ac Alternating current or voltage which changes direction and polarity rhythmically. The most common alternating current used in industry is sinusoidal.

Accumulator tank Enclosed volume that contains a compressible fluid (gas) to provide standby energy in the event of loss of system pressure.

Air manifold Enclosure containing inlet and outlet ports that route the flow of air to the active piston or diaphragm areas.

Air pressure Available air supply at the user site; maybe less than compressor setting.

Ampere Unit of measurement of electron flow or current. The rating on the motor nameplate gives the amount of current the motor draws from the power lines while operating at full load at rated voltage.

Analog Measurement system where the units and signals bear a direct linear relationship to functions.

Automatic system System which will function according to predetermined requirements without the intervention of human surveillance.

Auto transformer Static transformer having a single winding, part of which is common to both primary and secondary circuits.

Auxiliary switches Auxiliary circuit switches fitted and mechanically coupled to switching devices for the purpose of indicating the state of the main contacts.

Ball, V-notch Flow-controlling member for a popular style of throttling ball valve. The V-notch ball includes a polished or plated partial-sphere surface that rotates against the seal ring.

Ball valve Device that incorporates a ball with a center hole to provide shutoff (center hole perpendicular to pipe) or flow (center hole in line with pipe) through a pipe system. One of several types of 90o operation valves.

Bimetal strip Used in overload relays and overcurrent releases. It is a strip made up of two kinds of metal with different temperature coefficients. It is enclosed by a current winding. The heat due to an overload causes the strip to deflect and to actuate a tripping bar which, in turn, acts on the auxiliary circuit switch of the release or relay.

Black box Unit that converts input signals to appropriate output values without the need for the observer to know the internal workings of the device.

Breakaway (unseating) torque Published value depicting the turning force required to initiate rotation of the closure element of a rotary valve.

Butterfly valve Device that incorporates a circular disc to provide shutoff or modulating flow through a pipe system. One of several types of 90~ operation valves.

Capacitance The property of a capacitor device to store electric charge. The unit of measurement is the farad-coulomb per volt = ampere per second per volt.

Capacitor Device having two conductive plates separated by a dielectric insulation substance and thus possessing the property of capacitance. Capacity Rate of flow through a valve under stated conditions.

Cavitate Formation and subsequent collapse of voids or cavities in a valve resulting from increased fluid velocity through the restricted areas of the valve. It occurs in liquids when the valve operates near the vapor pressure of the liquid.

Characteristic Relation between flow through the valve and percent rated travel as the latter is varied from 0 to 100 percent.

Circuit Continuous path for an electric current to flow through.

Circuit breaker Switch with a making and breaking capacity, capable of controlling items of equipment in normal operation and under fault conditions.

Closure member Moveable part of the valve which is positioned in the flow path to modify the rate of flow through the valve.

Conductance Reciprocal of resistance. Measured in siemens.

Conductor Material in which free electrons are available to move under the influence of an electric field.

Contact Condition brought about by two conductive parts (e.g., contact pieces) touching. The expression is also used to denote a contact element or contact piece.

Contactor Remote-control switch actuated electromagnetically and capable of switching motor loads.

Control circuit Auxiliary circuit.

Control switches Power circuit switches for the direct control of electrical equipment, or auxiliary circuit switches for indirect control.

Control valve Power-operated device which modulates the fluid flow rate in a process control system. It consists of a valve connected to an actuator mechanism that is capable of changing the position of a flow-controlling element in the valve in response to a signal from the controlling system.

Corrosion Damaging effects of hostile media on control valve components resulting from material incompatibility.

Current Flow of electrons through an electric circuit.

Cg Flow coefficient commonly used for gases.

Cs Flow coefficient commonly used for steam.

Cv Flow coefficient commonly used for liquids. See flow coefficient.

dc Direct current or voltage which has a constant value related to zero value. Can be stipulated as a positive or negative value.

Deadband Range through which input can be varied without initiating observable response.

Diagram

  • Block Diagrammatic representation of a circuit or system in which the various component parts of the system are represented by a rectangle or block with no detail shown for the complexities contained in the system block.
  • Ladder or schematic Diagrammatic representation of a circuit or system where for the sake of clarity no relationship is made to actual physical location of components.
  • Wiring Diagrammatic representation of a circuit or system where actual physical location of components with regard to each other is shown.
Diaphragm Flexible seal area that provides for linear motion by the change of shape of the material.

Digital System that handles information as numbers.

Disc Flow-controlling member used in the most common varieties of butterfly rotary valves.

