Instrumentation Sensors Book

13.3Flow Control Valves

When a change in a measured variable with respect to a reference has been sensed, it is necessary to apply a control signal to an actuator to make corrections to an input controlled variable, via a valve bringing the measured variable back to its preset value. In most cases, any change in the variables (e.g., temperature, pressure, mixing ingredients, and level) can be corrected by controlling flow rates. In general, the actuators used to control valves for flow rate control, and can be electrically, pneumatically, or hydraulically controlled. Actuators can be self-operating in local feedback loops, in such applications as temperature sensing with direct hydraulic or pneumatic valve control, pressure regulators, and float level controllers. The two most common types of variable aperture devices used for flow control are the globe valve and the butterfly valve.

13.3.1Globe Valve

The globe valve’s cross section is shown in Figure 13.7(a). The actuator controlling the valve can be driven electrically (using a solenoid or motor), pneumatically, or hydraulically. The actuator determines the speed of travel and the distance that the valve shaft travels. The globe valve can be designed for quick opening operation, for equal percentage operation, or with a linear relationship between flow and lift, or any combination of these. In equal percentage operation, the flow is proportional to the percentage the valve is open, or there is a logarithmic relationship between the flow and valve travel. The shape of the plug determines the flow characteristics of the actuator, and is normally described in terms of percentage of flow versus percentage of lift or travel. Various valve configurations for quick opening operation, for equal percentage operation, or with a linear relationship between flow and lift, are shown in Figure 13.8.

The valve plug shown in Figure 13.7(a) gives a linear relationship between flow and lift. The valve characteristic is given in Figure 13.7(b). Shown in the graph are the characteristics for a quick opening plug and an equal percentage plug, illustrating some of the characteristics that can be obtained from the large number of plugs that are available. The selection of the type of control plug should be carefully

Figure 13.7 (a) Cross section of a globe valve with a linear flow control plug, and (b) different flow patterns for various plugs versus plug travel.

which is used to switch either of two incoming flows to a single output. Figure 13.9(b) illustrates a three-way valve. In the neutral position, both exit ports are held closed by the spring; when the valve stem moves down, the top port is opened; and when the valve stem moves up from the neutral position, the lower port is opened.

Other types of globe valves are the needle valve, which have a diameter from 1/8 to 1 in; the balanced cage-guided valve; and the split body valve. In the cage-guided valve, the plug is grooved to balance the pressure in the valve body, and the valve has good sealing when shut off. The split body valve is designed for ease of maintenance, and can be more cost-effective than the standard globe valve, but pipe stresses can be transmitted to the valve and cause it to leak. Globe valves are not well-suited for use with slurries [1].

13.3.2Butterfly Valve

The butterfly valve consists of a cylindrical body with a disk the same size as the internal diameter of the valve body, mounted on a shaft that rotates perpendicular to the axis of the body. The action is similar to a louver damper. The disk pivots to the vertical position to shut off any flow, and to the horizontal position when fully open. The valve is shown in Figure 13.10(a), and its flow versus travel characteristics are shown in Figure 13.10(b). The relation between flow and lift is

% flow

(b)

Figure 13.10 (a) Cross section of a butterfly valve, and (b) its flow versus travel characteristics.

approximately equal up to approximately 50% open, after which it is linear. Butterfly valves offer high capacity at low cost, are simple in design, are easy to install, and have tight closure. The torsion force on the shaft increases until the valve is open 70°, and then reverses. Butterfly valves have a limited pressure range, and are not used for slurries. The pressure produces a strong thrust on the valve bearings.

13.3.3Other Valve Types

A number of other types of valves are in common use, including the weir-type diaphragm, the ball valve, and the rotary plug valve. The cross sections of these valves are shown in Figure 13.11.

A weir-type diaphragm valve is shown in Figure 13.11(a). The valve is shown open, and can be closed by forcing a flexible membrane down onto a lower flexible membrane, using a pincer action. A flexible membrane also can be forced down onto a weir. Diaphragm valves are good low-cost choices for slurries and liquids with suspended solids, but tend to require high maintenance and have poor flow characteristics.

