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Current market demands for greater flexibility and efficiency mean that operators of conventional power plants have to operate their fleets in a more cyclic manner, as well as at higher temperatures and pressures. Paul Breeze explores how these changing operating conditions impact on critical components, such as valves and actuators.
The ability to control a power plant, be it natural gas-fired, combined-cycle, coal-fired boiler, solar thermal array, or even a wind turbine will depend on the reliable functioning of valves and the actuators used to operate those valves.
These components come in a range of types and sizes and the operations they perform are equally varied. Without them the management of modern power plants would be impossible. Today, the demands on these vital components are becoming ever greater.
Over the past 20-30 years the level of control required of all types of plants has increased greatly. This is partly a result of changes in the market structure such as the widespread privatization of power generation in Europe and elsewhere — when plants stopped making electricity and started manufacturing pounds, shillings and pence, as one industry expert wryly observed — and partly the changing nature of the power supply system, with increasingly large volumes of renewable energy being fed into grid systems that are operated within much lower tolerances than in the past to cater for the increasingly large base of sensitive electronic equipment that underpins modern life.
These changes have led to a need for more frequent and precise control of plants, an evolutionary process that has affected all aspects of plant component design including that of valves and actuators.
Ian Elliot, sales manager at Rotork Site Services in the UK, recalls the first of these changes taking place in the early 1990s when the UK industry was privatized. The private sector companies that took over plants demanded lower manning levels so valve operations that might have been carried out manually had to be automated. Meanwhile 400 MW coal-fired baseload plants that had previously started and shut down once a year suddenly started ‘two shifting’, that is coming on line early in the day to meet morning demand, shutting down and then coming on again in the evening. As a consequence, Elliot notes, “actuators had to move twice each day instead of twice each year.”
Step-change in procedures
With this change in duty cycle, both valves and their actuators suddenly had to be much more reliable.
One of Rotork’s responses was to redesign its actuators to reduce the number of moving parts. Today the company relies increasingly on components such as Hall-effect sensors and piezoelectric sensors where in the past it would have relied on electro-mechanical components. Changes of this type have been replicated across the industry.
Privatization, where it happened, led to a step-change in operating procedures. Since then there has been an even greater shift in the power generation landscape but its introduction has been more gradual. The result is that, 20 years on, ‘two-shifting’ is more likely to be ’20-shifting’, or even ‘continuous-shifting’. This is because the power networks in many regions have to manage larger volumes of renewables, primarily from wind and solar photovoltaic plants. Both of these sources are by their nature intermittent and in order to keep the grid in balance, it is necessary to have alternative generating stations in readiness to step in if the renewable output falls. The same plants must be capable of backing down, when renewable output increases.
This grid support role can be managed with hydropower and energy storage plants but often they are not available and then much of the responsibility falls onto conventional fossil fuel plants, particularly gas-fired combined-cycle, but also some modern coal plants. These plants are required to be able to start and stop frequently and ramp their output up and down often and rapidly. Achieving this requires precise control of the plant operating conditions and those conditions are maintained using valves that modulate the flows in all critical parts of a plant.
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A model to meet all demands
For valve and actuator manufacturers, these new operating conditions have changed the demands placed on their products in two important ways.
The first is that the valves and actuators are required to operate even more frequently than before. The second, highlighted by Udo Hess, power market sales manager at AUMA Riester, is that faster startups and faster output ramping mean that valves and actuators must move more quickly than before. To meet these conditions, manufacturers have been forced to adapt their designs.
Some valves, such as ball and butterfly, are designed to be either open or shut. These are often used as fail-safe on/off valves. Others are designed so they can be partly open, their position controlled with varying degrees of precision. This allows for modulation of the fluid flow through the valve. Controlling both types of valve may involve a linear motion or a rotary motion, and this is provided by an actuator.
Actuators are designed for specific valve types and for particular duties. Some provide a linear force to open and shut a linear valve. Others generate a rotary force. And as with valves, there are different types of actuator.
Many modern actuators are electro-mechanical, relying on electrical power to operate. Others are hydraulic or pneumatic. Each has its advantages and disadvantages.
Valves and actuators for use in demanding environments such as power plants are divided into found standard classes — A, B C, and D — depending on the type of duty they perform.
