Selective Course 1 (Computer Control Systems) PDF

Selective Course 1 (Computer Control Systems) By Assoc. Prof. / Ahmed M. ElMahalawy Dr/ Bassam Wasfi Communications and Computers Engineering Department  Introduction to SCADA System Chapter 6 Sensors, Actuators, and Wiring...

Selective Course 1 (Computer Control Systems) By Assoc. Prof. / Ahmed M. ElMahalawy Dr/ Bassam Wasfi Communications and Computers Engineering Department  Introduction to SCADA System Chapter 6 Sensors, Actuators, and Wiring By Dr / Bassam Wasfi Introduction to SCADA System  Master Terminal Units (MTUs) Learning Objectives When you have completed this unit, you should: 1. Be aware that the cost of process interface equipment can be significant. 2. Recognize that when sensors and actuators are being selected and installed the special considerations of the process and ambient conditions must be taken into account. Introduction to SCADA System  Table of Contents Agenda ❑ Overview ❑ A Forgotten Cost ❑ Special Considerations ❑ Standardization ❑ Maintenance Introduction to SCADA System  Sensors, Actuators, and Wiring Overview ❑ The details of any project can easily be overshadowed by its one or two most prominent or obvious highlights. ❑ With SCADA projects, for example, the features that attract the eye are the MTU, the communications system, or the RTUs. ❑ It may therefore come as a surprise to learn that such apparently simple and inexpensive parts as limit switches, valve actuators, and equipment analyzers usually represent the largest portion of the capital cost of a SCADA project. ❑ This is particularly true if the project involves the modification of an existing process so it interfaces with SCADA. Introduction to SCADA System  Sensors, Actuators, and Wiring A Forgotten Cost ❑ SCADA is used in a wide range of industries. however, it is forgotten that the sensors, the actuators, and the wiring between them and the RTU also cost money. ❑ In fact, these items frequently cost three to four times more than the RTU, UPS, and communications equipment at a particular site. ❑ Figure 10-1 shows the costs of SCADA at a simple RTU site. ❑ In the figure, there is only one RTU, one radio, and one power supply at a remote site. However, that site may easily have as many as 250 sensors, actuators, and analyzers. Introduction to SCADA System  Sensors, Actuators, and Wiring Example 9-1 ❑ This chapter has concentrated on some of the considerations to be kept in mind when applying sensors and actuators. ❑ As used in SCADA, these devices may not be significantly different from the instruments that fulfill similar functions in any highly automated plant in a similar industry. ❑ However, they will be different from instruments that are designed to be read directly by a human operator. ❑ Many years of engineering effort have been invested improving sensors and actuators so a person- operator can interface with the process. ❑ When a SCADA system is installed between the operator and the process, some additional engineering effort is required. ❑ Sensors that are cheap to buy if they are to be read by a human operator are usually more expensive if they are to be read by a machine. ❑ The thermometer to be fitted into a thermowell in a process vessel (shown in Figure 10-2) will cost about $50. However, a temperature transmitter to be fitted into the same thermowell and to output a 4-20 mA signal into an RTU (shown in Figure 10-3) will cost $600. Introduction to SCADA System Sensors, Actuators, and Wiring Communications Interface Introduction to SCADA System  Sensors, Actuators, and Wiring ❑ Besides purchase cost, there is a cost associated with connecting the sensor or actuator to the RTU. ❑ Copper wire is normally used because the signals are normally low-voltage electrical. ❑ In many applications, shielding must be added over the copper wire to prevent electromagnetic interference or noise from corrupting the signal. ❑ Some form of physical protection of the copper wire and shield is necessary; it may take the form of some type of flexible armor that is integral to the structure of the cable. It may be that the unarmored cable is fed through rigid pipes called conduit or laid in protective channels called cable tray. ❑ Finally, the cable is encapsulated in a jacket of fluid-proof, flexible plastic. One reason for this is to prevent moisture or corrosive atmospheres from reaching the conductors and shields. The other reason is to prevent toxic or flammable fluids from traveling through the cable from the process areas to areas that are normally free of these fluids. Introduction to SCADA System  Sensors, Actuators, and Wiring Cross-section of copper cable ❑ Figure 10-4 shows a cutaway drawing of such a cable. ❑ Many pairs of copper wires, each capable of carrying the signal from one sensor or to one actuator, can be integrated into a cable. Introduction to SCADA System  Sensors, Actuators, and Wiring junction boxes ❑ Process instruments are not large enough to allow multipair cables to enter their wiring box, pass through, and carry on to the next instrument; junction boxes or field termination boxes are used to overcome this. ❑ Figure 10-5 shows a large multipair cable coming from an RTU to a concentration of instruments, in this case status switches on ball valves. Introduction to SCADA System  Sensors, Actuators, and Wiring junction box ❑ At a convenient location near the concentration, a junction box (Figure 10-6) with terminal strips is mounted. ❑ The wires from the multipair cable are terminated in the junction box. ❑ Smaller cables, often with only one or two pairs of wires, are run from this junction box to the individual instruments. Introduction to SCADA System  Sensors, Actuators, and Wiring The marshaling panel ❑The physical configuration of the process may mean that several of these multipair cables will need to connect to the RTU. Often when this is done a large junction box, called a marshaling panel, is located at the RTU location (see Figure 10-7). ❑Terminal strips in this box allow all the wiring to the process instruments to be wired and checked for correctness before the RTU is even delivered to the construction site. ❑The marshaling panel also acts as the platform for power relays, control power fuses or circuit breakers, intrinsically safe barriers, and special calibration or testing circuitry. ❑If small numbers