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flow path

FIGURE 33 For a given pressure drop DP across a valve, the globe will show a higher pressure at the vena contracta, making it more likely that cavitation difficulties will be avoided when the vapor pressure is high.

flow path

FIGURE 33 For a given pressure drop DP across a valve, the globe will show a higher pressure at the vena contracta, making it more likely that cavitation difficulties will be avoided when the vapor pressure is high.

depends on valve type: ball and butterfly valves have higher recovery percentages than do globe and angle valves. To avoid cavitation, which is the formation of vapor bubbles near the vena contracta of the valve, followed by a sudden damaging collapse near the metal, the static pressure at the vena contracta must be above the liquid vapor pressure. This is easier to do with a low-recovery valve because the initial pressure drop need not be as high for a given downstream pressure. Several factors have been devised to indicate pressure recovery. One, Cf, the critical flow factor, is the ratio of pressure recovery, varying for different valve openings. Another, Km, the valve recovery coefficient, is the ratio of pressure drop across the valve to pressure drop between valve inlet and vena contracta at that instant when flow begins to be choked by bubble formation. Both of these factors will be higher for globe valves than for ball and butterfly valves, and the factors serve to indicate valve suitability for marginal cavitation service.

actuators The motion needed to change the valve orifice area and to close the valve tightly is produced by an actuator. The types of motion of the valve plug or disk are either linear or rotary (Figure 34), the latter being usually 90° but occasionally as low as 70°. These motions can be effected in several ways. The linear translating motion can result from a cylinder or diaphragm actuator working directly or through linkage (Figure 35). A screw thread at the stem top can convert a rotary motion to linear stem motion, or threads at the stem bottom can engage threads in the valve disk so rotation of the stem moves the disk. Geared electric motor drives (Figure 36), cylinders (Figure 37), and diaphragm-and-spring actuators (Figure 38) are common with ball, plug, and butterfly valves. The solenoid valve (Figure 39) relies on an electromagnetic force to move a disk directly or to initiate the piloting action that allows line fluid to open the valve. The piloted solenoid valve (Figure 40) relies on fluid pressures to open the main orifice.

The simplest actuator is the manually powered operator, which is a gear box. It provides enough mechanical advantage to overcome starting friction and to seal the valve tightly. Provision for an impact blow to initiate opening is found in some operators.

The choice of actuator depends first on whether the service is on-off or modulating. For on-off service, the actuator need have only enough force to overcome breakaway force or torque and sufficient stroke to open the valve fully. Speed of operation is rarely critical, and motion limits can be designed into valve or actuator. Pneumatically (Figure 41) and hydraulically powered actuators usually stroke rapidly but can be slowed in either direction by auxiliary valving or controls. On some pneumatic actuators, times to five minutes are possible. Electric-motor-driven actuators are slower than pneumatic or hydraulic types and require limit switches to stop the motor at the end of travel.

In modulating service, where the actuator must hold a control valve setting, demands are more severe. The speed of movement, expressed as stroking speed, is sometimes an important factor, especially in emergency shutdown or bypass. The stability of an actuator is partly its ability to hold the valve setting under fluctuating or buffeting

FIGURE 34 Rotary actuator with sealed blade (Vomox)
FIGURE 35 Linkage connects actuator and valve stem (Masoneilan International)
FIGURE 36 Electric-motor-driven actuator with mechanism for limiting torque (Philadelphia Gear)

loads from the fluid. Damping and high spring rate can help with this. The relation of the natural frequency of the actuator and its adjacent elements to the frequencies encountered in controlling the flow or those experienced from fluid buffeting can also be important.

Stroke length is also a factor. Although the disk in a globe valve or similar type need lift only one-quarter of the seat diameter to give adequate area for full flow, this distance in large valves will exceed the 2-in (5-cm) stroke of most diaphragm actuators. If linkage

FIGURE 37 Cylinder actuator and linkage for ball valve (Jamesbury)

FIGURE 38 Diaphragm-and-spring actuator, reversible type (Foxboro)

FIGURE 37 Cylinder actuator and linkage for ball valve (Jamesbury)

FIGURE 38 Diaphragm-and-spring actuator, reversible type (Foxboro)

FIGURE 39 Solenoid valve for three-way operation is direct-acting (Skinner Precision Industries).

FIGURE 40 Piloted solenoid valve relies on fluid pressure to open main orifice (Magnetrol Valve).

FIGURE 39 Solenoid valve for three-way operation is direct-acting (Skinner Precision Industries).

FIGURE 40 Piloted solenoid valve relies on fluid pressure to open main orifice (Magnetrol Valve).

with its lever advantage is needed to increase thrust, the problem becomes more acute. A cylinder or electric actuator is then necessary.

The source of power for the actuator influences choices too. One standard may be 3- to 25-lb/in2 (0.2- to 1.7-bar) instrument air pressure, whereas in other cases much higher air or oil pressure is available.

FIGURE 41 Opposed pistons drive rack-and-gear mechanism for 90° rotation in this pneumatic actuator (Worcester Controls).

