I became sensitized early in my career to flowmeter and valve rangeability when I inherited the responsibility of solving pH control problems from the late Vernon Trevathan. The 0-14 pH measurement scale covers 14 orders of magnitude of hydrogen ion concentration (14 orders of magnitude of strong acid or strong base concentration). Standard practice in the 1970s dictated the use of one neutralization stage for every 2 pH units away from the control band (allowable error around the pH setpoint). For a 7 pH setpoint, neutralization of influent pH at the extremes of the scale required three stages of neutralization with successively smaller valves, each with a real rangeability requirement of 100:1. These exceptional requirements led me to find that the real limitations to rangeability are important for nearly all applications.
I quickly learned that available details on flowmeter and valve rangeability were missing or misleading. Some suppliers of on-off rotary valves posing as control valves claimed a rangeability of 200:1 based on the ratio of the maximum to minimum flow coefficient (Cv) of the inherent flow characteristic. Another definition based on deviation from the inherent characteristic led to the conclusion that linear trim was best. The real rangeability for a valve is the ratio of the maximum controllable flow to the minimum controllable flow, taking into account valve precision and the slope of the installed characteristic.
Real rangeability depends upon valve backlash and stiction near the closed position and the percentage of the system pressure drop allocated as control valve pressure drop. The minimum Cv is more realistically the Cv for the deadband, resolution, and threshold sensitivity. For example, if the resolution is 0.5%, the minimum Cv is the inherent characteristic Cv at 0.5% signal. Stiction and consequently resolution and threshold sensitivity are often worse near the closed position, the result of seating and sealing surfaces. Supplier provided estimates of deadband, resolution, and threshold sensitivity are for 50% signals. The minimum controllable flow is then computed from the minimum Cv and the ratio of valve drop to system drop. Equations 7-19a through 7-19d in the ISA book Essentials of Modern Measurements and Final Elements provide the simple series of computations needed to provide a practical estimate of valve rangeability.
Sliding stem control valves designed for throttling service with diaphragm actuators and digital positioners generally have the least backlash and stiction. Fortunately for pH control, the reagent flows are so small that a sliding stem valve is economical and the valve pressure drop is more than 50% of the system drop due to low frictional losses in the piping system from low flow.
I also discovered that the real rangeability of differential head meters and vortex meters depends upon the signal-to-noise ratio, which in turn depends upon the piping geometry. Fluctuations in the velocity profile, and particularly swirling, from upstream valves, fittings, elbows, and changes in plane increase the noise. I found that manifold taps and flow nozzles or venturi tubes and straightening vanes offer less noise than orifices with single taps and just straight runs. The addition of a low range differential pressure transmitter can extend differential head meter rangeability if measurement noise is minimized.
Concept: Plant turndown depends on the ability to control the process at low flow rates. Valve and flowmeter rangeability statements in the literature are often optimistic and don’t take into account practical limitations such as backlash and stiction for valves and signal-to-noise ratio for flowmeters.
Details: Valves with minimum packing, seating, and sealing friction have the best threshold sensitivity and resolution (precision). Sliding stem valves with a direct connection of actuator shaft to plug stem have the least backlash and least friction. Diaphragm actuators and digital positioners have the best threshold sensitivity. The trend to allocate a smaller percentage of the system drop to valve drop to reduce energy use does not consider the accompanying reduction in valve rangeability. The drive to reduce upfront hardware costs does not take into account the loss in real rangeability. Flowmeters with the least sensitivity to fluid velocity profile have better rangeability from a better signal-to-noise ratio at low flows. For flowmeters with good signal-to-noise ratios, the threshold sensitivity becomes the limiting factor to rangeability. On the basis of these considerations, properly sized Coriolis meters and magmeters have a rangeability that is 20 and 10 times, respectively, better than the rangeability of the best differential head and vortex meter installations.
Watch-outs: If a valve manufacturer does not know what is meant by the terms deadband, resolution, and threshold sensitivity, the valve was probably originally designed for on-off applications and is not suitable for throttling service. Statements that only 5% of the system pressure drop needs to be allocated to valve drop do not take into account the loss of turndown and the distortion of a linear inherent characteristic into a quick opening installed characteristic. Very low flow rates and viscous flows can cause the Reynolds number to go from the turbulent to the transition region. In these applications, a roller diaphragm valve that forces laminar flow should be considered. See tips #77 – #85 for more details on poor valve package and system design. Vortex meter signals will become erratic if the velocity is too low. The signal must be forced to zero. An oversized vortex meter, where the maximum meter flow is much greater than the maximum process flow, can cause an unexpected drop out of the flow signal.
Exceptions: If a continuous process runs at one steady-state production rate, and there are no automated start-ups or transitions, turndown may not be a concern.
Insight: Flowmeter signal-to-noise ratio and valve backlash, stiction, and pressure drop determine the turndown capability of a plant.
Rule of Thumb: To increase plant turndown, use control valves and flow measurements with the best low flow response and precision.