Stories from the Field,Fundamental Questions and Solutions - Measurement While Drilling Signal Analysis, Optimization and Design - page 32

Figure 1.5f.2. Flow straightener construction.
Comments: Flow straighteners can be inexpensively fabricated from raw
PVC stock. Different combinations of diameter and length were tested to
evaluate their effectiveness and influence on wave propagation and pressure
drop. Generally speaking, twelve inch lengths with inner tube diameters as
shown in Figure 1.5f.1 suffice.
Figure 1.5g.1. Digital flowmeter.
Comments: Accurate flow rate measurements are critical to wind tunnel
testing, since air-based data must be extrapolated to mud conditions. While mud
flow rates can be accurately estimated, that is, calculated from piston
displacements and “strokes per minute,” those in air are generally inferred from
rotating impeller data obtained in different conduits with siren blockages. The
consequences can be serious. Suppose torque data from a wind tunnel test at
100 gpm is extrapolated to flow at 500 gpm. Under perfect conditions, the
torque would increase quadratically as (500/100) 2 or twenty-five-fold based on
velocity considerations alone. A 10% error in flow rate, that is, a measurement
range of 90-110 gpm, would replace the “25” by “21-31.”