RDK8 supercritical steam power plant in Karhslruhe contains over 1000 AUMA actuators
RDK8 supercritical steam power plant in Karhslruhe contains over 1000 AUMA actuators
Credit: AUMA

Rapid advances in the field of electronics have transformed the reach and remit of power plant central control systems, writes Penny Hitchin

Changes in power generation are being driven by politics and economics as well as advances in technology.

The political imperative to restrict carbon emissions is being felt across the developed world, leading to unprecedented investment in renewable energy technologies.

New intermittent sources of power mean that conventional thermal power plants must operate more flexibly, which places additional demands on processes and components. As power generation technology evolves to meet 21st century requirements, rapid advances in the field of electronics have transformed the reach and remit of central control systems.

A power station contains hundreds of valves in a range of types and sizes, each controlled by an actuator. Valves may be used to stop and start flow, reduce or increase flow, control the direction of flow, regulate a flow or process pressure or relieve a pipe system of a certain pressure.

The fundamental design of the valves has not changed much in the last decades, but the requirement to operate at increased pressure, temperature and/or frequency has been aided by advances in material technology and manufacturing techniques.

The actuator is an interface between the valve and the control system that wants the valve to move at a specific time or speed or opening rate. Plant efficiency requires that this process is controlled as accurately as possible. The basic design of the actuator – a motor mounted to a gearbox which drives the valve – has also not changed much in the last 30 to 40 years. What has changed is the control and operations of the actuator which has been transformed by continuing developments in electronics.

Valves and actuators suited to the demanding environments of a power station are classified by their level of complexity. The simplest arrangement is an on/off actuator for a valve that is either open or closed and that operates infrequently. A Class B valve can be shut, part-open or fully open and also operates infrequently. Class C combinations can provide frequent operation and modulation while class D valves are required to operate continuously. Older baseload power plants in continuous operation relied mainly on the lower class valves but modern plants required to operate more flexibly rely on more sophisticated modulating valves and actuators.

Modern actuators can be monitored and directed from distant control rooms and are capable of collecting and transmitting a host of operational data which informs both production and maintenance requirements. In line with many other components, the trend is for an increase in the volume and type of information exchanged.

Refining human machine interface

Developments in control systems have led to a rapid increase in the range of data available in the control room. Advances in the interpretation and presentation of this information have followed.

Steffen Koehler of SIPOS Aktorik, a global supplier of electric actuators and part of the AUMA Group, talked to PEi about the company’s approach to improving the human machine interface (HMI) in its newly launched SIPOS SEVEN electric actuator range.

He stresses that intelligent actuators need to be selected and configured for specific applications. His company was keen that the product replicated the accessibility and interfaces used in devices such as smart phones.

“Talking to control systems is almost like talking different languages, but though we are operating in an industrial environment, technology has infiltrated so many parts of our lives that we all expect intuitive, state-of-the-art interfaces.”

The aim was to develop clear accessible interfaces that can use be used by technicians and non-specialists alike.”We are talking user friendly. That was one of the big motivations. There used to be a lot of ‘press buttons’ but we decided to take a whole new look at this. We considered touch screens but opted instead for a big colour display with screens that are easily understandable.

“A large, programmable colour display means that the customer can have an easily read status screen. Different skins mean that different software functions can be easily viewed. Graphics, icons and animations make it much easier for the user in front of the machine to understand what they have to do.”

The graphic displays can be used to replace the instruction manual. They have the additional advantage that they are available in over 30 languages, including Chinese and Arabic. The computer graphics will notably come into their own during the complex process of commissioning, showing the sequence in clearly defined pictures.

The front end interface has been researched and designed for maximum ease of use. Extensive tests were carried out with a usability centre – for example eye trackers were used to establish where people look when they want information.

Discussions took place with sales people from across the globe and customers and colleagues from various plants were invited to try out the software before the design was finalised. The aim was to design a front end that would provide the information in an easily accessible and appropriate format.

Ease of use extends beyond the control room with a USB interface integrated into the system so that information can be uploaded, downloaded or copied directly to the actuator.

