Mill tolerance equation confusion 1

[Jomanchester]

Hi
My company got some previous data sheet based upon ASME B31.3 for wall thickness calculation of a pipe. But I am confused by two set of equations to include the Mill tolerance with the minimum required wall thickness. Please find those two equations.
T[sub]m[/sub]=t[sub]m[/sub]+ MT (expressed in mm)
or
T[sub]m[/sub]=t[sub]m[/sub]÷[1-(MT/100)] ,If MT is given in percentage

The 1st equation is pretty straight forward adding the tolerance with the calculated wall thickness.

then why it is not similar with MT tolerance expressed in percentage?

Thanks for the help

 

[DSB123]

Jomanchester,
For piping Mill tolerance is normally expressed as a % of the nominal thickness of a pipe. So when you calculate the required thickness to sustain the Design Pressure and add on the Corrosion allowance you end up with a thickness Tmin. The next step is to select a pipe thickness such that the nominal thickness Tnom less the Mill Tolerance is greater than Tmin.

i.e Tnom - MT >= Tmin or Tnom x (1 - MT/100) >= Tmin

So Tnom(or Required) > = Tmin / (1 - MT/100)

 

[Jomanchester]

That was perfect DSB123

CHEERS fellow

 

[LittleInch]

Its amazing how many people get this wrong, but always do a back calc, i.e. start with your nominal thickness, then subtract mill tolerance as a percent of nominal thickness, then subtract any fixed allowances such as corrosion and make sure you've got your design thickness.

You may only be fractions of a mm out but there's no excuse for getting this wrong.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way