• A limitation inherent to pneumatic valve actuators is the amount of air flow required to or from the actuator to cause rapid valve motion. This is an especially acute problem in all-pneumatic control systems, where the distance separating a control…

  • Valve actuators provide force to move control valve trim. For precise positioning of a control valve, there must be a calibrated relationship between applied force and valve position. Most pneumatic actuators exploit Hooke’s Law to translate applied air pressure to valve stem…

  • An important design parameter of a control valve is the position it will “fail” to if it loses motive power. For electrically actuated valves, this is typically the last position the valve was in before loss of electric power. For…

  • The purpose of a control valve actuator is to provide the motive force to operate a valve mechanism. Both sliding-stem and rotary control valves enjoy the same selection of actuators: pneumatic, hydraulic, electric motor, and hand (manual). 27.6.1 Pneumatic actuators Pneumatic actuators use air pressure pushing against either a…

  • 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…

  • 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…

  • 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.…

  • 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…

  • 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…

  • 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…

  • 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:…

  • 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…

  • 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…

  • 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…

  • 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…

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