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Stan: What’s the deal with the variable- speed drive (VSD), a.k.a. adjustable-speed or variable-frequency drive? Numerous articles and papers on their energy savings are out there, but we don’t see the VSD replacing the control valve any time soon.
Greg: Precious few presentations exist on the potential advantages of variable-speed drives for eliminating the backlash and stick-slip associated with control valves. The best one I’ve seen is in the presentation, “Flow and Pressure Control Using Variable-Speed Drives” by Greg Shinskey. In this paper, a VSD demonstrated a four-times faster set point response than a control valve, a rangeability greater than a magnetic meter, no measurable hysteresis, and no cogging (torque pulsing) at low speeds.
Stan: System design presents several practical problems. While for variable torque loads, such as liquid pumping, overheating is much less a concern at low speeds, the motor rating, frame and ventilation must be properly sized. When pumps feed downstream equipment with a static head (e.g., pressure controlled reactor), the pump speed must not drop the pump discharge pressure below the static pressure downstream, or there can be flow reversal, leading to some very nasty results. Automated on-off valves are required for interlocks and isolation, but even when a control valve is used, a separate automated blocking valve is added to increase reliability. In general, a throttling valve does not make a good isolation valve and vice versa.
Greg: Flow lab test results show a good globe valve with a digital positioner and a VSD provide about the same control loop performance. Details of particular cases can be cited to sway the argument for a valve or VSD. To ensure a perspective, it helps to know the normal range of key performance parameters. Control valves have a deadband (e.g., backlash) varying from 0.1% to 10% and a resolution (e.g., stick-slip) varying from 0.05% to 5%. The lowest number is for a sliding stem (e.g., globe) throttling valve with a diaphragm actuator, a digital positioner and low-friction packing. The higher numbers are generally for on-off valves disguised as throttling valves with high-friction packing, tight shutoff (e.g., high breakaway torque) and piston actuators.
Deadband and resolution increase loop dead time, create limit cycles and set the practical rangeability of the control valve via its impact on the minimum controllable flow. The stroking time of a control valve varies from 5 sec (small globe) to 50 sec (large butterfly or damper). Second-order responses have time constants of 0.1 sec to 0.5 sec and a pre-stroke dead time of 0.2 sec to 20 sec, depending again upon actuator size. The speed of the valve becomes important for fast loops such as flow, pressure and especially surge. Boosters can be added to speed up a control valve, but the combination of a booster in series with a positioner is unstable unless some of the air signal is bypassed around the booster so the positioner sees the larger volume of the actuator.
A VSD has a hysteresis ranging from 0.05% for tachometer feedback to 0.2%, and a resolution ranging from 0.05% for a 12-bit A/D input card to 0.2% for a 10-bit A/D input card. The lower numbers are improved on some drives by the use of special speed encoders and 16-bit A/D cards. The drive response dead time is negligible, but the time to ramp full scale for a VSD ranges from 10 sec to 100 sec, depending upon rotor inertia and imposed ramp rate limits. While even the lower time of 10 sec might seem long, it translates to 0.1 sec for a 1% change in speed. Most controller outputs don’t change more than 1% in a scan. Following are four checkpoints for getting the best dynamics.
Stan: While these issues are not well documented, they are all understandable, and you’d think drive manufacturers could provide the necessary guidance. Intelligent options do exist to auto-detect motor data for setting up the drive to operate correctly. Galen Wilke, vice president at Fisher Slidning Stem Valves, found the real reasons why users are not choosing VSDs.
Greg: Setting up a VSD is no more complex than setting up a home entertainment system, so why do engineers relish setting up the latest TV/stereo system, but avoid a VSD even when application economics show it might be a good deal?
10. 52" high-definition screen
9. Surroundsound of Eric Clapton, not inverter hum
8. An easy chair
7. Four remotes
5. Virtual rather than plant reality
4. Sports—unless you consider improving loops a sport
3. You can’t TiVo the plant
2. You can save the best and erase the worst
1. Only slip you is sliding into is your vinyl recliner.
For more McMillan on valves, go to ControlGlobal.com/CntrlTlkValves.html.
What are implications of an integrating or a runaway process response on control dead band, resolution and stroking time requirements?
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