Dynamic unbalance force: Generated on the spool at any specified opening due to process fluid pressure.

Effective area: In thin film actuators, the effective area is the area of the diaphragm that effectively produces the output force. The effective area of the diaphragm may change as it moves, usually being maximum at the beginning of the stroke and minimum at the end of the stroke. Moulded diaphragms have a smaller effective area change than flat diaphragms, so moulded diaphragms are recommended.

Shut-off: A condition in which the valve stopper moves to the closed position when drive power is lost.

Deaeration-open: A condition in which the valve stopper moves to the open position when drive power is lost.

Loss of air -- safety: A characteristic of the valve and its actuator that, in the event of an interruption of the drive energy supply, causes the valve closure element to move to fully closed, fully open, or left in the previous position, either of which is considered necessary to protect the process. Failure-safe modes of action may require auxiliary controls attached to the actuator.

Flow characteristics: The relationship between the flow rate through the valve and the percent rated stroke as the percent rated stroke varies from 0 to 100%. The term should always be expressed as either inherent flow characteristics or installed flow characteristics.

Flow coefficient (Cv value) : A constant (Cv value) associated with the valve geometry for a given stroke that can be used as a measure of flow capacity. It is the number of US gallons of 60°F water flowing through the valve per minute at a pressure drop of pounds per square inch.

High pressure recovery valve: A valve construction that disperses relatively little fluid energy due to a streamlined internal profile and minimal fluid turbulence. As a result, the pressure downstream of the valve retraction section returns to a high percentage of the inlet pressure. Dc through valves, such as rotary ball valves are typical of high pressure recovery valves.

Natural die pressure range: high and low pressures acting on the diaphragm to produce rated spool stroke when pressure in the valve body is atmospheric. This range usually refers to the spring setting range, as this will be the range of action of the valve when it is set to that operating range.

Natural flow characteristic: The relationship between flow rate and cut-off element stroke as the valve moves from closed position to rated stroke at constant pressure drop through the valve.

Diaphragm mounting pressure range: high and low values of pressure acting on the diaphragm to produce rated spool stroke under specified operating conditions in the valve body. The natural diaphragm pressure range may differ from the installed diaphragm pressure range due to the forces acting on the interceptor element.

Installation flow characteristics: The relationship between flow rate and cut-off element as the valve moves from closed position to rated stroke when the pressure drop through the valve is affected by changing process conditions.

Low pressure recovery valve: A valve structure that disperses a significant portion of the fluid energy due to turbulence generated by the fluid channel profile. As a result, the pressure downstream of the valve retraction section returns to a smaller percentage of inlet pressure than a valve with more streamlined channels. Although each valve structure is different, ordinary through valves usually have low pressure recovery.

Spring setting range: Adjusting range of valve actuator spring to balance the actual process force.

Flow capacity: Rated flow through a valve under specified conditions.

Gap flow: The flow rate below the minimum controllable flow rate when the interceptor element is not plugged.

Diaphragm pressure range: The difference between high and low values of the diaphragm pressure range. This can be considered an inherent or installed feature.

Double acting actuator: Actuator that can provide power in either direction.

Modified parabolic flow characteristic: A flow characteristic that provides an equal-percentage characteristic at the low stroke of the interceptor and a linear characteristic at the high stroke of the interceptor.

Push-down closing structure: A straight-through valve structure in which the interceptor is positioned between the actuator and the seat ring so that the ejector of the actuator pusher moves the interceptor toward the seat ring and closes the valve. The term may also be used for rotary valve construction. In a rotary valve configuration, the linear extension of the actuator push lever moves the ball or valve plate toward the closed position. Also known as positive action.

Push-down open structure: a straight through valve structure. Its seat ring is located between the actuator and the interceptor element, so that the push of the actuator lever removes the interceptor element from the seat, thus opening the valve. The term may also be used for rotary valve construction. In a rotary valve configuration, the linear extension of the actuator push lever moves the ball or valve plate toward the open position. (also known as reaction).

Adjustable ratio: The ratio between the maximum flow coefficient (Cv value) and the minimum flow coefficient (Cv value) when the deviation from a specified flow characteristic does not exceed a specified limit. A valve that is still well controlled will have an adjustable ratio of 100:1 when the flow rate is increased to the next 00 times the minimum controllable flow rate. The adjustable ratio can also be expressed as the ratio between the maximum and minimum controllable flow.

Rated stroke: The distance of the valve closure element from the closed position to the rated full open position. Rated full open position is the maximum opening recommended by the manufacturer.

Relative flow coefficient: The ratio between the flow coefficient (Cv value) at specified stroke and the flow coefficient (Cv value) at rated stroke.

Seat leakage: The amount of fluid flowing through the valve when it is in the fully closed position at a specified differential pressure and temperature.

