Instrumentation control loops

Process control instrumentation—control loops, analyzers, instruments... 

Instrumentation. Pilot plants are usually heavily instmmented compared to commercial plants. It is not uncommon for a pilot plant to have an order of magnitude more control loops and analytical instmments than a commercial plant because of the need for additional information no longer requked at the commercial stage. A discussion of all the specific types of instmmentation used on pilot plants is beyond the scope of this article. Further information on some of the more common instmmentation is available (1,51). 

Documents such as job descriptions, operating manuals, emergency procedures, accident, and "near-accident" records, can be useful sources of information about the task to be studied. Pipework and instrumentation diagrams can also be used to gain an insight into the complexity of the process, the type of control loops installed, and the process parameters to be manually controlled by the workers. 

All control loops and instruments, with an identification number. 

The symbols used to show the equipment, valves, instruments and control loops will depend on the practice of the particular design office. The equipment symbols are usually more detailed than those used for the process flow-sheet. A typical example of a P and I diagram is shown in Figure 5.25. 

Instruments are provided to monitor the key process variables during plant operation. They may be incorporated in automatic control loops, or used for the manual monitoring of the process operation. They may also be part of an automatic computer data logging system. Instruments monitoring critical process variables will be fitted with automatic alarms to alert the operators to critical and hazardous situations. 

It is desirable that the process variable to be monitored be measured directly often, however, this is impractical and some dependent variable, that is easier to measure, is monitored in its place. For example, in the control of distillation columns the continuous, on-line, analysis of the overhead product is desirable but difficult and expensive to achieve reliably, so temperature is often monitored as an indication of composition. The temperature instrument may form part of a control loop controlling, say, reflux flow with the composition of the overheads checked frequently by sampling and laboratory analysis. 

In this chapter only the first step in the specification of the control systems for a process will be considered the preparation of a preliminary scheme of instrumentation and control, developed from the process flow-sheet. This can be drawn up by the process designer based on his experience with similar plant and his critical assessment of the process requirements. Many of the control loops will be conventional and a detailed analysis of the system behaviour will not be needed, nor justified. Judgement, based on experience, must be used to decide which systems are critical and need detailed analysis and design. 

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. 

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. 

For now let us say merely that the control system shown in Fig. 1.5 is a typical conventional system It is about the minimum that would be needed to run this plant automatically without constant operator attention. Notice that even in this simple plant with a minimum of instrumentation the total number of control loops is lO. We win find that most chemical engrneering processes are multivariable. 

Let s start from the beginning of the control loop, at the sensor. Instruments for on-line measurement of many properties have been developed. The most... 

In addition to the basic control loops, all processes have instrumentation that (1) sounds alarms to alert the operator to any abnormal or unsafe condition, and (2) shuts down the process if unsafe conditions are detected or equipment fails. For example, if a compressor motor overloads and the electrical control system on the motor shuts down the motor, the rest of the process will usually have to be shut down immediately. This type of instrumentation is called an interlock. It either shuts a control valve completely or drives the control valve wide open. Other examples of conditions that can interlock a process down include failure of a feed or reflux pump, detection of high pressure or temperature in a vessel, and indication of high or low liquid level in a tank or column base. Interlocks are usually achieved by pressure, mechanical, or electrical switches. They can be included in the computer software in a computer control system, but they are usually hard-wired for reliability and redundancy. 

Several instrument vendors have developed commercial on-line adaptive controllers. Difficulties have been reported in two situations. First, when they are applied in a multivariable environment, the interaction among control loops can cause the adaptation to fail. Second, when few disturbances are occurring, the adaptive controller has little to work with and its performance may degrade drastically. 

The decision for each example is expressed as an "action-next state" pair. The "action" is a reference to executable Radial code, which consists of a sequence of Radial statements. These statements may contain references to external programs in various languages (this will be discussed further later). The "next state" describes the context to which control is to pass after the action is completed. For diagnostic expert systems, such as TOGA, the next state will usually be the "goal" state of the module. This passes control back to the calling module. For procedural expert systems, such as robotics and instrumentation control applications, the control will be transferred between several states within a module to Implement looping. 

For the process measurement and control instrumentation the loop schedule enables allocation of a unique identifier (tag number) to each instrument used in the operation of the plant. This will allow application details to be added to the schedule (e.g., range, accuracy, set-point tolerance, signal type, description, location and any other information thought necessary to provide a clear understanding of the requirements for each instrument). 

Instruments which can monitor the important process variables during plant operation must be specified. These instruments must be capable of measuring the variables and should have an acceptable accuracy and repeatability of measurement, usually the latter attribute is more important than the former on chemical plant measurements. The instruments may be used for manual measurements or included in automatic control loops. Automatic alarms may also be required to indicate deviations outside acceptable limits. If possible, direct measurement of the process variable should be made, however it is often easier to measure a dependent variable, e.g. temperature measured as an indication of composition for distillation column top product. 

Although many pressure relief devices are called SRVs, not every SRV has the same characteristics or operational precision. Only the choice of the correct pressure safety device for the right application will assure the safety of the system and allow the user to maximize process output and minimize downtime for maintenance purposes. Making the correct choice also means avoiding interference between the process instrumentation set points in the control loop and the pressure relief device limits selected. These SRV operational limits can vary greatly even when all are complying with the codes. 

Each instrument in this control loop has its tolerances, for instance +/ — 5%. To ensure smooth operation, tolerances should never interfere with each other. Also, the SRV should be selected so that it does not start to open under the highest pressure switch setting plus its tolerance. Therefore, it is important to know the tolerance of the pressure relief device, or in this case the SRV, and the same applies for the SRV closure. In short, tolerances should never interfere... 

Let s assume that a user has made an investment of an installation with a design pressure of 110 barg a typical traditional ASME V1II-API 520-type spring-loaded SRV is used and the instruments in the control loop have 5% accuracy. 

While an intensity profile at the detector as a function of retardation may be acquired in a step-scan mode, two major drawbacks affect this method of interferogram acquisition. First, the mirror(s) requires stabilization times with mirror inertia and time constants of the control loop determining this parameter in achieving a given optical retardation. Second, additional hardware and control mechanisms need to be incorporated into the spectrometer, thus increasing instrument cost and complexity. In certain cases, however, the utility of a step-scan instrument justifies this additional expense. Historically, the step-scan approach was favored with slow detectors. With the advent of fast detectors and electronics, step-scan interferometry became... 

Similar to GC instruments, HPLC instruments consist of an injection port, a separation column, a detector, and an instrument control/data acquisition computer. The use of liquid as a mobile phase influenced the design and construction materials of HPLC instrumentation elements. A sample extract or an aqueous sample is introduced into the separation column through an injection loop that can be programmed to receive various volumes of liquid (5 pi to 5 ml). 

At PPG, Class 1 Prooftesting also covers 250 Safety Instrumented System loops in the PSM Safety Systems. A Safety Instrumented System (SIS) is composed of sensors, logic solvers, and final control elements for the purpose of taking the process to a safe state when predetermined conditions are violated. SISs are normally controlled by a PLC with the sole function of monitoring a process to insure operation is maintained within the safe operating envelope. 

Loop testing of remote control loops is a two-person exercise, with one person located in the field and the other in the control room or instrument room. Each person must be provided with an adequate means of remote commuification (e.g., field telephones or two-way radios) as approved by the customer. 

The following test procedure should be carried out in order to test the correct operation of field instrumentation and equipment installed in a control loop, and to provide the necessary documentary evidence (test records) to satisfy the requirements of the IQ protocols ... 

---