Definition of Control Valves
Unlike valves in a piping system that primarily serve to shut off, drain, fill, or divert, control valves are a part of an automated control system. They are considered the ‘‘final control element’’ in an automated and usually very sophisticated ‘‘control loop.’’ Aside from the control valve, the ‘‘loop’’ consists of a transmitter that measures the variable to be controlled (usually pressure, flow, level, or temperature) and a controller (nowadays a computer of sorts). Following an error in the variable to be controlled (such an error being sensed by the transmitter), the controller sends a signal change to the control valve which, in turn, responds by altering the flow rate through the valve sufficiently to restore the desired variable (such as pressure, for example).
Control valves have basically three interactive components: (1) a valve body sub assembly (either with a reciprocating or rotating stem), (2) an actuating device (usually a spring diaphragm type), (3) a valve positioner (an instrument that converts an electronic control signal from a controller, or computer, into an air signal to control the position of the control valve stem), and (4) an airset or regulator to supply air pressure to the positioner (see Figure A10.34).
How to Specify Control Valves
The first step in specifying a control valve is to define its function in the given application. In some, it will operate as an on-off valve that opens or closes following the commands of a programmable controller on, say, a batch process. In others, it will be used to remotely set a flow rate in a process—that is, it will be used as a manually controlled variable orifice in a pipe (an open-loop application). Finally, in more sophisticated applications, the control valve will serve as the final control element in a process control loop and respond to the sometimes infinitely small variations of a signal coming from a controller (typically a computer). The signal will be generated in response to a deviation in the desired temperature, pressure, or level of a process fluid as measured by a transmitter.
In the first type of application, any on-off valve with a pneumatic or electrical actuator (say, for example, a ball valve) may suffice. The requirements are to provide tight shutoff (perhaps with a Teflon® seat) to withstand the pressure, temperature, and corrosiveness of the fluid, and, finally, to have sufficient flow capacity. No valve positioner is required (see Figure A10.35).
Open-loop control requires a higher level of sophistication, such as a character- ized valve plug and good repeatability. The latter calls for a valve/actuator combina- tion with low dead band (low friction). A valve positioner, a device that is essentially a stem position controller with an accuracy between 0.5 and 1.0 percent of stem position, may be required. Controlling the stem position may not always assure that the valve plug or ball moves the required amount unless the stem or shaft is pinned or welded to the ball, vane, or plug (unless fluid pressure assures constant contact). See Figure A10.36.
The modulating control valve that is part of a control loop is the most sophisticated device. Typical features are plug or ball with either linear or equal percentage flow characteristic, low-friction packing and actuating devices, and, if required, low-noise or anticavitation features. These are in addition to the previously stated requirements.
Figure A10.37 shows an eccentric rotary plug valve with a low noise restrictor in the valve outlet port. Part of the pressure drop at moderate to high flow rates occurs across this slotted device. The smaller jets created by the slots produce about 10 to 15 dBA less noise than the valve itself.
Flow control is only possible if the control valve can reduce some of the fluid pressure. Such pressure reduction (also used for valve sizing) typically amounts to 5 to 10 percent of the maximum pump pressure. This makes a streamlined valve trim (highly desirable for on-off valves) actually less desirable for control purposes. It takes much higher velocities with a streamlined trim or valve (hence, more noise or cavitation) to achieve a certain pressure drop than with a nonstreamlined valve. Signals from controllers to control valves are 3 to 15 psi (0.2 to 1.0 bar) if pneumatic or 4 to 20 mA if electronic. Digital signals will be used in the future
once the question of fieldbus standardization has been resolved.
Control Valve Styles.
Let’s take a look at the characteristics of some of the most commonly used control valve types:
Globe valve. The globe valve (see Figure A10.34), which is the most widely used type of control valve, has a screwed-on, integrally attached, or cage-supported seat ring, and typically a lathe-turned, single-seated valve plug. Larger valves or high-pressure valves may have designs such that the valve plug is cage-guided and pressure-balanced to reduce actuator force requirements. Globe valves are cost effective in sizes NPS 2 (DN 50) and below and are available in sizes as small as NPS ¹⁄₄ (DN 6) for research applications. End connections are flanged or threaded. High-pressure or high-temperature valves can be welded to the piping. NPS 2 (DN 50) and smaller can be provided with socket-welding or threaded ends. NPS 2¹⁄₂ (DN 65) and larger are generally butt-welded or flanged.
