Power Plant re-control - MOV wiring

Hello all,
I hate to make my first post here a big question, but I have done enough lurking to think that I won't quite be pounded into the pavement with this question. I've tried searching to no avail....

I am part of a team that is upgrading the DCS (can I say that here ? =) for a 1000+ MW power plant- Our DCS is just a big PLC really (input cards, output cards, processor cards...).

We have a number of limitorque/ motor operated valves that are currently controlled by panel-mounted switches and will soon be controlled by a DCS/PLC output (dry contact output on the DCS system we are installing.

I have a few motor operated valves, that have relatively complex permissive circuits wired in series with the opening and closing contactors coils. These permissive contacts are things like the overtorque switch in the valve actuator, the open/closed limit swtich (it will cut power to the open coil if the limit swtich indicates open...) and a permissive with another system (example - must have a cooling water pump running to admit steam...)

Since we are replacing the switches with relay outputs, I have the option of blowing away all the 'extra' contacts and just bring everything back to the PLC. (The postition LS's and torque switches become inputs which are tell the actuator to stop via logic and the positioning contactors become outputs

OR

I can retain the existing wiring, and just replace the single switch contacts with relay contacts from the output card and retain the existing hardwired interlocks. cheaper (and quicker?) to install.



The first case is more simple to understand/troubleshoot (to me) and gives me the option of putting LS/TSW status on an HMI screen somewhere. The 2nd case is more complex, and would involve more time sitting at the valve with a meter and schematic, trying to see where voltage is and isnt (in order to find out why it doesnt move when commanded)

Our I&C department is half old guys with 20+ years and half 20-somethings that txt on their phones all day =) So the issues of legacy knowledge are moot from what I can tell. This is a large capital project so they'll install/wire it however I draw it (my spec = MIL-T-FD41 =) - I'd like to do this "right" because we are still a few years out from install, and still in the drawing/design phase.

I have full confidence in the wiring and hardware (15+ years of operation so far...), all of this piping is inside of a closed loop system so personnel hazards (if the PLC were to fail and drive a motor closed or open past its limit switch) are negligible - or at least please assume such for the scope of this question. If there was an "equipment damage" or "personal hazard" issue, then it would trump any of my design complexity concerns no questions asked.

I will be consulting my local operations/maintenance people with this same question, but I wanted to get the community wisdom of other controls guys before going local - to see if you guys can find an angle that I've missed.


Have I made a good case ? do I keep the existing complex design, or wipe the slate clean, use the programmable controls to their fullest extent and clean up the local wiring ?

-John C. G.

I would lean towards keeping the wiring as is ... that would provide manual control the same as now (if the DCS were to fail).

I agree with Oakley. You might be violating codes for your type of plant (with which I have zero experience) if you change the hardwired interlocks. What you might be able to do, to get the best of both worlds, is to use auxillary contacts (if available) from some of those interlocking switches so that they can be monitored by the PLC/DCS. That way, when the system tells a valve to move, and it doesn't, the PLC will at least know why, and can trigger more intelligible alarms/warnings.

Without any knowledge of your industry's codes I will take the other side. I'd wire the permissives to the DCS and let the logic control the outputs directly. That's the way we would do a new machine - there wouldn't be any relay logic circuits out on the machine. Of course I also would defer this ideaif there were safety concerns I am not aware of.

If you decide to leave the current wiring and want to monitor it, I would not use the method described by OkiePC above - I'd modify his suggestion slightly. I would tap plc inputs off of the series devices and use them for status on the DCS.

To explain further, let's say there are four limit switches in series. Each limit switch has a N.O. held closed contact that is used for your permissive circuit to drive the motor operated valve. These limit switches have an unused N.C. contact just sitting there, and this is what OkiePC suggested using for the DCS status.

In my experience it has been common for the N.O. held closed contact in the permissive to fail but the limit switch doesn't move so the N.C. contact used for monitoring doesn't change state. That leaves you without status and back out at the machine with a meter going through divide-and-conquer troubleshooting.

