Hydraulic Actuators Gaining in Use

Pneumatics have been the mainstay of control valve manufacturers for the better part of the last century—just imagine how large the installed base has grown to be. Electromechanical designs, on the other hand, date back almost as far as pneumatics, but have only recently been seriously considered for throttling control applications.

But what about hydraulics? The basic technology also has been around a long time, but the technology was never considered mainstream because of the complexity, maintenance requirements and costs. Still, some people argue that today, hydraulically driven actuators are among the highest performing modulating designs with the widest range of thrusts and torques. And, over the last 20 years, advances in hydraulic actuator technologies have opened doors to applications previously categorized as either pneumatic or electromechanical opportunities.

THE DIFFERENCES

So, what changes have we seen over the last few decades? Here are some thoughts on that issue.

Operation has changed from “continuous” to “discrete”: Conventional hydraulic technology, whether a central hydraulic reservoir power unit or a valve-mounted individual reservoir, is based on a servo system that uses a continuously running unidirectional motor and pump. This technology has superior frequency response (small spool valves for directional changes) and offers extremely tight positioning control with high available stroking speeds. A potential drawback is that these systems use gravity-fed reservoirs that communicate with the atmosphere. This can lead to ingress of condensate and particulates, requiring significant filtration and maintenance.

Discrete operating technology utilizes a bidirectional motor and pump in a truly self-contained, sealed hydraulic system. This eliminates the need for an active reservoir. It also means the oil volume is a small fraction of what would be required for a continuous system. The motor and pump operate discretely, only running when required to make a position change. Power consumption is lower than any other control actuator technology.

While the frequency response available with a discrete system is high, it can’t compare to the continuous system. However, the hydraulic fluid filtering and maintenance are eliminated, and the positioning accuracy and control remains as good as a continuous system.

“Self contained, discrete” technology has gained more acceptance in the control actuator market: What this really describes is a form of electromechanical actuator with a hydraulic transmission instead of a gear train. Electromechanical actuators use reversible motors and operate discretely (just like the discrete type of hydraulic actuator). Most traditional electromechanical actuators are designed for isolating duty and as such lock in position and power down when set point position is satisfied. Unlike the self-contained hydraulics, traditional electromechanical actuators are usually limited to fail-in-last-position on loss of motor power. Some newer designs of electromechanical actuators can replicate the functionality of the hydraulic or pneumatic actuators; however, most traditional electromechanical actuators have limitations on duty cycle and life cycle when compared to hydraulics or pneumatics.

A COMPARISON

So what does the self-contained discrete hydraulic technology have in common with pneumatics? Pneumatics are revered by most control valve manufacturers as the foundation of control valve positioning. They are the puppet controlled by the puppet master, which is the smart positioner. Pneumatics have been proven performers, with about 100 years as the staple of control valve actuation. That’s why it’s hard to stack up this younger, self-contained discrete hydraulic technology against the legend that is pneumatics.

However, both types are capable of continuous movement. They both have a variety of failure modes—through the use of simple springs or stored energy devices such as accumulators and volume tanks. However, pneumatics is more akin to conventional hydraulic technology regarding the power source. Just as a conventional hydraulic uses a continuously running unidirectional motor and pump to maintain pressure, the pneumatic has stored energy (supply pressure) from a central air compressor. Both approaches consume much more power than either the electromechanical or the discrete electrohydraulic technologies.

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.

AN ALTERNATIVE

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.