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Spring setting range: The adjustment range of the spring in the actuator of the control valve, used to balance the actual process force. Flow capacity: The rated flow rate through the valve under specified conditions. Gap flow: The flow rate that is below the minimum controllable flow rate when the shutoff element is not in place. Diaphragm pressure range: The difference between the maximum and minimum values of the diaphragm pressure. This can be considered an inherent or installed feature. Double-acting actuator: An actuator that can provide power in either direction. Dynamic unbalanced force: The net force exerted on the valve stem at any specified opening, due to the pressure of the process fluid. Effective area: In a diaphragm actuator, the effective area is the part of the diaphragm surface that actually generates 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. Molded diaphragms exhibit a smaller change in effective area compared to flat diaphragms; therefore, molded diaphragms are recommended. Equal percentage flow characteristic: (see “Process Control Terminology”: “Equal Percentage Flow Characteristic”). ) Loss of pressure – closed: A state in which, when the driving energy is lost, the valve’s shut-off element moves to the closed position. De-pressurized – Open: This is a state in which, when the driving energy is lost, the valve’s shut-off element moves to the open position. Loss of air – safety: A characteristic of valves and their actuators; in the event that the power supply for actuation is interrupted, it causes the valve’s shut-off element to move to a fully closed, fully open position, or to remain in its previous position. Any of these positions is considered necessary to protect the process. The fail-safe operation mode may require auxiliary control connected to the actuator. Flow characteristics: The relationship between the flow rate through the valve and the percentage of rated stroke, as the percentage of rated stroke varies from 0 to 100%. This term should always be expressed as inherent flow characteristics or installed flow characteristics. Flow coefficient (CV value): A constant related to the geometric structure of the valve, specific to a given stroke, which can be used to measure the flow capacity. It is the number of US gallons of 60°F water that flow through the valve per minute under a pressure drop of 1 pound per square inch. High-pressure recovery valve: A valve design that, thanks to its streamlined internal geometry and minimal fluid turbulence, dissipates relatively little fluid energy. Therefore, the pressure downstream of the valve’s constriction section will return to a very high percentage of the inlet pressure. DC valves, such as rotary ball valves, are typical high-pressure recovery valves. Inherent diaphragm pressure range: The high and low values of pressure acting on the diaphragm to produce the rated spool travel when the pressure inside the valve body is at atmospheric pressure. This range usually refers to the spring setting range, because when the valve is set within this operating range, it will be the range of motion for the valve. Inherent flow characteristic: the relationship between flow rate and the stroke of the flow-blocking element as the valve moves from the closed position to its rated stroke, while the pressure drop across the valve remains constant. Installation diaphragm pressure range: The high and low values of pressure acting on the diaphragm to produce the rated spool travel, under the specified operating conditions of the valve body. Due to the forces acting on the shutoff element, the inherent diaphragm pressure range may differ from the installed diaphragm pressure range. Installation flow characteristics: The relationship between flow rate and the flow-blocking element, as the pressure drop across the valve is affected by varying operating conditions, while the valve moves from the closed position to its rated position. Leakage rate: (See “Seat leakage rate”. )Linear flow characteristic: (see “Process Control Terminology”: “Linear characteristic”). ) Low-pressure recovery valve: A type of valve structure in which turbulence generated by the profile of the fluid passage dissipates a large portion of the fluid’s energy. As a result, the pressure downstream of the valve’s constriction section returns to a lower percentage value of the inlet pressure compared to valves with more streamlined channels. Although the structure of each valve varies, ordinary straight-through valves generally have low pressure recovery capacity. Modified parabolic flow characteristic: a flow characteristic that provides an equal percentage characteristic at the low stroke position of the shut-off element, and a linear characteristic at its high stroke position. Normally closed valve: (see “Loss of pressure – Closure”). ) Normally open valve: (\"See Lost – Open.\" ) Downward push closure mechanism: A straight-through valve design in which the shut-off element is located between the actuator and the seat ring; thus, when the actuator’s push rod moves forward, it pushes the shut-off element toward the seat ring, thereby closing the valve. This term can also be used for rotary valve structures. In the structure of rotary valves, the linear extension of the actuator rod moves the ball or valve disc toward the closed position. (Also known as direct action. ) Push down to open the structure: a direct-acting valve structure. Its seat ring is located between the actuator and the shut-off element; thus, when the actuator’s push rod moves forward, it displaces the shut-off element from the seat, thereby opening the valve. This term can also be used for rotary valve structures. In the structure of rotary valves, the linear extension of the actuator rod moves the ball or valve disc toward the open position. (Also known as reaction.) Fast-opening flow characteristic: (see “Process Control Terminology”: “Fast-opening characteristic”). ) Adjustable ratio: The ratio between the maximum flow coefficient (CV value) and the minimum flow coefficient (CV value), provided that the deviation from the specified flow characteristics does not exceed the prescribed limits. When the flow rate increases to 100 times the minimum controllable flow rate, a valve that can still be controlled well has a control ratio of 100:1. The adjustable ratio can also be expressed as the ratio between the maximum and minimum controllable flow rates. Rated flow coefficient (CV value): The flow coefficient (CV value) of the valve at its rated stroke. Rated travel: The distance that the shut-off element of the valve moves from the closed position to the rated fully open position. The rated full-open position is the maximum opening degree recommended by the manufacturer. Relative flow coefficient: The ratio between the flow coefficient (CV value) at a specified stroke and the flow coefficient (CV value) at the rated stroke. Seat leakage rate: The amount of fluid that flows through the valve when it is in the fully closed position under specified pressure differences and temperatures. Spring constant: The change in spring force per unit change in spring length. In thin-film actuator control valves, the spring coefficient is typically expressed in pounds per inch of compression. Valve stem unbalanced force: The net force acting on the valve stem at any position due to fluid pressure. Constriction section: the part of the flow bundle where the flow velocity is highest, and the hydrostatic pressure and cross-sectional area are lowest. In a control valve, the throat section is usually located downstream of the actual physical limitations.