Published Analysis - Steel Forged Slip On Flanges - Pipe

[ATSE]

Subject is steel forged slip on flanges under axial tension load.
Conforming to ANSI B16.5 for American std pipes (sch 40 and sch 80). Pipe grade is API 5L X42 and ASTM A53 Gr 35.
These flanges are used across many industries, but I am interested in tower leg splices (non-fluid).
Are there published analysis of ultimate tensile strengths?

It is safe to say to the ratings for 150 flanges are based on 150 psi water pressure for a given pipe size, with a 1.5 factor.

However, my simplied plate bending analysis shows much more capacity (tensile strength) than that.

I am dealing with a plan checker that wants a 3-D FE analysis for many different sizes of pipe for this particular project. 4" std thru 10" std. That hurts.
He is semi-reasonable, and would probably accept documented tensile capacities (something published and more rigorous than my simple plate analysis).

These flanges have been used millions of times (without exaggeration) and I'm sure someone smarter than me has done the high powered analysis 10 dozen times over, but I cannot find it.

The AISC Hollow Structural Sections manual is no help.

 

[paddingtongreen]

Does this help, it is based on internal pressure in the pipe.

http://www.pumpfundamentals.com/help15.html

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[racookpe1978]

I would caution against crossing a "pipe" specification and standard directly over to a "structural steel" design requirement without very, very carefully considering mill accuracy, wall thickness tolerances, steel grades etc. Pipe forgings and pipe fittings are more accurately fabricated (have smaller deviations from the norm) than the pipe itself.

"Round structural steel members" may have the same nominal dimensions (wall thickness, diameter) as "pipe" but are more dimensionally stable and more reliable structurally.

That said, a slip on flange is mounted with two fillet welds: calculate the total shear strength and joint bend resistance using the inside and outside diameters at each fillet position, the true positions of the two fillet welds at each point and the two fillet weld sizes. (Inside and outside fillets should be the same.)

 

[dig1]

ATSE,

As a simplified approach why not look at the problem as a flange design problem but not using the pressure bolt load (Wm1) and for gasket seating use the m & y values for a solid flat metal gasket (gasket width equal to the width of the raised face) for the seating load (Wm2) and then add the axial tensile load to determine the design bolt load (W). Once W is calculated then proceed through an ASME Section VIII, Division 1, Appendix 2 design. Depending on the flange configuration Appendix Y may need to be followed.

I'm not saying this is correct because this arrangement may be considered to be outside the scope of Appendix 2 but it may be a reasonable method which is acceptable to the reviewer and SEOR.

My thought is that using the metal ring gasket m & y values will mimic the force needed to bring the connection together and then the bolts will be subjected to additional tensile load from the design forces. If this is also subject to member bending (which your description does not state) you would alos have to look at the bolt loads handeling that load as well

As a note of caution, standard ANSI flanges may not pass the Appendix 2 calculation depending on the circumstances.

In this case you could look at each flange size in less than 1 hour each so in 1/2 a day you are done.

This is my first thought as an alternate hand method instead to an FEA.

BR,

Patrick

 

[racookpe1978]

Why use gaskets at all? You'd want a fixed (inflexible!) connection between each flange, and the original writer did not imply any pressurized fluids internal to his/her "hollow round structural members" ...

 

[ATSE]

Thanks for the responses.

To be clear, the primary design question is flexure of the ring plate.

If you do some quick math, you will see that the "end thrust" from pipe pressure is substantially less than the bolt group tension capacity.
As an example, take a 8" std pipe.
A Class 150 slip on flange has (8) 3/4" dia A325 bolts, with an LRFD strength = 238 kips

If I understand ANSI/ASME B16.5 correctly, then the test pressure for Class 150 = 450 psi, which translates to
Test tension = 450psi x 28.9 sq in = 13.0 kips
That is, pipe pressure correlations are not useful for structural steel since the axial demand is so small compared to typical structural loads on pipe.

Using Roarks or other statics-based methods, I get a flange bending strength that correlates closely to the bolt strength. That is, for a Y2 dim = 1.75" thick, phi-Mn = 10.3 kip-in per inch of hub diameter.

That is, my calculations show that the flange bending is not the weak link in the connection.

What I'm looking for a summary of B16.5 slip-on flange tensions capacities (preferrably LRFD)- considering the limit states of welds, bolts, and flange flexure - for a given pipe size and yield, based on parametric FE analysis.

