing in service due to over stress.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[daily123214]

@LittleInch: I didn't know that about small bore piping thank you!

 

[1503-44]

2" pipe and less is often selected with XXS wall simply to provide mechanical strength. Otherwise they sometimes act as stress concentrators.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[KernOily]

What nobody has asked here yet, and which I am very surprised at: is this pressure piping, or structural pipe?

What is the governing design code? What is the fluid service? What are the pressure and temperature? What facility is it in?

All those things are Critical.

Or are you using pipe to build a structure, e.g. a carport or a swing set?

When you answer these questions, I can provide some answers and a go-forward plan. Don't fart around with pressure piping until you know what the governing code is. If something goes wrong and there's litigation, you will be hung out to dry if you did not design this to the proper code.

 

[1503-44]

All have to be satisfied, no matter what code, pressure or temperature.
Snickster addressed the rest.
OP was out so far in left field, nobody had any desire to get into calculations.
Are you sure you want to?

 

[KernOily]

1503-44: No. In fact I'm not even sure what you are talking about by that reply.

Until we get details, we're guessing. Snickster is generally correct in the most general engineering mechanics sense, but each code has different allowables, different design bases, different rules, etc.

 

[Snickster]

I believe the best approach is to answer the question asked and provide some additional relevant information that may help OP understand more in detail what factors are involved. Cannot give a complete thesis on every questioned asked.

 

[GD2]

Snickster has almost explained all. The only other thing I would like to add is the 'thermal Load' arising out of a restrained piping system that may play a critical load in a process piping system.
Wind loading will be selective in a process piping. Wind load generally becomes effective from 10 inch diameter onwards and is applied to specific locations.

GDD
Canada