B31.1 104.1.2 formulas?

[mejia198021]

I am trying to figure out the internal design pressure (P)allowed in a 6" sch. 40 SA106-B 150# CS straight pipe, 6" CS 300# SA105 RF WN Flange, and a 6" CS 150# Raised Face Weldneck Flange, that are all connected. I have applied the formulas (3A) and (4A) in which I have figured P, d, SE - from App. A, y - table 104.1.2(A) - (5), but I am having trouble finding the term A (additional thickness (A.6)).

 

[jte]

mejia-

In essence, your term "A" will most likely be an educated guess. If you have threaded pipe, you need to add the depth of the threads to the req'd thickness. This would be one part of your "A". If you have low pressures then you need to consider increasing the thickness for pure mechanical strength. After all, the pipe doesn't do you much good if you fill it with water at no pressure and it crumples between supports. Add this req'd thickness to "A". Add corrosion allowance based on past industry experience with the fluid and metallurgy involved to "A".

The Code will not provide you the values you need for "A". You have to determine it yourself.

jt

 

[ApC2Kp]

mejia198021,
The term 'A' is the allowance for removal of wall thickness for cutting threads. This would be somewhat like the corrosion allowance that reduces pipe wall thickness available for pressure design. It is good to be able to check each component for pressure capability. You need to realize that the flanges in a smaller diameter piping system are usually the limiting component. A CL150 flange has lower pressure limit than a CL300 flange, and that pipe is never "CL150".
The pressure limit of the CL150 and CL300 flanges are given in the standard ASME B16.5-1996 Table 2-1.1, so you do not need to do that calculation.

 

[NozzleTwister]

mejia198021,

You shouldn't be making guesses for the term 'A'. The system you describe doesn't have threads and shouldn't have errosion. What you will have is a corrosion allowance of most likely 1/16" or 1/8" as this is most common. If your design criteria doesn't give you this then ask the Process Engineer for the value. If that gets you no where use the same corrosion allowance used for the Boiler.

As stated by ApC2Kp, your 150# flanges will be your limiting component. A new version of B16.5 was published in 2003 which contains a number of changes in the temperature-pressure ratings. Unfortunantly the valve standard B16.34 hasn't caught up with these changes.

Remember that your 'tm' (t sub m) term is your minimum thickness so you need to remove the mill tolerance from the nominal wall thickness. Since your SA-106 has a mill tol. of 12.5%, the 'tm' for your 6" sch. 40 pipe would be: 0.280" x .875 = .245", so 'tm' = 0.245", subtract your corrosion allawnce from this and your new value will be the thickness used to determine the allowable pressure of the Pipe.


NozzleTwister
Houston, Texas

 

[jte]

ApC2Kp-

The "A" term is not limited to thread cuts. It also includes other necessary allowances such as corrosion allowance, so "A" is not "somewhat like the corrosion allowance."

NozzleTwister-

I agree that mejia shouldn't be randomly guessing at CA's. That's why I said he should be basing the value on past industry experience. Ultimately, though, the CA is a SWAG. I've seen many vessels and pipes which have far outlived their design life without using anywhere near their allocated CA, and also the flip side, where rates of 5 to 10 mils per year were expected and we're seeing 50 mpy.

Good point about the 2003 edition of B16.5. People need to start using it for new construction. I compared the values a while back for group 1.1 materials and found no changes to the CL-150 ratings, and many (54 of 70) values which changed on the CL-300, 600, 900, 1500, and 2500 ratings. Most of the changed ratings were increases. Nine were decreases.

jt