Pneumatic controller

For many years the pneumatic controller was preferred to its electronic counterpart due to its simplicity, its general ruggedness in the process environment, and the fact that its output could be used to operate directly the diaphragm of a pneumatic control valve. Although now largely superseded by software or hard wired electronic equivalents, pneumatic controllers are still employed in special circumstances, e.g. in explosive atmospheres. Furthermore, substantial numbers of pneumatic controllers can be found on older plant and thus an understanding of their principles of operation is necessary. 

The excessive sensitivity of the narrow-band mechanism leads to considerable instability in the control system. This sensitivity is reduced by introducing a feedback bellows as illustrated in Fig. 7.115. In this case, if P increases due to the movement of the flapper towards the nozzle, the bellows will expand. Thus, as pivot Y is moved to the left by a change in measured value or set point, X will move to 

Pneumatic wide-band proportional controller mechanism 

Integral action is added by the insertion of a restrictor and further bellows (Fig. 7.116). The rate of change of pressure in the integral bellows is proportional to the pressure driving force across the restrictor, 

the pressure of the output to the valve P is the sum of Pi and the pressure produced by the wide-band proportional action contributed by the proportional bellows and the flapper-nozzle system (cf. equation 7.3). Note that for t 0, P/ is no longer equal to P0 and thus equation 7.268 only strictly applies at t = 0. The value of 3 i (and consequently r/) depends upon the capacity C/B of the integral bellows and the the resistance to flow R/r through the integral restrictor. It is generally assumed that C/fi changes little and r/ is varied by adjusting R/r. 

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E. A. Mayer, Electro-Pneumatic Control Valve for EGRfATC Actuation, SAE 810464, Society of Automotive Engineers, Warrendale, Pa., 1981. 

With the exception of pneumatic controllers for special applications, commercial single-loop controllers are almost entirely microprocessor-based. The most basic products provide only the PID control algo-... 

Single-Loop Controller The single-loop controller (SLC) is a process controller that produces a sin e output. SLCs can be pneumatic, analog electronic, or microprocessor-based. Pneumatic SLCs are discussed in the pneumatic controller section, and analog electronic SLC is not discussed because it has been virtually replaced by the microprocessor-based design. 

Pneumatic Controllers The pneumatic controller is an automatic controller that uses pneumatic pressure as a power source and generates a single pneumatic output pressure. The pneumatic controller is used in single-loop control applications and is often installed on the control valve or on an adjacent pipestand or wall in close proximity to the control valve and/or measurement transmitter. Pneumatic controllers are used in areas where it would be hazardous to use electronic equipment, in locations without power, in situations where maintenance personnel are more familiar with pneumatic controllers, or in applications where replacement with modern electronic controls has not been justified. 

Process-variable feedback for the controller is achieved by one of two methods. The process variable can (I) be measured and transmitted to the controller by using a separate measurement transmitter with a 0.2-I.0-bar (3-15-psi pneumatic output, or (2) be sensed directly by the controller, which contains the measurement sensor within its enclosure. Controllers with integral sensing elements are available that sense pressure, differential pressure, temperature, and level. Some controller designs have the set point adjustment knob in the controller, making set point adjustment a local and manual operation. Other types receive a set point from a remotely located pneumatic source, such as a manual air set regulator or another controller, to achieve set point adjustment. There are versions of the pneumatic controller that support the useful one-, two-, and three-mode combinations of proportional, integral, and derivative actions. Other options include auto/manual transfer stations, antireset windup circuitry, on/off control, and process-variable and set point indicators. 

Pneumatic controllers are made of Bourdon tubes, bellows, diaphragms, springs, levers, cams, and other fundamental transducers to accomplish the control function. If operated on clean, diy plant air, they offer good performance and are extremely reliable. Pneumatic controllers are available with one or two stages of pneumatic amphfi-cation, with the two-stage designs having faster dynamic response characteristics. 

FIG. 8-64 Pneumatic controller a) example (h) frequency response characteristic,... 

The main shortcomings of the pneumatic controller is its lack of flexibility when compared to modern electronic controller designs. Increased range of adjustability, choice of alternate control algorithms, the communication link to the control system, and other features and services provided by the electronic controller make it a superior choice in most of todays applications. 

In a process loop with a pneumatic controller and a large process time constant. Here the process time constant is dominant, and the positioner will improve the linearitv of the final control element, Some common processes with large time constants that benefit from positioner application are liquid level, temperature, large volume gas pressure, and mixing,... 

