Understand VALVE DIAGNOSTIC SMART
HTML clipboardThe controller output (CO) is the input to the valve assembly and the process variable (PV) is the output as shown in figure 1-1. When the term Dead Band is used, it is essential that both the input and output variables are identified, and that any tests to measure dead band be under fully loaded conditions. Dead band is typically expressed as a percent of the input span. Dead Time: The time interval (Td) in which no response of the system is detected following a small (usually 0.25% – 5%) step input.
It is measured from the time the step input is initiated to the first detectable response of the system being tested. Dead Time can apply to a valve assembly or to the entire process. (See T63.) Disk: A valve trim element used to modulate the flow rate with either linear or rotary motion. Can also be referred to as a valve plug or closure member.
Equal Percentage Characteristic*: An inherent flow characteristic that, for equal increments of rated travel, will ideally give equal percentage changes of the flow coefficient (Cv) (figure 1-2). Final Control Element: The device that implements the control strategy determined by the output of the controller. While the final control element can be a damper, a variable speed drive pump, or an on-off switching device, the most common final control element in the process control industries is the control valve assembly. The control valve manipulates a flowing fluid, such as gasses, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point. First-Order: A term that refers to the dynamic relationship between the input and output of a device. A first-order system or device is one that has only one energy storage device and whose dynamic transient relationship between the input and output is characterized by an exponential behavior. Friction: A force that tends to oppose the relative motion between two surfaces that are in contact with each other. The friction force is a function of the normal force holding these two surfaces together and the characteristic nature of the two surfaces. Friction has two components: static friction and dynamic friction. Static friction is the force that must be overcome before there is any relative motion between the two surfaces. Once relative movement has begun, dynamic friction is the force that must be overcome to maintain the relative motion. Running or sliding friction are colloquial terms that are sometimes used to describe dynamic friction. Stick/slip or “stiction†are colloquial terms that are sometimes used to describe static friction. Static friction is one of the major causes of dead band in a valve assembly. Gain: An all-purpose term that can be used in many situations. In its most general sense, gain is the ratio of the magnitude of the output change of a given system or device to the magnitude of the input change that caused the output change. Gain has two components: static gain and dynamic gain. Static gain is the gain relationship between the input and output and is an indicator of the ease with which the input can initiate a change in the Chapter 1. Introduction to Control Valves 4 Figure 1-2. Inherent Valve Characteristics A3449/IL output when the system or device is in a steady-state condition. Sensitivity is sometimes used to mean static gain. Dynamic gain is the gain relationship between the input and output when the system is in a state of movement or flux. Dynamic gain is a function of frequency or rate of change of the input. Hysteresis*:
The maximum difference in output value for any single input value during a calibration cycle, excluding errors due to dead band. Inherent Characteristic*: The relationship between the flow coefficient and the closure member (disk) travel as it is moved from the closed position to rated travel with constant pressure drop across the valve. Typically these characteristics are plotted on a curve where the horizontal axis is labeled in percent travel and the vertical axis is labeled as percent flow (or Cv) (figure 1-2). Because valve flow is a function of both the valve travel and the pressure drop across the valve, conducting flow characteristic tests at a constant pressure drop provides a systematic way of comparing one valve characteristic design to another. Typical valve characteristics conducted in this manner are named Linear, Equal-Percentage, and Quick Opening (figure 1-2). Inherent Valve Gain: The magnitude ratio of the change in flow through the valve to the change in valve travel under conditions of constant pressure drop. Inherent valve gain is an inherent function of the valve design. It is equal to the slope of the inherent characteristic curve at any travel point and is a function of valve travel. Installed Characteristic*: The relationship between the flow rate and the closure member (disk) travel as it is moved from the closed position to rated travel as the pressure drop across the valve is influenced by the varying process conditions. (See Valve Type and Characterization in Chapter 2 for more details on how the installed characteristic is determined.) Installed Valve Gain: The magnitude ratio of the change in flow through the valve to the change in valve travel under actual process conditions. Installed valve gain is the valve gain relationship that occurs when the valve is installed in a specific system and the pressure drop is allowed to change naturally according to the dictates of the overall system. The installed valve gain is equal to the slope of the installed characteristic curve, and is a function of valve travel.
(See Valve Type and Characterization in Chapter 2 for more details on how the installed gain is determined.) I/P: Shorthand for current-to-pressure (I-to-P). Typically applied to input transducer modules. Linearity*: The closeness to which a curve relating to two variables approximates a straight line. (Linearity also means that the same straight line will apply for both upscale and downscale directions. Thus, dead band as defined above, would typically be considered a non-linearity.)
This is default description text on Padangan Themes, of course you can change this text via you profile administration.