Disc, eccentric Commonly referred to as a high-performance butterfly valve. See high performance butterfly valve.

Double acting Characteristic of a piston or diaphragm system in which the energizing pressure acts on both faces of the piston or diaphragm and operates the system in forward and reverse to open or close.

Duty cycle Percentage of time a device is allowed to operate over a given period of time. Expressed in percent, it equals time on divided by time off multiplied by 100.

Efficiency Ratio of useful output power to total input power. Electromagnet Device in which a core of ferromagnetic material is encircled by a coil of wire carrying an electric current. The core can be made magnetic only when current is passing through the coil.

Electromotive force Force which causes the movement of electric charges. Measured in volts.

Electronics That part of electrical engineering which specializes in the conductance properties of semiconductor materials.

Erosion Damaging effects of gritty or dirty fluids on control valve components. Erosion is forestalled with valve designs which separate flow stream from critical valve components and with hardened materials.

Explosion-proof Characteristic of a device or element that inherently contains or prevents an explosion.

Farad Unit of capacitance.

Fail safe Characteristic of a system that provides that a unit will be operated into a safe condition upon the failure of electric power, power source, etc.

Failure mode Upon electric power failure or air pressure loss to the actuator, the valve is operated to a predetermined position (fail open, fail closed, fail in last position).

Feedback Signal indicating the actual position or the electrical or pneumatic condition of an element in a control system.

Flangeless body Body style common to rotary-shaft control valves. Flangeless bodies are held between ANSI class flanges by through bolts. (Sometimes also called wafer-style valve bodies.)

Flashing Phenomenon observed in liquid service when the pressure of the medium falls below its vapor pressure and does not recover to a higher pressure.

Flow characteristic Relationship between flow through the valve and percent rated travel as the latter is varied from 0 to 100 percent. This is a special term. It should always be designated as either inherent flow characteristic or installed flow characteristic. Common flow characteristics are linear, equal percentage, and quick opening.

Flow coefficient Cv The number of U.S. gallons per minute of 60oF water that will flow through a valve with a 1 lb/in² pressure drop.

Four-way pilot valve Pilot valve used with double-acting actuators.

Fusible element A specially shaped copper conductor (silver is used in some cases) in the ceramic body of the fuse element. It melts on overload or short circuit. Its special features are the low-melting-point insert and the so-called necks. The insert melts on overload, and the necks will interrupt a short circuit.

Globe valve Valve construction style with a linear motion flow-controlling member with one or more ports, normally distinguished by a globe-shaped cavity around the port region. Two categories are commonly recognized depending on the method of plug guiding: cage-guided and stem-or plug-guided.

Ground Main body of the earth which is considered to be at a zero electric potential. The ground itself and any conductor connected to it are termed at zero potential.

Hard facing Process of applying a material harder than the surface to which it is applied. This technique is used to resist fluid erosion and/or to reduce the chance of galling between moving parts, particularly at high temperature.

Hazardous environment Any combination of combustible or potentially explosive mixtures existing in the surrounding atmosphere.

Hertz Unit of frequency Number of times per second that alternating current flows in one direction, reverses, and flows in the other direction. Power companies in the United States bold this figure constant at 60 cycles per second (hertz).

High-performance butterfly valve (HPBV) Common name for valve design in which the positioning of the valve shaft-disc connections causes the disc to take a slightly eccentric path on opening. (This allows the disc to be swung out of contact with the seal as soon as it is opened, thereby reducing friction and wear.) See disc, eccentric.

Horsepower Unit of power equal to 746 W, or 33,000 ft lb of work per minute. The relationship between horsepower and torque is horsepower equals torque times revolutions per minute divided by 5250.

Hydrodynamic torque Turning force exerted on a valve stem due to fluid flow through the valve and its effect on the closure element.

Inductance Property of a circuit to generate an electromagnetic force by reason of a change in the magnetic flux through the circuit.

Inherent flow characteristic Flow characteristic when constant pressure drop is maintained across the valve.

Inlet Body opening through which fluid enters the valve.

In-rush current 1. Current input of a motor on starting. 2. Current input of a solenoid or contactor.

Installed flow characteristic Flow characteristic when pressure drop across the valve varies as dictated by flow and related conditions in the system in which the valve is installed.

Insulation Nonconductive material, e.g., vacuum, glass, nylon, rubber.

Integrated circuit Semiconductor device containing circuit elements which are manufactured in a single piece of material and which are individably connected to perform a function.

Leakage Quantity of fluid passing through an assembled valve when the valve is in the fully closed position under stated closure forces, with pressure differential and pressure as specified. Leakage is usually expressed as a percentage of the valve capacity at full rated travel.

