Understanding Piping Wall thickness calculation

Assuming that the analysis shown in the picture is correct (which it is), the wall thickness of a piping system under internal pressure is a relationship of its internal radius, the internal pressure and the values of the axial and hoop stresses induced according to the material selected. Let’s break this down.

Design Conditions

An essential part of every piping system design effort is the establishment of the design conditions for each process. Once they are established, these conditions become the basis of that system’s design. The key components of the design conditions for the wall thickness calculation are the design pressure and the design temperature. Designers must be aware that more than one design condition may exist in any single piping system.

Design pressure is defined as the most severe sustained pressure which results in the greatest component thickness and the highest component pressure rating. It shall not be less than the pressure at the most severe condition of coincident internal or external pressure and maximum or minimum temperature expected during service.

Design temperature is defined as the sustained pipe metal temperature representing the most severe conditions of coincident pressure and temperature. But why it is important to define the Design temperature if this variable is not mentioned in the equations of the picture above? It is a critical parameter since the strength of a material varies with temperature.

Allowable Stress

Since we are not interested in obtaining the stress level of a given pipe but just the opposite, the minimum thickness required for a given stress level, it is essential to know the limit up to where we can stress the material, the allowable stress. The allowable stress of a piping system or a piping component material is based on a function of the yield or tensile strength of the material. Piping systems, among other mechanical equipment, must not work within the plastic deformation zone under any circumstances. Therefore, the allowable stress is established by the design code for each load condition, and it is always a percentage below the Yield Point.

What if the design conditions are exceeded?

Once the design pressure and temperature have been established for a system, the question could be asked: Can these conditions ever be exceeded?  The answer is yes, they can be exceeded. In the normal operation of a refinery or chemical plant, there is a need, on occasion, for catalyst regeneration, hydro-test, steam-out or other short term conditions that may cause temperature-pressure variations above design. Rather than base the design pressure and temperature on these short term operations, design codes allow dealing with occasional loads stresses that approach the yield point for short periods of time without causing a genuine concern for over stress.

If you want to know more:

Piping Systems in Industrial Plants: I