Spring setting range: adjusting valve (control valve) actuator spring adjustment range to balance the actual process force.

Circulation capacity: The rated flow through the valve under specified conditions.

Interstitial flow: When the shutoff element is not seated, the flow rate is lower than the minimum controllable flow rate.

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

Double acting actuator: An actuator that can provide power in any direction.

Dynamic unbalance force: The net force generated on the spool due to the pressure of the process fluid at any specified opening.

Effective area: In the membrane actuator, the effective area is the area of the diaphragm that effectively produces the output force. The effective area of the diaphragm may change with its movement, and is usually the largest at the beginning of the stroke and smallest at the end of the stroke. Molded diaphragms have a smaller effective area change than flat diaphragms, so it is recommended to use molded diaphragms.

Equal percentage flow characteristics: (See "Process control terms": "Equal percentage flow characteristics".)

Cut off the air-off: Such a state: When the driving energy is lost, the valve shut-off element moves to the closed position.

Cut off the air-open：Such a state: when the driving energy is lost, the valve shut-off element moves to the open position.

Cut off the air-safety: A kind of valve and its actuator

Characteristic：When the drive energy supply is interrupted, the valve shut-off element will be moved to fully closed, fully open, or left in the last position. Any position is considered necessary to protect the process. The fail-safe mode of action may require the use of auxiliary controls connected to the actuator.

Flow characteristics：When the percentage of rated stroke changes from 0 to 100%, the relationship between the flow through the valve and the percentage of rated stroke. This term should always be expressed as inherent flow characteristics or installation flow characteristics.

Flow coefficient (CV value)：A constant (CV value) for a given stroke related to the geometry of the valve can be used to measure the flow capacity. It is the number of US gallons of 60°F water flowing through the valve per minute at a pressure drop of 1 pound per square inch.

High pressure recovery valve：A valve structure that disperses relatively little fluid energy due to its streamlined internal contour and minimal fluid turbulence. Therefore, the pressure downstream of the valve's contraction flow section will be restored to a high percentage value of the inlet pressure. Direct flow valves, such as rotary ball valves, are typical high pressure recovery valves.

Inherent die pressure range：When the pressure in the valve body is atmospheric pressure, it acts on the diaphragm to generate the high and low pressure values of the rated spool stroke. This range usually refers to the spring setting range, because when the valve is set in the working range, this range will be the valve's operating range.

Inherent flow characteristics：When the pressure drop across the valve is constant, as the valve moves from the closed position to the rated stroke, the relationship between the flow rate and the stroke of the shutoff element. Installation diaphragm pressure range: Under the specified working conditions of the valve body, the high and low pressure values that act on the diaphragm to generate the rated spool stroke. Due to the force acting on the shut-off element, the inherent diaphragm pressure range may be different from the installed diaphragm pressure range.

Installation flow characteristics：When the pressure drop across the valve is affected by changing process conditions, the relationship between the flow rate and the shut-off element as the valve moves from the closed position to the rated stroke.　

Leakage：(See "valve seat leakage".) Linear flow characteristics: (See "process control terms": "linear characteristics".)

Low pressure recovery valve：A valve structure that disperses a large part of the fluid energy due to the turbulence generated by the contour of the fluid channel. As a result, the pressure downstream of the valve's contraction section will return to a smaller percentage of inlet pressure than a valve with more streamlined passages. Although the structure of each valve is different, ordinary straight-way valves usually have low pressure recovery capabilities.

Modified parabolic flow characteristics：A flow characteristic that provides equal percentage characteristics at the low stroke of the intercepting element, and linear characteristics at the high stroke of the intercepting element.

Normally closed valve: (See "Lost Air-Close".)

Normally open valve: ("See Lost Air-Open".)

Push down to close the structure：A straight-through valve structure in which the shut-off element is located between the actuator and the seat ring, so that the push rod of the actuator will move the shut-off element to the seat ring, and finally close the valve. The term can also be used for rotary valve structures. In a rotary valve structure, the linear extension of the actuator rod will move the ball or valve plate to the closed position. (Also called positive action.)

Push down to open the structure：A straight-through valve structure. Its seat ring is located between the actuator and the shut-off element, so that the push rod of the actuator will move the shut-off element away from the valve seat, thus opening the valve. The term can also be used for rotary valve structures. In a rotary valve structure, the linear extension of the actuator rod will move the ball or valve plate to the open position. (Also called reaction).

Quick opening flow characteristics: (See "Process Control Terminology": "Quick Opening Characteristics".)

Adjustable ratio: The ratio between the maximum flow coefficient (CV value) and the minimum flow coefficient (CV value) when the deviation from the specified flow characteristics does not exceed the specified limit. When the flow rate is increased to 100 times the minimum controllable flow rate, a valve that can still be well controlled has an adjustable ratio of 100:1. The adjustable ratio can also be expressed as the ratio between the maximum and minimum controllable flow.

Rated flow coefficient (CV value): The flow coefficient (CV value) of the valve under the rated stroke.

Rated stroke: The distance that the valve shut-off element moves from the closed position to the rated fully open position. The rated fully open position is the maximum opening recommended by the manufacturer.

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

Valve seat leakage: The amount of fluid that flows through the valve when the valve is in the fully closed position under the specified pressure difference and temperature.

Spring constant: The change in spring force when the length of the spring changes per unit. In the diaphragm actuator control valve (control valve), the spring coefficient is usually expressed in pounds force per inch of compression.

Stem unbalanced force: The net force generated on the valve stem at any position due to fluid pressure.

Shrinkage section:  The part of the stream where the flow velocity, hydrostatic pressure, and cross-sectional area are the smallest. In a regulating valve (control valve), the contraction flow section is usually located downstream of the actual physical limit.

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