Class A is an on/off actuator for a valve that is either open or closed, normally quarter-turn valves such as butterfly or ball. A class A valve only operates infrequently. Class B is a valve that can be shut, part-open or fully open, and the actuator will have a position sensor to provide feedback for a power plant control system, but will again only operate infrequently. A class C valve and actuator can provide very frequent operation and modulation, while class D valves are required to be able to operate continuously.
In the past, traditional coal plants would have required very few higher class valves but modern coal and gas plants need many more class C and D modulating valves and actuators.
Moreover, specifications for the latter are rising so that, for example, AUMA has increased the frequency of starts supported by its class C actuators from 1200/h to 1500/h.
While valves and actuators must operate more frequently, power plant operators do not want to be forced to maintain them any more frequently that previously. So both components must be more rugged and reliable. The actuators often have to generate more force or torque than before and move more quickly than before too. All of this puts them under much greater stress.
And there is more: when valves and actuators are operated they generate heat. The more frequently they operate, the hotter they get.
With a low-frequency duty cycle the actuator has a chance to cool down between operations. In modern power plants with high-frequency duty cycle, the actuators, in contrast, have little opportunity to cool, especially at an ambient temperature of up to 120°C. That places extreme demands on all the components that make up the actuator.
Higher efficiency, greater precision
Rising temperature is not only a matter of frequency of operation. The temperatures and pressures inside conventional fossil fuel plants are becoming more elevated in the quest to achieve higher efficiency and this means the components operating in these more extreme environments must be able to withstand higher temperatures and pressures.
Moreover, conventional plants are getting bigger. In 800 MW to 1000 MW power plants you can find 3-metre diameter butterfly valves. “Previously this size was only found in water systems,” Hess says. The torque required to shut or open these valves can be as high as 675,000 Nm.
With large valves, actuators must be bigger and travel further. Yet at the same time actuators have to be more precise and sensitive. For US company Automation Technology Inc (ATI) this means that the mechanical parts have to be machined more precisely, as its chief executive Cooper Etheridge explains.
Greater precision means that parts match more perfectly and friction is reduced while maintaining smaller tolerances so that more precise control is possible. Low friction is important because excessive friction can lead to ‘jumpy movement’.
For ATI’s speciality, linear actuators, this must be achieved while providing faster and larger movements and greater force. Other actuator manufacturers face similar problems.
Valve design modifications
Valve manufacturers also have to cope with a demand for larger components that are at the same time more precise and can operate under more stressful conditions. If anything, the conditions faced by valves are more extreme since they can be controlling the steam or air flows in coal and gas-fired plants where temperatures and pressures are at their highest.
And while the most stressful conditions are in new plants, the problems can be just as great in older plants where the valves were not originally designed for the sort of duty cycle now expected of them. This means expensive refitting of newer and more capable components.
Not only is more demanded of each component, but power plant companies are also demanding that their valve suppliers prove their valves will stand up to the rigors of this new duty. This means producing lifetime calculations to show how the component will perform over 25 years when the plant might start up and shut down 50,000 times. This has meant introducing advanced modelling and testing techniques.
The issue with valves is often one of materials and design. In the past, valves were designed for baseload operations and now when subjected to this new type of operational duty will often start to develop cracks and key stress points.
This problem is exacerbated not only by the increased number of cycles that the valve goes through, but by the increased thermal gradients that it is subjected too as the ramp rate increases. “It is not only the frequency of cycling but also the speed of temperature change,” says Martin-Jan Strebe, global product manager for control valves at Pentair.
The solution is to redesign valves, removing all the sharp corners where stress fractures often start and limiting the changes in thickness of the metal used to fabricate the valve as far as this is possible.
Careful redesign will reduce the potential for stresses to build up in the component. In addition, manufacturers are having to use new materials to replace traditional steels. To achieve better high-temperature performance they are exploring the use of nickel-based alloys similar to those developed and used in gas turbines, where similar — if even more extreme — operating conditions exist.
Where steel continues to be used, the demand on steel alloys has become greater. F91 is a standard stainless steel used in valve and pipe manufacturing. However the durability of the material can be weakened by small quantities of aluminium in the alloy. Therefore valve makers are demanding special batches of F91 with reduced aluminium content.
Another change taking place is the way in which valves are fabricated. Valves can be cast or forged, but today forged valves are preferred in high pressure systems, another change brought about by the extreme conditions under which they are required to operate. “The industry is grappling with cast versus forged, and which is most suitable for today’s conditions,” says Arvo Eilau, global marketing manager of Natural Gas and Renewable Power at Pentair.
