Hi. An equipment change is about to be implemented on a Fisher 585c and I’m hoping that the people here will be kind enough to help me think of all the things that can go wrong with this plan so that they may be addressed before hand by the implementing engineering group.
Basically the plan is to go from our current 1 DVC6200 positioner (which is monitored/ controlled by DCS) on the 585c actuator to 2 DVC6200 positioners side by side, both fed from the same 4-20 mA signal but with a relay ckt that will delay both positioners enough so they don’t fight each other. Both positioners will be “Live” but only 1 (positioner 1)’s output will be aligned to the actuator. In the event of a travel deviation of 5% a solenoid will swap from positioner 1 to positioner 2 (at which point DCS only continues to see the feedback from positioner 1 since nothing is being changed on DCS to account for positioner 2) (yes. I know. Crazy). I think that their expectation for us technicians is that after operations somehow realizes that a swap has occurred, we perform a “hot swap” of positioner 1. Which I don’t even understand how that’s supposed to happen safely with positioner 2 in charge? Maybe take valve to manual, gag it, pin it, and swap it? But how will that affect positioner 2? Won’t it be full of alarms at that point? Do I take them both out of service? Up to this point whenever we have replaced a positioner we have been isolated on a clearance. I don’t understand how this mod is efficient or beneficial. I’m worried that I’m not imaginative enough or know enough to think about all the things that can go wrong. Please help.

 

Sounds like a Friday 13th problem

 

1st. I'm with Dave, Friday the 13th disaster story.
DCV's were put in to service to monitor control Valve performance with lots of data but No backup. (Old Positioners had a Pilot Bypass) They can show you Travel, Stroke, Last Bench set and some more. In the Power industry when I started in 1985 our Control Valves had bypasses around them. If the Main Valve failed, Op's used the Bypass to control the process. I/C was called (Me and the other Guys in the Shop) to fix it.
The jest of this story is this, DCV's are so smart they forgot then when they fail you lose everything. Your plant goes down. All that fancy stuff is just Jiber-Jabish that mostly gets ignored.
If it was me, I would have them put in a Bypass Valve with a Positioner on it and you can control it in the DCS with a Bypass Controller that OP's can use when that Main Valve goes on the Skitz. It doesn't have to be a fancy DCV either. A Fisher 3580 Positioner or something equivelant.
You can turn the output signal in to the input for Feedback. All you or (Op's)is looking for is Flow to the Process out of that Bypass until you get the Main Valve Fixed..
The old Configuration on critical installs was 2 block Valves on the main and a Bypass around them so you could work on the main. It had to all be done very Slowly and Carefully so you didn't totally upset the process. Slowly open the Bypass while slowly closing the Main valve downstream shut off valve.
***That install with 2 positioners and keeping the Faulty Positioner Feedback with a relay swapover is what's known as a Cluster Flock (Insert your own words) All that Swap over and bad positioner feedback IMO= a Trip.
I Hopefully explained it to you what we did on 5 Power Generation units. Boiler Controls, Condensate, Feedwater, Steam Sprays all have that Setup. Good Luck..