Mechanical Design of External Pressure Vessel | Vacuum Vessel | Finding Factors A and B Using Graphs

Mechanical Design of process equipment is a key subject for the Chemical
Engineers are often not taught in university courses in chemical engineering. Without a general how-know of the subject, chemical engineers can't move in the design phase of process equipment. The chemical engineering industrial academy will try to solve this problem.

In a previous video (link given below) on the design of a Pressure Vessel, we have seen how can we calculate L/Do and Do/t from a given or proposed geometrical data. In today's video, I will try to show you how can we find factors A and B from charts, before the determination of these factors, let's briefly define these factors.

Definition of Factor A

We can define the strain as equal to uni-axial stress over Young's modulus, and also we can put A as equal to strain, and the plot of this resultant equation will give rise to the curves for the factor A, and here K’' is the buckling coefficient with pressure on sides and ends and r is the radius of the vessel. This brief definition is, for the sake of concept. A detailed discussion and derivation will be provided in the next video.

Definition of Factor B

In the inelastic region, ASME uses a chart expressing axial compression stress versus factor A. This relationship is obtained by calculating the tangent modulus Et at a point on a stress-strain curve, and then calculating strain factor A at a specific stress value using the tangent modulus. The chart is plotted on a log-log paper, and the B stress is traditionally taken as one-half of the compressive stress. One such chart for carbon steel is shown in the video. The chart for factor B is different for different materials.

The complete method of factors finding is illustrated by animation in detailed in the video however if the value of factor A fall to the left of the material lines, and there is no curve in this region, then how can we find Factor A, for this case, there is an equation available, and we can directly find out allowable pressure for the vessel. In this case, We do not need to factor B at all.

See the first part of the video
Design of External Pressure Vessel | Vacuum Vessel | Shell Length and Thickness Calculation

   • Design of External Pressure Vessel | ...  
and 2nd part of the video

Mechanical Design of External Pressure Vessel | Vacuum Vessel | Finding Factors A and B Using Graphs

   • Mechanical Design of External Pressur...