Compendium of Function Terms and Characteristics for Control Valves

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Compendium of functional and characteristic terms for control valves. Additional information from fellow marine engineers: handshake, dynamic unbalance force: the force that arises on the valve element at any given opening degree due to the pressure of the process fluid; effective area: in diaphragm actuators, the effective area refers to the part of the diaphragm 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 it. Molded diaphragms exhibit a smaller change in effective area compared to flat diaphragms; therefore, molded diaphragms are recommended. Air loss – closed: A state in which, when the driving energy is lost, the valve’s shut-off element moves to the closed position. De-pressurization – Open: 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 of a disruption in the power supply, 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. Failure–safety operation modes 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 described 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 pounds of pressure drop per square inch, multiplied by the number of U.S. gallons of 60°F water that flow through the valve per minute. High-pressure recovery valve: A valve design that, thanks to its streamlined internal profile 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. Intrinsic diaphragm pressure range: The maximum and minimum pressures acting on the diaphragm to produce the rated valve stem stroke, 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 characteristics: The relationship between flow rate and the stroke of the shutoff element, as the valve moves from the closed position to its rated stroke, while the pressure drop across the valve remains constant. Operating pressure range of the diaphragm: The high and low values of pressure acting on the diaphragm to produce the rated spool travel, under the specified operating conditions applied to 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. Low-pressure recovery valve: A type of valve structure in which turbulence generated by the flow channel profile dissipates a large portion of the fluid 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 capability. Spring setting range: The adjustment range of the actuator spring for 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 force in either direction. Modified parabolic flow characteristic: A flow characteristic that provides an equal-percentage characteristic at the lower range of the valve travel, and a linear characteristic at the upper range of the valve travel. 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 a rotary valve structure, the linear extension of the actuator pushrod moves the ball or valve disc toward the closed position. Also known as positive action. Push down to open the structure: a direct-acting valve design. 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). 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 effectively has a tunable ratio of 100:1. The adjustable ratio can also be expressed as the ratio between the maximum and minimum controllable flow rates. Rated travel: The distance that the valve’s shut-off element 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. Valve seat leakage rate: The amount of fluid flowing through the valve when it is in the fully closed position under specified pressure difference and temperature. Spring constant: The change in spring force per unit change in the length of the spring. 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 vena contracta is usually located downstream of the actual physical restrictions. ANSI: Abbreviation for the American **Standards Organization. API: Abbreviation for the American Petroleum Institute. ASME: Abbreviation for the American Society of Mechanical Engineers. ASTM: Abbreviation for the American Society for Testing and Materials. Automatic control system: A control system that can operate without human intervention. Bode plot: A graph of the logarithmic magnitude ratio and phase angle values of a transfer function on a logarithmic scale. This is the most common form of graphical representation for frequency response data. Verification curve: A graphical representation of the verification results. The steady-state output of a device is expressed as a function of its steady-state input. This curve is usually represented in the form of a percentage output range versus a percentage input range. Verification cycle: Within the range of the instrument, in an upward then downward direction, known values of the variable being measured are used, and the corresponding output readings are recorded. The verification cycle curve can be obtained by first increasing and then decreasing the input to the device. It is usually expressed in the form of a percentage output range relative to a percentage input range. It provides a measurement of hysteresis. Gap flow: The flow rate that is lower than the minimum controllable flow rate when the shutoff element is not seated. Controller: A device that operates automatically to adjust the controlled variable. Enthalpy: a thermodynamic quantity that is the sum of the internal energy of the valve body and the product of its volume and pressure: H=U+pVo (also known as heat capacity). Entropy: A theoretical measure of energy in a thermodynamic system that cannot be converted into mechanical work. Feedback signal: The return signal obtained by measuring the directly controlled variable. For a control valve with a positioner, the feedback signal is usually a mechanical indication of the position of the valve plug’s connecting rod, which is fed back to the positioner. FCI: Abbreviation for Fluid Control Organization. Frequency response characteristics: the frequency dependence between a steady-state sinusoidal input, expressed in terms of amplitude and phase, and the resulting fundamental sinusoidal output. The amplitude and phase shift of the output can be regarded as a function of the input test frequency, and are 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 indentations). The ability of plastics and rubbers to resist sharp points from penetrating their surface. Oscillation: A vibration with a noticeable amplitude that persists even after the external excitation has ceased. Oscillation is sometimes referred to as a cycle or limit cycle. Oscillation is evidence of operating at or near a stable limit. In control valves, instability in the control system or valve positioner can cause fluctuations in the actuator’s loading pressure, leading to oscillations as a result. ISA: Abbreviation for the Institute of Instrumentation of the United States. It is now known as the International Society for Measurement and Control. Gauge pressure: The output pressure provided by an automatic control valve to operate the valve. Loading pressure: The pressure used to position the pneumatic actuator. This is the actual pressure acting on the diaphragm or piston of the actuator. If a valve positioner is not used, the loading pressure can be the instrument pressure. NACE: stands for the National Association of Corrosion Engineers. As the organization’s scope became increasingly international, this term was changed to International NACE. NACE is no longer an abbreviation. 0SHA: Abbreviation for the Occupational Safety and Health Act (United States). Working medium: This refers to the fluid, usually air or gas, that is used to power the operation of valve positioners and automatic controllers. Operating limit: The range of operating conditions that a device can withstand without suffering permanent damage to its operating characteristics. Range: The area between two limits; the distance within this range can be measured, accepted, or transmitted. It is represented by upper and lower range values (e.g., 3 to 15 Psi) ； —4 to 212°F, —40 to 100°C). Repeatability: The degree of closeness among a series of consecutive output measurements, for the same input value, over the entire range of operation, in the same direction and under the same operating conditions. It is usually measured as non-repeatability, but expressed in terms of a percentage range. It does not include hysteresis. Sensitivity: The signal whose change in output amplitude, after reaching a steady state, corresponds to a change in the input that causes such a change; it is a physical variable whose one or more parameters carry information about another variable represented by that signal. Signal amplitude sorting (multiplexing): A mode of operation in which two or more signals are generated, or two or more terminal control elements are driven by an input signal, with each terminal control element responding continuously to the amplitude of that input signal, with or without overlap. Range: The arithmetic difference between the upper and lower limit values (e.g.: Range=0 to 150°F, Range=150°F ； Range=3 to 15 PSig, span=12 PSig). Gas source pressure: The pressure at the gas supply outlet of a device. The typical supply pressure value for control valves is 20 Psig for spring settings in the range of 3 to 15 Psig, and 35 Psig for spring settings in the range of 6 to 30 Psig. Zero error: The error of a device under specified operating conditions when the input is at low values; it is usually expressed as a percentage of the ideal range.