A wider reagent delivery capability can be obtained by using the sequenced valve approach (Figure 7.40.20). The arrangement is virtually the same as that shown in Figure 7.40.19, except that the controller output can be switched to either valve by a pressure switch (PS) or its electronic equivalent. Valve positioners must be used, since each valve must be calibrated to stroke over only a portion of the controller output signal range. Figure 7.40.21 illustrates various combinations of different pairs of sequenced valves. Table 7.40.1 lists the various flow rangeabilities for some valve pairs, assuming a constant pressure drop across the valves (equivalent to 9.5 feet, or 2.85 m, of 66° Bé sul-furic acid) and assuming an individual valve rangeability of 35:1. The valve size coefficients (Cvs) are 1.13, 0.14, 0.08, and 0.04, respectively, for CV-1, 2, 3, and 4.
The overlap between each valve pair becomes smaller as the rangeability increases. The pressure switch to transfer the valves can be set anywhere in the overlap region, because in this region the process loads can be satisfied by either valve.
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