CONTROLLING TURBOMACHINERY —PART 1

USER CHOICES INCLUDE PLC, DCS, DEDICATED CONTROLLERS AND SOFTWARE-ONLY APPROACHES. WHICH IS RIGHT FOR YOU?

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. 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.

“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,” says Dan Levin, General Manager, Dresser-Rand Control Systems (D-RCS).

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-offthe- 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 a perceived stigma associated with it. Earlier PLCs were found lacking in some turbomachinery applications and so “automation controller” or “open platform” are the preferred names 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 & paper plant. PLCs, on the other hand, tended to be employed 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.

For example, even though its primary turbomachinery control solution is PLCbased, 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.

He added that the target turbomachinery applications’ physical controllers rate of response (maximum acceleration and deceleration) to the final control element position changes and 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.

A turbomachinery application with a relatively slow physical rate of response, for instance, such as a reheat steam turbine with a large amount of rotating inertia and plenty 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, he 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.

Scan times in DCSs could be as long as one second, fast enough for most control loops and slow processes, but pretty useless for turbomachinery, said Kuzmichev. However, modern DCS systems offer controllers such as the Honeywell C300, Yokogawa Stardom or Allen Bradley that can handle high speed with the help of special software routines.

CCS’s control applications are software- based, designed as custom function blocks that contain proprietary algorithms, and are implemented on any hardware platform capable of handling fast processes. This might be a standalone PLC with its own third-party HMI or a modern DCS controller.

One advantage of PLCs is that they are “open platforms,” which means they are compliant with standards such as IEC 61131 and able to support multiple programming languages.Aswell as the limitation of speed, earlier DCSs only supported one primary programming methodology, which gave them reduced functionality as a turbomachinery controller. Today, though, the DCS has largely addressed the speed issue and offers a broader range of programming environments. This has enabled CCS to implement its software-based controller on DCS as well as PLC platforms..

Kuzmichev is dismissive of most available turbomachinery controllers, which are expensive and tie the user to the vendor for service. At end of life, the user then has to buy another black box using proprietary hardware.

Those buying CCS software, he said, can service their control hardware as it works with any PLC. Scan rates are about 20 ms, which is fast enough for most applications. “PLCandDCS companies are hardware specialists that are used to addressing an entire plant,” said Kuzmichev. “They are generally not experts in software and turbomachinery, which is a tiny portion of thewhole package.”

Dedicated control options

Some vendors caution against using DCSs in place of dedicated turbine control systems. According to Rich Kamphaus, market manager for steam turbine and safety systems at Woodward, dedicated controls are designed to operate at high speeds to ensure rotating equipment safety. Endusers will often use their DCS as the “window” through which to operate their turbine control — starting and stopping the turbine controller, or changing speed of the rotating equipment based on demand.

“Regardless of claims by DCS and PLC vendors, tests by several turbine OEMs show dedicated controls to be faster and more reliable,” said Kamphaus. “The employment of DCSs to control rotating equipment as well as perform process control means that the same hardware and software is used throughout a facility, resulting in training and support cost benefits.While this may be attractive, in the past it has created major problems when simple changes made to DCS software accidentally affected or disabled the turbine controller and its protective functions.”

Kamphaus also points out that scan rates of some DCS systems could be as slow as 50-100 milliseconds (ms) under certain circumstances, which is much too slow for safe operation of high-speed rotating equipment such as turbines. He added that, due to lack of software-to-hardware synchronization, DCSs and PLCs have performance limitations which dedicated controls do not have.

Lack of synchronization has the biggest effect on performance of the core controller algorithmknown as the Proportional Integral Derivative (PID). If a PID is utilized in a DCS or PLC control loop which does not have software-to-hardware synchronization, users are required to de-tune (slowdown) the PIDs to achieve loop stability. In a typical process loop, slower PID controller settings work fine. However, with high-speed rotating equipment, such settings can limit a turbine’s or compressor’s responsiveness to fast system load changes or rejections.

Improper use of DCSs and PLCs, then, can lead to problems. “An example is a DCS controlling a polypropylene compressor in Brazil which had a scan rate of 160 ms,” said Kamphaus. “Due to this slow rate, the compressor surged six times before the DCS detected the surge event.”

With a dedicated controller, software and hardware are synchronized, and scan rates are uniform and short, allowing PID controllers to dynamically respond to slow or fast operating conditions, he said.