The diaphragm-and-spring actuator (Figure 38) is a very common type and has several important advantages. The spring can be pre-loaded to cause the valve to either close or open fully (be fail-safe) if control air fails. The spring also opposes the force generated by the control signal, doing so in a manner giving proportional control. Of course, the spring's opposition negates much of the force available on the diaphragm, but the simplicity and low friction of this actuator have made it very popular. Most modern types are reversing: The fail-safe action can easily be changed from open to close by turning the diaphragm enclosure upside down and reassembling.

positioners With actuators that lack an internal spring, a positioner is needed to adjust the valve position to the desired value. A positioner is a small feedback system that receives an input signal (usually air pressure but sometimes an electric signal) from a controller and adjusts a valve stem position to a prearranged corresponding value. The valve stem position, which is the output, need not vary linearly with input pressure; cams in the mechanism can give a wide range of stem position functions and thus apparently change the characteristic of the valve.

The positioner is a necessity for actuators in which the valve stem position is not a function of the actuator fluid pressure or electric current magnitude. Examples are pneumatic and hydraulic cylinders and electric motors. Even though positioners are not inherently necessary on the diaphragm-and-spring actuator, they are sometimes applied. The reasons for the application hold for other types of actuators, too.

Friction in the actuator diaphragm cylinder or valve stem packing is one reason. The positioner can cut the dead-band from values such as 5 to 15% to less than 0.5% and can give repeatability of 0.1% of full span.

Need for more force to close a single-seat valve tightly is another reason for using a positioner. If loading pressure must be increased above a standard 15 lb/in2 gage (1-bar) value, the positioner can control air at a higher pressure and thus greatly increase the stem force.

Split-range operation, in which different valves operate over different parts of the controller output pressure range, calls for positioners. Reversal of valve action, too, is easily achieved with positioners. A positioner can also speed up valve response because the low-volume positioner will act faster than the high-volume valve actuator and can open a larger air supply than that in the controller. A pneumatic amplifier or booster is an alternative way to do this. Finally, change in control valve characteristic, such as from linear to equal-percentage, is also possible through a positioner cam.

Because a positioner is another control loop added to a system, its effect under dynamic conditions may worsen overall performance. If changes or oscillations are slow, the positioner and actuator will follow them accurately and correct for them. For rapid changes, however, the effect of the positioner can be harmful. Evidence shows that if the natural fre-

FIGURE 42 Force-balance positioner with spool and sleeve pilot valve (Masoneilan International)

quency of the complete process control loop is more than 20% of the frequency at which the gain in the positioner-actuator system is attenuated 3 dB, the positioner will impair system performance. Liquid level control systems are more likely to benefit from positioners than are flow or pressure control systems.

Pneumatic positioners may be classified as force-balance or motion-balance types. in the force-balance type, the force in the range spring inside the positioner is balanced against control air pressure inside a bellows or double-diaphragm assembly. in the force-balance positioner of Figure 42, the feedback spring, which can be adjusted for range of pressure and initial actuation pressure, is attached to the actuator diaphragm plate at the bottom and to a double-diaphragm assembly at the top. The upper diaphragm has twice the area of the lower; introduction of signal air from the controller into the space between the two diaphragms forces the assembly upward very slightly but enough to lift a pilot valve at the positioner top and allow supply air pressure to flow through and downward past the feedback spring to press the actuator diaphragm down until forces balance. A reduction in signal pressure allows the double-diaphragm assembly to move downward, first closing the pilot valve and then exposing a hole through the pilot valve stem. Air then bleeds out from the actuator to atmosphere until forces are again in balance.

In the force-balance positioner of Figure 43, flexure strips and a bell crank convert the vertical actuator motion to a horizontal motion in the double-diaphragm assembly at the top left and the supply valve at the right.

A motion-balance positioner showing the application of a cam to impart a characteristic is shown in Figure 44. The cam at the lower right is pivoted and caused to rotate by the

FIGURE 44 Motion-balance positioner with cam and diaphragm comparator (ITT Hammell Dahl Conoflow)

actuator stem motion. Supply pressure enters the valve assembly block at the left and goes to both booster valves. It also bleeds through a restrictor and nozzle at the bottom of the valve assembly block. The position of the flapper before the nozzle determines the pressure in the diaphragm comparator at the right of the valve assembly block. An increase in signal air pressure to the bottom bellows moves one end of a balance beam and pushes the flapper closer to the nozzle. This builds pressure in the diaphragm comparator and moves it to the right. A linkage transforms this motion into a motion that opens the booster valve

FIGURE 45 Motion-balance positioner with flexure assembly (Foxboro)

to supply air to the cylinder actuator top and permits air to exhaust from the cylinder bottom. The actuator stem moves down until the feedback cam, aided by the comparator linkage, has repositioned the flapper in front of the nozzle.

In another motion-balance positioner (Figure 45), signal air pressure from 3 to 15 lb/in2 (0.2 to 1 bar) gage in a bellows opposes a flexure assembly on a shaft that is rotated by the valve stem motion. An increase in signal air pressure to the bellows expands it and moves the lower end of the flexure away from a flapper, permitting the flapper to move toward a nozzle. The resultant buildup of air pressure on the diaphragm of the control relay at the lower right closes the exhaust port and opens the supply port to allow air at fully supply pressure to pass to the actuator. The valve stem motion rotates the flexure and thereby shifts the tip touching the flapper. The flapper assumes an equilibrium position proportional to the signal air pressure.

The pneumatic amplifier or booster is a special kind of regulator valve that develops an output air pressure proportional to the input signal pressure. It can be used to boost pressure on an actuator for faster action in cases where the instrument tubing is small-bore and long and the actuator volume is large.

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