Pneumatic Control: Not Dead Yet
Still alive in many process industries, pneumatic control technology has adapted and changed to extend its usefulness.
Dick Johnson, CONTROL ENGINEERING -- Control Engineering, 7/1/1999
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Pneumatic control may be considered a dinosaur, but it is still walking this Earth very much alive. The predecessor of the 'beloved' 4-20 mA standard, pneumatics has remained 'active' in many process industries, even though it now often shares the spotlight in hybrid analog control systems. Even though control has slowly edged its way into the digital era, it has adapted to newer surroundings. The death knell has not yet been sounded.
The cry for 'smaller, faster, cheaper' control components and systems has driven much of the technology that is available to the process industries today. However, when an old standby like pneumatic control refuses to die, there must be some inherent advantages that rival newer technologies.
Although pneumatic components are not known for their small size (though there are so-called miniature valves and fittings), pneumatic transmitters, controllers, indicators, recorders, and panel-mount instruments are not as pricey as their electronic counterparts. Owing partially to their old, established technology and their relatively simple mechanical construction, pneumatic instruments have remained solidly entrenched in many industries.
Cheap, at what price?
Pneumatic controls can require more attention than standard 4-20 mA units. Apart from routine calibration functions, pneumatics must be checked for break-downs in their mechanical systems, including wear, leaks, and damage from airline contaminants. Al-though these may be rare occurrences, proper system maintenance still requires that these checks be made routinely.
One big factor in this maintenance issue is additional attention must be paid to the air system required for operation. Suitable compressed air systems are required. This entails the maintaining of compressors, regulators, air dryers and oil removal equipment (dry, oil-free air is a usual requirement), and air lines and drops. Although many pneumatic final control elements run on 'standard' 100-psi air, field devices often use 25- to 50-psi air, requiring additional regulation. However, with pneumatic devices costing as little as 25% of the least-expensive electronic/electrically operated device equipment, overhead issues loom much smaller on the horizon.
Longevity in the field adds to their economy. According to Jim Smith, marketing manager, actuators and positioners at ABB Instrumentation, Bailey Controls (Wickliffe, O.), Bailey did many complete pneumatic control systems in such industries as coal-fired utilities (forced- or induced-draft fan and pulverizer control), chemical, petroleum, and grain processing. 'Even though pneumatic control has a long history, its hey day [as far as Bailey was concerned] lasted from the 1950's to the mid-1960's. Finally, as control systems made the transition to analog, installation of pneumatic systems waned,' Mr. Smith adds. 'It is interesting to note that a majority of these original installations remain pneumatic to this day.' Bailey Controls and its sister division, ABB Instrumentation, Fischer+Porter (Warminster, Pa.) still support the technology by offering a complete line of pneumatic or pneumatic hybrid final control elements, including valve positioners and damper drives.
No need for speed, only safety
Securely entrenched in the process industries where response time is much less of an issue, pneumatic instruments have not been caught in the 'speeding up' process that effected its 4-20 mA and digital counterparts. Staying in the 'boomable' industries has also the allowed straight pneumatic instruments and final control elements to play their strongest hand. They are, by their very nature, rated as 'passive devices' by the National Electrical Code and are intrinsically safe. Pneumatic instrumentation is usable in any industry where Hazardous Area Classification Location is required and usually represents the lowest installed-cost option for Class 1, Division 1 service.
According to Richard Wolf, senior application specialist at ABB Instrumentation (Rochester, N.Y.), the use of pneumatic controllers and chart recorders also offers another advantage. In the case of a power outage, these devices continue to run and record process conditions. This information can be used to get processes safely on line manually in the event it is necessary. Additionally, it provides continuously collected data required by the U.S. Environmental Protection Agency, the Food and Drug Administration, and other regulatory agencies.
Beside intrinsic safety, measurement versatility is also an advantage in pneumatic instrumentation. ABB's 440R Series indicating controller has large indicating scales with easily read fluorescent orange process and set pointers for quick error recognition. However, the complete line of measuring elements available for use with the controller ensures application versatility. Temperature, pressure, level, flow, and differential pressure elements are available. The indicator also has the ability to receive electronic inputs.
A wide range of applications
Although the number of control manufacturers offering pneumatic control has dwindled, the range of products available is still impressive. Pressure, differential pressure, flow, temperature, and level transmitters are still available from a number of sources. Various controller types are also available including one-, two-, and three-mode versions. Panel-mount versions can use a variety of indicators, many of which are scaled to be visible in the large, dirty, hostile, and often poorly lit environments in which they must serve.
Large case instruments that can be field mounted on pipe stands, in panels, or on flush surfaces as required by the process are also available. Used for recording, indicating, controlling, totalizing, and transmitting, these familiar circular-chart and scale-indicator devices remain a staple in many industries.
Mostly available in NEMA 3 construction, these devices are designed to hold up to the elements when used out-of-doors, and refinery, chemical, wastewater remediation, and other large campus, process applications. Pneumatic control also requires a myriad of accessory items to assemble and maintain a properly operating system. Items like amplifying, reducing, boosting, and computing relays, repeaters, and no end of primary measuring elements are also readily available.
An interesting application of a pneumatic device is the pressure repeater. According to Wolf Musow, application consultant at The Foxboro Co. (Foxboro, Mass), this standalone pneumatic pressure transmitter--which provides an output signal directly proportional to process pressure-can be used as an accessory to electronic dp pressure or level transmitters. Instead of using the customary mechanical remote pressure seal, the repeater is used to transmit the process output pressure signal either to the high or low side of a dp transmitter. The repeater has a much higher operating temperature than the remote pressure seals, allowing accurate pressure measurement at process temperatures up to 200 °C.
