FDT takes the ‘guesswork’ out of paper machine maintenance
12 June 2009
Implementing a condition-based monitoring system with FDT/DTM technology gave these Danish paper machine engineers information they never had before.
Paper manufacturer Dalum Papir A/S in Odense, Denmark, uses the Metso Paper machines PM 6 and PM 7 to produce high-grade office and printing paper of 100 per cent recycled paper. Since 1999 the mill has been gradually modernised to boost its efficiency. The key measures include the change from time-based to condition-dependent (preventive) maintenance of the PM 7 through the introduction of FDT/DTM technology.
Paper production consists of a chain of mechanical, chemical, and thermal processes. The ‘paper- making machine’ is at the very end of this process chain. Its job is to create the final product—paper—from the aqueous suspension of fibrous material known as pulp. The individual steps in this process take place in rapid succession (see Figure 1).
In the pulp feed section the pulp is applied evenly to the endless moving wire screen, where its fibres become felted to form a wet web.
In the screen section excess water runs off through the moving screen. The felted web is passed into the wet press, where the water content is further reduced at about 50 per cent.
In the drying section the remaining moisture is drawn out of the paper web by up to 100 steam-heated drying cylinders.
MAINTENANCE STRATEGY WITH FDT
Paper machines work at extremely high speeds. The PM 7 in Dalum, for example, operates at 750 metres per minute, which corresponds to 45 km/h. It has an hourly paper output of 15 tonnes.
Because of its high operating speed and the various subprocesses taking place within the plant, stopping and restarting a paper machine is usually a critical process. It is therefore vital to minimise the number of planned standstills and as far as possible, prevent unplanned downtimes altogether. For the PM 7 a standstill of five hours incurs costs of about € 50,000.
A planned stop is scheduled every three weeks for cleaning and replacement of faulty or worn parts and subassemblies. Previously this was time-consuming because no information was available about the actual condition and operability of the parts, so that replacements had to be made based on ‘educated guesses.’
The aim in retrofitting the PM 7 was to create a system for real-time monitoring of device conditions. This was an essential step towards an effective preventive maintenance programme. A key element of this maintenance concept was a change in strategy away from time-driven and towards status-driven scheduling (see text box Condition Monitoring). FDT/ DTM technology offers an excellent solution for this task.
The PM 7 is controlled and monitored using five control systems with four operating stations. Several hundred process devices (valves with positioners, sensors for flow, pressure, fill level and moisture, etc.) are connected to remote I/O in close proximity to the machine.
The machine’s drying section contains about 150 control valves and positioners (Figure 2), which control the steam for heating the drying cylinders. The devices support device diagnostics and the HART protocol. They possess the device drivers (DTMs or Device Type Managers) needed for FDT.
Because the existing remote I/O is not capable of transmitting HART signals, a separate Pepperl+Fuchs HART multiplexer was fitted to get the HART signals independently of the control system. The Endress+ Hauser Fieldgate FXA520 provides the connection to the company’s intranet and FieldCare plant asset management. Because FieldCare is FDT-based, third-party devices can also be fully operated through DTMs.
The implementation consists of the following components:
• HART multiplexers provide access to the HART data from intelligent field devices and form an independent service layer which enables the transmission of digital information.
• Fieldgates poll field devices for readings, configuration parameters, and diagnostics and activity data. They manage this information and—through a connection to a higher-level Ethernet network—make it available to a wider user community, for example the control room and maintenance.
• FieldCare is an FDT-based software application that offers an open solution for device and plant administration. FieldCare can communicate with all plant devices, from any manufacturer. Instrument Condition Monitoring allows diagnostic information to be output at any point within the user network, for example in the control room and the maintenance workshop. The real-time data flow from the devices is displayed with a colour-coded alarm system and selected status messages (OK, Warning, Alarm, Unknown). This allows faults to be detected and preventive maintenance activities to be planned on time.
• A DTM contains all device features—both the standard configuration parameters and the device-specific functions. FieldCare and other Frame Applications display these DTM contents and the data associated with them in a clearly structured form.
The volume and type of transmitted data naturally depends on the scope of the devices’ diagnostic properties and on the subset of properties described in each device DTM.
Figure 3 illustrates the data flow of the PM 7’s retrofitted asset management system, which allows a condition-based preventive approach to maintenance.
It has many long-term benefits for the user:
• Continuous insight into the current state of valves under actual process conditions as basis for preventive maintenance;
• Acquisition and visualisation of trends to allow on-time detection of faults;
• Information output at any point of the plant for remote measures;
• Cost reduction through cutting or preventing plant downtimes;
• Access to highly specific device functions; and
• Especially suitable for retrofitting existing plants, such as the PM 7.
—Sandra Gisy Endress+Hauser Process Solutions and Vagn Kudsk, Dalum Papirfabrik
SIDEBAR BOX: CONDITION MONITORING
To optimise plant maintenance, many industry sectors today use a condition-based rather than a time-based asset management. Essentially, this means that parts are no longer replaced at fixed service intervals or after their vendor specified service life, but based on the actual current or forecast plant condition. This means that machine parts or devices have to be replaced only when a malfunction or breakdown is imminent. This requires reliable condition monitoring of the plant and its components. This condition driven maintenance strategy aims to utilise components’ service life as fully as possible while at the same time preventing unforeseen standstills.
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