THE WAY FORWARD FOR TURBOMACHINERY CONTROL

PROPRIETARY AND BLACK-BOX APPROACHES OF THE PAST ARE BEING REPLACED BY SOFTWARE-ONLY SOLUTIONS

The original Digital Control System (DCS), Honeywell’s TDC 2000, began a revolution in industrial process control. What had previously been the realm of operators and pneumatic control systems slowly began to migrate into a microprocessorbased control chassis. The history and evolution of turbomachinery control was linked to this progression, but it also took its own course, with specialists emerging who focused solely on control, regulation and protection of the turbomachinery.

With the rapid advancement of microprocessor technology, we are seeing a shift in process control and turbomachinery control toward software-based solutions.And the emergence of fast and flexible Programmable Logic Controllers (PLCs) means that firms specializing in turbomachinery control will gradually move away from proprietary hardware and instead focus on software and know-how.

Historic perspective

This history of turbomachinery control can be traced back to the initial development of steam engines, which required effective speed control. Watt’s governor (also known as the flyball governor) of 1788 was the first centrifugal and fully automatic speed control which utilized a system of levers and gravity to control the opening of the aperture on throttle valves in steam engines.

The need to regulate speed became even more essential over time, and various versions of the original flyball governor were enhanced to manipulate steam (steam turbines) or fuel valves (gas turbines). The mechanical set up became more advanced over time, with hydraulic systems replacing the flyball. Prior to the computerization of turbomachinery systems, the control of valves was firmly based in mechanical engineering, via a set of hydro-mechanical governors, each responsible for some aspect of control or limiting. All these elements were connected to a common hydraulic system, with each element controlling pressure based on given control parameters. The total change in pressure created by all working governors resulted in manipulating the throttling valve, which in turn affected the steam or gas supply to the turbine in question.

These systems continued to improve and develop along with the increased use of turbomachinery for gas transportation and power generation, until the 1970s when the governors were replaced with I/O modules, and control philosophies were firmly planted within controllers.

Instead of relying on the aforementioned system of mechanical governors as the “brain” of turbomachinery controls, these actions soon shifted into controllers sending out rapid signal outputs to electrohydraulic actuators.

Requirements to decrease energy costs and boost efficiency of industrial processes required a different approach to control; one which continued to protect turbomachinery from excessive speeds and allow it to operate continuously while also taking into account the processes being driven.

Over time, after all, it wasmost important to protect the process, and not just the process drivers. Microprocessor-based control, therefore,was the natural progression from mechanical- hydraulic control.

Dedicated controllers became more important. Algorithms began to be developed that focused on a variety of conditions beyond simple protection of equipment:

1. Control and protection of not only the machinery, but also linking the regulation of these units to plant technology to optimize processes, minimize losses and increase output

2. Load sharing between units working together in parallel, in series or in mixed pattern

3. Control of increasingly sophisticated turbomachinery, such as multi-shaft turbines that required precise control to match shaft operation

4.Accounting for the increased operating temperatures in combustion chambers, and the resulting necessity to operate close to temperature limits where rapid changes in temperature would need to be effectively monitored

5. Operating turbomachinery on the border of allowable set points in order to expand the operational envelope of the machines. Pneumatic and mechanicalhydraulic controls were insufficient to incorporate these needs.

Honeywell’s TDC2000

Honeywell’s TDC2000 was one of the first control systems to utilize function blocks, where self-contained “blocks” of code that emulated analog hardware control components performed tasks that were essential to process control. Developing this hardware coincided directly with developing the complementary software.

DCS systems, however, were much too slow to be utilized for effective turbomachinery control. For most monitoring and information gathering processes, scan times could be processed over seconds and, in some cases,minutes. The operation of heavy rotating equipment at the narrowest tolerance limits of temperature and speed required scanning transmitter inputs over the course of milliseconds.

Rotating machines, such as centrifugal compressors at the border of their surge lines, necessitated the development of proprietary controllers. Measuring a compressor’s behavior as it approached its surge line not only required speed, but also advanced mathematics. This was mainly due to the nonlinearity of a compressor’s performance curve for any given speed, where the operating point accelerates along the curve toward the surge limit.

The initial computerization of turbomachinery control then progressed naturally and into two key providers: OEMs and independent control and automation firms. As OEMs were manufacturing the turbomachinery, it fell in line to offer a turn-key solution that included turbine and compressor anti-surge control. However, OEMs were rarely concerned with protecting the entire process; they were more inclined to protect the turbine or compressor they had under warranty.

