Emerson manual Double Block and Bleed, Rosemount 848L, Reference Manual

Rosemount 848L

Reference Manual

00809-0100-4696, Rev AA September 2004

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:

848L/DOUBLEBLOCK.TIF

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);

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