Electraulic Actuators Optimize Valve Response
By Kevin M. Hynes, REXA/Koso America Inc.
The Limitations and Drawbacks Associated with Electromechanical and Electrohydraulic actuators
explain to some extent the amazing longevity of the pneumatic actuator. These same limitations also led to the development of “electraulic” actuation technology. These actuators are self-contained, pressurized hydraulic units, containing a minimal volume of conventional motor oil. They are being applied in numerous power plant applications where extreme service conditions, close control or high reliability is warranted. To appreciate the value of electraulic technology, it is important to understand the history of actuation technology and the emergence of the “smart” positioner.
Much has been said and written about the latest technology in field instruments, the “smart” valve positioner. Most major control valve manufacturers have introduced their version of these digital interface devices, which are designed to open communications between a distributed control system (DCS) and the final control elements. Product design has focused on establishing two-way digital communications that allow the DCS to monitor the function and “health” of the control valve. This facilitates pro-active response to potential maintenance problems, calibration and various other data gathering functions. All of these features have merit, but the question remains: “Do these devices actually improve process control?”
A process loop or, for that matter, an entire process plant, functions in ways analogous to living organisms. We have sensors, decision-making microprocessors and actuators to respond to the wishes of the decision-maker. Our actuators (muscles) control our physical response to the electrical stimulus generated by our decision-maker (the brain). The key word here is physical. Actuators perform physical work. They move an object against a load and position it in accordance with the instructions sent to it by a controller. The simple truth is:
Sensors (primary elements) do not control. They sense (see, feel, hear, smell, taste) the process variables, with ever-increasing sensitivity, and send this information back to the controller.
Controllers (or DCSs) do not control. They make decisions based on often-complex input and send out commands to a multitude of final control elements to physically adjust and control various process parameters.
Control valves do not control. They serve merely as an adjustable orifice in a pipe. The actuator makes the adjustments to these “slave” devices.
Actuators control the process. They are the muscles–the final control elements–in a process plant, responding to commands sent by the controller. The condition or “fitness” of these muscles determines their ability to control the process. Soft muscles respond slowly and with limited accuracy. Soft muscles often miss their target or get pushed around by opposing forces. Until significant steps are taken to improve the operating performance of the actuator, the major technological strides in instrumentation achieved over the last 20 years will remain under-utilized.
Pneumatic actuators, of the spring-opposed diaphragm or double-acting piston types, account for about 80 percent of all actuators currently installed and operating on industrial control valves. Pneumatically powered actuators have enjoyed this widespread acceptance for more than half a century. In this time, however, there have been only modest advances in the core technology. Control valve manufacturers market most pneumatic actuators as integral parts of the control valve, and have focused the lion`s share of their product development on issues relevant to the process engineer. An almost infinite variety of valve body and trim styles have been introduced to address concerns related to the peculiarities of the process or flowing medium. For example, valve styles address problems such as tight shutoff, erosion, corrosion, cavitation, flashing and noise. These are process concerns, not control issues. On issues that directly affect control, manufacturers have still opted to address them through modifications to the valve body and trim. Only after control engineers (not process engineers) finally realize that the actuator and not the interface device, or the valve, is the controlling element, will the demand be generated for a high-resolution final response to a high-reso lution digital control signal.
Poor control performance is inherent in the pneumatic actuator. The compressibility of air or other gas, in conjunction with friction, result in an unavoidable weakness in positioning. Friction force must be overcome for a valve to operate. This is not a major concern. The inherent problem is the significant difference in the coefficient of friction when the mating surfaces are static and when they are in motion. In effect, it takes more actuator output force to move a valve stem from a static condition. Once the static friction is overcome the resistance to movement drops, often dramatically. This will cause an inherent step change in valve position, often referred to as “stiction” or the “stick-slip” effect. The degree to which this occurs determines the actuator`s positioning resolution. Pneumatic actuators are particularly susceptible to stiction.
Consider a 50-in2 spring-opposed cylinder actuator with a 2-inch stroke and a 35 psig operating pressure. Referring to the figure, for the piston to move upward, the positioner must vent air from the cylinder until the pressure has decreased enough to overcome static friction. For this example, assume that the force differential between static and dynamic friction is 250 lb, corresponding to a pressure change from 35 psig to 30 psig.
1. The volume of air at 35 psig (50 psia) that must be vented is calculated as follows:
Change in volume = volume x [1 – (final absolute pressure/initial absolute pressure)1/k ]
where k = the ratio of specific heats = 1.4
Volume = 50 in2 x 2 inch (stroke) = 100 in3
Change in volume = 100 x [1 – (45/50)1/1.4 ]
Change in volume = 7.3 in3
2. The time required to vent this volume at 60 F (520 R) through a typical positioner exhaust port (capacity) is calculated as follows:
Time (sec) = change in volume / [(5885 x CvCf)/(temp)1/2 ]
where 5885 = a consolidation of constants, and CvCf , the output capacity index, is 0.2.
Time = 7.3 / [(5885 x 0.2)/(520)1/2]
Time = 0.14 seconds
This value represents the inherent lag time introduced by the compressibility of air.
