Latest valve tech saves companies cash

Modular electrically controlled solenoid valve terminals can save huge amounts of money in treatment plants. But, writes Martin Hunt of Festo, the benefits of these latest-generation terminals encompass more than just capital expenditure

Water and wastewater treatment plants are potentially some of the largest benefactors of the cost savings that can now be made by using modular electrically controlled solenoid valve terminals to actuate pneumatic process valves.

By adopting this integrated system approach, instead of using individual solenoid control valves, operators can implement decentralised control schemes very cost-effectively.

But the benefits of these latest-generation fieldbus-equipped solenoid valve terminals go way beyond reducing capital investment. They also enable operators to shorten plant commissioning times, and to implement advanced, internet-based, monitoring and remote diagnostics facilities to reduce maintenance costs and to all but eliminate unscheduled system downtime.

Significant cost savings are possible with integrated system solutions, compared with individual solenoid valve solutions. This applies in particular to installations that have a large number of solenoid valves and process valves positioned very closely to each other, such as in filter systems and in the putrefaction basins of community wastewater treatment plants.

But it is important to choose a system supplier with a complete product portfolio. Festo, for example, produces automation and integrated system solutions encompassing everything from the industrial process valves right up to the fieldbus. These include:

  • Process valve actuators with position feedback
  • Fieldbus connectivity
  • Tubing and connectors
  • Compressed-air preparation units (including sensor technology for monitoring consumption and operation)
  • Modular solenoid valve terminals with fieldbus connection, or complete control cabinet solutions
  • Technical advice on commissioning, installation and compressor packages

The solenoid valve terminal is the key element of an integrated system solution – it allows for the quick and easy set-up of decentralised automation. The Festo CPX-MPA solenoid valve terminal is completely modular. It can control the coils of up to 128 solenoid valves, process digital as well as analogue electrical signals, and communicate with PLC or standalone master automation systems via a fieldbus. In addition to this, the solenoid valve terminal can be equipped with a controller, so that portions of the system can be operated independently of the process control system.

These elements make it possible to create custom-tailored, consistent automation concepts, from the process valve right up to the control level. But it is important to use components that have been carefully matched to each other to ensure that all of the elements within the control sequence function ideally.

Problems are often caused by minor details. For example, signal transmission is delayed if tubing or connectors are used that have diameters that are either too large or too small, or if they are made of materials that are not suitable for the application.

Although cost differences with regard to initial investment may be minimal, incorrect planning may become very expensive during operation of the system if replacement should become necessary.

The integrated system solution has only a single, clearly defined, standardised interface from the solenoid valve terminal to the fieldbus – for example, Profibus DP. There are no interfaces to the industrial process valves because Festo co-operates closely with the industrial process valve manufacturers and is able to implement this interface, both technically and commercially, on its own. The advantages of a system solution for decentralised automation include:

  • Flexibility thanks to the fieldbus, facilitating integration and expandability
  • Time-savings resulting from simultaneous engineering of assemblies
  • Improved reliability due to previously, autonomously tested, assemblies
  • Reduced installation expense
  • Maximised system availability through better diagnostics

Commissioning is often neglected during the planning phase. Take a piping installation for a filter system in a waterworks, for instance. In the event of dry commissioning with individual process valves, the entire system is wired and connected with tubing.

Compressed air is applied and the solenoid valves are actuated with the manual override, in order to make sure that the individual process valves have been correctly connected. In the case of the system pictured, many of the process valves are difficult to access – often requiring use of a ladder – and the allocations list cannot be checked on-site.

A similar situation often exists in wastewater treatment plants. With the pipe gallery of two putrefaction basins where sludge treatment takes place for a community wastewater treatment plant pictured, there are a lot of pipes with process valves underneath the ceiling.

In the case of this particular application, control cabinets with solenoid valve terminals have been installed instead of individual process valves.

The control cabinets can easily be accessed by the operators, resulting in an enormous reduction of distances that have to be covered during dry commissioning. In addition to this, it is easy to implement a systematic commissioning procedure.

Process valve position is indicated by means of travel limit switches – the signals are connected to the I/O modules at the left-hand side of the valve terminal. Each input and output signal (for example from the solenoid valve as well) is displayed directly by means of light-emitting diodes (LEDs).

Complex systems

An additional advantage offered by solenoid valve terminals is the fact that function modules or portions of the system can be grouped together and represented within the solenoid valve terminal. This makes it easier to automate complex systems in a clean-cut, modular fashion, and provides the operator with clear and unambiguous system-wide visibility.

