Feature

Pneumatic Advantage in Water Treatment

Posted on 01 February, 2010 | Tags: Pneumatic Technologies

Pneumatics presents a valuable and extremely reliable alternative to electrical automation systems, especially when it comes to meeting the requirements of water and wastewater treatment. Many system planners favour pneumatic drives for opening and closing gate for process valves because of their durability, long service life and safety under overload conditions.

- Armin Muller

Water is a precious commodity all over the world, one which humanity needs to be even more careful in using as consumption levels rise. Industrial automation of potable water and wastewater treatment plants ensures customers have access to this high quality resource, whenever it is needed.
Pneumatics presents a valuable and extremely reliable alternative to electrical automation systems, especially when it comes to meeting the requirements of water and wastewater treatment. Pneumatics, one of the indispensable core technologies for automation, has the additional advantages of robustness and cost- effective system solutions.

Automation In Water Treatment Plants
A pneumatic automation system reduces the costs of investment, installation and operation compared with conventional electrical installations. Many system planners favour pneumatic drives for opening and closing gate process valves because of their durability, long service life and safety under overload conditions.
Compressed air provides a high degree of safety, since it performs opening and closing movements without any need for electrical power. This means that in practice the system is easy to operate and not prone to failure, resulting in excellent system availability. If one or more process valves are required for dosing a flow of material into the system, the pneumatic actuators are also capable of advancing the process valves to intermediate positions. So- called position dependent actuators are used to this end.
In addition to the actuator itself, actuator systems of this sort also include the following function and modules

• Electro-pneumatic positioning controller
• Displacement encoder for actual position
• Limit switch for sensing the drive's end-position
• Tubings and fittings
• Complete air preparation unit with pressure sensors
• Modular valve terminal solution with fieldbus

connection or complete control cabinet solutions (Figure 2)
The most important components of a water treatment plant are the filters. They're used to treat and filter raw water, thus processing it into drinking water. Regardless of whether closed or open filters, or ultra-filtration systems are used, the processes are automated.
 The butterfly valves are equipped with pneumatic actuators to this end and are controlled by the valve terminal. The filter's fill-level, differential pressure and flow rate are defined by its current status.
These measured values are also acquired at the valve terminal. If a filter needs to be backwashed, the PLC controls the valve terminal and thus the order in which the butterfly valves are actuated, in accordance with the backwashing programme. After the back-washing programme has been completed, normal filter operation is once again initialised. The valve terminal is configured in accordance with the number of pneumatic actuators, limit switches and measuring instruments. The valve terminals may be fitted in a modular structure, depending on the number of valves.
This system solution has just one clearly defined and standardised interface between the control system (PCS) and the valve terminal - the fieldbus.

Pneumatic Actuators
Pneumatic actuators require little or no maintenance throughout their entire service life. With a service life in excess of a million operating cycles, pneumatic actuators outlive any device. Since the butterfly valves in a water treatment plant are only operated occasionally, their safety under overload conditions is of great importance.
This is because sediments or incrustated material can result in a steep increase in breakaway torque. Pneumatic actuators are 'overloadable', while electric actuators trigger the torque limiter when torque or the need for power increases. With pneumatic actuators (Figure 3), increasing the operating pressure and forces and thus torque is a simple matter. Since most devices in water systems are operated in open/closed mode or are designed as manual controls, compressed air systems offer significant potential for rationalisation.
In particular for economical and technical reasons, compressed air should only be purified to the extent necessary for the respective application. Preparation must be executed such that long service life and error-free operation of the pneumatic control components are not impaired.
In each individual case, the manufacturer's specifications regarding compressed air quality must be adhered to for the operation of pneumatic control components. This is the only way to assure error- free operation, and to assure long service life for the utilised components.

Diagnostic Concept
With regard to total cost of ownership (TCO), reliable system operation is more important than the initial investment, ie system construction up to the point in time of commissioning. Just how successfully the 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 production process. Possible diagnostics concepts can be subdivided into various stages in this respect:
Stage 1: Simple monitoring functions (detection of malfunctions)
Stage 2: System diagnostics (localisation of faults within the system)
Stage 3: Subsystem diagnostics (error identification)
By incorporating just a few pressure and flow sensors into the pneumatic control loop in a manner which is suitable for the respective requirements and by taking advantage of existing electrical control signals, conclusions can be drawn regarding the status of the system by simply comparing setpoints and actual values as in  Stage 1.
Stage 2 involves the evaluation and automated interpretation of process parameters from stage 1. A diagnostics controller which can be utilised in a decentralised fashion, as well as model- based software solutions, evaluate and interpret the data. 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. The foundation for preventive maintenance is thus laid.
Stage 3 is above all advisable for especially critical process functions. Additional sensors monitor critical drives and detect, for example, mis-adjusted flow control valves, worn seals and pinched supply lines. Evaluated diagnostic information can be visualised at master control systems. Required action can then be implemented in a targeted fashion and carried out effectively.
Pneumatic Open-Loop Control
The typical open-loop control in a pneumatic system consists of the air supply (air preparation with filter and pressure control), the solenoid valves/ valve terminals as control element, the exhaust throttles to control the actuation velocity and the actuators (linear cylinder or quarter-turn actuator).
As signals normally the control or limit switch signals are available. For process-sensitive actuators the position-indication sensors, pressure or volume flow sensors are applied. With this open-loop control, several faults can be classified.
The pneumatics itself are very rarely the cause of a system failure, errors in the process, communication errors or sensor failures are more frequently. Errors in the process are often becoming evident in the system behaviour of the pneumatic system, so the pneumatic actuator is applied as a sensor for the process diagnostics, whereas failures in the pneumatic system are manifested in leckages or changing actuation velocity.
The analysis of the monitoring of a Scotch-Yoke- Actuator shows the diagnostic concept. For the diagnostics the pressure difference of both pressure chambers is evaluated.
When the actuator has reached the end position, one chamber is exhausted, in the other chamber supply pressure is determined, so that the maximum pressure difference can be measured. If the actuator is moved, depending on the required force, a characteristic pressure profile is monitored (Figure 4) dependent on the valve type and the pressure/volume flows.
The difference between the pressure profile during the movement and the maximum pressure difference is a degree of the torque reserve of the actuator and can be used for diagnostics. (trend monitoring or defintion of a minimal torque reserve).
In addition to that the pressure profile is an indicator of the closing time of the process valve. And as a final result the position of the process valve is indirectly determined with the pressure/volume flow signal and helps to identify leckages within the actuators.

Crux Of The Matter
Valves and drives have mechanical parts which are subject to wear and must be serviced or readjusted at regular intervals for this reason.
Preventive maintenance functions 'remind' the operator of the respective maintenance intervals in a timely fashion or are transmitting warnings via e-mail or SMS.
A consecutive switching cycle counter is normally integrated into the electronic module for each solenoid coil to this end, which generates a message when the selected setpoint has been reached. An integrated diagnostics memory saves these messages along with a time-stamp.
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 wasn't detected until the individual components failed.

Offsite Diagnostics
State-of the-art systems can be monitored from the office and configured and diagnosed via Internet using the established direct Ethernet interface for the communication between automation and information technology.
The integrated direct communication not only makes it possible to network automation components amongst one another, it also makes all common IT services accessible to system operators:
This allows the operator to keep an eye on their systems from anywhere via remote maintenance, and to change configurations by means of online access.

Contact details: Shashank. M. Dangi Manager - Marketing  S_M_Dangi@IN.FESTO.COM