SubscribeThe valve actuator plays a key role in the reliable, efficient and safe operation of water and waste treatment processes, particularly now that plants are predominantly automated and computerised.
The duty of the valve actuator can be divided into three areas:
- Valve operation
- Communication with the controller
- Diagnostics
Using the Rotork IQ intelligent electric actuator as an example, we can see how the advent of the digital age has increased the functionality and usefulness of the actuator in all three areas.
Valve Operation
The basic function of the actuator is to operate the valve mechanically. Simplicity is the answer to achieving long term reliability here, in Rotork's case utilising a single worm and wheel arrangement in an oil bath.
Most valve actuators are inactive for over 90% of the time and during these periods their function is to communicate the position of the valve and their availability for operation. However, when commanded to change the valve position the actuator must respond with accuracy and reliability, especially since some valves only open or close for safety reasons and others are required to be precisely positioned to correct the flow in the process stream.
It is also essential that the actuator communicates a signal to confirm that the requested action is being performed, or generates an alarm if it is not or cannot be completed. All these functions are generated by the control and instrumentation elements inside the actuator, which must be protected from the outside environment to ensure long term reliability.
The majority of valve locations are exposed to environmental challenges to a greater or lesser degree, often involving the effects of water - spraying, flooding and even submersion. It is essential therefore that the electrical components in particular are protected from the corrosive effects of the outside atmosphere. Rotork utilises 'O' ring seals and the actuator's terminal compartment is separately sealed (double-sealed), so that even with the terminal cover removed water cannot reach the internals of the actuator.
Reliability is further enhanced by the 'non-intrusive' actuator design pioneered by the IQ actuator design. With this technology actuators now use an intrinsically safe, hand held setting tool - rather like a TV programmer - with a secure, dedicated wireless infra-red link to perform all the switch setting and commissioning functions that were previously only achieved by removing electrical covers.
There are significant advantages inherent in this technology. Using the menu on the actuator display screen it is quick and easy to commission the valve, in any weather - day and night - without jeopardising the actuator's integrity and with complete safety even in hazardous areas. The same tool may also be used to download this data and upload it to other actuators with similar commissioning requirements to simplify and speed up the operation, or to download actuator operating data for analysis, as explained later.
'Non-intrusive' commissioning and communication would not be possible using conventional electro-mechanical switch mechanisms, limit switch counters and torque sensing devices, so it is the replacement of these devices with solid state electronic developments that has facilitated this advancement in reliability and user-friendliness.
Communication with the controller
Digital technology and the advancement of solid-state electronics have given the valve actuator the ability to store and communicate a proliferation of control and indication data. Traditionally, communicating this data to the control room was performed by 'hard-wiring', whereby a separate wire or two was required for each control or monitoring function. Typically, up to twenty-six separate terminal connections were necessary on an electric actuator, but the development of digital bus control technology has radically changed the situation.
Digital bus systems replace the multitude of conventional hard wired control cables with a single loop or strand of two or four wire cable linking multiple numbers of actuators together. Basically, each actuator is given a unique address code in order to recognise or ignore the data instructions that are transmitted along the bus from the controlling element. Furthermore, digital communication fully exploits the information gathering potential inherent in intelligent actuators whilst enabling rapid plant upgrades and simplifying the work involved in altering plant functionality. Engineering work is greatly simplified and commissioning times are reduced.
The last two decades have witnessed the development and introduction of many public or 'Open' digital systems including Modbus, Profibus, DeviceNet and Foundation Fieldbus. These various standards have been passed into the public domain, documented and controlled by the IEC and other authorities in the USA, Europe and other parts of the world. The advantage promoted by open systems is the ability to mix and match equipment from several different manufacturers on the same bus highway. For example, a Profibus network will often support actuators, flow sensors and motor drives on the same data highway.
There are, in addition, a number of proprietary systems that are designed for a particular item of plant equipment, an example of which is Rotork's Pakscan. The advantage of a proprietary system is that the network can be optimised to exactly match the needs of the elements attached to it. For example, Pakscan can cope with extremely long data highways - up to 20km - without the need for any repeating devices, facilitating installation in the spacious environments of large sewage treatment plants where actuators are typically used.
Pakscan is also able to cope with the removal of power from any of the actuators on the system at any time, and protects against cable failures by providing dual communication paths (both directions) at all times. Open systems can do this, but only if they are extensively modified from their existing design.
There are therefore many factors that need to be taken into account when deciding between the use of a proprietary or open system of digital control. These are often predetermined by industry preferences. For example the water and waste treatment industry has adopted the Profibus protocol in many areas and Pakscan has found favour in a large number of tank farm installations that have adopted two-wire digital control technology. In all cases, using the technology has improved functionality through the ability to both control and receive/exchange/update operating data with the valve actuators in the plant.
Diagnostics
Unlike valve operation and communication, diagnostics is an area of valve actuation functionality that has been introduced, rather than enhanced, by digital technology. In Rotork's case the inclusion of a data logger within the actuator enables an event-by-event history of valve activity to be generated, including the torque profiles produced during each opening and closing of the valve. These can be compared with the valve torque signature profile logged during the commissioning process to identify the trend of valve operating wear.
Using the hand held setting tool, the user can extract and store this information, which can be downloaded and viewed on a PC running Rotork IQ-Insight software. Alternatively, a laptop running the software an be directly linked to the actuator on site. The data collected is analysed to identify potential valve problems, tight spots and changes in the torque profiles which enable preventative maintenance to be planned and unexpected interruptions to plant operation avoided. These abilities also enable plant utilisation to be optimised by the implementation of 'just in time' valve maintenance schedules.
The two case studies below illustrate how users can quickly and conveniently investigate the operation of actuated valves for purposes such as preventative maintenance, fault diagnosis and asset management.
In the first, a plug valve operated by an IQ actuator is displaying an intermittent failure to fully close.
Analysing the data logger event log on IQ Insight confirms that the actuator sometimes torque trips before reaching the fully close position. The screen shows a torque trip event (Event 177) on 18 February 2006 at 00.49 hours.
Analysing the data logger torque log on IQ Insight shows the valve torque profile. The relatively high closing torque profile (green) indicates a valve problem, confirmed by the valvemaker.
After draining the plug valve body cavity the valve was stroked again and the data logger reading taken. The new closing profile proves that the operating torque has dropped significantly from the previous operation, as indicated by the torque reference profile shown as a blue dotted line on the graph.
The second case study involves an IQM modulating actuator on a steam control valve in a power station. Valve and actuator operational parameters are investigated for process control analysis and maintenance requirements.
Analysing the data logger starts log shows the opening and closing operation (starts) against the valve position. This indicates where the process control requires the valve to be positioned by the modulating actuator over the operational cycle of the boiler unit.
The statistics tab of the data logger shows that the actuator has completed over 74,000 close positioning starts and over 66,000 open positioning starts in the nine months since installation. This is well within the maintenance cycle life of the actuator, indicating that it can continue without requiring attention.
The actuator configuration is downloaded and saved to allow its type and setup to be recorded as part of the station's asset management system. The file can be viewed for maintenance purposes and uploaded to replacement actuators to replicate the setup configuration.