Checking the maximum allowable pressure for the shell area of the thin walled pressure vessel is important from the vessel sizing or design point of view. We will see the calculation methodology in line with the ASME section viii division 1 for our example.
Required equations according to the American Society of Mechanical Engineers Standard Sec.8 div.1 are:
Longitudinal pressure applied to the pressure vessel (PV) shell,
P1= (S*E*nt)/(Ri1+0.6*nt) ……………..Eq.1
Circular pressure applied to the PV shell,
P2= (2*S*E*nt)/(Ri1-0.4*nt) ……………..Eq.2
S = maximum allowable stress for the vessel material
E = Welding joint efficiency
nt = Nominal thickness of the shell = Actual shell thickness – Corrosion allowance
Ri1 = Inside radius of the pressurevessel considering the corrosion allowance = actual inside radius – corrosion allowance
The ASME section eight division one guidelines say that,
Maximum allowable pressure inside the vessel, Pm = Minimum of (P1, P2)
Now, if the actual working pressure inside the vessel is less than the maximum allowable pressure calculated above then it can be concluded that the PV is safe.
Or in other words, the vessel to be in safe working condition,
Please refer the problem statement explained in part-1 of this pressure vessel design calculation tutorial series.
S=103 MPa (refer part-3)
E=0.85 (refer part-3)
nt= 6 (refer part-2) –0.02 (Corrosion allowance is assumed according to the application) = 5.98 mm
Ri1=1349.98 mm (refer part-3)
Internal pressure for the pressure vessel, P=75 Pa = 0.000075 MPa (Refer Part-1)
By applying the above given data to the Eq.1 and Eq.2, we get,
P1= 0.386791798 MPa
P2= 0.777016417 MPa
So, Pm=MIN. OF (P1, P2) =0.386 MPa
We can see that P<Pm for the above example.
So it can be concluded that the pressure vessel is safe from the maximum permissible pressure calculation point of view as per the ASME section 8 division 1 codes.