Figure 1119

Level measurement with two flush diaphragm transmitters and a summing relay

Other important but non-control-related decisions include those of selecting mechanical and metallurgical features relative to corrosion, temperature, pressure, and fire safety.14

In trying to make these decisions, the engineer must recognize three significant factors that bear on control-valve selection:

1. There are today many new types of valves, including those with anti-cavitation or low-noise features. There are many disk and other rotary valves with much larger Cv (flow coefficients), lower price, and greatly reduced weight, as compared with globe valves. Some of the newer designs, however, have a greater tendency to cavitate in liquid service, or are noisier in gas service.

2. There are newer methods of calculating the normal or operating C„ that are much more accurate than the old FCI (Fluid Controls Institute) equations.6-8

3. In pumped systems there is a trend toward minimizing valve pressure drop to reduce pumping costs.16

General Approach

A general approach to choosing body size, trim size, and valve type has been developed that is based primarily on equations for Cr developed by an

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