-21/PAND+I.http://origin.library.constantcontact.com/download/get/file/1101862056399pdf
Many clients require redundant transmitters, valves, and switches. In numerous cases both transmitters and interlock switches are redundant and always on-line. A transmitter may go bad and have multiple schemes for voting to keep the process on-line. In the case of switches used for safety, redundancy is for making sure that one failure will not cause a catastrophic event. But, one area of redundancy that could provide cost savings for both batch and continuous processes may need further examination.
Problem Statement and Previous Options
A client wanted to add a redundant valve for level control on a vessel. Manual valves were provided for maintenance, allowing operation of one valve while the other valve was off-line. This is the normal practice for many production facilities and we felt it would lead to unnecessary shutdowns, because of equipment failures. The valve was to serve as a redundant valve and manually switched, if the on-line valve failed or required maintenance.
Solution
Using the existing PLC we were able to provide an on-line solution that avoids unnecessary shut-downs for maintenance. See "Attachment A" for a Process and Instrument Diagram representation of this control. By adding a second PID loop and setting setpoints SPA and SPB at slightly different values, the level controller with the higher setpoint will become the primary and the controller with the lower setpoint will become the backup. The off-line controller will integrate shutoff as the primary maintains setpoint. To provide maintenance for the online valve the operator would reverse the setpoints (i.e. make x = -0.5% instead of +0.5%) If a hardware failure closes the online valve, it will not maintain setpoint and the backup valve will open, maintaining a slightly lower setpoint.
Benefits
1 Cost savings for services and facilities that want to minimize shutdowns,
providing a means of maintaining valves while keeping the process active.
Interlocks remain active, however, this method may be used to prevent the
interlock from reaching its shutdown setpoint on failure (ie low alarm on failed
valve).
2 Quick maintenance without necessity of a shutdown.
Implementation
This technique may be implemented easily within the software of a DCS or PLC by adding a second controller. A transmitter may be used as an input to both controls or other redundancy methods, such as selection or voting methods may be used. Any safeties required should still be maintained to protect environment , personnel, or equipment independent of this control. Additional wiring may be required for the second valve if not already planned. Tune each controller independently, but each should have similar tuning parameter for the same fluid. Interlocks should be duplicated for each valve if required and should not interfere with maintenance of the off-line valve.
Summary
Once the decision has been made for using a redundant valve, this technique should be investigated to see if further cost savings may be achieved or if would avoid any unnecessary shutdowns. All safeties should remain in place based on plant standards.