3. The piston “jump” is calculated as follows:
Jump (inches) = change in volume / piston area
Jump = 7.3 in3 / 50 in2
Jump = 0.15 inches
For the 2-inch stroke actuator in question, the “jump,” which is the resolution of the actuator, is a full 7.5 percent of the span. This is a typical performance value for a pneumatic actuator and could easily cause what is called a “friction hunt.” A friction hunt will put a great deal of stress on the valve positioner, requiring it to continuously correct position.
Control valve manufacturers have taken steps over the years to reduce the jump effect. Higher actuator operating pressures will marginally reduce the jump effect. Improvements in packing (reduced packing friction) also can help, at least temporarily. With the introduction of smart positioners some manufacturers have demonstrated units with highly polished valve stems, low-friction packings installed only finger tight, and no process load. Under these test conditions, resolution better than 0.5 percent can be achieved. In the real world, however, tight packing, process loads and stem wear (and/or deposits on the stem) will increase friction considerably.
By now it should be painfully clear that the source of weakness in pneumatic actuators is the compressibility of air. The technological solution is an actuator utilizing an incompressible (or virtually incompressible) driving fluid. This actuator must operate on a continuous basis, provide spring failure capability on loss of power and exhibit long-term reliability. In addition, the flexibility of digital control and valve monitoring should be available. Do the other major actuator technology alternatives, mechanical and hydraulic, meet these criteria?
Most gear motor, mechanical actuators of any appreciable size have limited duty cycles, typically about 25 percent, and self-imposed time delays on motor reversals. They also have inherent inertia problems requiring high starting torques and current draw as well as braking systems to reduce motor coast when stopping. Positive spring failure options are limited to very small sizes.
Hydraulic actuator systems, on the other hand, can modulate continuously, are virtually immune to dynamic instability and provide precise positioning. They can also readily accommodate spring fail safe action. To demonstrate the dramatic performance difference between air (compressible) and oil (incompressible) as a driving medium, consider the same scenario studied earlier and substitute oil for air in the actuator. Assuming the same 5 psi pressure change to overcome static friction, the calculations result in a volume change of 0.0019 in3, a time lag of 0.00037 seconds and a jump of 0.000038 inches. For the 2-inch stroke actuator in question, the “jump” is a meager 0.0019 percent of the span, as compared to 7.5 percent for the pneumatic equivalent.
Conventional hydraulic actuators have been harshly criticized on cost, maintenance and reliability issues. Initial costs can run 3 to 10 times those of pneumatics. Gravity fed hydraulic systems “communicate” with the atmosphere, resulting in contamination with corrosive condensate and particulate matter. This results in the need for monitoring and maintenance of the hydraulic fluid. Also, these hydraulic systems require motors to run 24 hours a day, and the extensive network of hydraulic tubing and fittings tend to leak an environmentally unfriendly fluid.
In the development of electraulic actuation technology, every effort was made to create a universal actuator incorporating the advantages of pneumatic, mechanical and hydraulic devices, while eliminating their drawbacks. The electraulic actuator capitalizes on the benefits afforded by hydraulic actuation using conventional motor oil, packaged in a self-contained, simple unit.
Like pneumatic actuators, electraulic actuators are capable of 100 percent modulating duty cycle (continuous duty) and can be supplied with fail-safe springs. They are also simple and reliable. Unlike pneumatics, they provide extremely precise, “stiff” positioning control.
Like electromechanical actuators, electraulic motors only operate when positioning movement is required, and “lock in place” on power failure is standard. Unlike electromechanicals, electraulic actuators are rated for continuous duty service, are simple and reliable (only 8 moving parts), and can easily accommodate spring failure options.
Like electrohydraulic actuators, electraulic actuators provide quick, precise response with inherently stiff positioning. They do not have continuously running motors nor do they require elaborate filtering systems. Also, electraulic actuators require little or no maintenance and are price competitive.
Severe service conditions, critical reliability and/or tight control requirements (or any combination thereof) typically trigger the need for electraulic actuation. Spray valves of any kind, deaerator level control valves and boiler feedwater valves require continuous modulation and tight control. Boiler/turbine start-up valve systems, especially on supercritical boilers, require reliable control under extremely harsh service and environmental conditions.
Control complexities are often simplified through the use of electraulic actuators in such applications as turbine speed control and steam seal regulation. Damper applications including forced draft, induced draft and primary air fans also require good control and stiff drives because of typically high-friction (and high maintenance) linkage systems. p
Many pneumatic-activated valves, suchas this Leslie severe service control valve, are being upgradedwith advanced electraulicactuation for improved valveperformance.
This REXA Xpac Model L8000-6-B-P electraulic actuator is installed on a boiler feedwater valve at a supercritical power plant. The actuator generates 8,000 lb thrust with a six-inch stroke.
Kevin M. Hynes is president and CEO of REXA/Koso America Inc. He is the former owner and operator of CFM Inc. and the vice president of marketing and sales for Masoneilan. Hynes holds a bachelor`s degree in mechanical engineering from Northeastern University.