The independents were largely comprised of experts in turbomachinery control and protection, as well as the technology of the processes they were designed to work in. However, due to the slow controllers available on the market at that time, they were forced to design and develop their own faster controllers.

Focusing on software

One such company, Compressor Controls Corporation (CCC), where I worked in various positions over the course of 16 years, became the industry standard in turbomachinery control. The company focused on the development of advanced control algorithms and their successful implementation in proprietary software.

The fact that CCC had to design its own controllers was a necessary evil; it created the industrial standards for compressor and anti-surge control (and, later, turbine control) through its know-how in compressor control technology. However, none of the available controllers on the market at that time was fast enough to implement this software. Thus, CCC developed its own controllers. As the 1970s turned into the 1980s and 1990s, microprocessor technology continuously evolved, allowing for increasingly rapid scan and execution times. Moore’s Law, or the idea that the number of transistors that can be placed on an integrated circuit doubles approximately every two years, was proven over and over.

Most processes for turbomachinery control did not change over time; in otherwords, the control strategy remained more or less constant (to prevent surge and operate continuously over time). However, increased controller processing speeds spearheaded the development of ingenious methods of control inclusive of the ability to run turbomachinery much closer to set limits than ever before.

Costly burden of proprietary hardware

Due to greater speeds, and with the development of PLCs capable of 5- to 10-millisecond scan times, specialized turbomachinery control firms were moving further away from carrying the heavy load of hardware development on their R&D budgets.

The negatives of continuing to rely on proprietary hardware development included the need to deal with:

• Operating systems and hardware issues that were not the core strength of turbomachinery control firms

• Carrying costs of servicing older hardware platforms

• Proprietary hardware requiring extra training for the customer, and “black box” solutions typically requiring greater integration with existing DCS

• Start-up engineers setting up their own proprietary operating systems on customer sites rather than focusing on the technology and software implemented on customer turbomachinery.

The positive was that by supplying both hardware and software, a control firm could set market prices for integrated solutions. And the combination of hardware and software provided a greater inventory of products, which needed servicing and spare parts, and this translated into increased revenue.

In the case of many black box solution providers, their reputation for superior products and proven effectiveness over time created good will and net worth, providing attractive profit margins as long as thirdparty controllers continued to operate too slowly for effective turbomachinery control.

However, hardware also required continuous improvement, with cash flow dedicated to products that could compete with PLCs, and that left less money for the advancement of control strategies (algorithms and software).

In essence, proprietary hardware became the monkey on the back of firms offering black box solutions. Too much time ended up being spent on hardware development and manufacturing, and not enough devoted to turbomachinery control.

More importantly, today’s users were more knowledgeable and had their own hardware specifications. Increasingly, they were showing a preference for open hardware platforms and flexible control algorithms implemented in software that was portable to multiple platforms.

The narrative became a tale of two strategies: One focused on control, and one on control and hardware development.

By 2012, the game had changed. PLCs became more affordable; their standardization required less time for learning and programming. And the uniqueness of specialized controllers requiring special approaches and special spare partswas gradually turning into a burden.

The complexity of rotating machinery control and protection algorithms has not decreased with the creation of appropriate programs. However, start-up, commissioning and debugging should be done by experts specializing in control, not necessarily in hardware. Standard PLCs, already providing solutions to almost any problem of control, will continue to improve.

The user wins

Thus, everyone will do what they do best, and the user will be able to select both PLC suppliers and suppliers of various turbomachinery applications. Of course, these changes will not occur over a matter of days; PLCs will not capture the entire market of turbomachinery control next week.

But technological trends and customers who have become increasingly sensitive to costs and black box solutions will be more inclined to standard PLC solutions, and this trendwill progress just as rapidly as the development of the technologies themselves. Competition of algorithmic and software solutions, both in terms of control quality, completeness and depth, will only intensify.

 Author

Leonid “Lenny” Shcharansky is the founder and former Chairman of the Board of Continuous Control Solutions (CCS), a turbomachinery controls company located in Des Moines, IA. He currently holds the position of Head of Russian and CIS Operations at CCS. Lenny has a Masters Degree in Mechanical Engineering from the Moscow Power Engineering Institute in Russia. For more information visit, www.ccsia.com or contact Boris Shcharansky at 515-278-9655 ext. 110.