Emerson FX-455 Electronic Component Hardware Architecture

Description

VPs in a redundant subsystem can be calibrated as a pair or independently. The

same commands are used to perform either calibration.

If you want to calibrate the VPs independently, the FX-455 easiest method is to pull one of

the VPs out of the mounting unit. In this case, calibration is the same as for a non

redundant VP (see Section 25-6). When a VP is calibrated and the Backup is not in

service, and the RDNDNT flflag is set, a warming message is printed to inform the

operator that he is calibrating only one VP of the pair.

In any case, only the Primary VP or a pair can be calibrated. If you attempt to

calibrate with the terminal FX-455 connected to the Backup VP, the VP will print a message

stating that the request is refused.

If you request a calibration of the FX-455 Primary VP, and the Backup VP is also in service,

the Backup is calibrated unconditionally. During calibration, the Primary disables

the redundancy software state machine so failovers are eliminated.

The message packet exchange between the Primary and Backup VPs includes

command/status bits related to calibration. The following table lists those

commands and descriptions.

In normal, local, and calibrate modes the Valve Positioner is always controlling

valve position. The PI routine runs unconditionally every 10 msec. Typically, it will

execute the PI equation as described below. The FX-455 PI routine has some conditionally

executed parts to handle seating and backseating.

Seating and backseating are important features of valve position control. The

purpose of seating is to ensure that the hydraulic system is forcing the valve closed

and admitting no steam into the turbine. Backseating is desirable because it can

affect effificiency, and because, for certain valve assemblies, a leakoff into other

steam systems are designed, assuming the valve is fully backseated.

The demand position from the Controller to the Valve Positioner is designed for a

range of -5% to 105% and resolution exceeding 13 bits. Seating is in effect when

the position demand is equal to or less than a programmable value, typically 0%,

and actual position feedback is less than 5%. When seating is in effect, the output

is driven to the maximum possible value in the direction that closes the valve. A

timer is started. After 10 seconds, the output is reduced to 50% of the maximum

value in the direction that closes the valve. A similar limit and function is provided

for backseating.

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