Spring coefficient: Change in spring force per unit change in spring length. In thin-film actuator regulator valves, the spring coefficient is usually expressed in pounds of force per inch of compression.

Stem unbalance force: The net force exerted on the stem at any position due to fluid pressure.

Contractile section: The shunt beam at the highest velocity, hydrostatic pressure, and lowest cross-sectional area. In a regulator, the contractile section is usually downstream of the actual physical limit.

ANSI: Abbreviation for American National Standards Organization.

API: Short for American Petroleum Organization.

ASME: American Society of Mechanical Engineers.

ASTM: American Society for Testing and Materials.

Automatic control system: A control system that works without human intervention.

Bode plot: A plot of the logarithmic amplitude ratio and phase Angle values of a transformation function on a logarithmic baseline. This is the most common form of graphically representing frequency response data.

Check curve: Graphical representation of check results. The steady-state output of a device is expressed as a function of its steady-state input. The curve is usually expressed in terms of the output range of a percent versus the input range of a percent.

Check cycle: Using the known values of the variable being measured and recording the corresponding output reading values in the direction of rise and fall within the range of the instrument the check cycle curve can be obtained by first increasing and then decreasing the input to the device. It is usually expressed in terms of the output range of a percentage versus the input range of a percentage. It provides a measure of the return difference.

Gap flow rate: The flow rate below the minimum controllable flow rate when the interceptor element is not fastened.

Controller: A device that automatically operates to adjust controlled variables.

Enthalpy: A thermal dynamic quantity that is the sum of the internal energy of the body and its volume and pressure: H=U+pVo(also called heat capacity).

Entropy: Theoretical measure of energy that cannot be converted into mechanical work in a thermodynamic system.

Feedback signal: The return signal obtained by measuring the direct controlled variable. For a regulator with a positioner, the feedback signal is usually a mechanical indication of the position of the connecting rod of the interceptor element fed back to the positioner.

FCI: Abbreviation for Fluid Control Organization.

Frequency response characteristic: Frequency dependence between the steady-state sinusoidal input, expressed in amplitude and phase, and the fundamental sinusoidal output it causes. The amplitude and phase shift of the output can be viewed as a function of the input test frequency and used to describe the dynamic behavior of the control device.

Hardness: The ability of a metal to resist plastic deformation, usually in the form of dents. The resistance of plastics and rubber to prongs penetrating their surfaces.

Oscillation: A vibration of appreciable magnitude that persists after the disappearance of an external excitation. Oscillations are sometimes called cycles or limiting cycles. Oscillations are evidence of operation at or near the limit of stability. In the regulator, the instability of the control system or the valve positioner will cause the loading pressure of the actuator to fluctuate, and the oscillation will follow.

ISA: American Institute of Instrumentation. It is now called the International Society of Measurement and Control.

Instrument pressure: The output pressure provided by an automatic regulating valve to make the valve work.

Loading pressure: Pressure used to position the pneumatic actuator. This is the actual pressure acting on the actuator diaphragm or piston. If no valve positioner is used, the loading pressure can be meter pressure.

NACE: Used to stand for American Society of Corrosion Engineers. As the organization has become more international in scope, the term has been changed to international NACE. NACE is no longer an acronym.

0SHA: Abbreviation for Occupational Safety and Health Act (United States).

Operating medium: This refers to the fluid, usually air or gas, used to power the operation of the valve positioner and automatic controller.

Operating limit: The range of operating conditions within which a device can withstand without causing permanent damage to operating characteristics.

The region between two limits, the spacing of which can be measured, accepted, or transmitted, and expressed as upper and lower range values (e.g., 3 to 15Psi; -- 4 to 212°F, -- 40 to 100 ° C).

Repeatability: The closeness of a series of continuous output measurements for the same input value, in the same direction, under the same operating conditions, over the full range of travel. It is usually measured as non-repeatable, but expressed as a percentage scale. It does not include retractions.

Sensitivity: Between the change in output amplitude after a steady state is reached and the change in input that causes the change

Signal: A physical variable whose one or more parameters carry information about another variable represented by the signal.

Signal amplitude sequencing (range) : A mode of action in which two or more signals are generated or two or more terminal control elements are driven by an input signal, and each terminal control element responds continuously, with or without overlap to the amplitude of the input signal.

Range: Arithmetic difference between upper and lower range values (e.g. Range =0 to 150°F, range =150°F; Range =3 to 15PSig, range =12PSig).

Air source pressure: The pressure at the air supply port of a device. Common regulator air source pressure values range from 20 Psig for springs from 3 to 15 Psig to 35 psig for springs from 6 to 30 psig.

Zero error: The error of a device under specified service conditions when the input is a low range value. It is usually expressed as a percentage of the ideal range. Bring positive effects.