Pipe Wall Thickness Calculation
Typical Pipe Wall Thickness Calculation :
1) Pipe Wall Thickness calculation for Internal Pressure :
It is performed as per formula given in piping code ASME B31.3
Internal Design Pressure Thickness t = PD/2(SE+PY) ---[1]
t = Pressure Design Thickness (cm)
P = Internal Design Gauge Pressure (Kg/cm^2g)
D = Outside Diameter of Pipe (cm)
S = Allowable stress value of pipe material at the design pressure.This is given in ASME B31.3 Table A1
E = Quality factor from Table A1 B of ASME B31.3
Y = Quality factor from Table 304.1.1 of ASME B31.3
"t" calculated as per the formula is the minimum thickness required to withstand pressure.
The minimum required thickness "Tm" is calculated by adding the corrosion allowances to value 't'.
Tm = t + corrosion allowance ---[2]
The corrosion allowance depends on the corrosive nature of the fluid in service
NOTE : a negative milli tolerance of 12.5% is considered at Tm.
Example :
Find the wall thickness for 10'' pipe with following condition.
Design pressure = 20 kg/cm^2
Design Temperature = 80 C
Corrosion Allowance = 1.5 mm
Pipe material = API 5LG.B (Seamless)
Solution :
1) We need to find OD of the pipe from pipe wall thickness table
for NPS 10" it is 273mm (refer table)
2) to find Allowable Stress S Check the ASME B31.3 Table A1
Design Temperature in Farenheit = 176 F.
API 5L B is red line.
Since there is deviation in Maximum Allowable stress only after 400 F, We can surely say that value of S is 20000 psi.
3) Convert 20,000 psi to kg/cm^2g. we get 1406 kg/cm^2g (approximate)
4) Quality factor E = 1 (for seamless pipe) from TABLE A1 B; Y = 0.4 from TABLE 304.1.1
5) Substitute the values in the formula [1]; We get t = 0.193 cm = 1.93mm.
6) Adding corrosion allowance, t = 1.93 + 1.5 = 3.43 mm.
7) Now to find Tm, a 12.5% negative milli tolerance is considered
8) Tm*(0.125-1) = 3.43
9) Tm = 3.92 mm.
10) From wall thickness chart NPS is 10'' & We got 3.92 mm minimum wall thickness;from chart we take Sch10(since 3.40 is less than 3.92,we take next highest value 4.19).
Exercise:
Find the wall thickness of 12" Pipe having
Design Pressure = 39 kg/cm^2
Design Temperature = 670 F
Corrosion Allowance = 3mm
Pipe Material = ASTM A106 Grade C (Seamless).
Comment if you have find the answer or if you have any doubts in this topic.
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