So what I'm suggesting is tapping each side of the N.O. held closed contacts in the permissive circuit back to the PLC as individual inputs. That leaves all of those safety devices in series to the motor operated valve AND gives you status of each safety device. Moreover, it gives you the actual status of the contact used by the motor operated valve and not that extra auxillary contact. So if a contact fails because its dirty - you know.

I used to use this method on doorswitch circuits and it worked great. Then it became a violation of the safety circuit wiring when we started going to the European style circuits. I tell you this so you can be aware of it, but I wouldn't think it would apply on your motor-operated valve design.

I have worked on a gas production platform which got a re-fit like this.
The maintenance crew might say that they wanna keep the old. Thats what they know, thats what they can fix.
How ever, will the parts of the old system still be available in the next 15 - 20 years.
I say; Out with the old, in with the new. Easier to trouble shoot (as soon as the maintenance crew figures out how it works) and less down time, which makes the client happy goz it'll save him some dollars.
It's not what the re-fit will cost, but it'll be the money they safe on downtime.
Why do they want a re-fit?
Because the old system runs smooth and has plenty of control?

I'm gonna weigh in with agreement for OkiePC's method. I understand Paul's point (in theory) that examining the ACTUAL contact would let you look at what is really going on, but the problem is that you MIGHT compromise the circuit by doing so. This is why safety circuits don't allow it any more (as Paul noted) and why I can't recommend it.

As OkiePC said, using auxillary contacts is a really good compromise. Not only do you prevent the potential of compromising your circuit, but it allows the legacy guys to continue working with a system they already understand and gives the hi-tech guys the annunciation they need.

OkiePC also hit on what I think is the biggie here - switches/sensors that are needed to meet your industry code. One example might be the over-torque sensor. In many conveyor belt applications there is a sensor that monitors the physical location of the drive motor with respect to the frame. If the drive motor winds up too much, it will physically twist the conveyor frame enough to break the sensor. Often these sensors are hardwired into the motor circuit to guarantee the motor stops. If your application is especially dependent on such a conveyor, this is one sensor that might need to stay like it is.

Steve

Considering that it is a power plant, you need to do a failure analysis and identify what could happen in other parts of the process in case of failure.
Sometimes the hardwired circuits are redundant and putting them in a PLC increases the risk to the rest of the equipment and possibly even personnel.

Hello Johnster,

I work for a coal power plant. We always want to make things as simple as possible. This is why we avoid complex wiring. In my opinion it is always the best practice to take advantage of modern DCS. I don't know what is your DCS but most modern DCS systems today have built in standard motor valves libraries that enables efficient control of motor and drives. Our system is SPPA-T2000, which is made for controlling power plants. Most of valve interlocks are transferred to PLC software.

To choose optimal solution, you need to know your DCS possibilities. Since it is a large power plant I imagine that all PLCs come in redundant pairs (redundant CPU, power supply units, communication processors and similar) so the probability of "DCS fail" is negligible.
For example, for the DCS, a motor driven valve is just a black box like show in attachment. For controlling the motor valve, what is important is: limit switch positions open and close, torque switches open and close and commands for open and close.

Therefore all technological interlocks you can implement in software. If some condition is true just remove the open or close command. Most actuators support this interface. It is really very simple. Our experience that the system is very robust and reliable. You don't need to program torque switch information. That logic is implemented in the I/O modules itself, so the system will terminate the "drive" command in case torque switch is active.
Again, you need to know your DCS possibilities well.
For me, it is a pity not to use everything you can. This way wiring will be very simple and same for all drives, but interlocks and protection logic is implemented in PLC software.

A fail safe PLCs are used to implement boiler and turbine protections since these are safety critical applications.

Best regards,

Pandiani

All,
Thank you for the responses. You have given me plenty to think about- Safety, the equipment and the process really need to come first.

For background, we are getting an Ovation DCS from Emerson Process control. It has Dual/Redundant CPU's and power supplies. D/R ethernet (!) comms and some pretty decent background hardware. While I don't like the idea of ANY windows-boxes involved with control anywhere, Emerson seems to have mitigated most of its shortcomings and doesnt sell a bad product. Plus, We are still in the "everything is great" phase of the project =)

I think that Emerson does sell a "valve control" card, and my parent company is developing standardized programming blocks for such, but I need to research its capabilities and at least be aware of those tools that are available.

