July/ August 2013
Fieldbus for steam-drain control
Combining simple devices and fieldbus devices in a common system
By John Petzen and Larry O'Brien
As intelligent field devices are applied with more frequency throughout plant design, design cases occur where native bus-technology devices and simple field devices exist in the same control system. This article illustrates one specific case, steam-drain control, which can benefit from intelligent diagnostics on the control valves, but also works quite effectively with simple level switches. It is possible for users to combine more advanced Foundation Fieldbus devices with simple devices, such as level switches, to create a truly effective control solution.
Many instrumentation and control engineers think of the field device as an instrument, and the distributed control system cabinet as the control system. With Foundation Fieldbus technology, however, the net effect of deploying configurable field devices is to extend the boundary of what we traditionally think of as the control system into the field devices. This change in philosophy is most frequently discussed as the opportunity to deploy control strategies into the field devices themselves, labeled "control in the field." While there are undoubtedly control applications where control in the field offers advantages, using only fieldbus-enabled devices in a field-control strategy limits the overall possibilities for using intelligent devices as an extension of the control system.
A case study: Steam-drain control
Steam lines are typically fitted with low-point drains, which are implemented using a piping device called a drain pot. This is basically a section of pipe about the same diameter as the main line, connected via a tee connection and extending downwards from the main line. Liquid is formed when heat loss through the wall of the pipe causes steam to condense. Large amounts of liquid are formed during startup, when the bulk metal temperature of the piping components is below the saturation temperature for the steam flowing through the piping system. If this liquid is not removed from the piping system, the water can be entrained with the steam moving at high velocity down the pipe, causing mechanical damage termed "water hammer." To alleviate this problem, liquid water is trapped in the drain pots located throughout the piping system and drained through remotely operated valves.
One traditional control strategy for drain-pot-valve control involves a pair of level switches in the steam line drain pot. When the high switch detects liquid water, the control system sees the change in the switch signal and sends a command to open the valve. Depending on the line pressure, this may cause a partial open command or a full open command. When the high switch and the low switch both detect no liquid present, the control system commands the drain valve to close.
The fieldbus design
Typically, applying fieldbus technology to this control system function would involve replacing the valve with a fieldbus-enabled valve controller and replacing the level switches with a fieldbus-enabled transmitter. This would usually be implemented with a Foundation Fieldbus analog output block for the valve control, with position feedback derived from the BKCAL_OUT of the block, with the "Use PV for BKCAL_OUT" parameter. PV is the process variable. BKCAL_OUT is the value and status sent to an upstream block to prevent reset windup and to provide bumpless transfer to closed-loop control. An analog input block for the transmitter would be added, with logic in the control system to switch the valve to either the partially or fully open (depending on pressure) position when the level exceeds the set point.
Control in the field is not appropriate for this application since the algorithm involves signals from transmitters on different segments. What brings this approach into question is that the level switches are relatively inexpensive and can take the system operating temperature (1100 °F) and pressure (2400 pounds per square inch, gauge) without requiring additional piping components. Most continuous level technology would require additional piping components to protect the transmitter from the process conditions, which would mean added expense. The end user in this case also determined that the level switches possessed sufficient reliability and would benefit only incrementally from the additional diagnostics that fieldbus would provide.
The intelligent field device solution
Fieldbus-enabled valves are available that have extended capability, including the capability to interrogate external switches. While these were intended for use with external limit switches for valve position, these valve capabilities provide an interesting solution to the drain-valve control problem laid out above. The level switches can be wired to the valve controller, and switch position can be reported back to the control system using discrete input blocks. While this allows the wiring reduction typically associated with digital device installation, it avoids the higher device and installation cost of level transmitters.
This approach requires a large number of function blocks, which in other control applications could create performance issues. Because the speed required for system response for this application is relatively slow (approximately 1 second), a sufficient number of devices can still be connected on the same segment to make the digital field device wiring cost benefit easily apparent.
Today direct integration of simple devices like the limit switches into the local fieldbus valve is a cost-effective solution and a great example of how you can combine conventional sensors and switches with a fieldbus control solution. Even more direct integration of simple devices is coming in the near future as the valve suppliers add higher density discrete blocks. Some intelligent valve suppliers have already begun using the bitwise capabilities of the discrete data type described in the Fieldbus Foundation standards to reduce the number of blocks required to transmit a number of valve conditions. Ultimately, when you minimize signals, you improve the overall efficiency and performance of the network. The Foundation Fieldbus technical specification defines different ways for valves of all types, including actuators and positioners, to communicate more than just open/close data.
ABOUT THE AUTHORS
John Petzen (email@example.com), a senior systems engineer at GE, began his career in 1983 as a reactor operator in the U.S. Navy. In 1991, he moved to quality assurance in the operations department at the Department of Energy Rocky Flats weapons complex near Denver, Colo. Starting in 1996, he led instrumentation-and-control design efforts for combined cycle, fossil-fired energy, and oil and gas projects at the power plant engineering firm Harris Group, Inc. based in Denver. He moved to Greenville, S.C. in 2002 to pursue safety-system engineering with AE Solutions, LLC, becoming a certified functional safety expert. He is currently engaged in power island instrumentation and control integration for GE Power and Water.
Larry O'Brien (firstname.lastname@example.org) is global marketing manager at the Fieldbus Foundation. Before his job at the Foundation, he was research director for process automation at ARC Advisory Group, covering the topics of process fieldbus, distributed control systems, process safety, the automation services business, and intelligent field instruments. He has a B.A. from the University of Massachusetts at Lowell.