Book - School of Science and Technology

632 Automatic controls and building management systemswhereN ˆ valve authorityP 1 ˆ pressure drop across the open valveP 2 ˆ pressure drop across the remainder of the circuit:The most common applications are represented in Figure 22.13. For stable control, thevalve authority should be not less than the following values.Mixing ˆ 0:3Diverting ˆ 0:5Throttling ˆ 0:5Application of the above criteria would normally lead to control valves being selectedat one, or maybe two, pipe sizes smaller than the pipeline diameter (depending upon thefluid temperature difference being used). Three-port valves should always be fitted in themixing mode, that is with two inlets and one outlet. As illustrated in Figure 22.13, suchvalves may be applied to either a mixing or a diverting duty but many of the patternsavailable are not suitable for use in the diverting mode, that is with one inlet and twooutlets.P1P2SourceLoadSourceLoadP2P1(a) 3-port mixing(b) 3-port divertingP2SourceLoad Valve authority =P1P+ 1 P2P1(c) 2-port applicationFigure 22.13 Basic control valve applications