MAWP for NPT fittings 1

[grosenbalm]

I have been trying to put this all together for a little while now and I could use you guys' help to get through this.
I am trying to figure out how to calculate the maximum allowable working pressure for NPT threaded fittings. I know the formula P=2St/(D-2yt) can give you the pressure but I am having trouble matching it exactly to some tables I am comparing it to. Let's take for example a 2000#, 1/2", API-5L-B seamless pipe which has an allowable stress of 20,000 psi, OD= .84, nominal thickness=.147, max thread depth of .05714, and assume no corrosion. I get 4680 for MWAP while the chart I am comparing it to comes out with 3678! the formula they use to find t is t=tn*(.875)-C-A where tn is nominal thickness C is corrosion allowance and A is additional thickness required for: threaded pipe=depth of thread. Please help me figure out what I else I need to fix this descrepancy. Thanks, Greg.

 

[BigInch]

First of all, I don't see that equation in my edition of B31.3.

Second, not seeing the table you're looking at, I don't know, but

Is it because you're using t = specified wall thickness - maximum thread depth, but they're calculating (?) the minimum wall thickness at 87.5% of the nominal thickness as mill tolerance and not deducting thread? Are they specifically deducting thread depth somewhere?

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[grosenbalm]

Well I'm getting the equations and the tables from a Technical bulletin (No. 128) and it back references B31.1, B16.11, and B31.3. It's kind of old (1981) so I don't know if some information has changed or not. I guess what I'm really getting at is, for high pressure piping, how do you solve for the MAWP with the main focus on forged steel tapered threads. What all do you need to solve? There is a male and a female thread depth, along with a truncation depth, and tolerances all along the way. How do you put all of this together for an accurate assessment?

 

[BigInch]

Busted! I think its time to update codes. I thought I was bad, my B31.3 is from 2002.

p 304.1.2 Straight Pipe
t = P*D / (2 (Y*P + S*E))
t = (P*d+2c) / (2 (SE-P (1-Y))
Y = (d+2c)/(D+d+2c) for t>D/6
other conditions apply for t<D/6

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[grosenbalm]

O.K. well I have B31.3-2000 now. What do you know about the pressure and wall thickness formulas from Chapter IX K304.1?

 

[code1]

Is the MAWP of the fitting also limited by the yield of the threads (other than the failure of the cylinder)? If yes, how can this be calculated.

 

[grosenbalm]

From my knowledge, a 10,000# rated fitting is rated for its ability to withstand the pressure equivalent of a 10,000# rated pipe. I have been wondering myself if the MAWP is limited by the yield of the thread. I'm still looking for the answer. I know I saw something that calcualted the shear area on a thread...
Also, what about the pressure and wall thickness formulas found in B31.1 104.1?

 

[JohnBreen]

Hi grosenbalm,

It is important to recognize that NO ASME CODE provides equations for determining the required thickness for fittings. Fittings are rated by proof tests and not by equations.

The B31 Code calculations for required wall thickness REALLY DO include allowances for erosion, corrosion, threading, and grooving (for using graylock type fittings). Also, mill tolerances are to be considered for seamless pipe.

Fittings and flanges are rated for coincident application of pressure and TEMPERATURE for various materials.

Regards, John.

 

[grosenbalm]

So, is the only way to find out the MAWP of a pipe fitting to test it under pressure until it fails? Do all manufacturers do this and is it made public? Is there another way to predict the outcome mathematically?

 

[JohnBreen]

Hi grosenbalm,

The ASME B31 Pressure Piping Codes address pressure design for pipe and refer to other associated Codes and Standards for pressure design of fittings. If you are using a "Standard listed" fitting then the appropriate Standard details the methodology for the fitting's MANUFACTURER to pressure-rate the fitting.

If you were using the ASME B31.3, Process Piping Code you would go to table 326.1 and if the fitting is "listed" there you need do no further calculation - this is one of the reasons for "Standards". The fitting (if it is listed) will have a pressure-temperature rating and if that rating is appropriate for the service of the system you are designing you can use it in systems with the same P?T rating.

Regards, John.

 

[dcasto]

please note B31.3 Table 314.2.1 on minimum thicknes regaurdless of calculated ticcknes required for threaded male ends (of pipe)

 

[grosenbalm]

O.K., I am putting together a formal report on the subject: So, how would you explain the process of determining the maximum allowable working pressure of something (pipes, fittings, flanges, valves, etc.)? What process do manufacturers go through to determine an exact number for its product's maximum allowable working pressure?

 

[eliebl]

If you look at ASME B31.3 paragraph 304.7.2 there is a list of several options.

EJL

 

[grosenbalm]

Does anybody have the write up from 16.9 on proof testing of components in piping systems? I don't have or know anybody with a copy. Thank you.