How close is too close? Tensile vs. Yield

[TXLSUCHE]

I am considering using some 7" OD x 1" W.T. 4130 tubing for a 10,000# piping application. The T.S. is 109,000 and the Y.S. is 93,000. Is there a disadvantage of having the T.S. and Y.S. this close together? That is the only thing I can see that could be detrimental, but I'm not a mechanical engineer.

Secondly, does anyone have recommendations for what to use for the allowable stress if the material is not listed in B31.3 and is considered high pressure piping? I've read the K Section of 31.3 and it says I can use 2/3 SMYS (operating temp. < 100 F), but this seems less conservative than the requirements in the lower pressure piping section (lower of 1/3 T.S. or 2/3 Y.S.). Everyone I have spoken with on the subject that has calculated W.T. for 10,000# piping uses 1/3 T.S. and I don't know if it is a carry over from doing low pressure piping or some other reason I have been unable to find.

Thanks in advance for your help.

 

[metengr]

Answer to question 1). I see no detriment in using higher strength materials that have a higher YS/UTS ratio. You are designing to avoid macro-yielding in service.

Regarding the use of Chapter IX (Piping for High Pressure Fluid Service) in B31.3, read thru the entire Chapter regarding additional piping analyses and the formula(s) used to calculate minimum wall thickness. You just can’t compare the methodology for determination of allowable stress values for Chapter II and Chapter IX. The Code is very clear that when pressure piping is deemed “High Pressure”, the formulas cited in Chapter IX require the use of Table K-1 for allowable stress values, not the allowable stress values from Table A-1.

 

[TXLSUCHE]

Thanks metengr for the response. I understand your explanation of Chapter IX. Unfortunately, 4130 (A519) isn't listed in Table K-1. If I compare the other Cr-Moly materials for tubing that are listed in the table, I see that the allowable stress for <100 F are calculated using 2/3 the Y.S.

I have read through the entire Chapter IX and I feel that the only loads my calculations need to be designed for are those in tension / internal pressure. I don't expect any external loads (shear/bearing, compression, etc.) on this pipe. It will be adequately supported and not associated with an rotating/vibrating equipment.

 

[JStephen]

Not a piping guy, but my thoughts:
1) Does the code prohibit materials that aren't listed in it? (Think B&PV code is set up that way)
2) Is the material weldable, assuming you're going to be welding it? & can you get weld qualifications and all to meet code?
3) Does code require pipe be hydrotested and is that material hydrotested? IE, is it even intended for pressure applications?
4) Is the material seamless? If not, how 'bout that seam?
5) Is the material actually cheaper than thicker less-strong stuff?
6) Are there impact test requirements for the material or HAZ that you'd need to meet?

On having the UTS and YS close together- that's not necessarily good, but it's typical of high strength materials.

 

[TXLSUCHE]

Thanks JStephen.

To answer your questions:
1)The code allows you to use an unlisted material if it has an associated spec that governs it (i.e. A519 in this case).

2)Material is weldable and there are existing qualifications and procedures.

3)Pipe is to be hydrotested to 1.5 times. If I choose a stress with some saftey factor (maybe 85% Y.S.) and calculate the internal pressure rating of the pipe, it is well over the hydrotest pressure.

4)It is seamless.

5)In this case, I can not go thicker, cheaper because the large W.T. will hinder my flow conditions (velocity, erosional velocity, deltaP, etc.)

6)Impact tests are generally done by the supplier as an acceptance of the material from the steel mill.

 

[StoneCold]

Can I ask what are you building that runs at 10,000psi?
I can't think of anything other than very large hydraulics that are even close to that.

Regards
StoneCold

 

[TXLSUCHE]

Stone,
The piping is on a Gulf of Mexico oil & gas platform. The shut in pressure of the well is 8900 psig, so the piping is being designed for 10,000 psig. It will probably never see that high of a pressure because of pressure safety devices, but the piping needs to be designed for that just in case.

 

[metalonis]

StoneCold,
See

http://www.highpressure.com

for equipment designed for pressures up to 150,000 psi