TechnoCrazed

Category: Education

  • 27.5 Valve seat leakage

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    In some process applications, it is important that the control valve be able to completely stop fluid flow when placed in the “closed” position. Although this may seem to be a fundamental requirement of any valve, it is not necessarily so. Many control valves spend most of their operating lives in a partially-open state, rarely…

  • 27.4 Valve packing

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    Regardless of valve type, all stem-actuated control valves require some form of seal allowing motion of the stem from some external device (an actuator) while sealing process fluid so no leaks occur between the moving stem and the body of the valve. The general term for this sealing mechanism is packing. This mechanical feature is not unlike…

  • 27.3 Dampers and louvres

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    A damper (otherwise known as a louvre) is a multi-element flow control device generally used to throttle large flows of air at low pressure. Dampers find common application in furnace and boiler draft control, and in HVAC (Heating, Ventilation, and Air Conditioning) systems. Common damper designs include parallel and radial. Parallel-vane dampers resemble a Venetian blind, with multiple…

  • 27.2 Rotary-stem valves

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    A different strategy for controlling the flow of fluid is to insert a rotary element into the flow path. Instead of sliding a stem into and out of the valve body to actuate a throttling mechanism, rotary valves rely on the rotation of a shaft to actuate the trim. An important advantage of rotary control…

  • Chapter 27 Basic Principles of Control Valves and Actuators

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    One of the most common final control elements in industrial control systems is the control valve. A “control valve” works to restrict the flow of fluid through a pipe at the command of a remotely sourced signal, such as the signal from a loop controller or logic device (such as a PLC), or even a manual…

  • 26.4 Analytical Measurements of Chemical Compositions

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    Temperature measurement devices may be classified into two broad types: contact and non-contact. Contact-type temperature sensors detect temperature by directly touching the material to be measured, and there are several varieties in this category. Non-contact temperature sensors work by detecting light emitted by hot objects. Energy radiated in the form of electromagnetic waves (photons, or light) relates to…

  • 26.3 Material volume measurement

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    A variety of technologies exist to measure the quantity of stored material in a vessel. For liquid applications, hydrostatic pressure, radar, ultrasonic, and tape-and-float are just a few of the more common technologies: These measuring technologies share a common trait: they infer the quantity of material stored in the vessel by measuring height. If the vessel…

  • 26.2 Flow measurement in open channels

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    Measuring the flow rate of liquid through an open channel is not unlike measuring the flow rate of a liquid through a closed pipe: one of the more common methods for doing so is to place a restriction in the path of the liquid flow and then measure the “pressure” dropped across that restriction. The…

  • Chapter 26 Signal Characterization in Control Systems

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    Mathematics is full of complementary principles and symmetry. Perhaps nowhere is this more evident than with inverse functions: functions that “un-do” one another when put together. A few examples of inverse functions are shown in the following table: Inverse functions are vital to master if one hopes to be able to manipulate algebraic (literal) expressions. For…

  • 25.13 Auxiliary and lockout (86) relays

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    An important type of “accessory” relay, especially for legacy electromechanical protective relays, is the so-called auxiliary or lockout relay, designated by the ANSI/IEEE number code 86. The purpose of an 86 relay is to serve as an intermediary element between one or more protective relays and one or more control devices, both expanding the number of control elements actuated…

  • 25.12 Distance (21) protection

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    A form of protection against faults on long-distance power lines is called distance relaying, so named because it is actually able to estimate the physical distance between the relay’s sensing transformers (PTs and CTs) and the location of the fault. In this way, it is a more sophisticated form of fault detection than simple overcurrent (e.g. 50…

  • 25.11 Directional overcurrent (67) protection

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    While 50 and 51 (instantaneous and time overcurrent) relay functions monitor line current magnitude and guard against excesses, there are applications where the direction of line current is just as relevant as the magnitude. In such cases, we need a protective relay function able to discriminate between current in one direction versus current in the other direction.…

  • 25.10 Differential (87) current protection

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    One of the fundamental laws of electric circuits is Kirchhoff’s Current Law, which states the algebraic sum of all currents at a circuit node (junction) must be zero. A simpler way of stating this is to say “what goes in must come out.” We may exploit this principle to provide another form of protection against certain…

  • 25.9 Instantaneous and time-overcurrent (50/51) protection

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    Perhaps the most basic and necessary protective relay function is overcurrent: commanding a circuit breaker to trip when the line current becomes excessive. The purpose of overcurrent protection is to guard against power distribution equipment damage, due to the fact that excessive current in a power system dissipates excessive heat in the metal conductors comprising that…

  • 25.8 ANSI/IEEE function number codes

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    In the United States, the ANSI and IEEE organizations have standardized a set of numerical codes referring to different types of power system devices and functions (IEEE C 37.2). Some of these codes refer to specific pieces of equipment (e.g. circuit breakers) while other codes refer to abstract functions (e.g. overcurrent protection). Two partial listings…