Woodward controllers, for instance, sense rotor speed and acceleration and deceleration every 100 ms and perform a CPU scan every 5-10 ms. This enables detection and response to changes in rotor speed that are fast enough to protect equipment from overspeed and surge events. To furthermake his case, Kamphaus cited API 612 6th Edition, which names a dedicated turbomachinery controller as a best practice for petrochemical applications.

According to Woodward, about 85% of existing turbomachinery use dedicated controllers. About 30% of those controllers are mechanical governors, particularly overseas. Analog governors comprise another 5% while digital governors make up the rest.

PLC progress

Wood Group GTS is a believer in the PLC open-platform approach. In the past, turbomachinery- specific control and I/O modules for gas turbines in the power and oil & gas markets were non existent, said Clark Weaver, Manager of New Product Introduction, Wood Group GTS. Today those product portfolio holes have largely been filled.

It used to be true, he said, that off-theshelf control platforms were not real-time and therefore not a good fit for high-speed reciprocating machinery. “Today, almost all open platformcontrollers have real-time operating systems. There is no longer justification to utilize a proprietary turbine or BOP system.Any gaps or justifications are quickly being addressed.”

By leveraging the RockwellAutomation platform, Wood Group GTS seeks to offer users improved life cycle costs and support infrastructure with direct applicability to turbomachinery control, boiler, HRSG and BOP control.

LikeWood Group, Invensys is an advocate of the PLC open-platform approach. Its Triconex systemis IEC61131TUVcertified and modularized to prevent obsolescence of the underlying processor or other hardware.

“An open platform is the best match for a turbomachinery controller,” said Brown. “Dedicated purpose devices assume that every future application was anticipated and has been accounted for even before that device was put into service. Every turbomachinery application system typically has unique requirements which change over time.”

Emerson Process Management is a supporter of the DCS philosophy. Its Ovation DCSworks for all power generation applications, including boilers, scrubbers, BOP, steam turbines and gas turbines. “Using one DCS platform for all generation assets streamlines maintenance, training, spare parts and system support,” said Laurence O’Toole, Director of Turbine Business Development for the company’s Power & Water Solutions. “We also see many plants operating with fewer resources. Therefore many power generators want to reduce the number of control systems to achieve efficiencies in reduced training and reduced spare parts inventory.”

A DCS typically has one engineering workstationwith a common database used to program and monitor the entire system. By comparison, PLCs often require separate engineering workstations and databases for theHMI and PLCcontroller,O’Toole added. .

Tri-Sen takes amiddle ground approach — it sells both PLCs (such as its TSx platform) as well as configurable turbomachinery controllers (such as its TS300 and 310SV). “For non-critical, simple turbomachinery controls applications, a stand-alone configurable controller is often the best solution,” said Tri-Sen Vice President, Jim Jacoby. “Applications that are more critical, including integrated compressor or generator control, multiple extraction and more complex ancillary control and monitoring, such as lube oil skid, vibration and boiler level control, almost always require a PLC type controller.”

Specific applications

Processes vary and require control solutions tailored to meet specific requirements. Recognizing this, Dresser-Rand Control Systems (D-RCS) custom engineers each control system to meet the specific application. It takes a general approach, selecting hardware with enough performance to adequately control and protect the machinery. However, the company is largely not in favor of a black-box approach.

“We have provided control systems using six different PLCs, three client-specified black boxes and controls using four different Distributive Control Systems,” said Dan Levin. “But the use of designs available from a single source should be avoided as it has proven to have an expensive life cycle because of its reliance on a single provider. We believe COTS PLCs or high-spec DCS should be the preferred route.”

He urged users to realize that hardware platforms have continued to standardize.The advent of digital controls has enabled better machinery protection. However, COTS hardware did not meet control requirements at that time, which led to the development of proprietary black box processors.

As time passed, the performance of PLCs has improved to the point that they could be considered for turbomachinery controls. “Scan rates and throughput times of certain improved PLCs meet the stringent requirements necessary,” said Levin.

“The Holy Grail of standardization would be using plant control DCS hardware for all controls, plant-wide,” he said. “Recent DCS improvements have reached the point that select DCS hardware with specially developed D-R software has the necessary performance capability to control turbomachinery.” To date, the company has confirmed satisfactory performance of four different DCS systems for certain turbomachinery applications.

“Some applications are still too demanding for the DCS hardware currently available,” said Levin. “That said, DCS controls will continue to improve and D-RCS is committed to evaluate those improvements and offer the benefits of further plant-wide standardization.”