Category: Education
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9.5 Pressure switches
A pressure switch is one detecting the presence of fluid pressure. Pressure switches often use diaphragms or bellows as the pressure-sensing element, the motion of which actuates one or more switch contacts. Recall from section 9.1 that the “normal” status of a switch is the resting condition of no stimulation. A pressure switch will be in its “normal” status…
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9.4 Proximity switches
A proximity switch is one detecting the proximity (closeness) of some object. By definition, these switches are non-contact sensors, using magnetic, electric, or optical means to sense the proximity of objects. Recall from section 9.1 that the “normal” status of a switch is the resting condition of no stimulation. A proximity switch will be in its “normal” status when it…
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9.3 Limit switches
A limit switch detects the physical motion of an object by direct contact with that object. An example of a limit switch is the switch detecting the open position of an automobile door, automatically energizing the cabin light when the door opens. Recall from section 9.1 that the “normal” status of a switch is the resting condition of no stimulation.…
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9.2 Hand switches
A hand switch is exactly what the name implies: an electrical switch actuated by a person’s hand motion. These may take the form of toggle, pushbutton, rotary, pull-chain, etc. A common form of industrial pushbutton switch looks something like this: The threaded neck inserts through a hole cut into a metal or plastic panel, with a matching…
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Chapter 9 Discrete process measurement
The word “discrete” means individual or distinct. In engineering, a “discrete” variable or measurement refers to a true-or-false condition. Thus, a discrete sensor is one that is only able to indicate whether the measured variable is above or below a specified setpoint. Discrete sensors typically take the form of switches, built to “trip” when the measured quantity either exceeds…
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8.4 Fiber Optic Data Communication
Light has long18 been used as a long-range signaling medium. While communication by light through open air is still possible using modern technology, it is far more practical in most cases to channel the light signals through a special strand of optically transparent material called an optical fiber. When packaged in a protective sheath, it is known as…
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8.3 Electrical signal and control wiring
There is much to be said for neatness of assembly in electrical signal wiring. Even though the electrons don’t “care” how neatly the wires are laid in place, human beings who must maintain the system certainly do. Not only are neat installations easier to navigate and troubleshoot, but they tend to inspire a similar standard…
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8.2 Tube and tube fittings
Tube, like pipe, is a hollow structure designed to provide an enclosed pathway for fluids to flow. In the case of tubing, it is usually manufactured from rolled or extruded metal (although plastic is a common tube material for many industrial applications). This section discusses some of the more common methods for joining tubes together…
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Chapter 8 Instrument connections and communication
All instruments connect to their respective processes and to each other by means of pipe, tube, and/or wires. Improper installation of these connective lines can make the difference between success or failure in an installation. Safety is also impacted by improper connections between instruments and the process, and from instrument to instrument. 8.1 Pipe and pipe…
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7.5 Instrument and process equipment symbols
This section shows some of the many instrument symbols found in different types of technical diagrams used to document instrument systems. 7.5.1 Line types Note: the single backslash signifying a “discrete” or “binary” signal type has been removed from the ISA standard as of the 2009 ANSI publication. Regular pneumatic and electrical line symbols may represent…
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7.4 Functional diagrams
A unique form of technical diagram for describing the abstract functions comprising a control system (e.g. PID controllers, rate limiters, manual loaders) is a functional diagram4 . This form of document finds wide application in the power generation industry to document control strategies. Functional diagrams focus on the flow of information within a control system rather than on…
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7.3 Loop diagrams
Finally, we arrive at the loop diagram (sometimes called a loop sheet) for the compressor surge control system (loop number 42): Here we see that the P&ID didn’t show us all the instruments in this control “loop.” Not only do we have two transmitters, a controller, and a valve; we also have two signal transducers. Transducer…
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7.1 Process Flow Diagrams
To show a practical process example, let’s examine three diagrams for a compressor control system, beginning with a Process Flow Diagram, or PFD. In this fictitious process, water is being evaporated from a process solution under partial vacuum (provided by the compressor). The compressor then transports the vapors to a “knockout drum” where they condense…
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Chapter 7 Instrumentation documents
Every technical discipline has its own standardized way(s) of making descriptive diagrams, and instrumentation is no exception. The scope of instrumentation is so broad, however, that no one form of diagram is sufficient to capture all we might need to represent. This chapter will discuss three different types of instrumentation diagrams: Process Flow Diagrams (PFDs)…