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Reference Manual
00809-0100-4696, Rev AA
September 2004
F-3
Rosemount 848L
Double Block and Bleed
If the valve absolutely must not leak into the process, then two valves are put
in series and the short pipe between them is vented (bled) to an appropriate
place. The bleed valve must be shut before the main valves can be opened,
and the main valves must both be closed before the bleed valve can be
opened.
Motorized Valve
The actuator is a reversible motor that turns a lead screw that moves the
valve stem. Two confirms are required because the motor is only free to turn
while the valve stem is travelling. Outputs are required for the Forward and
Reverse motor directions. If a big motor-driven valve takes a minute to
change position, that’s a long time to find out that it didn’t move. The crack
time is a period of time in which the previously closed contact must open, to
confirm that the actuator is moving and the valve is not stuck or powerless.
Heat Exchange Medium Selection
Batch heat exchangers have to use different media to heat and cool. If the
media are compatible, like steam and chilled water, then a simple four valve
manifold can handle the selection. The four valves are independent because
it is necessary to drain one medium from the exchanger before using the
other. There are many variations on this theme, for incompatible media or
more than two choices.
Boolean Expressions The motor control descriptions used ladder logic. Another method that takes
less room on the back of an envelope is the Boolean expression. The
following is a comparison of Boolean and ladder operators (math operators
are +,-,*,/). Only three operators are used in the examples:
Functions are the same as 848L functions. The examples use TON, TOF and
TP.
Basic Valve Control Since very few applications exist for local valve control that are more than a
simple toggle switch (electric or pneumatic), all examples use a DO block to
take a command from Fieldbus. The DO point is on for open and off for close
in all cases.