Internal Pressure and Minimum thickness of pipe.

The inspector should be able to determine the internal pressure and minimum thickness of the pipe. 

  1. The minimum required thickness or pressure design thickness of a straight pipe section for internal pressure only (ASME B31.3 Section 304.1.1 and 304.1.2)
  2. The minimum required thickness of a permanent blank for a given design pressure (ASME B31.3 Section 304.5.3);
We will start from Clause 304.1.2. There are two equations (3a) and (3b).

We will study to calculate t from each of these equations.

(3a) : t=PD/2(SEW+PY)
t = Pressure Design Thickness
P = Internal Design Gage Pressure (Pressure to a pipe can be internal or external. API 570 exam contains only internal pressure scenarios)
D = Outside diameter of pipe as per standards or spec. or as measured.
S = Stress Value for material
E = Quality Factor
W = Weld joint strength reduction factor
Y = Coefficient (We will discuss in detail about this.)

(3b) : t=P(d+2c)/2[SEW-P(1-Y)]
d = Inside diameter of pipe. Maximum inside diameter allowable as per purchase specifications has to be considered.
c = mechanical allowances + corrosion allowance + erosion allowance

Try to memorize the above two equations
Now we will see how to get values for each of the parameters in (3a) and (3b)

We have to calculate the thickness 't' required for the pipe. 

P = Internal Design Gage Pressure.

This we will get from process data sheet. The design pressure of each component in a piping system shall be not less than the pressure at the most severe condition of coincident internal or external pressure and temperature (minimum or maximum) expected during service.

The most severe condition is that which results in the greatest required component thickness and the highest component rating.

Don't get confused with these explanations, for API 570 exam the design pressure will be given in the question if thickness is to be calculated or if thickness is given then maximum design internal pressure can be calculated from the equations (3a) or (3b)

D & d; The diameter's of pipe

'D' is the outside diameter and 'd' is the internal diameter.

'D' , the outside diameter of pipe as listed in tables of standards or specifications or as measured.

'd' is the inside diameter of pipe. For pressure design calculation, the inside diameter of the pipe is the maximum value allowable under the purchase specification. This means the value of 'd' is to be taken by adding maximum value for allowance's for inside diameter, if allowance's are given in the question.

From API 574 Table 1, 'D' and 'd' can be taken

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