Orifice plates often provide flow measurements in plants. If manufactured to ISO or equivalent standards, they generate specific pressure drops for a given flow rate. However, their accuracy varies with the flow profile (see: "Think Straight About Orifice Plates"). Depending upon the situation, getting accurate readings may require from 10 to 90 diameters of straight pipe run upstream of the orifice plate and an additional five diameters downstream. Flow conditioners can reduce the necessary straight piping runs. With proper conditioners, upstream piping runs of 10–15 diameters often will suffice.
However, many installation locations still will fail to meet this requirement. So, what can you do in such situations? Consider a v-cone meter. It requires as little as 2–3 diameters of straight upstream piping.
A v-cone meter uses an obstruction to generate a head difference between two points in the flow path. As Figure 1 depicts, the first pressure measurement is upstream of the cone obstruction while the second most commonly is at the apex of the trailing cone shape. As with all head meters, the measured pressure drop translates into a volumetric flow rate with proper accounting for Reynold’s number and physical properties. Density corrections are required to get a mass flow rate.
Such meters now are beginning to become more common because many of the underlying patents have started to expire. Nevertheless, they’re still relatively unknown. So, let’s examine them in more detail.
Orifice plate flow meters, so long as manufactured and installed to ISO or equivalent standards, generally are accepted as being accurate. They don’t need calibration on a meter-by-meter basis. V-cone meters lack such manufacturing and installation standards. So, each meter requires individual calibration, which can be relatively difficult and costly.
The practical implication of this varies with the application. The question the plant engineer must ask is: "Does the meter need precision or accuracy?" Precision means the meter gives the same reading for identical flow rates. Accuracy means the meter’s readings are statistically random around the actual value.
Many applications for trend analysis require precision more than accuracy. If the plant wants to track changes in flow rate between different operations or over time, precision is the key need. Field experience and references show that v-cone meters provide precision equivalent to that of orifice meters. If you’re looking mainly for precision, you may be able to use a v-cone meter without calibration.
In contrast, if accuracy is necessary, you almost certainly must perform individual meter calibration. Relatively little disinterested test work has been published comparing the accuracy before calibration of v-cone meters to each other. The data available indicate flow measurement errors of up to 8.5% between nominally identical flow meters from different manufacturers. Separate work reviewed by the U.S. Bureau of Land Management (BLM) on using v-cone meters for custody transfer showed errors of less than 3% for a specific style of meter produced by one manufacturer. The BLM review concluded that those specific v-cone devices were no worse than orifice meters for custody transfer.
Without individual meter calibration, you won’t know either the overall accuracy or biases in the accuracy of measurement. One key in calibration is that the flow test must cover the entire Reynold’s number range expected. Test results reveal many v-cone meters have nonlinear calibration curves — so don’t rely on extrapolated calibrations. Once calibrated, a properly installed v-cone meter should provide measurement accuracies of 0.7% or better.
We can’t ignore maintenance. Ongoing meter accuracy depends upon having a clean meter, installed on the centerline (or the same distance from it at all times), and with an unvarying angle of the cone. Mechanical cleaning shouldn’t move or twist the v-cone. If the cone body is deformed or misaligned by cleaning, the meter will need recalibration.
V-cone meters offer an attractive option for tight piping layouts. However, you must understand whether or not the application requires a calibrated meter. If not, ensure all users understand the data’s limitations.
ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can e-mail him at ASloley@putman.net.