Stem position

The key parts of the positioner/actuator system, shown in Fig. 8-74 7, are (1) an input-conversion network, (2) a stem-position feedback network, (3) a summing junction, (4) an amplifier network, and (5) an actuator. 

The stem-position feedback network converts stem travel to a useful form for the summer. This block includes the feedback linkage. 

Limit Switches and Stem-Position Transmitters Travel-limit switches, position switches, and valve-position transmitters are devices that, when mounted on the valve, actuator, damper, louver, or other throtthng element, detect the component s relative position. The switches are used to operate alarms, signal hghts, relays, solenoid valves, or discrete inputs into the control system. The valve-position transmitter generates a 4-20-mA output that is proportional to the position of the valve. 

A simple example of an area meter is a gate valve of the rising-stem type provided with static-pressure taps before and after the gate and a means for measuring the stem position. In most common types of area meters, the variation of the opening is automatically brought about by the motion of a weighted piston or float supported by the fluid. Two different cyhnder- and piston-type area meters are described in the ASME Research Committee on Fluid Meters Report, op. cit., pp. 82-83. 

Fracture of cement transversely near the distal end of the prosthetic stem or in the area of the medial neck results in a slight downward or medial shift of stem position and further increases the forces upon the stem ... 

A0 The rate constant for adding a new stem to the substrate, that is, the probability per possible stem position per unit time that a new stem will be added. 

Figure 10-18 Flow rate versus stem position for various trim functions.

Stem Position Horizontal Vertical Vertical Vertical Vertical... 

Gas is drawn off the top of the drum through a control valve whose stem position is set by a pressure controller (Fig. 3.7). Liquid comes off the bottom of the tank on level control. 

B. SIZE. Sizing of control valves is one of the more controversial subjects in process control. The flow rate through a control valve depends on the size of the valve, the pressure drop over the valve, the stem position, and the fluid properties. The design equation for liquids (nonflashing) is... 

C. = valve size coelllcient X = valve stem position (fraction of wide open)... 

Chamcteristics. By changing the shape of the plug and the seat in the valve, different relationships between stem position and flow area can be attained. The common flow characteristics used are linear trim valves and equal-percentage trim valves as shown in Fig. 7.7. The term equal percentage comes from the slope of the /Ji, curve being a constant fraction off. 

If constant pressure drop over the valve is assumed and if the stem position is 50 percent open, a linear-trim valve gives 50 percent of the maximum flow and an equal-pcrcentagc-trim valve gives only 15 percent of the maximum flow. The equations for these valves are ... 

TIME FULL OPEN MIN M K 10 VALVE STEM POSITION ... 

Gain Scheduling Adaptive Control is a special application of this procedure. For example we may have a control valve whose characteristic (input signal/valve stem position relationship) is non-linear. In this case, the valve stem position would be measured in order to obtain the gain of the valve (the appropriate relationship must be known) and the valve gain is used then to adjust the gain of the controller. If the auxiliary variable relationships are more complicated then it may be necessary to employ a Programmed Adaptive Control procedure. 

The relation between the valve stem position and the flow through the valve at constant pressure drop is termed the valve characteristic. Two characteristics must be evaluated for valve selection, the inherent and the installed characteristics 67. ... 

The different inherent characteristics (Fig. 7.124) are determined by the geometry of the valve orifice and valve plug and describe the volumetric flowrate through the valve as a function of the stem position with constant pressure drop across the valve. 

Devices mounted on the control valve that interface various forms of input signals, monitor and transmit valve position, or modify valve response are valve control devices. In some applications, several auxiliary devices are used together on the same control valve. For example, mounted on the control valve, one may find a current-to-pressure transducer, a valve positioner, a volume booster relay, a solenoid valve, a trip valve, a limit switch, a process controller, and/or a stem position transmitter. Figure 8-80 shows a valve positioner mounted on tne yoke leg of a spring and diaphragm actuator. 

The stem position feedback network converts stem travel to a useful form for the summer. This block includes the feedback linkage which varies with actuator type. Depending on positioner design, tKe stem position feedback network can provide span and zero and characterization functions similar to that described for the input conversion block. 

Figure 4 Calculation results of integral of secondary helium gas supply and isolation valve stem position...

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