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Reference Manual
00809-0100-4696, Rev AA
September 2004
Rosemount 848L
F-6
Double Block and Bleed Certain materials must not leak through a valve that is supposed to be shut.
Three valves are arranged in a leak-proof configuration as shown:
All three valves are spring return. V1 and V2 return to closed, V3 returns to
open. All 3 valves must have closed confirm switches, which allows two
instances per 848L. If open confirms are also used, the alarm logic is different
and only one instance per 848L is possible. V1 and V2 must both confirm
closed in order to open the bleed valve by removing power to it. V3 must be
closed (powered) to allow V1 and V2 to open. Since V1 and V2 operate
together, they are both powered by the same output. The second output
operates V3. The close confirms take inputs of the same number as the valve.
A second instance takes inputs of the same number as the valve plus four.
Open confirms take inputs of the same number as the valve plus three. DO1
is still the open/close command.
The outputs are the same whether or not there are open confirms.
OUT1 contains AND(DO(1),IN(3));
OUT2 contains NOT(AND(NOT(DO(1)),IN(1),IN(2)));
For single closed confirms, the valve assembly is confirmed open if V1 and V2
do not confirm closed and V3 confirms closed. The assembly is confirmed
closed if V1 and V2 confirm closed and V3 does not confirm closed. The
alarm is true if any of these conditions is false after the travel time has
expired. The equation will not fit on one line, so two must be used:
EQ1 contains AND(NOT(DO(1)),OR(NOT(IN(1)),NOT(IN(2)),IN(3)));
EQ2 contains
TON(OR(AND(DO(1),OR(IN(1),IN(2),NOT(IN(3))),EQ(1)),110);
For both confirms, the valve assembly is confirmed closed if V1 and V2
confirm closed and V3 confirms open. The assembly is confirmed open if V1
and V2 confirm open and V3 confirms closed.
EQ1 contains AND(NOT(DO(1)),NOT(AND(IN(1),IN(2),IN(6))));
EQ2 contains
TON(OR(AND(DO(1),NOT(AND(IN(4),IN(5),IN(3)))),EQ(1)),140);
848L/DOUBLEBLOCK.TIF