Emerson manual Boolean Expressions Basic Valve Control, Rosemount 848L, Reference Manual

Reference Manual

00809-0100-4696, Rev AA September 2004

Rosemount 848L

Double Block and Bleed

Boolean Expressions

Basic Valve Control

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.

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.

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.

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