Emerson manual Alarms Variations on Valve Control, Rosemount 848L, Reference Manual

Rosemount 848L

Reference Manual

00809-0100-4696, Rev AA September 2004

Alarms

Variations on Valve Control

There are at least three permutations of any valve circuit:

1.Steady or pulse output to the pilot solenoid (or whatever, piezoelectric bars are in use). Steady requires one output, pulse requires two.

2.Confirmed position switches for open, closed or both, using one or two inputs.

3.Automatic control or a local selector for Open-Auto-Close using no or two inputs (these are not common).

Interlock and permissive may be additional permutations.

If a valve has one or both position switches then it is possible to alert the operator to the fact that the valve is not where it should be. This is not a permuted choice because the main reason for having position switches is to alarm this condition. It is not a simple alarm because time must be allowed for the valve to complete its stroke after it receives a command. An On Delay timer set for the travel time is required.

All numbers in 848L equations are examples. The user will want to change them.

Interlock

There may be a process condition where it is not safe to open the valve. If this condition can be detected and transformed into the change of state of a contact, then the normally closed contact may be inserted in series with the control output. If the interlocked condition occurs then the valve will close if open or stay closed. An example is the drain valve of a batch reactor, which may have two interlocks. One prevents opening the drain if any feed valve is open. The other will not let material in the reactor drain into a tank that isn’t ready for it.

Permissive

There may be a process condition that is required to be present when a valve is opened, but is not required once it has been opened. A contact that is closed when the permissive condition is true is placed in series with the open command. A latch is required because the permissive may go false after the valve is opened. One application for a permissive involves a gas storage tank. The pressure must be above a certain amount to allow the valve to be opened, but once opened, the pressure will fall below the permissive level.

Open-Auto-Close

An operator may be required to perform some function near the valve, such as unplugging a pipe or locally directing material flow. The valve is normally controlled by the central system but must have a local station to allow the local operator to control it. The local station has a three position switch for Open-Auto-Close selection. The control room has control when the switch is in the Auto position. If the switch is turned to Open then the valve will open, possibly bypassing interlocks, and the same for Close. There is no bump going through Auto because the command is either Open or Close.

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