Limit switch Set of electric contacts that are activated mechanically at a preselected position.

Linear flow characteristic Inherent flow characteristic which can he represented ideally by a straight line on a rectangular plot of flow coefficient versus percent rated travel. (Equal increments of travel yield equal increments of flow coefficient change at a constant pressure drop.)

Line voltage Voltage existing between the two lines of an electric supply.

Manual override Capability provided to actuators that allows the manual positioning of the actuator.

Modulate Function of a controller which causes a valve to respond to an infinite number of positions between the closed and full-open positions.

Moment arm Linear distance from the center of rotation upon which a force may act to produce torque.

Multiple-spring design Unique design where two or more springs may be used in parallel to provide a variable spring rate within a fixed space.

NEMA classification Code established for the construction of electrical components by the National Electrical Manufacturers Association.

Noise, control valve Generally refers to aerodynamic noise associated with flow-stream turbulence in compressible fluids. Noise levels can be reduced to safe levels defined by OSHA and the EPA with noise-abatement trim (source treatment) and with silencers and diffusers (path treatment).

Normally closed control valve One which closes when the actuator pressure is reduced to atmospheric.

Ohm Measure of resistance. Even in a good conductor, such as copper wire, there is some resistance to current flow. Three factors determine this resistance: the conductor’s substance, its diameter, and its length. Large-diameter wiring offers less resistance than smaller-diameter wiring, and the farther the current has to flow from the power source to the equipment, the greater the resistance and the greater the voltage drop.

Ohm’s law Explains the relationship of amperes, volts, and ohms. One am- pere of electric current will flow when pushed by 1V against 1 W of resistance. Ampere=volts/ohms.

On-off valve Any of a number of valve types used for either full-open or shutoff service. Terminology indicates an automated assembly.

Outlet Body opening through which fluid exits the valve.

Output torque characteristics Equation or graph that shows torque versus position, usually 0 through 90o operation. Overload Conditions in a healthy electric circuit that result in an overload of current flow. A sustained overload may cause the circuit to suffer damage.

Overload relay Relay which operates as a function of the current. Used for the protection of electrical equipment (e.g., motors) against overload. Its mode of operation is to actuate a built-in auxiliary switch by thermal or magnetic means. The auxiliary switch causes the switching device to be tripped remotely. Overload relays must be protected by fuses to prevent them from being destroyed by a short circuit.

Packing box (assembly) Cavity in the valve body or the part of the bonnet assembly used to seal against leakage around the valve plug stem or shaft.

Parallel Two or more circuits which are said to be connected in parallel when the total current flow is divided between them.

Pilot valve Device acting between the source of air pressure and the actuator that directs air flow to the required actuator air inlet ports.

Piston area Net cross-sectional area upon which air pressure may act to provide an output force.

Plug, eccentric Flow-controlling member of the eccentric rotary plug valve. Because of its eccentric action, it clears its seat soon after opening.

Plug valve Device that uses a truncated V-shaped element to provide tight shutoff or flow through a pipe system. One of several types of 90o operation valves.

Port Fixed opening, normally the inside diameter of a seat ring, through which fluid passes.

Power supply There are two types of power supply: direct current and alternating current. The latter is the most widely used power supply.

Pressure drop Difference between upstream pressure and downstream pressure that represents the amount of flow-stream energy that the control valve must be able to withstand.

Pressure drop, maximum allowable Maximum flowing or shutoff pressure drop that a control valve can withstand. While maximum inlet pressure is commonly dictated by the valve body, maximum allowable pressure drop is generally limited by the internal controlling components (plug, stem, disc, shaft, bearings, seals). Maximum allowable pressure drop may apply to the pressure drop while flowing process fluids or at shutoff.

Process control Act of maintaining or correcting a process within preset limits using a measurement of the actual process conditions in comparison to an established, desired condition.

Proportional controller Device that provides an output signal based on a valve(s) or a measured process parameter(s) and an input set-point command.

Quick-opening flow characteristic Inherent flow characteristic in which there is maximum flow with minimum travel.

Rack Linear gear that operates in conjunction with a pinion gear to provide rotary motion.

Rangeability Ratio of maximum to minimum flow within which the deviation from the specified inherent flow characteristic does not exceed some stated limit. Rangeability is expressed as the ratio of the maximum flow co-efficient to the minimum usable flow coefficient of a control valve.

Rated Cv Value of Cv at the rated full-open position.

