Keeping it simple

Pneumatics are a more simple alternative to electric drives. Martin Hunt of Festo looks at the case for pneumatics, and analyses some example applications

Electrical drives are traditionally used for process valve automation in the water and wastewater sector. Modern pneumatic actuation offers a simple and economic alternative for some common applications - particularly where valves are sited in Atex zones or are prone to submersion, and are used for precision flow control or very high-duty cycle operations.

Rotary and linear actuators are used to drive butterfly and ball valves, gate valves, and penstocks. Depending on the process, actuation requirements are usually either a simple fixed on/off operation or a control mode where speed control is employed to deliver a timed on/off valve cycle. Electric actuators employ quite different designs for on/off and controlled cycles. Conversely, the same pneumatic actuator can be used for both modes, with adjustable timed operation available by simply restricting exhaust air in conjunction with an electro-pneumatic controller.

With an open/close time of two seconds for a 200Nm knife gate valve, the pneumatic actuator is faster than its electric drive counterpart, which has a typical time of 30sec due to its speed-reducing gearbox. Electric drive actuators can have 50 or more components and require regular planned maintenance. As well as a single- or three-phase supply, the electric actuator can require up to 12 control cables to cater for temperature detectors, heating elements, torque switching, limit switches and indicators. Specialist installation is required and troubleshooting usually necessitates dedicated equipment.

In comparison, pneumatic actuators have far fewer components, require only semi-skilled installation, have simple troubleshooting procedures and require almost zero maintenance. With no high voltage required, the system is intrinsically safer. In addition to a 6 bar, ring-main air supply, full control and diagnostics can be achieved with a two-core cable for 24V DC supply and a six-core cable for limit switches and solenoid valves. System upgrade and scalability is easily achieved with pneumatic actuator systems where individual valves can be simply upgraded to control mode variants, and additional valves may be added without costly downtime for high-voltage electrical isolation (no hot work permits required).

Duty cycle times for electric drive actuators can be as low as 25%, and motors may stall on overload and over-temperature, particularly in unfavourable climatic or environmental conditions. In comparison, pneumatic systems can be safely and reliably overloaded by simply increasing the air pressure - with the added advantage of a 100% duty cycle rating.

Pneumatic actuators are also particularly suited wherever valve modulation is required. Their intrinsic speed of operation (operation times of less than 1sec for a full stroke for example) can help to control flow precisely. They may equally be configured for slow operation to prevent water hammer - by the simple means of controlling air exhaust rate, or even a servo pneumatic control system if the application demands it.

Considered from a life-expectancy point of view, pneumatic actuators offer an estimated life of more than 2,000,000 cycles. For electrical control failure conditions, pneumatic actuators may be configured to run in safety modes for open position, closed position, or stop in current position. This is considerably more difficult to achieve with electrical drives. It is also very easy to interlock two pneumatic actuators for synchronous or asynchronous operation without complex control.

Wastewater treatment areas usually mandate at least an IP67 ingress protection rating, necessitating sealing rings for electric actuators. Pneumatic valve actuators are inherently rated for IP68 (submersion), and the natural pressurisation resists the ingress of contaminants.

Certain areas of wastewater treatment such as anaerobic processes demand full Atex explosion-proof certification, pneumatic drives require no electrical power connections, and the standard pneumatic alternatives are Atex-compliant for Zone 2 GD (Gas and Dust) applications. Condensation formation inside electric drives can become problematical in sub-zero temperatures and necessitate constant heating and monitoring. Undried air can pose an icing problem for pneumatic systems operating at sub-zero temperatures, the solution involves air-drying equipment.

Valve terminal in pneumatic systems using Profibus-DP and other fieldbus control solutions are used to control groups of up to eight actuators, sharing a single fieldbus interface card. Valve terminals for pneumatic actuators also have a facility for localised manual control. Push buttons may be used to select a specific valve actuator and perform override functions. The terminal can also be configured to directly interface Profibus-DP to Hart protocol diagnostic equipment, as well as control pump motors and other electric devices.

Among the common applications for pneumatic actuation are valves in Atex areas such as gas chlorination, methane recovery and sludge fermentation, valves in underground channels or pits prone to flooding, and valves used for flow control - for all sizes of requirement from small units in filter beds to large butterfly valves on works outlets to ring mains.

A recent application that illustrates this capability is Ashford Common. It is one of the larger treatment plants, clustered on the Thames above Teddington Weir. It is supplied by five fixed-speed pumps, and employs a 1,600mm butterfly valve as a gravity bypass and to control flows onto the works. The valve is located halfway down a 30m shaft.

Towards the end of last year, Thames Water commissioned a replacement for this main gravity valve because the existing electrically actuated required frequent movement to provide adequate flow control. Although numerous attempts had been made over the years to optimise the valve's performance, it still required actuating every 10min. Also, the valve constantly oscillated under load conditions and required frequent servicing, which was proving inconvenient and costly, due to its relatively inaccessible location.

The valve manufacturer Erhard came up with a suitable alternative - choosing to base the new valve design on a pneumatic actuator. This offered a much more robust control solution, with minimal maintenance requirements. The valve's pneumatic actuator is a custom size produced by Festo, which is coupled directly to the butterfly vane without gearing or spindles. The system is controlled by a remote PLC, with local operator override facilities for routine inspection.

Pneumatic actuation technology offers the water and wasetwater industry a valuable range of performance characteristics that can deliver considerable benefits.

Faster actuation times and pneumatic actuation's intrinsic overload capability, can also reduce the force requirement for a given operation, allowing more compact processing stations to be built. Pneumatic actuators such as the Copar range comply with the ISO 5211 standard for process valve interconnectivity, providing interchangeability with electrical drive solutions and other actuator brands.

It needs to be noted that the pneumatic solution requires air-processing services. But as long as there are a number of actuators on the network (the break-even point is typically around five), then considerable capital cost savings can be made. Festo estimates these at 20-30% in its target application sectors.

For some applications - such as large flow control valves and Atex zones (as cited here) - the particular constraints mean that pneumatic actuation could be preferable even for quantities of one. Substantial additional cost benefits in the areas of plant design, installation, maintenance, and lifecycle costs can also be made.

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