Scope

Valve size often is described by the nominal size of the end

connections, but a more important measure is the low that

the valve can provide. And determining low through a valve

can be simple.

This technical bulletin shows how low can be estimated

well enough to select a valve size—easily, and without

complicated calculations. Included are the principles of low

calculations, some basic formulas, and the effects of speciic

gravity and temperature. Also given are six simple graphs for

estimating the low of water or air through valves and other

components and examples of how to use them.

Sizing Valves

The graphs cover most ordinary industrial applications—from

the smallest metering valves to large ball valves, at system

pressures up to 10 000 psig and 1000 bar.

The water formulas and graphs apply to ordinary liquids—and

not to liquids that are boiling or lashing into vapors, to slurries

(mixtures of solids and liquids), or to very viscous liquids.

The air formulas and graphs apply to gases that closely

follow the ideal gas laws, in which pressure, temperature,

and volume are proportional. They do not apply to gases or

vapors that are near the pressure and temperature at which

they liquefy, such as a cryogenic nitrogen or oxygen.

For convenience, the air low graphs show gauge pressures,

whereas the formulas use absolute pressure (gauge pressure

plus one atmosphere). All the graphs are based on formulas

adapted from ISA S75.01, Flow Equations for Sizing Control

Valves.

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