Control Valve Primer

1

What Is a Control Valve and How Does It Affect My Control Loop?

Control valves may be the most important, but sometimes the most neglected, part of a control loop. The reason is usually the instrument engineer's unfamiliarity with the many facets, terminologies, and areas of engineering disciplines, such as fluid mechanics, metallurgy, noise control, and piping and vessel design, that can be involved depending on the severity of service conditions.

Any control loop usually consists of a sensor of the process condition, a transmitter, and a controller that compares the process variable received from the transmitter with the set point, that is, the desired process condition. The controller, in turn, sends a corrective signal to the final control element, the last part of the loop and the "muscle" of the process control system. If the sensors of the process variables are the eyes, and the controller is the brain, then the final control element is the hands of the control loop. This makes it the most important, alas sometimes the least understood, part of an automatic control system. This comes about, in part, due to our strong attachment to electronic systems and computers causing some neglect in the proper understanding and proper use of the all-important hardware.

Control valves are the most common type of final control elements; however, there are other types, such as

devices that regulate (throttle) electric energy such as silicon-controlled rectifiers;

variable-speed drives;

feeders, pumps, and belt drives; and

dampers.

Some of these devices perform functions similar to those of control valves and could be used as an alternative. For example, to control the pH level, a variable stroke-type metering pump may be used to inject acid into wastewater (instead of using a control valve lined with polytetrafluorethylene [PTFE]).

What, then, is a control valve? This is a difficult question because there is considerable overlap with other types of valves. For example, a valve operating strictly in the on-off mode (such as the hydronic solenoid valve in your home heating system) could be replaced by a simple ball valve operated by a pneumatic cylinder, a type usually referred to as an automated valve.

The distinction between automated and control valves is usually considered to be the ability of the latter to "modulate," that is, to assume an infinite number of "throttling" travel positions during normal control service.

There are three basic physical components of a control valve:

1.The valve body subassembly-This is the working part and, in itself, a pressure vessel.

2.The actuator-This is the device that positions the throttling element inside the valve body.

3.Accessories-These are positioners, I/P transducers, limit switches, handwheels, air sets, position sensors, solenoid valves, and travel stops.

A more detailed breakdown of the various types of valves, actuators, and positioners is shown in Figure 1-1.

Figure 1-1. Basic control valve terminology from ANSI/ISA-75.05.01-2000 (R2005).

Now let us discuss what the control valve should do. Referring to Figure 1-2, which shows a somewhat simplified process control loop diagram, we see three important function blocks above the control valve symbol.

Figure 1-2. Schematic block diagram of controller, control valve, and process in a control loop.

The first one is control valve gain (Kv). This is determined by the "installed flow characteristic" of the valve (quite different from the characteristic shown in the vendor's catalog). Kv tells you how much the flow through the valve is changing per a given signal change. For more information, see Chapter 8, "What About Fail-Safe?"

The second block shows the control valve dead time (TDv). This is the time it takes before a valve moves following a controller signal change. This is usually determined by the valve and actuator friction but may include time lags due to long pneumatic signal transmission lines and the time to build the pressure ina diaphragm case, for example.

Finally, the third block shows the time constant of the valve (TCv). This is simply related to the stroking speed of the actuator or actuator/positioner combination (see Chapter 9, "Why Most People Choose 'Equal Percentage' as a Flow Characteristic"), that is, how fast the valve is responding to an upset in the controlled variable. All these function blocks interact, and each one should be considered in evaluating a control valve application.

The "ideal" valve should have a constant gain throughout the flow range, that is, a linear "installed" flow characteristic, no dead time with packing tightened, and a time constant that is different from that of the process by at least a factor of three.

Need I tell you that there is no such thing as the "ideal" control valve? So, let us attempt to develop a workable compromise.

There are many valve types on the market most (about 80% are used for shutoff purposes). What distinguishes control valves most is that they can continuously modulate flow in a predicted and controlled manner.

What to Look for in a Good Control Valve Design

Besides the obvious, such as good quality workmanship, correct selection of materials, and noise emission, special attention should be paid to two areas:

1.Low deadband of the actuator/valve combination (with tight packing)

2.Tight shutoff, in cases of single-seated globe valves and some rotary valves (if required)

The prime concern of the operator of a process control loop is to have a loop that is stable. (Nothing makes people more nervous than a lot of red ink and scattered lines on a strip of paper from a recorder.) The final control element will influence the stability of a loop more than all the other control elements combined.

The biggest culprit here is dead time.1 This is the time it takes for the controller to vary the output signal sufficiently to make the actuator and the valve move to a new position. In other words, dead time (TDv), is the time it takes for the pneumatic actuator to change the pressure in order to move to a different travel position. It is most commonly related to the deadband1 of the actuator/valve combination, or in the case where a positioner is used, the deadband of the valve divided by the open-loop gain2 of the positioner plus the positioner's deadband. (Deadband keeps the valve from responding instantly when the signal changes, which, in turn, causes dead time.) The valve itself should never have a deadband of more than 5% of span, that is, less than 0.6 psi for a 3 to 15 psi signal span or 0.8 mA for a 4 to 20 mA signal. The positioner/valve combination should have no more than 0.5% of signal span. Ignoring process dynamics, a positioner may, therefore, improve matters by an order of magnitude. However, positioners can raise havoc with the dynamics of a control loop (see Chapter 9). The ideal valve is still the one in which the operating deadband with tight stem packing is less than 1%. There you have maximum stability without the extra cost and complication of having to use a valve positioner. Unfortunately, the majority of valves cannot operate without a positioner. Luckily, modern positioners have electronic tuning capabilities that can help to drive the deadband down (see Chapter 9).

The first control valves recognized the value of low deadband. The valve in Figure 1-3, for example, features ballbearings around the actuator stem to avoid twisting the spring and thereby reducing friction (circa 1932). Low seat leakage can be beneficial. First, it saves the extra expense of a shutoff valve in case the system closes down. (Note: For safety reasons, never rely on the control valve for absolute tight shutoff.) Second, it saves energy. Third, it is an unqualified requirement in temperature control applications in a batch process, especially when you are supplying a heating fluid to a chemical that can undergo an exothermic reaction.

Figure 1-3. When it all began: This is a cross section of an early spring-diaphragm-actuated control valve, circa 1932, made by the Neilan Company of Los Angeles, CA. The actuator signal was 2 to 15 psi. The double-seated valve was developed to reduce actuator forces. The fluid pressure forces the upper plug up and the lower plug down, thereby achieving a near cancellation of fluid forces.

Another feature is good rangeability. As you probably already know, valves are usually terribly oversized. That is, they operate at perhaps only 30% of their rated Cv under normal flow conditions. Furthermore, it is not wise to operate a conventional valve trim at less than 5% travel.3...