Upgrading pneumatic control loops


I WORK for a Nuclear Power Operating company and am working on a project to upgrade the present pneumatic control loops for our secondary steam cycle feed water heater level control at one of our Nuclear Power Plants. The driver for this upgrade is Maintenance. They want to get rid of the problematic pneumatic level controllers and replace them with electronic controllers.

Rather than replace the pneumatic controllers with a standard 4-20mA control loop I would like to use a Foundation Fieldbus solution in order to gain some foothold into a modern control scheme and use it as a test case for proof of concept for a more extensive implementation of Foundation Fieldbus at the plant. Since I will have to provide signal wiring and power for the new level instruments, level controllers and valve positioners I could just as easily install a fieldbus segment instead at a similar cost.

Foundation Fieldbus is very rare in the U.S. power industry so this has been a hard sell. Therefore, the closer I make the loops to a standard 4-20ma stand alone single loop controller the better, at least initially.

Each heater shell has two level control loops a primary control for normal level and a high level dump control as a backup, both are standard PID. I intend to specify the Magnetrol 705FF GWR probes for level because they will fit into the existing displacer geometry and have a Link Active Scheduler (LAS) included in the device. I will specify for the valve controller, the Fisher DVC6000f. I want to use the LAS and AI Function Blocks in the Magnetrol and the PID and AO Function Blocks in the DVC6000f for my single loop control scheme. I will start on a single FW heater using two segments one for the Normal and one for the Dump control, each with only two devices, one level instrument and one valve controller operating without a permanent host. During initial installation I will be using a laptop, an HSE interface and the NI-FBUS configurator. I plan to use the Fisher Field calibrator for tuning the loop after installation, the Magnetrol GWR will display level. The result is a very similar local single loop PID level control.

If all goes well I will ultimately add up to 8 devices per segment, 4 level instruments and 4 valve controllers. This stepwise installation will allow the plant to become comfortable with Foundation Fieldbus. Eventually, I hope to move all the segments to a real Host/HMI to take advantage of he diagnostics and loop tuning capabilities of FF.

I know that this is not the best way to implement a Foundation Fieldbus strategy but will it work? Will this segment work without a master scheduler? What are some potential problems? Any advice?

Ray Herb, Southern Nuclear, Component Engineering, Electrical / I&C


YES, IT will work. Control in the field works. Small systems can be operated without linking device or proprietary controller. We have a power station here in Singapore that had a loop operating in that manner for several months. However, they selected the SMAR FY302 valve positioner primarily because it has a configurable digital display allowing them to see what happens in the loop and to make adjustments. The PID was done in the positioner. We set it up for them in such a way that they could adjust the setpoint, tuning constants, and manually override the output from this local display as necessary. Once setup we brought the linking device back to our office. This positioner was like a controller and local operator panel as well. Some months or a year later they purchased the linking device as well as some other devices as well and slowly started building a larger system.

For more details, take a look at the book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance."

Jonas Berge

BOTH THESE devices have passed the ITK 4.6 test and will work as you describe though you will need some way to monitor and configure the loops as well as change the set point. There are products available to provide local indication and with some engineering the set point. Configuration will likely have to be done with a laptop based tool that you connect in the field. You will also need to have an FF power supply/conditioner as well which means wires.

Check out the engineering and technical guides on the Foundation Fieldbus website for a bit of information to get you started.

When installing this direct replacement, Ray will also have to be sure that he uses his configuration tool in the "slave" mode since doing otherwise could cause a potential conflict over who is the Link Active Scheduler (LAS). Ray is on a fixed set point, so once the system is operational it should run without any additional viewing other than periodic checks. We discussed the need for terminators and that Ray will also be able to connect several systems to a single power supply if he wishes, and probably should to save money (unless it costs more to run the extra cable than to purchase the power supplies). It appears he has a good understanding of what is required and should not have any difficulties with his proposed plan.

Ian Verhappen, ICE-Pros

I WOULD recommend installing a field instrument Data Highway (Fieldbus type to be selected) throughout the plant. The Data Highway should be triple redundant. Once this installation is completed and fully tested and FCC validated, one should proceed to a step by step conversion from pneumatic to electronic analog / digital instrumentation. On should start with non critical control loops, one at a time.

On the first and second level control loops I suggest to make them electronic analog stand alone PID with the provision to be easily converted to digital and connected to the Highway with the controller part of a DCS which ultimately will control the units under consideration to be converted, namely secondary steam cycle feed water heaters. On should add digital converter at control valves and connect them to the highway also. I did not dwell on specific hardware you should use although I like Magnetrol. This require a quite detailed analysis.

By testing these few loops which will make the transition from pneumatic to digital one should accumulate enough experience and build confidence to execute a full conversion.

Herold Hertanu, P.E., President, HLP Associates, Inc.


  Need an Expert Opinion?
Send your knotty control and automation questions to Béla Lipták at liptakbela@aol.com. Lipták will either answer the questions himself or bring in specialists from his cadre of Instrument Engineer’s Handbook, Fourth Edition co-authors. Questions and answers will be published online on www.ControlGlobal.com. Each month, Lipták will pick the best question, and publish it, along with the answer in CONTROL. If your question is chosen, a prize will be awarded.
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