A one-piece ball valve is shown in Figure 13.11(b). The valve is a partial sphere that rotates, and the valve tends to be slow to open. The ball valve is available in other configurations with various shaped spheres for different flow characteristics. The valve is good for slurries and liquids with solid matter because of its self-clean- ing operation. Ball valves have tight turnoff characteristics, are simple in design, and have greater capacity than similar-sized globe valves.

Compressor

Flexible membrane

Flow

Flow

Valve shapes

Ball valve

Flow

(b)

Figure 13.11 (a) Diaphragm valve, (b) one-piece ball valve, and (c) rotary plug valve.

have lower current handling capability than electronic power devices, and have a limited switching life. Relays typically are used to switch low-power signal lines, whereas contactors are used to switch higher powers, such as power to motors. In electronic devices, the problem of electrical isolation between drive circuits and output power circuits can be easily overcome by design or with the use of optoisolators. Electronic power devices have excellent longevity and many advantages due to their high switching speeds in variable power control circuits.

13.4.1Electronic Devices

A number of electronic devices, such as silicon controlled rectifiers (SCR), TRIAC, and MOS devices, can be used to control several hundred kilowatts of power from low-level electrical signals. Electronic power control devices fall into two categories. First, a triggered devices such as the SCR and TRIAC, which are triggered by a pulse on the gate into the conduction state. Once triggered, these devices can be turned off only by reducing the anode/cathode current to below their sustaining current (i.e., when the supply voltage/current drops close to zero). These devices can block high reverse voltages. They are used extensively in ac circuits, where the supply regularly transcends through zero, automatically turning the device Off. The second group of devices include: Darlington Bipolar Junction Transistors (BJT), Power MOSFET, Insulated Gate Bipolar Transistors (IGBT), and MOS-Controlled Thyristors (MCT). These devices are turned On and Off by an input control signal, but do not have the capability of high reverse voltage blocking. This group of devices is more commonly used with dc power supplies, or is biased to prevent a reverse voltage across the device.

The SCR is a current-operated device, and only can be triggered to conduct in one direction. When used with an ac supply, it blocks the negative half cycle, and only will conduct on the positive half-cycle when triggered. Once triggered, the SCR remains On for the remaining portion of the half-cycle. Figure 13.14(a) shows the circuit of an SCR with a load [2]. Figure 13.14(b) shows the effects of triggering on the load voltage (VL). By varying the triggering in relation to the positive half-cycle, the power in the load can be controlled from 0% to 50% of the total available power. Power can be controlled from 50% to 100% by putting a diode in parallel with the SCR to conduct current on the negative half-cycles. Light-activated SCRs also are available.

Figure 13.14 (a) SCR circuit with load, and (b) its waveforms.

R2 VC

SCR

C R3

VL

Figure 13.16 Bridge circuit for SCR control (a) using full wave rectification, and (b) corresponding waveforms.

the SCR only sees positive half-cycles. The SCR is triggered during every half-cycle, and is turned Off every half-cycle when the supply voltage goes to zero. As shown, the system is controlled by a low-level signal coupled by an optoisolator. The potentiometer R and capacitor C set the triggering point, and since the SCR only sees positive voltages, the diode is not required. For cost savings, the zener diode is omitted. As in the previous figure, resistor R can be connected to either side of the load.

The DIAC is a semiconductor device developed for trigger control, primarily for use with TRIACs. Figure 13.17(a) shows the symbol for the device, and Figure 13.17(b) shows the device’s characteristics. The DIAC is a two-terminal symmetrical switching device. As the voltage increases across the device, little current flows until the breakdown voltage VL is reached, at which point the device breaks down and conducts as shown. The breakdown occurs with both positive and negative voltages. The breakdown voltage of the DIAC is used to set the trigger voltage for the TRIAC. When the device breaks down, the TRIAC triggers.

TRIACs can be considered as two reversed SCRs connected in parallel. They can be triggered on both the positive and negative half-cycles of the ac waveform. A circuit for triggering a TRIAC is shown in Figure 13.18(a), with the associated waveforms shown in Figure 13.18(b). The TRIAC can be used to control power to the

V

Figure 13.17 DIAC used in SCR and TRIAC triggering circuits: (a) symbol, and (b) characteristic.