Michael Herbstritt, AUMA, Germany
|New generation actuators support power plant asset management|
The management of a power plant’s physical assets can have a significant affect on its operational performance. These assets – components that are relevant to a power plant’s industrial processes, such as vessels, machines, piping, process control technology instruments and equipment – clearly need to be well maintained to maximize the power plant’s output.
On-line plant asset management plays a vital role in determining maintenance intervals. In this context, the use of the term ‘online’ refers to a system that is connected to a network. The objective is to preserve or enhance the plant’s value through its maintenance.
Asset management is growing in importance. It has achieved the status of a science, enabling plants to achieve whole-life optimization. As a result, large numbers of people in the power industry are currently developing guidelines for implementing systems. The oil and gas industry has, to a large extent, been at the forefront of adopting asset management, but the power industry is now also embracing such management tools. The new coal fired power plant RDK 8 in Karlsruhe, for example, which is currently under construction, has managers who recognize the business benefits of asset management and there are plans to implement a system incorporating actuation technology.
Primarily, an asset management system comes under the jurisdiction of plant engineering, since it provides on-line information about the technical assessment of plant components. Process control tends to be the responsibility of the plant operator and, as a general rule, on-line plant asset management uses elements of a process control infrastructure, but under separate management.
When defining asset management systems, it is useful to highlight two primary tasks. The first is signalling conditions that cause a failure, so that measures can be taken to prevent such a situation. The second is the supply of complex information relating to the maintenance requirement of a device to eliminate the static (pre-defined) maintenance intervals.
The summer is traditionally the power industry’s maintenance period and the time when plants are switched off to maintain components. The procedure is carried out regardless of whether there is a need for it – this is because the technician does not have reliable information available about the degree of wear of devices: actuators that open and close valves only twice a day are monitored in the same manner as products that operate valves dozens of times daily.
The latest generation of actuators have been designed to address asset management requirements and provide information that will negate unnecessary maintenance. This enables the actuator supplier to offer a more informative, pre-emptive and effective method for servicing the plant – and gives the actuation engineer an opportunity to enjoy a short holiday in the vacation period!
Actuators and automation
Automation is at the heart of a power plant’s asset management and actuators have a key role in operating valves that are vital to the station’s final control processes. Actuators are used in a range of applications that are critical in optimizing plant efficiency and they are found in every type of power generation solution, including gas and coal fired plants. It is useful to highlight that, although actuators are adopted in the nuclear power industry, comprehensive asset management actuation technology cannot be adopted as the electronic components required to achieve this are not permitted in nuclear applications.
One significant task performed by actuators is their ability to monitor the development of torque requests and to update the distributed control system (DCS) with the status of both the actuation equipment and the valve. Because actuators can play an important contributory role in the asset management process, it is important that the science is addressed by electric actuator manufacturers. Before providing more detail of how actuators can support successful asset management systems, it is useful to give a historical overview of actuators detailing how they perform in process applications and providing some working examples of power plant installations.
Electric actuators are used for valve automation in a multitude of process applications. They comprise a gearbox with a flange-mounted electric motor and integral controls, housing, the switchgear for motor control and the communication interface to the DCS. Interfaces for all conventional process automation fieldbus systems are generally available including the various Profibus DP protocols. The Ninghai coal fired power plant in China is an excellent example of a plant that has embraced fieldbus technology.
Benefits of the digital actuation solution have included reducing cabling to a two wire solution and expanded functionality. Additionally, the utility has used the actuator parameters for remote modification to achieve continual optimization without a service technician physically attending to the device.
While the widespread trend is towards fieldbus communication, traditional parallel control systems continue to be adopted in a number of power plant installations. The Ventanilla power plant in Peru, for example, is a good illustration of an application where actuator functionality was required without recourse to fieldbus technology. The brief for this application made it clear that digital control technology could not be incorporated.
The demands of the Peru plant were met with a motor switch integrated actuator solution. The ability to commission a device without direct contact with a colleague in the control room was important and this was achieved via integral controls for local device operation. Additionally, a plug-in electrical connection to the actuators was a key feature as this facilitated rapid mounting and removal of an actuator to the valve.
