Process control pneumatic systems

Retention of a given solids particle in the system is on the average veiy short, usually no more than a few seconds. This means that any process conducted in a pneumatic system cannot be diffusion-controlled. The reaction must be mainly a surface phenomenon, or the solids particles must be veiy small so that heat transfer and mass transfer from the interiors are essentially instantaneous. 

Control systems, speed control, pressure control, process control If remote control, will it be pneumatic or electric And, what speed or pressure variation can be tolerated, and how fast must the system respond ... 

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. 

Basic Process Control System (BPCS) - Pneumatic, electronic, hydraulic or programmable instruments and mechanisms that monitor and/or operate a facility or system to achieve a desired function, i.e., flow control, temperature measurement, etc., which are supervised by human observation. 

An even greater pitfall into which many young process control engineers fall, particularly in recent years, is to get so involved in the fancy computer control hardware that is now available that they lose sight of the process control objectives. All the beautifiil CRT displays and the blue smoke and mirrors that computer control salespersons are notorious for using to sell hardware and software can easily seduce the unsuspecting control engineer. Keep in mind your main objective to come up with an effective control system. How you implement it, in a sophisticated computer or in simple pneumatic instruments, is of much less importance. 

The original GC control system took the form of a central room which monitors the flowllne6, oil, water, and utility sections, plus a smaller satellite control room monitoring the gas compression and gas conditioning section of the plant. Closed loop process control, such as separator liquid level, pressure, flow and temperature control were handled by local pneumatic analog controllers. The key process variables are displayed in the control room via electronic instrumentation. All the key process and equipment trouble alarms are annunciated m the control rooms, plus the on/off status of key machinery and open/close status of key valves are displayed. 

Since the 1970s, process controls have evolved from pneumatic analog technology to electronic analog technology to microprocessor-based controls. Electronic and pneumatic controllers have now virtually disappeared from process control systems, which are dominated by programmable electronic systems based on microprocessor technology. 

If operated on clean, dry plant air, pneumatic controllers offer good performance and are extremely reliable. In many cases, however, plant air is neither clean nor dry. A poor-quality air supply will cause unreliable performance of pneumatic controllers, pneumatic field measurement devices, and final control elements. The main shortcoming of the pneumatic controller is its lack of flexibility when compared to modem electronic controller designs. Increased range of adjustability, choice of alternative 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. Controller performance is also affected by the time delay induced by pneumatic tubing mns. For example, a 100-m run of 6.35-mm ( -in) tubing will typically cause 5 s of apparent process dead time, which will limit the control performance of fast processes such as flows and pressures. 

Reactor process monitoring, measurement and control systems Reactor process monitoring, measurement and control systems, sub-systems and components. All analog and digital process control computers and hydraulic and pneumatic process monitoring and control instruments and equipment. 

Transmission lines in process control systems rarely make any significant contribution to the overall loop characteristics. Where signals are transmitted electrically, there is no detectable signal attenuation for any frequencies characteristic of the process components, and even for pneumatic transmission lines as long as 200 feet there is little loss of signal. Transmission lines have distributed properties, but according to Bradner (B3) who has studied pneumatic transmission lines extensively, they can be approximated as second-order systems. 

The ultimate characteristics of a process control system are obtained readily by closing the loop through a proportional controller, and increasing the gain on the controller to the minimum proportional gain at which the system oscillates steadily. Johnson and Bay (J4) describe tests of both the ultimate gain approach and the reaction curve approach applied to pneumatic analog systems. 

Process temperatures. The process temperature control is less critical in the hydraulic process than in the pneumatic system. 

Multiple-column systems were previously explored in the petroleum industry and some process-control situations. In the former case, typical petrochemical samples share some similarities with biochemical samples in terms of complexity while the GC column typically receives a total sample, only certain portions of it may be of interest. Thus, selected parts of a column effluent can be pneumatically switched over to a second column for an optimum analysis, while the residual uninteresting substances (heavy ends) are being rapidly removed through backflushing. In the case of process GC analysis, such backflushing is essential to the speed of analysis required from these industrial analyzers indeed, a similar situation is often found in a clinical laboratory. 

Following development of pneumatic field control systems came the idea that a control house was needed to incorporate all controls for a process into a central room. Pneumatic transmitters, including the development of dP cells for flow measurements, provided the next step in instrumentation and moved the local process control room into a central control house for the entire plant. 

Some familiarity with control hardware and software is required before we can discuss selection and tuning. We are not concerned with the details of how the various mechanical, pneumatic, hydraulic, electronic, and computing devices are constructed. These nitty-gritty details can be obtained Ifom the instrumentation and process control computer vendors. Nor are we concerned with specific details of programming a distributed control system (DCS). These details vary from vendor to vendor. We need to know only how they basically work and what they are supposed to do. Pictures of some typical hardware are given in Appendix B. 

Pneumatics is used in applications in which a hydraulic fluid leak is undesirable such as food-processing plants. Pneumatics is clean, and the fluid required for the system—air—is readily available. Pneumatics is typically used in control systems. 

For engineers to use pneumatics, they must design a pneumatic system. As we noted in Chapter 2, a system is a group of related components that work together to achieve a process. A typical pneumatic system consists of a compressor, a reservoir, transmission lines, control valves, and actuators. These components are shown in Figure 11-13. 

One way to minimize process generated dust from accumulating in the plant is to install an effective and safe dust control exhaust system or upgrade an existing one. Pneumatic conveying equipment and dust control exhaust systems that transport combustible solids and particulate need to be protected from fire and dust explosions. If the concentration of combustible dust suspended in air is at or above the minimum explosible concentration (MEC) and there is a source of ignition, an explosion may occur within the... 

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