pipe welded

Get your material specification , for instance ASTM 333 gr. 6 seamless or whatever material you have whicj h is not seamless , but whicg is allowed by the Code

If you work with B31.1 or B31.3 , based on your material find out the appendix at the end of the standard which gives you the value of=
E = Joint efficiency , This E has a maximum value of = 1 ,
The paragraph of the code defining the required minimum wall thickness will tell you where to look exactly.

Introduce his value E in your formula ....

Note that your allowable stress will decrease when the temperearuture gets hotter , say from 200 celsius,

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Now we have talked about a STRAIGHT pipe
And so far you have the minimum required wall thickness for a straight pipe ONLY
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but the maximum allowable pressure might depend on any other possible fitting in your entire piping system, this can be a tee or a flange,
the reason is that for relatively hight pressures the required thickness for a tee, a reinforced tee , or an elbow must be thicker that the matching pipe
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Check also and again the flange rating , because it might dictate the maximum allowable pressure

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If you do not understand the above 100 % , Please indicate the:
Material
The Code
The design pressure
the Maximum design TEMPERATURE
the MINIMUM design temperature
The corrosion allowance

And if you yave flanges , tees,


Take care