“Everyone now is used to using a USB stick or USB connection. If you want information direct from the actuator you do not want to have to have a special tool or computer – you want to go to it and use the USB stick. We have integrated a USB interface which is accessible from the outside and is submersible so that, even if the actuator gets flooded, you don’t have a problem. If you want to change settings and the actuator has not been connected electrically to the mains power, you can use a USB battery pack to enable you to make the process control settings.”

AUMA electric actuators communicating via PROFIBUS DP with
AUMA electric actuators communicating via PROFIBUS DP with a Siemens DCS at ERZ Zurich combined heat and power plant
Credit: AUMA

System integration

The primary function of an actuator has always been to open and close the valve. However development of increasingly sophisticated control systems in combination with intelligent actuators has introduced vital additional roles for the actuator in communicating information used for diagnostic and maintenance purposes to the distributed control system (DCS).

An actuator has different interfaces. The mechanical interface with the valve defining torque, flange size and output speed, and its established engineering practices is well documented. The interface with the environment means meeting specifications and standards for vibration and temperatures encountered, for example. The connection with the DCS is provided by an electronic interface for communications.

The family of industrial computer network protocols used for real-time distributed control is called fieldbus. This was designed to replace point-to-point links between field devices such as actuators and their control systems with a digital single link capable of transmitting all the information.

Intelligent actuators have more capability than traditional actuators and can provide diagnostic information to monitor conditions and help diagnose and locate device faults. Integrating fieldbus into such actuators enables the devices to provide additional benefits such as remote diagnosis but also remote device configuration and testing.

In modern power plants a large number of different data and information packages need to be exchanged between control systems and field devices. Reliable and smooth system integration is vital to ensure the communication protocols and data interfaces operate accurately.

Years ago before standardised systems and standards were internationally agreed, the first protocols were devised by companies pressing ahead with their own designs. A few of these are still in use, but they exist as closed and proprietary systems, relying on one dedicated single supplier. Manufacturers and operators are converging on the use of standardised systems which are being increasingly used worldwide. This gives end users and consultants freedom of choice of suppliers.

System integration of communication protocols has become a key issue when customers select actuator suppliers for modern plant installations. It is no longer just the mechanical design that matters: system integration is centred on effective communication between the actuator and the host system and suppliers’ ability to ensure this capability is increasingly important.

As a global supplier of electric actuators with a strong reputation for providing comprehensive integration support, AUMA is well placed to comment on actuation advancements.

Werner Laengin, senior product manager for Fieldbus Systems at AUMA, says: “Power stations tend to be in service for up to 50 years. Technology needs to be available as a long-term solution. We recommend using only standardised solutions, as these are supported by a vast number of vendors worldwide.”

He explains: “We always take into account standardised fieldbus protocols and DCS integrations that ensure inter-operability with field devices. Standardised solutions use fieldbus protocols such as PROFIBUS, DeviceNet or HART. Taking the example of the HART interface, this combines conventional 4–20 mA wiring with additional digital communication, which makes it possible to have a combination of conventional wiring with digital communications.”

He cites the advantages of standardised systems. “Operators can use and mix different suppliers’ devices; operational expertise can be found throughout the world and communications protocols ensure interoperability with other devices.”

SIPOS SEVEN's advancements include a large programmable colour display
SIPOS SEVEN’s advancements include a large programmable colour display
Credit: SIPOS Aktorik

Comprehensive testing

Ensuring that new devices are interoperable requires implementation testing to ensure they meet specifications before submission for registration. Often a dedicated DCS integration is added to the control system. For example, the standardised EDDL (Electronic Device Description Language) is one method used to describe a unit’s parameters and minimum and maximum values for diagnosis of a field device.

An EDD is basically an ASCII text file which can be read by an EDDL interpreter which then presents the field device diagnosis or parameter information on the central maintenance screen.

Laengin reflects on the process required to ensure harmony in configurations within the DCS system and the field devices.

“The current interpretation of EDDL language varies slightly between different interpreters. Therefore we tend to recommend dedicated integration tests in order to ensure that once things come together, there is flawless device integration. Flawless device integration of each and every parameter, of each and every menu, of each and every maintenance and diagnostic information is essential in order to ensure there is no issue when it comes to commissioning.”

