K-Patents DD-23 - Manual (Page 20)

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5.2 Divert decision rules
Two refractometers in operation:
BLRBAC states the following: For the solids measurements, two refractometers in se-
ries must be used. When both refractometers are in service, the requirement for an au-
tomatic black liquor diversion can be satis󰅳ied by either of the following options:
1. If either refractometer reads dissolved solids content 58% (default) or below,
an automatic black liquor diversion must take place.
2. When both refractometers read dissolved solids content 58% (default) or be-
low, an automatic black liquor diversion must take place.
Either option is satisfactory.
The rules are listed in decreasing order of security. Only rule 1 satis󰅮ies the high safety
requirements set by BLRBAC, August, 1982. On the other hand, the probability of false
trips decreases with decreasing safety. It means that if false trips are too frequent, the
rule 2 may be preferred at the cost of safety. The rule 2 has later been accepted by
BLRBAC.
The divert decision rule is selected by the rule switch inside the divert control unit
cover (Figure 5.1). The state of the switch is indicated by a yellow indicator LED for
rule #2 and a green one for rule #1.
Rule switch Rule
1-of-2 Rule #1
2-of-2 Rule #2
Table 5.1 Divert decision rule selection.
The logical description of the safety decision logics is given Tables 5.3 and 5.2 below.
A
B
under 58% over 58%
under 58% divert divert
over 58% divert safe
Table 5.2 Two instruments in operation, rule 1-of-2
A
B
under 58% over 58%
under 58% divert unsafe
over 58% unsafe safe
Table 5.3 Two instruments in operation, rule 2-of-2