of special instruments are required, such as flowmeter totalizers, pollution-monitoring electronics, or process moisture analyzers, they are often housed in the marshaling panel. Introduction to SCADA System  Sensors, Actuators, and Wiring The marshaling panel Introduction to SCADA System  Sensors, Actuators, and Wiring Data Storage ❑Very often, the marshaling panel even houses a few status lights and manual controls. ❑If local, manually read meter outputs are mandated, they also are usually located here. Figure 10-8 shows the front of the marshaling panel. Introduction to SCADA System  Sensors, Actuators, and Wiring Engineering cost of SCADA system ❑ All of this technical equipment must be specified. ❑ Installation drawings and instructions must be developed. ❑ Documentation of the system, individual equipment, licenses, software, and operating instructions must be gathered or written. ❑ In short, the engineering of the SCADA field installation must be performed. ❑ Because of the high relative complexity of a SCADA system, the ratio of engineering cost to total project cost may often be three to five times as high as would be expected for a non-SCADA project. ❑ Engineering, development, and documentation costs may reach 50 percent of the total SCADA project cost compared to 10 percent to 15 percent of the cost of nonautomated process projects. Introduction to SCADA System  Special Considerations ❑ Each industry has process considerations that make it if not unique then certainly distinct. ❑ Production processing and the transportation of hydrocarbon fluids require that the sensors, actuators, and wiring used in these processes not initiate fires and explosions. ❑ Chemical processing may face these same issues, but it will also be concerned with corrosion caused by the process fluids and the pollution of the surrounding atmosphere. ❑ Electric power generation and transmission must deal with the issue of isolating instruments from strong electrical fields and high voltages. ❑ Food processing has to be concerned that field equipment that comes into contact with the food will not contaminate it. ❑ Each of these special considerations can be met by existing equipment if there is enough of a market for the equipment and if the purchasers are prepared to pay enough. But there will be an extra charge. Introduction to SCADA System  Standardization ❑ Very few sensors and actuators are so unusual that they are made by only one manufacturer. ❑ As market conditions change, the prices of one manufacturer's product relative to another's will vary, and there may be a temptation to shift from one supplier of the process end devices to another. ❑ This temptation should be resisted. ❑ Changing suppliers because the product is of inferior quality or because it is no longer available is acceptable. ❑ But small price savings can easily be overshadowed by the increased costs of warehousing, training personnel, the time lost replacing the old product, changes to drawings and other documentation, and calibration procedures. ❑ If the SCADA system will be installed in an existing facility, you should consult local maintenance personnel to learn which suppliers have provided good service in the past. ❑ A survey should be made of potential suppliers of sensors and actuators early in the planning stages of a SCADA system. ❑ In addition to the fitness of the manufacturer's sensors and actuators to the purposes of the device, consideration should be given to the financial stability of the manufacturer and the manufacturer's history of supporting equipment that is no longer marketed. Introduction to SCADA System  Standardization ❑ Ideally, spares for each part that forms your SCADA system will be available until you decommission it. ❑ However, an acceptable second-best situation might be that replacement parts for the sensors and actuators will continue to be available for at least ten years after they cease to be marketed. ❑ The reason that this is more important for sensors and actuators than for MTUs and RTUs is that MTUs and RTUs do not have very many moving parts. ❑ They may suffer from a lot of failures when they are first installed (so-called infant mortality) but, unless subjected to long-term overheating, they can be expected to run without trouble for a long time afterward. ❑ On the other hand, sensors and actuators do have moving parts that wear out. ❑ They are exposed to process chemicals, shifts in ambient temperature, vibration, and a host of other conditions that contribute to failure. ❑ The planning for a SCA DA project should include provision for maintenance, and those maintenance plans should concentrate on sensors and actuators. Introduction to SCADA System  Maintenance ❑ The maintenance requirements in a SCAD A system are not significantly different than those in most other high-technology control systems. ❑ However, the communications equipment, radios, modems, and protocol drivers are the exception. ❑ The calibration, validation, and servicing of these devices will require special equipment and manpower training that is not generally available. ❑ Depending on the size of your SCADA system and the availability of contract service companies with these particular talents and resources, you may have to make sure your company provides for this maintenance. ❑ When you are establishing the proper level of service for your SCADA system you should do it with the expectation that requirements will be high initially and then will drop off to a lower plateau (see Figure 10-10). ❑ As noted earlier in this chapter, the maintenance requirements of the sensors and actuators will have different profiles. ❑ There will be a high initial maintenance level as new equipment goes through its early mortality stage and as loose contacts and pinched wires from the manufacturing and construction phases make themselves known. Introduction to SCADA System  Maintenance ❑ But after this early phase is over, the process interface devices will continue to wear and fail at a higher rate than will the solid-state equipment (see Figure 10-11 ). ❑ To anticipate this, the facility's design should allow for ease of access, temporary isolation from the process, and manual monitoring and control while the sensors and actuators are being calibrated or replaced. Introduction to SCADA System  Maintenance Introduction to SCADA System