With increasing amounts of intelligence included in field devices, DCS integration is a fast moving field.

Laengin is excited by the possibilities. “Smart actuators can trigger alarms on minimum or maximum readings. Increasing intelligence means they can also interpret their own history.

“They not only count the number of switch offs or the number of open and close cycles, they are also able to record the environmental conditions, the torque applied to the valve and are able to make interpretation of availability for the future. They can make an indication about which kind of maintenance has to be done at which time for the specific application.”

Maintaining continuity

The operational life of a power station can be measured in decades, with thermal and nuclear stations remaining in production for as many as 50 years. Operators are keen to extend the life of existing thermal and nuclear stations where this is feasible.

Rapid developments in electronics have driven advances in control systems and state-of-the-art actuator/valve combinations are now installed in new plants.

However it may not be appropriate to retrofit these devices to an established plant. A lot of the control systems in older power stations may not be suited to modern electronic systems and the best course of action might be to overhaul and refurbish the original electro/mechanical valve actuators to maintain plant continuity.

Weir Engineering Services actuator workshop in Alloa, Scotland, is an engineering centre of excellence that provides services to the power generation sector, including the refurbishment and upgrade of valve actuators which may have been in operation for decades.

Workshop supervisor Phil Russell explains: “We see many actuators which were originally supplied in the mid 1960s and have been in service ever since. During 40 odd years of operation some have already taken advantage of overhaul and service exchange programmes, some have been partially maintained, and others may not have been maintained at all.”

Electro/mechanical actuators consist of a gearbox with an electric motor and electrical switchgear. Such actuators were often supplied to power stations with a design life of 25-30 years as ‘fit and forget’ items. Soft parts such as oil seals and O rings had a useable shelf life of between 10 and 15 years but in many cases there were no recommended service intervals for the actuator. Weir is confident that a major overhaul meets the aspirations of any life extension programme by returning the actuator to the customer in an “as new” operational condition.

Operators of the older nuclear fleet have a general understanding that remaining with the original design intent is less problematic than retrofitting an equivalent because design and engineering change is not required and controls and HMI interfaces are not compromised.

When comparing the cost of overhauling an existing unit with the supply and retrofitting of an equivalent unit of different design, the apparent cost benefit of the new unit is often outweighed by the cost of the technical justification, installation and plant control system alterations.

Phil Russell sees a great opportunity for re-energising old installed product rather than replacing with a different design.”A WES workshop refurbishment involves a full strip down, evaluation of any damaged or obsolete components, upgrade to the current design specification, rebuild and test. As much of the original electrical equipment is now obsolete, we will either perform a complete re-wire and fit all new electric components or leave the wiring looms intact and just fit new electrics.”

A power station may have hundreds of electric actuators installed. Russell says in a traditional power station this is made up of four or five main different types, each with any number of sub types, amounting to 30-40 different actuators with minimal differences.

“One way we have been looking at keeping up with reduced customer outage times is by offering service exchange programmes. A pool of new, spare or reclaimed actuators is available. We don’t need to keep 30 different spare actuators in stock as the appropriate model can be easily selected from a smaller pool and configured for a specific application to fit a plant where a requirement has been identified.

“The actuator(s) can be prepared ahead of an outage or maintenance window which means that instead of removing a faulty actuator and trying to fix and replace it in a tight time-frame, the unit can simply be exchanged which may take as little as two to three hours. The unit that comes off is returned to us for refurbishment and then goes back into the pool of spare actuators in preparation for future use.”

We see actuators which have been in service since the 1960s, says Weir's Phil Russell (right)
“We see actuators which have been in service since the 1960s,” says Weir’s Phil Russell (right)
Credit: Weir Engineering Services

Keeping pace with technology

The basic design of valves and mechanical actuators is established, but as power plant technology moves forward, devices must be designed to operate more accurately, more flexibly and under central rather than local control.

Devices must operate with precision and speed in the increased temperatures and pressures found in big modern plants.

Actuators and valves are an essential and established component in power plants. As power plants evolves to operate with greater flexibility and efficiency, developments in electronics will play a key role in enabling state of the art actuators to keep pace.

Penny Hitchin is a writer specializing in energy and power generation.

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