Suppliers often differ on the best way to control turbomachinery. Some advocate plant-wide controls operated by Programmable Logic Controllers (PLC) and Distributed Control Systems (DCS), while others opt for proprietary black box or software-based solutions.
(Modern gas turbine control systems)
In any case, choosing a controller is a challenge for users because the applications are becoming more demanding as the technology races, to keep up. Dan Levin, General Manager, Dresser-Rand Control Systems (D-RCS) says, “The future will require greater software portability with a focus on software development rather than hardware, and fewer boundaries between rotating equipment and balance of plant (BOP) control.”
For every user who belongs to the PLC or DCS camp, there appears to be another who prefers dedicated turbomachinery controllers, designed as a unified hardware and software package to address the specific needs of a given turbine or compressor.
While solutions used to be far apart in terms of functionality, the advent of faster processors, better software and commercial-off-the-shelf (COTS) hardware has obscured some of the differentiating features. “The reality is that almost all modern control systems are hybrids of PLCs and DCSs, and it is difficult to distinguish them nowadays,” said Klaus Brun, Program Director the Mechanical Engineering Division of Southwest Research Institute.
Even the term “PLC” is being avoided by many manufacturers due to the stigma associated with it. Earlier, PLCs were found lacking in some turbomachinery applications, and therefore “automation controller” or “open platform” is the preferred name in some camps.
DCS vs. PLC
For years, DCSs were used primarily in continuous process operations, where precise sequences of control were necessary, for example, to optimize a petroleum refinery or a pulp and paper plant. PLCs, on the other hand, tended to be found more often in discrete manufacturing, where variable control based on temperature or pressure was usually not an issue.
Recently, the lines have blurred. Both DCSs and PLCs are now found in gas turbine applications. Even though its primary turbomachinery control solution is PLC based, Invensys can also provide turbomachinery control applications within its Foxboro IA Series DCS platform. One factor that would affect the applicability of a DCS for turbomachinery control is the physical ability of the platform to deliver an acceptable (fast enough) complete closed loop control cycle time, said David Brown, Consulting Application Engineer, Invensys Operations Management.
Brown added that the target turbomachinery applications’ physical controllers rate of response (maximum acceleration and deceleration) to the final control element position changes, ultimately determines the maximum allowable controller throughput time between the sensed input signal and the achievement of the last controller requested position for the final control element. For example, a turbomachinery application with the relatively slow physical rate of response, such as a large reheat steam turbine with a very large amount of rotating inertia and a large amounts of stored energy could be properly controlled by a slower control platform throughput time.
“A small aero-derivative gas turbine application with a relatively small amount of rotating inertia and almost no stored energy, on the other hand, would require a turbomachinery control application with the fastest physical rate of response time, which given the current state of the technology, would be a PLC,” said Brown.
Speed of performance
As noted above, PLCs and DCSs are often distinguished by speed. “The PLC is used, generally, for start up and shut down activities, especially in safety applications because of the faster response,” said Alex Kuzmichev, Product Development and Sales Support, Continuous Control Solutions (CCS). “The DCS is harnessed for continuous control because the cost of implementing continuous control in a PLC would be high and the response time slow.”
Evolution wise, Kuzmichev said, the PLC is the successor of hardwired relay logic, while the DCS succeeded Single Loop Process Controllers (SLPCs). Operationally, a Human Machine Interface (HMI) is an integral part of DCS, while there was no HMI to start with in a PLC. Today, many third party suppliers offer HMI software for any PLC.
(More in the November-December issue of Turbomachinery International)