As microprocessor-based controls displaced hardwired electronic and pneumatic controls, the impac t on plant safety has definitely been positive. When automated procedures replace manual procedures for routine operations, the probability of human errors leading to hazardous situations is lowered. The enhanced capability for presenting information to the process operators in a timely manner and in the most meaningful form increases the operator s awareness of the current conditions in the process. Process operators are expected to exercise due diligence in the supervision of the process, and timely recognition of an abnormal situation reduces the likelihood that the situation will progress to the hazardous state. Figure 8-88 depicts the layers of safety protection in a typical chemical jdant. 

Gas compressor anti-surge (GM-OFF) control circuit, comprising transmitters, computers and pneumatic control valve Reverse flow protection (on axnal compressors only) as supplementary protection device against surging, working independently of the control circuit Expander emergency stop valve with pneumatic actuator and solenoid valve... 

Gas storage cabinets were originally developed for the semiconductor industry in the 1970s. These early storage cabinets consisted of a box that enclosed the tank and connections they were operated under negative pressure and exhausted to the outside. Gas storage cabinets have become more sophisticated, adding gas detection, fire sprinklers, alarms, and pneumatic controls. " Some cabinets have point-of-operation air cleaners such as scrubbers. 

Pneumatic control A control system that operates on compressed air as the operating medium for the control of valves and dampers, etc. 

Compressed air is needed for general use and for the pneumatic controllers that usually seiA e for chemical process plant control. Air is often distributed at a pressure of 100 psig. Rotary and reciprocating single-stage or two-stage compressors are used. Instrument air must be dry and clean (free from oil). 

During the 1960s, air conditioning system control began to shift from simple electrical or pneumatic control to electronic and rudimentaiy computer control. This new technology was applied to commercial, institutional and commercial buildings where the high cost and complicated nature justified its use. 

Pneumatic Control Systems. The first widely adopted automatic control systems used compressed air as the operating medium. A transition to electronic... 

Figure 15.29 The layout of a typical diesel generator engine room. 1 Diesel generator set 2 jacket water header tank 3 lubricating oil service tank 4 air receiver 5 diesel-driven compressor 6 batteries and charger 7 engine control panel 8 pneumatic control panel 9 fuel oil control panel 10 engine exhaust silencer 11 charge air filter 12 daily service fuel oil tank 13 three-section radiator...

The basic purpose of an oil separator is to clean the pressurized air of any oil contamination, which is highly detrimental to pneumatically controlled instrumentation. A separator consists of an inlet, a series of internal baffle plates, a wire mesh screen, a sump, and an outlet. The pressurized air enters the separator and immediately passes through the baffle plates. As the air impinges on the baffle plates it is forced into making sharp directional changes as it passes through each baffle section. As a result, the oil droplets separate from the air and collect on the baffles before dropping into the separator s sump. 

Few plant operators need to be told of the problems caused by water in compressed air. They are most apparent to those who operate pneumatic tools, rock drills, automatic pneumatic powered machinery, paint and other sprays, sandblasting equipment, and pneumatic controls. However, almost all applications, particularly of 100-psig power, could benefit from the elimination of water carryover. The principal problems might be summarized as ... 

Pneumatic controllers, which may include part of the sensing instrument, are supplied with compressed air at 1 bar gauge which is allowed to escape from an orifice controlled by a detector. The resulting pressure modulates about 0.4 bar and is used in a servo... 

Pneumatic controls are used widely in hazardous situations such as chemical plants and oil refineries. The same risk of chafing applies as with capillary tubes. Pneumatic tubing is more usually in copper and is correctly secured. 

Luyben (1973) (see simulation example RELUY) also demonstrates a reactor simulation including the separate effects of the measuring element, measurement transmitter, pneumatic controller and valve characteristics which may in some circumstances be preferable to the use of an overall controller gain term. 

Signal transmission limitations of pneumatic control systems made it necessary to limit the distance between the control house and the transmitter or control valve. As a result, early control houses were located within or at the periphery of the process unit. 

The death knell for pneumatic control equipment has been predicted for at least the past 15 years. So far this has not happened, but it is still predicted. The major reason why pneumatic equipment is so popular is that the pneumatic control valve is cheap and requires little maintenance. The pneumatic system also has the advantage of posing no problems in the presence of flammable substances. (Extreme care must be exercised if electrical signals are used in such environments.) One major problem with pneumatic systems is the delay encountered in sending a pneumatic signal over 300 ft (90 m). However, this can usually be avoided by mounting the controller next to the unit instead of in the control room. This does not affect the monitoring of the process, which can still be done in a remote location. 

For slow reactions, the shallow fluid beds have been organized into a cocurrent multistage fluid bed (MSFB) reactor as shown in Fig. 33 (Yan, Yao, Wang, Liu and Kwauk, 1983). In this reactor, solids are carried up by the flowing gas stream, and once they reach the top, they are collected through a funnel and recirculated to the bottom by means of a pneumatically controlled downcomer. 

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