Meanwhile, German company PS Automation is exploring the use of plastics. Injection-moulded plastic components can be very accurate, according to Michal Kral-Serrato, field sales manager.
However, any plastic component needs to be carefully load-tested to ensure it can meet the demands imposed on it. The company introduced its first plastic components around two years ago.
Risk management, an integral part of plant operation
A further key issue for both actuator and valve manufacturers is to control the risk associated with their products. Part of the driver behind this is to achieve greater plant safety but beyond that, risk management is becoming an integral part of modern plant operation and management.
At its most basic the issue means that investors, owners and operators want a number that tells them the level of risk associated with each power plant.
In order to gauge risk for each power plant component, an assessor needs to know its associated risk level — how likely is it to fail, and in what way will it fail? To meet this requirement, manufacturers are increasingly asked to provide components that are rated against the standard Safety Integrity Level (SIL) measure of performance. Typically, valve and actuator manufacturers seek to achieve SIL level 3 or above for components that are used in power plants.
As ATI’s Etheridge explains, obtaining a SIL rating means allowing an outside assessor to examine a component and explore all the ways it can fail. Simplicity in this case can be absolutely key.
For example, many of ATI’s actuators contain a spring, a simple fail-safe component that will ensure a valve moves into its fail-safe position if the actuator fails in any way. “A spring is a very simple device,” Etheridge says. “For safety, the simpler the component, the safer and more reliable it is.”
Another way of increasing security is to constantly monitor and record the operation of valves and actuators. Rotork uses monitoring to record the operation of its valves and actuators over their operational lifetime, and then compare the daily performance with the baseline performance established when the unit was new. By comparing original and current performance it is possible to detect when problems such as wear or valve sticking begin to arise, allowing predictive maintenance to be carried out before the unit fails. The same monitoring and logging system will also record the number of operations and compare these to the expected lifetime number, another way of gauging the status of the component.
At the same time, the digital revolution is bringing changes that affect all aspects of valves and actuators. Manufacturers need to provide standardized digital interfaces to all their components so they can be integrated into modern control systems. PS Automation is using software in its controllers to customize its actuators for different applications.
New opportunities with renewables
While the biggest power sector market for valves and actuators remains conventional plants, for some companies the nuclear market is important, while a new but growing sector is renewables – and this is likely to become a more important market in the future.
As an example of this, Young & Franklin has recently designed a single-cylinder operated parabolic trough actuator for concentrated solar plants. As company marketing specialist Jason Dyer points out, this is opening new design and manufacturing opportunities for both new and established companies.
Rotork has also been supplying valves to solar thermal plants where they are used to control the temperature of the fluid in the heat transfer circuit to ensure it is maintained within operational limits. Flow rate is increased when the solar input is high and reduced as the heat input falls.
There are around two actuators for each megawatt of solar thermal generating capacity. In a conventional fossil fuel plant, the number is generally around one quarter the number, so this is potentially a large market.
Another section of the solar thermal market where specialist valves are needed is to manage flows in plants that use molten salt, either as the heat collection fluid or for energy storage, or for both.
|The industry is grappling with cast versus forged valve design, and which is most suitable for today’s conditions
The mixture of molten nitrates that is often used in these plants is generally maintained at between 300ºC and 400ºC. This is a hostile and corrosive environment and requires valves that are particularly corrosion resistant.
Solar thermal plants are akin to conventional thermal plants in concept, even if the energy source is different, so it is not surprising to find that they make extensive use of valves and actuators.
However, even wind turbines need these components too. One particular application is within the hydraulic control system that operates the blade pitching for speed control.
Most modern turbines have moveable blades or blade sections that are used to control rotational speed as the wind speed changes. These may also have to perform a critical fail-safe function, shutting down the turbine if wind speeds become too high. As turbine technology advances, so the need for valves and actuators here is likely to increase too.
Across the power sector, the changes that are taking place are challenging manufacturers of valves and actuators to find new solutions to old problems.
Valves and actuators are needed to perform more complex operations, but at the same time they need to be simpler to satisfy risk requirements.
Meanwhile, new markets are arising that require different solutions, so the challenges are accompanied by a range of opportunities. Companies across the board are responding.
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