Electric actuators are always equipped with a handwheel and an activation mechanism to allow the valve position to be moved in the event of power failure. The integral controls either manage the actuator via the binary operation commands ‘open – stop – close’, or a valve position to be approached is defined by the DCS. The controls perform actuator positioning. On reaching a valve end position, the actuator is switched off automatically, either via a position feedback signal or when reaching a pre-set torque limit.
Modern actuator controls are equipped with microcontrollers. Since their introduction around 20 years ago, the functionality of actuators has been extended significantly and, in the 1990s, the basic facility of preventative maintenance by logging and assessing operating data of field devices was identified.
Due to the development of fieldbus systems, the technical capabilities were created to transmit higher volumes of status data from the field device to the DCS than could be handled via parallel communication. However, the first steps in this direction were not very successful; this was due to the fact that a technical standard for data transmission was not sufficiently defined.
The additional data flow reduced transmission speed of information relevant to process control, which overstrained the DCS, which in turn impacted the plant operators. Device operating data was mainly used for service purposes supporting the device specialists with fault diagnosis. However, the failure had already occurred, which was unhelpful as the objective of preventative maintenance is to anticipate failures.
Addressing this situation, modern electric actuators have evolved to offer a field device solution with internal status monitoring – as such, they independently monitor that specified operation conditions are met. This includes pre-defined operation times or recording the number of starts.
These variables are easy to record with microcontroller technology. Monitoring other variables has required additional hardware within actuators, including torque monitoring, sensors for continuous recording of temperatures (motor, gearbox and electronics) and vibration sensors.
The actuator industry has, therefore, embraced the needs of asset management as it is possible to monitor device-specific service conditions, such as permissible ambient temperature. Additionally, the new era of actuator design also facilitates monitoring permissible vibration, maximum number of starts or operating times and the definition of customer-specific limit values. For example, a reference torque characteristic can be recorded after commissioning.
However, while it is clear that actuators have a role to play in asset management for the power industry, to be effective these devices need to comply with defined schemes. Exceeding predefined limits are spontaneous events that must appear on the plant operator’s display to enable immediate corrective actions. Because plants consist of numerous components supplied by different manufacturers, it has become apparent that device feedback must comply with defined schemes. The objective is that the plant operator can take appropriate action to address the problem. One approach is to classify status signals into four categories in compliance with NAMUR recommendation NE 107, as follows:
Out of specification: Deviations from the permissible application conditions determined by the device itself through self-monitoring. The device can still be controlled from the control room.
Maintenance required: The device can still be controlled from the control room and must be inspected to avoid any unscheduled failure.
Function check: Due to ongoing work on the device, it cannot be controlled from the control room at that moment in time.
Failure: Due to functionality failures within the device or the peripherals, the device cannot be controlled from the control room.
AUMA demonstrates wireless connection between programming device and actuator
The manufacturer must classify the status signals of its devices using these categories. For example, if an actuator senses an ambient temperature out of device specification, the plant operator will receive the respective symbol with the device identification so the plant’s engineering team can be informed and the source of the problem investigated.
A similar procedure will occur if the torque is outside the tolerance band width around a reference characteristic. The maintenance symbol will be displayed on the plant operator’s display and the service staff can be informed accordingly about the appropriate measures to be taken.
Asset management functionality has had a strong influence on the development of AUMA’s new range of Generation .2 actuators and controls. With the new product series, the company has provided an actuator solution that can monitor actuator-specific service conditions and limits.
Additionally, the indication of events according to NE 107 categories is implemented to help prevent actuator failures and downtimes. In a next step, a continuously available ‘maintenance key’ will be defined to supply information on the actuator status. The available data obtained from lifetime tests will be fed into formulas stored within the actuator firmware.
The actuation industry is embracing asset management to take data management beyond mere capture to analysis to achieve full plant control to pre-empt plant critical problems. A co-ordinated approach to the subject is emerging and disciplines, including compliance to defined schemes, are being implemented. The power sector, supported by forward-looking manufacturers of field devices, is moving to a new era of integration with regard to its plant systems.