Angle valves. Angle valves are a special variety of globe valves typically having an inlet port at a right angle to the valve stem and a discharge port in line with the valve orifice. Typical applications include flashing and erosive fluids.
Three-way valves. As the name implies, three-way valves are globe valves (or some rotary valves) that have three access ports and two plugs and orifices opposed to each other. Depending on the flow direction, three-way valves may serve as either mixing valves (where two different fluids enter the valve through two of the ports, and discharge as a mixture through the third), or diverting valves around heat exchangers (for example, where a fluid enters at one port and discharges through either the second or the third port).
Eccentric rotary plug valves. Eccentric rotary plug valves are designed especially for modulating control (i.e., they have solid stem connections, low or constant operating torque, a good flow characteristic, and tight shutoff). They feature a lower weight than globe valves and, therefore, have a cost advantage in sizes NPS 3 (DN 80) and above. They are either flanged or wafer-style for installation between flanges (Figure A10.37).
Characterized semispherical ball valves. The characterized semispherical ball valve is another form of ‘‘designed for modulation’’ rotary control valve with a backlash-free stem connection. Here, the seal is a thin metal or plastic ring that engages a segmented rotating ball. A V-notch in the ball surface gives a good repeatable flow characteristic. This valve type is popular in the paper industry and is available in either flanged or wafer-style (Figure A10.36).
Ball valves. Ball valves have a good shutoff characteristic and high flow capacity. As a result, they are a good choice for on-off or sequencing control. End connec- tions are flanged or wafer-style. Metal-seated ball valves are designed for high temperature applications and can be provided with welded connections. Soft- seated ball valves are used for normal liquid or gaseous fluids up to 482°F (250°C) and where tight shutoff is required (Figure A10.35).
Butterfly valves. Except for some special designs with low-torque and low-noise features, butterfly valves for modulating control have to be selected with care. This is because their high-torque (both seating and dynamic) and high-pressure recovery tend to encourage noise and cavitation. A lower-cost valve choice in sizes NPS 6 (DN 150) and above, butterfly valves are typically wafer-style due to their narrow profile.
Actuators. More than 90 percent of all control valves use pneumatic actuating devices—either spring-opposed diaphragm types or piston actuated. The spring/diaphragm actuator is by far the most popular due to its simplicity and ability to fail-safe (that is, the spring force will drive the valve either to close [fail- close] or in the open position [fail-open], depending on process safety requirements, should the air pressure be lost). Piston actuators provide more dynamic stiffness. In addition, because they use higher air pressures, they are more compact than spring-opposed diaphragm actua- tors. Other forms of actuation are electric or hydraulic. They are used more for special applications and their use is limited due to higher cost and limited reliability.
Materials of Construction. For noncorrosive use, the material of choice is carbon steel (ASTM A216 Grade WCB, if cast; and A105 when forged). Valve plugs and seat rings are typically ASTM A 351, CF8M (316 stainless steel). For mild, corrosive applications, valve housings are made from type CF8M (316 stainless steel). However, Teflon®-lined housings and exotic alloys, such as Hastelloy®, monel, or titanium are available for highly corrosive fluids. For additional information, refer to: Hans D. Baumann, Control Valve Primer, A User’s Guide, ISA, Research Triangle Park, 1998.
How to Size Control Valves
The flow capacity of control valves is expressed by the coefficient Cv. This is a combination of valve flow area and the valve’s headloss coefficient K. It is ex- pressed as
where A is the ‘‘vena contracta’’ area of the valve’s orifice, typically 70 percent of the orifice area. Cv is expressed in the flow of U.S. gallons per minute of water when the pressure drop is one psi. N1 is a numerical constant = 0.059 if A is in
mm2, or 38.1 if A is in inch2. For example, if K = 1 and A = 25 mm2, then the
Cv = 25 × 0.059/C1 = 1.475.
While Cv was initially a liquid flow coefficient, this term can also be used for gases or steam with the proper conversion coefficients as shown below.
We have to distinguish two modes of flow in a control valve which, in turn, governs the use of the correct equation.
Normal Flow. This occurs when the pressure drop across the valve lies below the following limits
where Aplim is the limited pressure drop across the valve (see equations), p1 is the valve’s inlet pressure, and pv is the vapor pressure of the respective fluid and at the flowing temperature (all pressures absolute).
Choked Flow. This occurs if the actual pressure drop exceeds Aplim. CAUTION: Such conditions could cause cavitation in valves handling liquids, or high sound levels with gas or steam. Consult your control valve supplier.