What the future holds
Even though the ranks of total pneumatic control have thinned over the years, air-operated components and systems (whether whole or in part) have persisted. The pneumatically operated valve positioner or control valve is perhaps the best-known electronic/pneumatic hybrid in service in the process industries. Offered universally by final control element manufacturers, this simple device interfaces an electronic signal (4-20 mA, HART, or all digital depending on type) with an air-operated positioning device. And depending on the signal supplied and the power of the onboard electronics, these hybrids can offer anything from PID tuning, remote calibration and setpoint control, and onboard diagnostics. Because the actuator end of the device remains air-powered, these devices are safe for operation in explosive environments, such as petroleum/chemical processing, pharmaceutical manufacturing where ether-based processes abound, and in processing operations where airborne dust is a problem.
Flexible solutions
All major companies that presently offer pneumatic products have pledged to support them into the new millennium. There are some very valid reasons for this.
Pneumatics remain in service, and new installations occur due to;
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Real or perceived fear of introducing electricity into a control scheme.
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Continued ease of maintaining and monitoring an existing system.
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Prohibitive cost of changeover to newer control technology (4-20 mA, Hart-based, etc.); and
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Convenience of a readily available air supply.
According to Mark Stiefbold, product manager, pneumatic and valve control products for Moore Process Automation Solutions (Spring House, Pa.), 'Moore recognizes that pneumatics options are an important part of delivering flexible control solutions. We expect them to remain an integral part of our measurement and control product line. However, with decreasing costs and continuing advances in loop controllers and distributed control systems for applications with safety concerns, the need for pneumatic instruments in critical applications will decrease. There is, however, current and expected growth in pneumatic control valve applications where volume boosting relays and other signal conditioners optimize valve performance.'
So where does all the emphasis by the control industry on available of flexible solutions leave pneumatics? Certainly not dead yet.
Using pneumatic instruments for modern control applications
Because they are easy to understand and operate, offer precise, reliable operation, long installed life, and electrical interference immunity, pneumatic instruments provide great application versatility.
The force-balance transmitter is widely used in pneumatic
control systems because it is one of the simplest means of
converting process variables to air pressure.
A direct-nozzle system, which uses the force-balance principle (see the pneumatic transmitter diagram), is the basis for many pneumatic instruments. The source of the process pressure applied to the upper side of a flexible diaphragm varies and can include:
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Process impulse lines;
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Gas expansion from pressure changes;
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Adjustable spring; and
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Signals from another pneumatic device.
The process pressure can be further weighted by applying the source to different diaphragm areas. The fixed orifice and nozzle form a pressure-divided circuit that detects and corrects errors between the process and transmitted pressures, while the differential pressure between the signals acts on the diaphragm. The resultant diaphragm displacement then meters the nozzle airflow to increase or decrease the transmitted pressure and the system reaches equilibrium. The result is a linear, high-gain relationship between process and transmitted pressures. To provide a higher degree of safety, most devices separate the transmitted pressure from the rest of the loop via booster stages.
Batch reactor example
A typical batch reactor temperature cascade scheme can be implemented using force-balance pneumatic instruments. In the application shown in the floating limits diagram, a high temperature differential across the reactor wall could lead to local burning or deposition of the ingredients on the inside.
To control this, a reactor temperature controller (TIC-1) changes the setpoint to a jacket temperature controller (TIC-2). This limits the setpoint for the jacket controller to a function of the reactor temperature, which prevents a high temperature differential.
The input to the reactor temperature controller is a 3-15 psig signal from a temperature transmitter, which includes a sealed-gas filled sensing element and a force-balance transmitter. The transmitter's flapper-nozzle design converts pressure changes in temperature to 3-15 psig output signals. The full 12 psig span can be achieved with less than 0.001in. of flapper-nozzle movement, improving accuracy and minimizing hysterisis.
The temperature signal from a temperature transmitter (TT-1) goes to the reactor temperature controller (TIC-1), a direct-acting proportional, plus reset pneumatic controller. It senses any difference between the process variable and the controller setpoint signal via an error detector similar to the pneumatic circuit previously described, which adjusts the controller output through proportional and integral action until the process variable signal equals the setpoint signal. Diaphragm areas and nozzle diameters balance the pressures and provide a 3-15 (0-100%) psig output signal based on the incoming signal.
The jacket temperature high limit is set using a positive biasing relay and a low-pressure selector (LPS). The biasing relay takes the pressure output from the temperature transmitter and adds pressure (bias) to it. Thus, a mid-span output from the reactor temperature controller of 9 psig into a biasing relay set for 3 psig would have a 12 psig output. The bias can be adjusted by compressing a spring that acts on the flapper side, which increases pressure. The low-pressure selector then sets the bias for the allowable temperature differential by selecting between the 'biased' temperature output signal and the output from the controller (TIC-1).
Since the reactor has a cooling capability, a negative biasing relay and a high-pressure selector (HPS) establish a low limit on the jacket temperature. In this case the 'bias' comes from a spring on the nozzle side of the flapper, which decreases pressure. The result is a 3-15 psig setpoint to a direct acting proportional plus reset controller (TIC-2) that is compared to the 3-15 psig signal transmitted from the controller (TT-2). Here, the controller sends a 3-15 psig signal to air-to-open (A/O) steam valves and air-to-close (A/C) cooling water valves after comparing the setpoint and process pressures.
Mark A. Stiefbold is product manager, pneumatic and valve control products for Moore Process Automation Solutions.
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