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    Control Valve Primer, Fourth Edition
    Control Valve Primer, Fourth Edition
    Control Valve Primer, Fourth Edition
    Ebook293 pages2 hours

    Control Valve Primer, Fourth Edition

    By Hans D. Baumann

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    About this ebook

    Includes insights on valve sizing, smart (digital) valve positioners, field-based architecture, network system technology, and control loop performance evaluation. Hans Baumann, a holder of more than 150 patents, and author of over a hundred publications in control valve technology, shares his expertise on designing control loops and selecting final control elements. The easy-to-read text provides shortcuts through complex sizing and noise calculation formulas including for liquids and cavitation, and gives practical advice on how to apply control valves for safety, reduced energy costs, loop stability, and easy maintenance. - See more at: https://www.isa.org/store/products/product-detail/?productId=116240#sthash.et1aSrm9.dpuf
    LanguageEnglish
    PublisherInternational Society of Automation
    Release dateFeb 15, 2016
    ISBN9781941546048
    Control Valve Primer, Fourth Edition
    Author

    Hans D. Baumann

    Hans D. Baumann received an industrial engineering education in his native Germany and studied under U.S. Government sponsorship at Western Reserve University and later at Northeastern University, culminating in a Ph.D. in Mechanical Engineering from Columbia Pacific University. In addition, he is registered as a Professional Engineer in four states. During his professional career, he personally designed or directed the development of over 28 valve lines. One of them, the famous “CAMFLEX” valve and its derivations, is produced in eight countries where over three million units have been sold. He is credited with over 150 U.S. and worldwide patents and has published 115 papers and articles in addition to co-authoring seven handbooks on valves, instrumentation, and acoustic. He also was named by InTech magazine one of fifty most important innovators and wrote the acclaimed business book The Ideal Enterprise. Prior to founding his own control valve manufacturing company in 1977, he was an International Consultant, Corporate Vice-President of Masoneilan-Inter-national, Inc., and Manager of R&D at Worthington S/A in France. After selling his company to Emerson Electric, he worked for Fisher Controls as Senior Vice President. Usually ahead of his time, his “critical flow factor” and “pipe reducer correction factors” (FL & Fp) for valve sizing, introduced in the early sixties, later became part of ISA sizing standards in 1972, and his proposal in 1970 to utilize modified jet noise theories for aerodynamic valve noise prediction, became the basis for the ISA-75.17-1989 and IEC standard 60534-8-3. He served as a director of the ISA Standards & Practices Department Board, Chairman of the ISA75.11 Committee, U.S. Technical Expert for IEC Committee SC/65B/WG9, was a Member of the ASME Bioprocessing Equipment Executive Committee, and Chairman of the Equipment Subcommittee on Seals, and was the former Standards Chairman for Control Valves for the Fluid Controls Institute. As a well received guest speaker in the U.S. and abroad, he has also been invited as a guest Professor to Kobe University in Japan and to the Korean Advanced Institute of Technology in Korea. As a Life Fellow Member of ISA and ASME, he has been honored for his “many contributions to the science and technology of control valves” with the “Chet Beard” and the “UOP Technology” awards; he was named Honorary Mem-ber of the Spanish Chemical Engineering Society, and Honorary Life Member of the Fluid Controls Institute. His many valve designs have been honored with a gold medal from Germany, prizes from France and Japan, and seven U.S. “Vaaler” awards. He is a member of Sigma Xi, the scientific research society.

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      Book preview

      Control Valve Primer, Fourth Edition - Hans D. Baumann

      society.

      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, i.e., 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. While the sensors of the process variables are the eyes, the controller 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 control valves and could be used as an alternative. For example, in order 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 since 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, i.e., to assume an infinite number of throttling travel positions during normal control service.

      Physically, there are three basic components of a control valve:

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

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

      •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.

      Now let’s 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.

      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 input see Chapter 8.

      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 up in a 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), i.e., how fast the valve is responding to an upset in your 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, i.e., 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’s attempt to develop a workable compromise.

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

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

      WHAT TO LOOK FOR IN A GOOD CONTROL VALVE DESIGN

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

      •Low dead band of the actuator/valve combination (with tight packing).