-John C. G.

Johnster,
Emerson Ovation is a very powerful DCS system. I don't have hands on experience myself with this system, but I have heard a lot about it. I'm sure that it has standard libraries and I/O cards for the purpose you need.

You should investigate and if possible consult with Emerson automation engineers. I'm sure they have very robust and standard solutions.

Good luck.

Pandiani - What an excellent contribution. I really got a lot out of your posts. I don't work with DCS systems but I have been on the fringes of PCS7 installations and your post summed up my novice impression of their extreme capability. Thank you for your excellent explanation!

I have worked with the Westinghouse WDPF system and now the Ovation processors via the old Q-line IO in a large WWTP for about 15 years. MOVs are common in Water and Wastewater. Limitorque, Rotork, EIM etc are common electrically operated actuators in this industry. The limit switches and overtorque switches mentioned are built into these types of actuators and protect the motion whether in the LOCAL or REMOTE(DCS) modes.

The actuator self protects against errant DCS output commands wired to the REMOTE mode inputs. Also, in my experience it would be pretty difficlut to actaully bring these internal protective contacts outside of the actuator for series wiring as you described with your DCS contacts. Usually there are very few extra contacts available!

As for other interlock contacts from associated machinery, then a choice of hardwired interlocks or DCS logic interlock has to be made. I would normally choose DCS interlocking unless there is any need for local manual operation by a plant operator, and depending on what type of machine it is. For example if we're talking about a large centrifugal compressor, and its startup-bypass-air valve actuator, then I would opt probably for local hardwired interlocks between the switchgear and the actuator/DCS contacts. This is because of the possibility of local startup of the compressor at the switchgear and the absolute need to avoid compressor surge conditions. Each scenario has to be assessed for potential damage, safety, reliability, ESD emergency shutdown, etc.

Just food for thought.

TJS said:

...in my experience it would be pretty difficlut to actaully bring these internal protective contacts outside of the actuator for series wiring as you described with your DCS contacts. Usually there are very few extra contacts available!

As for other interlock contacts from associated machinery, then a choice of hardwired interlocks or DCS logic interlock has to be made. I would normally choose DCS interlocking

Click to expand...

TJS - excellent food for thought.

I snipped your quote for 2 reasons.

1) I agree that there are normally not that many spare contacts available- but in many cases, (at least one the valves that I've reached) there ARE enough contacts to TSW limit the coil AND provide a TSW input. The question then is- do I have (enough) spare wires run to the actuator or not ? =) (so far, the answer is yes, credit the overbuilt AEP design)

2)The second point I quoted was the crux of my question - When no other case takes priority (operator safety, equipment protection, process safety...) should I default to DCS or hardwired ?

I tried to ask this on the context of an I&C repair crew that is half-experienced with the current, hardwired way of doing things. it has slightly surprised me that no one really addressed this point (even if to say - "screw the tech's, make it YOUR way" =) I would assume that operator/tech knowledge/legacy would figure into ones design...even of only sometimes.
...or...a more accurate statement would be - Figure operator/tech legacy design practice AFTER the issues of Personell, equipment and process safety have been addressed.

TJS - PM me if you'd like, I have some friends in the Cincinnati and Canton OH municpal water agencies...

-John C. G.

The most common way I have seem MOVs wired is with 2 outputs and 2 inputs. The control system outputs are maintained on for the postion desired, the internal limit switch removes power from the coil when the desired position is reached. These internal limits switches are generally form c contacts with the NO contact available to wire back to the control system inputs. The outputs have to remained powered on to provide power to the limit switch.

I have seen people re-wire these so you could separately power the limit switches. Now you have to turn off the outputs when the desired position is reached. It requires different logic in the control system. They will both work. However, do you really want to modify the manufacturers wiring inside the MOV? What happens when you buy a replacement? I'd argue strongly to keep the MOVs in standard configuration.

Interlocks outside the MOV package can certainly be wired to the control system as inputs if they are not required to be hardwired.