With regard to total cost of ownership, reliable system operation is more important than the initial investment, i.e. system construction up to the time of commissioning. Just how successfully the integrated system concept has been implemented does not become apparent until it is placed into operation. With the help of a consistent, well functioning diagnostics concept, users are able to reduce unscheduled downtime for the process. Possible diagnostics concepts can be subdivided into various levels in this respect:

  • Level 1: Simple monitoring functions (detection of malfunctions)
  • Level 2: System diagnostics (localisation of faults within the system)
  • Level 3: Subsystem diagnostics (error identification)

By incorporating just a few pressure and flow sensors in the pneumatic control loop, and by taking advantage of existing electrical control signals, it is possible to provide operational feedback on various system parameters – such as the open/close status of individual process valves and flow modulation – simply by comparing set-points with actual values (level 1).

Level 2 diagnostics involve the automated evaluation of process parameters from level 1, conducted locally using a control panel or some form of handheld diagnostics device, such as Festo’s CPX-MMI terminal.

Trends involving the change of process parameters can be recognised early by comparing targeted and actual values. On the basis of this knowledge, maintenance measures can be implemented before failure occurs, thus laying the foundation for true preventive maintenance.

Level 3 diagnostics use field-based sensors to provide feedback on critical process parameters such as a change of actuator speed or the time that it takes for a particular process valve to operate – both of which would be indicative of a potential problem. Additional sensors monitor critical drives and detect, for example, misadjusted flow control valves, worn seals, and deterioration of process valve performance.

Evaluated diagnostic information can be visualised at master control systems, and the required action can then be implemented in a targeted fashion and carried out effectively.

Latest-generation pneumatics products such as the CPX-MPA solenoid valve terminal concept – a combination of remote I/O terminal and solenoid valve terminal – opens up new horizons for modular diagnostic services. The CPX-MPA can be precisely custom configured so that only those diagnostic features that are actually required by the system are enabled at the modules.

These can include module- or channel-oriented diagnostics for the I/O, per-channel solenoid valve diagnostics, on-site plain text displays and an additional ethernet interface, as well as remote maintenance and e-mail or SMS alarms.

The CPX electrical section includes the digital and analogue I/O modules, fieldbus or ethernet interface and optional integrated, programmable signal pre-processing. The MPA pneumatic section consists of a modular solenoid valve terminal with variably selectable flow rate. Both systems communicate via a single internal bus. The overall system can be easily configured in an individualised fashion with the help of configuration software, and the entire solenoid valve terminal package is assembled, 100% tested and shipped directly to the plant, ready to install. The CPX-MPA terminal can be assembled in the field – it has IP 65/67 protection – and is connected to the master controller via fieldbus.

Together with the MPA solenoid valve terminals, the CPX modular electrical terminal increases system availability, by providing advanced maintenance and diagnostic capabilities. The MPA contains its own on-board ASIC (application-specific integrated circuit) and the electrical module allows for diagnostics for each solenoid coil. It is capable of monitoring coil current, as well as detecting over-voltage, short-circuits and line interruptions.

An integrated diagnostics memory saves these messages along with a time-stamp. The data can be interpreted in the field by means of a multi-colour LED on the solenoid valve or by using a handheld CPX-MMI terminal, as well as via the fieldbus or ethernet (web server).

Most process valves and actuators have mechanical parts that are subject to wear, and therefore must be serviced or readjusted at regular intervals. Preventive maintenance functions remind the operator of the respective maintenance intervals in a timely fashion.

The CPX-MPA is even capable of transmitting warnings via email or SMS; a consecutive switching cycle counter is integrated into the electronic module of the MPA for each solenoid coil, which generates a message when the selected setpoint has been reached. Operators can reduce their overall maintenance requirements significantly by using pneumatic cylinders and actuators instead of electrical drives.

For example, by using Festo pneumatic linear actuators for driving key process valves such as penstocks, the lead screw and drive nut – which constitute a weak link in the system because they often wear due to insufficient lubrication – are eliminated.

Integrated features

Further integrated features for preventive maintenance include diagnostics trace with error memory, as well as continuous monitoring of solenoid valve coil currents and voltages. Any increase in the occurrence of sporadic errors can thus be detected without delay. In the past, this usually was not detected until the individual components failed.

Systems can be monitored from the office, and configured and diagnosed via the internet – the CPX-MPA valve terminal, expanded to include the FEC front-end controller, establishes direct lines of communication between automation and information technology.

The integrated ethernet interface not only makes it possible to network automation components to each other, it also makes all common IT services accessible to system operators. This allows them to keep an eye on their systems from anywhere – facilitating true remote maintenance – and to change configurations online.

Furthermore, they can use online facilities such as an E plan to obtain manufacturers’ information about every device in the plant.

New applications can be conveniently downloaded via the internet. The integrated web server visualises diagnostics information and device status. Evaluation via TCP/IP ensures process data integrity, and ensures that operators are always kept up to date regarding exceeded limit values and other faults, thanks to email or SMS alarms.

Martin Hunt is industrial specialist at Festo. T: 01252 775000

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