Single loop

A polymer is adsorbed in the form of loops, tads, and trains (Fig. 9). Sufftciendy long tads or loops provide stabilization. The action is extremely efficient (20,21) a single loop or tad gives a barrier of approximately 20 kT. 

While the single-loop PID controller is satisfactoiy in many process apphcations, it does not perform well for processes with slow dynamics, time delays, frequent disturbances, or multivariable interactions. We discuss several advanced control methods hereafter that can be implemented via computer control, namely feedforward control, cascade control, time-delay compensation, selective and override control, adaptive control, fuzzy logic control, and statistical process control. 

In addition to single-loop process controllers, products that have benefited from the implementation of fuzzy logic are ... 

The ISA symbology provides different symbols for different types of actuators. Furthermore, variations for the controller symbol distinguish control algorithms implemented in DCS technology from panel-mounted single-loop controllers. 

Remote control units. These units are used to control unit processes. Basic control functions such as the PID algorithm are implemented here. Depending on other hardware components used, data acquisition capability may be required to perform digital control. They may be configured to supply process set points to single-loop controllers. Radio telemetiy may be installed to communicate with MUX units located at great distances. 

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 controllers provide both the process control functions and the operator interface function. This makes them ideally suited to very small applications, where only two or three loops are required. However, it is possible to couple single-loop controllers to a personal computer (PC) to provide the operator interface function. Su(m installations are extremely cost effec tive, and with the keen competition in PC-based produc ts, the capabilities are comparable and sometimes even better than that provided by a DCS. However, this approach makes sense only up to about 25 loops. 

Initially, the microprocessor-based single-loop controllers made the power of digital control affordable to those with small processes. To compete with these products in small applications, the DCS suppliers have introduced micro-DCS versions of their products. As a PC-based operator interface is usually a component of the micro-DCS, there is sometimes little distinction between a micro-DCS and a system consisting of single-loop controllers coupled to a PC-based operator interface. 

More microprocessor-based process equipment, such as smart instruments and single-loop controllers, with digital communications capability are now becoming available and are used extensively in process plants. A fieldbus, which is a low-cost protocol, is necessary to perform efficient communication between the DCS and these devices. So-called mini-MAP architec ture was developed to satisfy process control and instrumentation requirements while incorporating existing ISA standards. It is intended to improve access time while... 

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. 

Presently, fieldbus controllers are single-loop controllers with 8- and 16-bit microprocessors and are options to digital field-control devices. These controllers support the basic PID control algorithm... 

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. 

L2 = length of single looped section L3 = length of double looped section dp = d 1 — do... 

A design pressure of 545 kPa, gage is normally specified for water disengaging drums. The water outlet system is designed to seal the drum and prevent entrainment of hydrocarbon or air into the sewer. Figure 5 indicates the normal layout incorporating a single loop seal. 

Emergency Power Generators-Diesel board (Single Loop) Screw... 

Partclboard (Single Loop) 3.5.1.2 Valves-Non-Operated-Check... 

Taxonomj No. 2.2.1,1 Equipment Description CONTROLLERS - ELECTRONIC PANELBOARD (SINGLE LOOP) ... 

Three different forms of EPR test can be employed, designated as the single loop, double loop and reactivation ratio methods in Figure 19.20. 

Single Loop EPR Test The single loop method requires the sample to be polished to a 1 tm finish then passivated at -F200 mV (S.C.E.) for 2 min following which the potential is decreased at 1-67 mV s until the corrosion potential of approximately —4(X) mV (S.C.E.) is reached. The reactivation process results in the preferential breakdown of the passive film in the... 

Fig. 19.20 Schematics of reactivation polarisation curves, (a) Single loop EPR test method, (<>) double loop EPR test method and (c) reactivation ratio EPR test method...

Pitting caused by the dissolution of non-metallic inclusions can increase the value. Consequently, the microstructures of specimens with a high value must be examined to identify the source of the elevated value. In general, P values below 0-10 are characteristic of unsensitised microstructures, while sensitisation is indicated if P, exceeds 0-4. Single loop tests are sensitive to mild degrees of sensitisation but do not readily distinguish between medium and severely sensitised materials. 

Figure 6. A wound single loop 45-degree helix pattern.

Regulatory / Control / Seconds/Minutes Single Loop/... 

Shinskey (1984) has shown that there are 120 ways of connecting the five main parts of measured and controlled variables, in single loops. A variety of control schemes has been devised for distillation column control. Some typical schemes are shown in Figures 5.22a, b, c, d, e (see pp. 234, 235) ancillary control loops and instruments are not shown. 

Design and Tuning of Single-Loop Control Systems... 

In a simple single-loop system, we measure the outlet temperature, and the temperature controller (TC) sends its signal to the regulating valve. If there is fluctuation in the fuel gas flow rate, this simple system will not counter the disturbance until the controller senses that the temperature of the furnace has deviated from the set point (Ts). 

When both G12 = G21 = 0, the system is decoupled and behaves identically to two single loops. When either G12 = 0 or G21 = 0, the situation is referred to as one-way interaction, which is sufficient to eliminate recursive interactions between the two loops. In such a case, one of the loops is not affected by the second while it becomes a source of disturbance to this second loop. 

No interaction. Controller design is like single-loop systems. Strong interaction if 8 is close to 1 weak interaction if 8 1. One-way interaction... 

After proper pairing of manipulated and controlled variables, we still have to design and tune the controllers. The simplest approach is to tune each loop individually and conservatively while the other loop is in manual mode. At a more sophisticated level, we may try to decouple the loops mathematically into two non-interacting SISO systems with which we can apply single loop tuning procedures. Several examples applicable to a 2 x 2 system are offered here. 

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