Instrumentation single loop

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... 

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. 

Safety Instrumented Systems, The Logic of Single Loop Logic Solvers Moor Industries Worldwide, Brochure, Moore IndustrieS International, Inc., January 2014-... 

The cost of process control, as a fraction of the total construction cost of the process, has risen substantially since the early 1960s. Then is it was around 5 %, now it is closer to 25 %. In the 1960s the view was that some instrumentation was necessary but costs should be kept low. As a result plants had the minimum of measurements - just enough for safety and operability. Much of the instrumentation was local to the process, not repeated in the control room, and most of the controllers were single loop with the occasional cascade controller. 

There is a traditional pattern of what is called instrumentation in the chemical and petroleum industries based on single-loop control (sometimes called SISO—single input, single output). Each process operation has a number of independent or single loops for feedback control of temperatures, pressures, flows, liquid levels, and sometimes compositions. The term single loop means there is one measurement, one controller, and one final control element, usually a valve. Many, but not all, of these loops are represented in the central control room (CCR) by control stations. 

The process control subsystem was developed to allow control of the moderator level and temperature, in order to perform reactor physics experiments. It was made up of "conventional" instruments, which means AP transmitters (to measure level), thermocouple sensors (to measure temperature), and a single loop controller. 

Systems are designed to function normally even when a single instrument or control function fails. This is achieved with redundant controls, including two or more measurements, processing paths, and actuators that ensure that the system operates safely and reliably. The degree of redundancy depends on the hazards of the process and on the potential for economic losses. An example of a redundant temperature measurement is an additional temperature probe. An example of a redundant temperature control loop is an additional temperature probe, controller, and actuator (for example, cooling water control valve). 

Occasionally an incident occurs that results in a common mode failure. This is a single event that affects a number of pieces of hardware simultaneously. For example, consider several flow control loops similar to Figure 11-4. A common mode failure is the loss of electrical power or a loss of instrument air. A utility failure of this type can cause all the control loops to fail at the same time. The utility is connected to these systems via OR gates. This increases the failure rate substantially. When working with control systems, one needs to deliberately design the systems to minimize common cause failures. 

To illustrate the concept, consider a single distillation column with distillate and bottoms products. To produce these products while using the minimum amount of energy, the compositions of both products should be controlled at their specifications. Figure 8.13u shows a dual composition control system. The disadvantages of this structure arc (1) two composition analyzers are required, (2) the instrumentation is more complex, and (3) there may be dynamic interaction problems since the two loops are interacting. This system may be difficult to design and to tune. 

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