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Greg: Pinch (boot) valves used for sanitary applications and slurries had a quick-opening characteristic, crude regulating capability and excessive capacity. The roller mechanism diaphragm valve offers precision throttling of flow. The diaphragm eliminates the need for stem packing, and provides a small, smooth path for fine adjustment of extremely small flows that meets sanitary valve requirements. The very low friction of the packless and roller bearing linkage design enables the valve to respond to incredibly minuscule changes in signal. These valves can be used for fine control of additives and nutrients in food and drug production. The flow varies with the third power with stroke. The exceptional resolution and flow characteristic offers extraordinary rangeability of better than 15,000:1 for a stroke range of 25:1. The flow is forced to be laminar. The pressure drop is small compared to available pressure. As a result, the installed characteristic is close to the inherent characteristic, enabling an accurate linearization of the controller output in the DCS, provided there's at least a 16-bit output card. pH control systems have incredible reagent delivery rangeability and precision requirements, and reagent demands so low for strong acids and bases (particularly for the last stage) that the flow is laminar. For higher flows, I still use Baumann sliding stem valves because the low-friction design and multiple spring diaphragm actuator offered the best precision, particularly below 10% stroke. This was shown in "Improve Loop Control Loop Performance," Oct 2007, in Chemical Processing. Most of the manufacturer tests and statements as to resolution (actually threshold sensitivity) are for tests near mid-throttle range, so that seating friction is not seen.
Stan: Hans, what are some of your more notable innovations besides the roller diaphragm valve?
Hans: Due to economics, users want to buy butterfly valves. However these valves tend to have poor flow characteristics, excessive flow capacity, stick-slip from high seal friction and dead band from backlash in shaft connections and linkages. There tends to be a terrible breakaway torque near the closed position and a torque reversal around 70 degrees. To help meet the demand for rotary valves without sacrificing controllability, I developed the Camflex valve at Masoneilan. I developed the low-noise and low-torque "Low-T" butterfly at Baumann. Later, I helped develop the Control-Disk butterfly valve at Fisher that has a contoured disk, low torque, an inherent equal percentage characteristic and better throttling at low flow rates. Butterfly valves with teeth cut down on cavitation. This can be done with a Sharktooth attachment from Arthur Yeary Associates in Chicago. The low torque requirement eliminates shaft windup seen in rotary valves, where the shaft twists and finally breaks free, jumping to a position beyond the desired position, a response worse than stick-slip.
Greg: How is backlash eliminated in rotary valves?
Hans: A solid clamp is used between the actuator shaft and valve stem. Spline shafts are also used rather than pinned shafts. The translation from linear to rotary motion is eliminated by the use of actuators with rotary shafts.
Stan: Why do we have so many rotary valves with excessive dead band?
Hans: Many of the rotary valves marketed as control valves were originally on-off valves. With on-off valves, you don't care about dead band. The valves were already in the piping spec, and the price was low. Users didn't know the consequences of dead band, so they installed them.
Greg: There is very little data offered by on-off valve manufacturers on dead band. When I asked a presenter of a valve with a piping valve heritage about dead band at a recent ISA conference, he had no clue as to what dead band was and why it is important. What is your view on positioners, considering that signal characterization, if needed, is preferably done in the DCS?
Hans: For the small sliding stem valves, I designed multiple spring diaphragm actuators. Positioners were not necessary. The pneumatic positioners were notorious for being difficult to keep calibrated, and the use of cams for signal characterization gave a false sense of improvement from linearization because the change in output signal was extremely small on the steep slope of the valve characteristic. I designed a small pneumatic positioner for globe valves with only eight parts. With the development of digital positioners with advanced diagnostics and position read-back, the additional information for predictive maintenance is advantageous. Flexible tuning enables the positioner to be optimized for actuator and valve type and operating conditions. Digital signal characterization and 16-bit I/O cards have reduced the concern of loss of resolution or threshold sensitivity during linearization. You still need to reduce the gain if you encounter high valve friction.
Stan: What is your latest innovation?
Hans: I have just developed a compact three-way valve having only a single vane that can be used for bypass, as well as for mixing service. The valve has a single curved vane with double shutoff at one-third the cost. I'm presently designing a butterfly valve for beer processing with a sanitary liner and virtually no breakaway friction.
Greg: There are a lot of different types of actuators. Excluding those originally developed for on-off valves, (e.g., link-arm and rack-and-pinion pistons), which are really bad news, and what do you see as choices?
Hans: I prefer diaphragm actuators because their pistons have sliding fraction. Diaphragm actuators have been developed for operation at actuation air pressures up to 100 psig instead of the previous 30-psig limit, extending their application to larger valves and higher pressure drops. Ballscrew drives are capable of 0.002-in. resolution with a response time of a few tenths-of-a-second limitation primarily due to inertia. Electrical actuators fail to the last position, whereas pneumatic actuators use springs to force a desired fail position (e.g., fail open to prevent surge, high-pressure or high-temperature trips). In addition, electro-hydraulic actuators with a stepping motor driving the oil pump and a spool valve reversing flow between the pump and the actuator are also capable of very fast responses. However, problems with leaks, ambient temperatures and maintenance make these packages unattractive.
Stan: A lot can be said about the use of valves as a final control element, but the last word comes from.
Greg's Top 10 list.
The Top 10 Things I Would Rather Do than Use an On-Off Valve as a Control Valve
(10) Get a root canal
(9) Sit in an all-day meeting
(8) Attend an all-evening team building exercise
(7) Watch a soap opera
(6) Watch a morning news show
(5) Watch a reality show
(4) Watch a cooking show
(3) Watch a daytime talk show
(2) Listen to another "time share" pitch
(1) Retire to Marshalltown, Iowa.