Page 242 - Chemical engineering design

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CHEMICAL ENGINEERING
Loads will arise from:
1. Thermal expansion of the pipes and equipment.
2. The weight of the pipes, their contents, insulation and any ancillary equipment.
3. The reaction to the fluid pressure drop.
4. Loads imposed by the operation of ancillary equipment, such as relief valves.
5. Vibration.
Thermal expansion is a major factor to be considered in the design of piping systems. The
reaction load due to pressure drop will normally be negligible. The dead-weight loads
can be carried by properly designed supports.
Flexibility is incorporated into piping systems to absorb the thermal expansion. A piping
system will have a certain amount of flexibility due to the bends and loops required by
the layout. If necessary, expansion loops, bellows and other special expansion devices
can be used to take up expansion.
A discussion of the methods used for the calculation of piping flexibility and stress
analysis are beyond the scope of this book. Manual calculation techniques, and the appli-
cation of computers in piping stress analysis, are discussed in the handbook edited by
Nayyar et al. (2000).

5.5.5. Layout and design

An extensive discussion of the techniques used for piping system design and specification
is beyond the scope of this book. The subject is covered thoroughly in the books by
Sherwood (1991), Kentish (1982a) (1982b), and Lamit (1981).

5.6. PIPE SIZE SELECTION
If the motive power to drive the fluid through the pipe is available free, for instance when
pressure is let down from one vessel to another or if there is sufficient head for gravity
flow, the smallest pipe diameter that gives the required flow-rate would normally be used.
If the fluid has to be pumped through the pipe, the size should be selected to give the
least annual operating cost.
Typical pipe velocities and allowable pressure drops, which can be used to estimate
pipe sizes, are given below:

Velocity m/s P kPa/m
Liquids, pumped (not viscous) 1 3 0.5
Liquids, gravity flow 0.05
Gases and vapours 15 30 0.02 per cent of
line pressure
High-pressure steam, >8bar 30 60

Rase (1953) gives expressions for design velocities in terms of the pipe diameter. His
expressions, converted to SI units, are: