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.