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

      The prime concern of an 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. What we are talking about here is that the 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 dead band¹ of the actuator/valve combination, or, in case a positioner is used, the dead band of the valve divided by the open loop gain² of the positioner plus the positioner's dead band (dead band keeps the valve from responding instantly when the signal changes, which, in turn, causes dead time). The valve itself should never have a dead band 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 4-20 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 dead band 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 can not operate without a positioner. Luckily, modern positioners have electronic tuning capabilities that can help to drive the dead band down (see Chapter 9).

      The very first control valves recognized the value of low dead band. The valve in Figure 1-3, for example, features ball-bearings around the actuator stem in order to avoid twisting the spring and thereby reducing friction (circa 1932). Low seat leakage can be very beneficial. First, it saves you the extra expense of a shut-off 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. Cross section of an early spring-diaphragm actuated control valve, circa 1932, made by the Neilan Co. of Los Angeles, CA. Actuator signal was 2-15 psi.

      Another feature is good rangeability. As you probably already know, valves are usually terribly oversized. That is, they operate only at perhaps 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.¹ The reason is that the controller might be slightly unstable at the low flow rates, and the actuator, following the sinusoidal output of the controller signal, will push the plug against the seat. When this happens, the actuator and positioner will bleed all the air out, and you end up having a really big dead time, (before the valve gets moving again) upon signal reversal. In short, you have a big mess and no control.

      Using the 5% travel limit, we find that the Cv is only about 7% of the rated Cv with a linear characteristic and not much less with most equal percentage characterized plugs. This gives us a useful range for the above-normal Cv of 30% divided by 7%, which is 4.5:1. However, in most loops the pressure drop across the valve always varies inversely with the flow, that is, a low DP at maximum flow and a high DP with low flow. We, therefore, might find that the actual controllable ratio of maximum to minimum flow is perhaps only 3:1. Incidentally, this ratio is the so-called installed rangeability, which, as you can see, is quite different from the inherent rangeability shown in a manufacturer's catalog for the same valve. A simple way to remember is that the inherent valve rangeability is the ratio of maximum to minimum controllable Cv, while the installed rangeability is the ratio of maximum to minimum controllable flow rate in your loop.

      REFERENCES

      1.McMillan G. K., and Weiner S., How To Become an Instrument Engineer - The Making of A Prima Donna. Research Triangle Park, NC: ISA (1987).

      1 Without a positioner and with high packing friction, such as graphite packing, the minimum travel position should be higher than the dead band caused by the packing friction (usually 7% to 10%)!

      2 Usually 50 to 200. Consult vendor’s published positioner catalog.

      1 This is often confused with hysteresis, a dead band produced by moving the actuator up and down through 100% travel. Hysteresis is not very important in a closed-loop system.

      2

      WHY NOT USE A SPEED-CONTROLLED PUMP?

      When the means to electronically control the speed of an electric motor became available in the mid-1970s, a thought immediately occurred: Why not vary the speed or number of revolutions of pumps in order to control the flow rate of liquids in process control systems? As a matter of fact, this seemingly ideal solution led to the purchase of a large electronics company by the owners of a major oil company.

      Alas, things did not turn out to be that simple. For one thing, modern sizing of control valves, coupled with the introduction of rotary control valves having higher inherent flow capacities, led to the use of pumps with smaller head pressures, hence, less waste of energy, i.e., pressure drop across the control valve. Energy seems wasted to us because the throttling action of a valve is a thermodynamically nonreversible process and, therefore, increases the entropy of the fluid; thus, the energy within the fluid is at a lower, less available state. In practical terms; we consume more electric power than we should.

      Unfortunately, as it turned out, reducing the speed of an electric motor driven pump was not as economical as first thought. Following a flow reduction, most of the energy savings at the pump is converted into useless heat within the motor, and more importantly in the variable frequency, and/or variable voltage converters. Most energy saving arguments center around the electric power that can be saved at the motor. However these arguments invariably omit the low efficiency of the current or voltage converters which can be as low as 10% at low motor speed¹,² Variable frequency drives have efficiencies of

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