Well, the trees are now bare in Southern Ontario and people are gearing up for the winter once again. Things, project wise are quiet right now in the process and petrochemical disciplines and we seem to be in a holding pattern awaiting some big contracts. Whereas, the mining discipline is at last seeing a turnaround.
This months’ tip

STRESS CATEGORIES
The B31 codes provide design guidance for two stress categories, PRIMARY and SECONDARY STRESSES.  Although the code reader will not find these terms listed or discussed in the code text, the understanding of these terms is very fundamental in applying the code equations to determine the stress levels in piping systems.
The definitions are:

A PRIMARY STRESS is a PRINCIPAL STRESS, SHEAR or BENDING generated by imposed loadings which are necessary to satisfy the simple laws of equilibrium of internal and external forces and moments. The basic characteristic of a PRIMARY STRESS is that it is not self limiting. As long as the load is applied, the stress will be present and will not diminish with time or as deformation takes place.

Examples of a PRIMARY STRESS are CIRCUMFERENTIAL STRESS due to internal pressure and LONGITUDINAL BENDING STRESSES due to gravity.   
The failure mode of a PRIMARY STRESS is gross deformation due to rupture.
   
A SECONDARY STRESS is a PRINCIPAL STRESS, SHEAR or BENDING caused by structural restraints such as flexibility controls, or by self constraint of the pipe itself. The basic characteristic of a SECONDARY STRESS is that it is self-limiting. As this stress condition develops in a piping system, local yielding will occur, thus reducing these stresses.

An example of a SECONDARY STRESS would be the bending of an elbow the joins two lengths of pipe subjected to a temperature increase. The bending in the elbow will reach a maximum value as the temperature stabilizes at its new increased value. Thus, the condition causing the stress to increase now stops. The elbow, now in this new higher stress state, will experience local yielding or deformation which will reduce the stress in the elbow.

One can see from this example that SECONDARY STRESSES are associated with cyclic conditions such as temperature increase or decrease in a piping system as the plant starts up or shuts down.

The failure mode of a SECONDARY STRESS is a crack initiation and propagation through the pressure boundary resulting in a leak.