BUTT WELD TEE

[JJAV1983]

Hello,

I was hoping you could answer this simple question.

According to ASME B16.9-2003 for factory made wrought butt welding fittings, the allowable pressure may be calculated as for straight seamless pipe...

So for example if the pressure rating for a 4" A53B pipe sched.40 is 2581 PSI (from charts), does that mean that a 4 inch butt weld TEE sched. 40 will have the same pressure rating? if not, what would be the best way to calculate the pressure rating on butt welded TEE's?

Thank you,

JAV1983

 

[XL83NL]

Yes, provided the fitting is seamless as well. For welded fittings, calculate as if the pipe is welded also.

 

[LittleInch]

Agree that the same material, same wall thickness or schedule and size fitting to ASME B 16.9 has the same design pressure as the connecting pipe, but be careful where you get your pressure rating from - "charts" will be for a specific duty and design code only.

You should be doing this the other way around and from your design pressure and using the factors and allowable stress from your design code, calculate the min thickness considering corrosion allowance, manufacturing tolerance etc. a lot of times someone will have already done this for you and produced a pipe spec which defines the requirement, but it's good to know where that data came from.


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

 

[r6155]

JJAV1983
ASME B16.5-2003 is an old edition, use 2012 edition

Regards
r6155

 

[JJAV1983]

Thank you all for your answers, and yes the charts I was just using to start my design, I validated it after using the code, I should get the new edition though.

Regards!

JJAV1983

 

[JJAV1983]

One more question about this, I want to make sure my calculations are correct:

1. To get the minimum thickness I used tm=t+c as per ASME B31.3-2008
2. To get t I used 301.1.2 (3a) using the following data:
P=350 PSI, D=4.5 IN, S=16,000 PSI, E= 0.85, W=1, Y=0.5

After plugging those numbers into the formula I got t=0.057in and for C (sum of mechanical allowances) I have .16 (.100 in for thread depth and .06 for corrosion allowance) so tm= .217

If I wanted a Factor of Safety: 2, would I have to double that thickness on my pipe?

One thing that is making me confuse is that if I re-write the formula in ASME B31.8-2008 (304.1.2-(3a))to find the pressure it gives me an allowable pressure of 1087.5 PSI (already subtracted the 25% allowance for connections and fittings)

How do you determine the FOS of the system?

This is the first process pipe system I have been involved with, that's why I have so many questions... I would appreciate your help!!!

Thank you,

JJAV1983

 

[r6155]

JJAV1983

1) Sorry my mistake. Change ASME B16.5-2003 to read ASME B16.9-2003. See 2012 edition
2) ASME B31.3-2008 is an old edition. See 2014 edition. Also see ASME B31.8-2014

Regards
r6155

 

[every_name_is_taken]

JJAV,
By using the allowable stresses from ASME B31.3 you have an inherent FOS. See 302.3.2 for the Bases for Design Stresses. The bottom line is if you are meeting the code you probably have an adequate FOS, but good engineering judgment should always be exercised. Remember that you are only calculating the thickness for pressure retention and will also need to consider the stresses due to thermal expansion. Since for you Y=.5 and W=1 I'm guessing you are probably using SS and at a fairly elevated temperature, so thermal expansion will certainly be something to consider.

 

[LittleInch]

jjav,

B31.3 is a piping code. It provides a max stress value to use (S) which takes account of factor of safety and temperature de-rating in its calculation. As e n i t says, you then need to consider other stresses and B 31.3 also requires manufacturing tolerance to be added.

B 31.3 inherently applies a larger FOS than pipeline pipe because it is recognised it has other loads on it and is usually supported in defined locations, stepped on, used as ladders and generally abused. Also piping wall thickness is a very small part of the cost of a facility and there is no reason to minimise it to as low as possible.

B 31.8 on the other hand is a pipeline code where the pipe is buried ( fully supported) and where cost of pipe forms a substantial part of any project and hence needs to be minimised to the lowest possible thickness commensurate with safety and long life.

It has a specific and variable Design factor (1/FOS) for different locations.

Each code is different and each system has its own way of calculating things.

Screwed joints are not common on pipelines so the 25% figure is a little odd to me.

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

 

[JJAV1983]

LittleInch,

Thank you for your answer, the reason I am taking that 25% off is because the customer wants some valves to be threaded instead of welded/bolted. Do you or e_n_i_t happen to know what the inherent FOS is?

Thanks,

JJAV

 

[XL83NL]

Its not A factor of safety, but more a design margin against a certain failure mode.

 

[moltenmetal]

Do people really thread stuff at 4" NPS? Really? I've seen NPT threads this large, but never on anything subject to a process piping code like B31.3...

 

[SnTMan]

I wouldn't want to make that joint up

Regards,

Mie

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[racookpe1978]

If you are including threaded fittings, the premis of the answer above (that the two are equally pressure capable) is wrong.