 

[JStephen]

If there's not some pressing reason to maintain the interior clear, it seems to me it would be simpler and easier to use full plates across the area rather than flanges.

And if I remember right on standard raised face design, much of the flange load is due to bolt preload, which is required to seal the gaskets, so you get some overkill there.

 

[ATSE]

JS,
The welding for a slip on flange is different than a blind flange.
A slip on flange has two all-around welds.
However, the main reason to use a forged flange is price. You can get a ductile steel flange (Class 150) ready for welding on a 6" std pipe for under $30.
Also, for outside steel, all weldments are hot dip galv. For pipe weldments, this means inside and outside walls. If your base plate or splice flange doesn't have at least a 1" diameter hole in it, the galvanizer will provide a somewhat undefined and irregular shaped hole (usually burned with a torch) for free.

 

[dhengr]

ATSE:
I would look around at some of the tower design literature, or the forums on E-Tips for transmission towers, antenna towers and the like, and see if anyone has used these flanges and how they handled the flange stress problem. Maybe ask some of your flange suppliers this very same question, maybe they can do that analysis for you, or have already done it for the ultimate strength of their flanges.

You might be better off using the ‘lap joint’ flanges since they would more closely compare to the a typical structural bolted joint. You are just adding another level of complexity to the analysis by using the slip-on flange. Your problem, with that slip-on flange, is a bit different than a normal structural joint which has several plates (elements) bearing on each other, at faying surfaces, due to the preload or tension in the bolts. The thing I am thinking about is that the total gap of .125" (2 x 1/16") btwn. flanges on your slip-on flange, allows bolt tensioning (preload) which could start to overstress the flange in trying to close the gap. Then, the real structural tension in a tower leg will add tension to that bolt circle, adding more flexural stresses to the flanges and more tension to the bolts. If you use the lap joint flange, preload in the bolts will not really stress the flanges in flexure during assembly. You might start to approach a slip critical joint condition however. Now, as a tower leg is loaded in tension it does start to stress the flange plate in flexure, but it doesn’t really change the tension in the preloaded bolt until the leg tension gets high enough to over come total bolt preload in that bolt circle. You do still have prying effects on the bolts and potential shear loading in the bolts to consider.

The 1/8" oversized holes (your flanges) for structural connections are actually called ‘oversized,’ while the standard hole dia. clearance is 1/16"+ bolt dia., this is per AISC or RCSC. How are you going to transmit shear through these joints? Through the bolts or could you put a short, saw cut, piece of pipe in the gap between the end welds on the main leg pipes to take the shear?

Given the slip-on flanges shown, what bolt tension (not torque, but that’s the way M.E’s. like to spec. it at times) is required in a normal ASME, API, ANSI Specs. for the normal pipe applications? You might argue that that bolt tension causes a certain set of stresses in the flange, per those specs., to compress gaskets, resist pipe pressures and forces, etc. And, that bolt tension is not causing any flange stress problems in that intended use. If your structural application causes no greater tension on that particular bolt circle than those required above, the flange will be stresses no higher than above. I assume you are thinking of using these flanges on pipes which make up the structural legs of a tower or some such. That tower and tower leg analysis and design itself might be every bit as complex as a FEA of the various flanges, so you might just want to bite that flange stress bullet too. I would assume you have the software and computer power to handle both of these design problems, if you are designing towers, however distasteful the extra effort might be. I would just give plan checker the calcs. for the pipe sizes your design is using, unless he/she wants to pay for the full set of calcs. for his/her own future reference.

 

[ATSE]

dhengr,
Good point about different behavior of slip-on vs. lap.
The fabricator has used slip on for 25 years. Inertia.
The 1/16" gap at the bottom surface that prevents clamping and direct contact during bolt tensioning makes the behavior and modeling (hand calcs or FEA) much more challenging.

 

[dhengr]

ATSE:
Fabricator inertia or not, that added analysis complexity and the potential of starting to over stress the flange plates for no good reason at erection, may be a good reason for you to suggest the change since you are being called upon to prove their design. What’s the difference in cost btwn. the two different flanges, can’t be much, if any. And, the fab. cost would not change at all, while you were improving the structural action of the connection. For your application the forged flange will be slightly better than a cast flange, assuming equal Fy and Fu values, for the combined stress and fatigue reasons which would concern me. The grain orientation will be improved at the transition radius, flange plate to pipe weld neck. The lap joint flange does require a little more attention to detail at fit-up and welding to keep the flange plate face perpendicular to the pipe, but that’s just minor fixtureing changes during welding.