Chapter 18 - Piping Stress Analysis Logic

  • The designer, after careful consideration, prepares the most economic layout he can undertake within the parameters available to them - only to find that alterations are required by the stress engineer.
  • If the stress engineer is asked why the layout must be altered - in most cases the reply will be the well known stock phrase "It's too tight". This is another way of saying that for one reason or another, the layout has not enough flexibility.
  • The Designer then has the task of meeting the stress requirements by altering the layout such as:
    (a)    Increasing the length of the offset(s) in a piping configuration.
    (b)    Altering the position, type or number of restraints in the piping.
    (c)    Moving the equipment in order to meet with the flexibility parameters.

Types of Stress
B31 stresses can be categorized into two types.  The categories correspond to the types of failure which the Code specifically protects against:
Sustained (primary) stresses are caused by loads which may lead to unacceptably large deformations or rupture.  Examples of loadings which cause sustained (primary) stresses are:

  • Sustained loads of pressure and weight, and
  • Occasional (short-term sustained) loads of water and steam hammer, fluid slugging, and earthquake [inertia forces].

Thermal expansion (secondary) stresses are caused by loads which are repeated and lead to fatigue failure.  Examples of loadings which cause fatigue are:

  • Startup shutdown of the piping system,
  • Earthquake anchor movements [however, earthquake anchor movements are not thermal loads], and Vibration.


Above is an excerpt from the chapters of the book: Detail Engineering and Layout of Piping Systems 5th Edition.
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