SHELL DESIGN BASICS
© Wilkinson Coutts Engineering Training Australia
TERMINOLOGY FOR CYLINDERS
IMPORTANT: Note how AS 1210 Section 3.7.3 gives 3 different equations depending on
the diameter used. Other diameters are Dm – Mean diameter and Do – Outside
diameter
Internal Diameter D
P = Internal pressure
P
f = Maximum allowable stress value
t = Minimum calculated
thickness of shell
AICIP exam questions only use mean diameter equations
© Wilkinson Coutts Engineering Training Australia
REMEMBER
Hoop Stress is normally twice that of Axial Stress, so HOOP STRESS
RESISTANCE is the main design parameter. Hoop stress is trying to split
the vessel along it’s length, ie acting on a longitudinal weld.
The thickness necessary to withstand the Circumferential or Hoop Stress
is TWICE that required to withstand the Longitudinal Stress
© Wilkinson Coutts Engineering Training Australia
SHELL DESIGN BASICS
Principal stresses are: Hoop Stress and Axial
Stress
© Wilkinson Coutts Engineering Training Australia
Sect. 3.7.3 (a) BASIC EQUATION
For Hoop Stress (acting on longitudinal joints)
t=
PDm
2fη
This transposes when you want to find P
P = 2fηt
Dm
P = Internal pressure
Dm = Mean diameter REMEMBER
f = Design (allowable) stress
η = Joint efficiency factor
© Wilkinson Coutts Engineering Training Australia
The pressure in the cylinder is
trying to split the cylinder
along its length:
Sect. 3.7.3 (b) BASIC EQUATION
For Longitudinal (Axial) Stress acting on the circumferential joints
(e.g. head-to-shell)
The pressure on the ends is trying to elongate the cylinder
t=
PDm
4fη
Again, this transposes when you want to find P
P = 4fηt
Dm
If you compare the two equations, you will notice that the thickness required for
Axial Stress is roughly half that for Hoop Stress. Ie multiply t(hoop) x ½ ~ t(axial)
© Wilkinson Coutts Engineering Training Australia
SPHERICAL VESSELS
Section 3.7.4 also covers spherical vessels
For spherical vessels, the stresses are the same in all planes
and directions so there’s only one set of formulae to
consider
t=
PDm
4fη
Again, this transposes when you want to find P
P = 4fηt
Dm
You will notice that this is the same equation as for Axial
Stress, as spheres do not experience Hoop Stress.
© Wilkinson Coutts Engineering Training Australia
CODE CALCULATED THICKNESS
The minimum thickness calculated from the AS 1210 Section 3.7.3
equations is referred to as:
Minimum Calculated Thickness
It is the minimum thickness required for pressure retainment due to the
governing stresses (ie normally Hoop Stress).
It does not contain corrosion allowances, mill under-tolerance
allowances, nor material manufacturing tolerances.
Do not confuse it with other AS 4942 thickness definitions as;
•
Minimum Required Thickness
•
Nominal Thickness
•
Required Thickness
© Wilkinson Coutts Engineering Training Australia
OVERRIDING THICKNESS
REQUIREMENTS
There are a few overriding thickness requirements in Table 3.4.3. These set absolute
minimum allowable thicknesses (irrespective of what the calculations say).
Look at AS 1210 Table 3.4.3 and note the following minimums:
 2.0mm minimum MMA and SAW welded vessels
 2 x any minimum for lethal contents
 2.4mm for larger (Do greater than 1000mm) vessels regardless of welding method
© Wilkinson Coutts Engineering Training Australia