The real “chink in the armor” of the pneumatic dynasty is positioning performance: pneumatics (air) is a compressible substance that will limit the positioning performance of the actuator. Boyle’s Law in physics simply states that P1 x V1 = P2 x V2. There is no getting around that reality; but think what that means in relation to static and dynamic forces on actuator stems.
Effectively, what’s taken place gradually over the last 20 years is emergence of a viable alternative to both electromechanical and pneumatic actuation. By modernizing the solid, but obsolete hydraulic-based actuation technology, with performance more in line with market needs, new application opportunities have opened up for use of discrete electrohydraulics. Some interesting applications include: steam temperature control or spray valves, boiler feedwater control valves, power plant damper controls, mining separator level control valves, water treatment plant filter level control valves, and more.
Such applications have historically been pneumatically or electromechanically actuated. The advantages of electrohydraulic actuators were outweighed by the capital and maintenance expense of older hydraulic actuator technology. With advances in electrohydraulic technology, there are now applications that can benefit economically from electrohydraulic through process efficiency improvements.
The conventional hydraulically operated valve (HOV) and electrohydraulic actuator (EHA or EHV) applications are also being fit with newer discrete hydraulic actuators. These applications historically were specified HOV, EHA or EHV for one of three reasons:
- The control requirement was known to exceed the capability of compressible pneumatics (i.e. steam turbines).
- The torque was too large for pneumatic or electromechanical actuator.
- Compressed air was not available.
Today, these applications are being retrofit with today’s discrete high performance electrohydraulic actuators to gain the benefits mentioned above.
The level of requirements for actuator sophistication do vary greatly across applications. Some specification breakpoints to consider when evaluating electrohydraulic actuator technology versus other technology are: valve operation (on/off, modulating, fail-position); communications requirements (feedback, bus networks); power consumption/availability; and the critical nature of the location and application.
This last consideration can be construed many ways, but a general way to put it is: How critical is the availability of a valve to the process? Is it acceptable to take this valve out of service annually? Is the location an environment that is easily accessed, inhospitable or hazardous?
The answers to these questions are vital to the type of electrohydraulic actuator technology selected. The answers can also influence which overall technology to select—whether it is pneumatic, electromechanical or electrohydraulic.
Kevin Hynes is president and CEO of KOSO America (www.koso.com). Reach him at khynes @rexa.com.
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