Rating Specified operating conditions or performance of a piece of equipment by way of its input-output values.

Recovery Relative term used to describe how much flow-stream pressure is reduced because of the design of the control valve; the ratio of maximum (valve fully open) downstream pressure to upstream pressure.
   High-recovery valve: A valve design that dissipates relatively little flow- stream energy due to streamlined internal contours and minimal flow turbulence. Therefore, pressure downstream of the valve vena contracta recovers to a pressure significantly higher than the pressure at the vena contracta. (Straight-through flow valves, such as rotary-shaft ball and butterfly valves, are typically high-recovery valves.)
   Low-recovery valve: A valve design that dissipates a considerable amount of flow-stream energy due to turbulence created by the contours of the flow path. Consequently, downstream pressure recovery is reduced when compared to high-recovery valves. (Although individual designs vary, conventional globe-style valves generally have low-pressure recovery capability.)

Relays May be integral parts of, or used as accessory items for, individual switching devices. They operate as a function of the current or voltage or of time to control other circuit elements connected through auxiliary circuits.

Resistance Opposition to the flow of electric current exhibited by matter.

Rotary actuator Device that provides torque or rotating movement to a rotary valve.

Rotary-shaft control valve Valve style in which the flow closure member (full ball, partial ball, or disc) is rotated in the flow stream to modify the amount of fluid passing through the valve.

Scotch yoke Device that translates linear motion to rotary motion. The linear motion is defined by a pin or roller element trapped within a channel in the rotary element.

Seat That portion of the seat ring or valve body which a valve closure member contacts for closure.

Seating torque Value depicting the turning force required to set a valve into its closed position.

Series Method of connecting the elements of an electric circuit so that the same current passes through them.

Shaft Portion of a rotary-shaft control valve assembly corresponding to the valve stem of a globe valve. Rotation of the shaft positions the disc or ball in the flow stream and thereby controls the amount of fluid which can pass through the valve.

Short-circuit current Prospective current which will flow under the conditions of a terminal short circuit

Shutoff See leakage.

Solenoid valve Electrically powered device with a movable element that switches flow as a method of controlling actuator position.

Spring compression force Characteristic of a spring expressed as the restoring force available when the linear spring dimension is at minimum length.

Spring end torque Published value depicting the potential turning force of a spring-return actuator at the end of travel, when air pressure is zero.

Spring-return actuator Device that contains a spring element having the capability of returning a valve-actuator system to its normal position in the absence of air pressure, electric power, etc.

Spring-start torque Published value depicting the initial turning force of a spring-return actuator when air pressure is zero.

Starter Switch and/or resistance unit for running motors up to speed either steplessly or in steps.

TEFZEL Thermoplastic [25 percent ethylene, 75 percent tetrafluoroethylene (TFE)]. Used as lining and seat material in valves.

Three-way pilot valve Pilot valve used with single-acting (spring-return) actuators.

Throttling Action of a control valve in motion as it modulates flow.

Torque Product of a force acting upon a moment arm to produce rotary motion.

Transducer Device that senses a process parameter, such as temperature and pressure, and converts this information to an electric or pneumatic signal for use in control systems.

Transformer Basic transformer consists of two inductors (termed primary and secondary windings) wound onto a ferromagnetic core. If an alternating current or voltage is passed through the primary winding, a corresponding output will appear on the secondary windings in a direct relationship to the windings ratios.

Trim Internal parts of a valve which are in flowing contact with the controlled fluid.

Turning force Characteristic of a rotary valve resisting its change of position, or the output of a rotary actuator. Also known as torque.

UHMWPE Ultrahigh-molecular-weight polyethylene. Seat material offers high abrasion resistance superior to TFE. Temperature limit is 200oF (93oC).

Unseating torque See breakaway torque.

Vapor pressure Pressure at which a given liquid begins to vaporize.

Variable speed Characteristic of the actuator providing for revised (slower) operating time over the range of operation.

Vena contracta Location where cross-sectional area of the flow stream is at its minimum. (The vena contracta normally occurs just downstream of the actual physical restriction in a control valve.)

Volt Unit of potential difference or electromotive force. The final velocity of an electron which starts from rest and traverses a space across which a potential difference of 1 V exists is 593 km/s.

Voltage Potential force available to move electrons (current). When you start equipment which uses a great deal of electricity, the voltage may drop. However, if your power service and wiring are properly sized, the voltage drop will not he enough to affect the motor.

Wafer-style valve body Flangeless type of butterfly or gate, short face-to- face valve body. Also called a flangeless valve body; it is clamped between pipeline flanges.