Page 449 - Chemical process engineering design and economics

430 Chapter 8

and the required pressure drop across the valve must be determined. The formula
required for valve sizing depends on the properties of the fluid and the flow re-
gime. These factors are:

1. liquid or gas flow
2. laminar or turbulent flow
3. flashing
4. cavitation
5. incompressible or compressible flow
6. choked flow
7. non-ideal gas effects
8. effects of piping arrangement
9. limit on outlet velocity to prevent shock waves and noise

We will only consider turbulent flow of an incompressible fluid, which also
includes the flow of gases - if the pressure drop is small - as well as the flow of
liquids. Formulas for other cases are discussed in References 8, 9, and 20. Refer-
ence 20 summarizes valve-sizing formulas in an attempt to standardize them.
Figure 8.12 shows the various pressure drops through a throttling valve.
When the fluid enters the valve, there is a small drop in pressure cause by fric-
tional losses. As the fluid passes through the small opening of the valve, the fluid

Inlet
Pressure Loss Orifice
Pressure Loss

Distance Along Flow Path

Figure 8.12 Pressure profile across a throttling valve.

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