How Pitot tubes shape up
All manufacturers’ multi-port averaging Pitot tubes depend upon the measurement of DP, even though devices may have dissimilar mounting arrangements and accessories to integrate them with secondary elements (transmitters) and controllers.
Differences in averaging Pitot tubes frequently are evident in the cross-section of the probes, advertised turndowns and costs. To make a cost-effective selection, you need to know the reasons for those differences.
As the probe separates the fluid stream, vortices are created at a certain point along the side of the probe. This creates a vacuum on the downstream side of the probe. Depending upon the shape of the probe, the velocity of the flow media and, to some extent, its density, these vortices are shed from various points around the perimeter of the probe.
Because the multi-port averaging Pitot tube senses the static pressure on the downstream side of the probe, these shedding vortices can cause variations in the static pressure over the range of velocities the probe is likely to encounter. This could affect the accuracy of the differential pressure sensed by the probe if accurate K versus Reynolds number data are not available.
Manufacturers have introduced several shapes over the years with different advertised turndown ratios. (Note that only shapes symmetrical in the upstream/downstream direction with sensing ports facing directly upstream and downstream allow bi-directional measurements for flow that changes direction.) The turndown ratios range from 4:1 for conventional round probes to 10:1 for diamond and bullet shapes to 17:1 for the elliptical shape. Because there is little difference in the accuracy levels of various shapes, the price premium for a non-round probe essentially is buying greater turndown. For many process streams, a turndown of 4:1 far exceeds what actually will be encountered. For large turndowns, the small available DP at low flow must be considered.
It does cost more to produce probes in non-round shapes, with the premium dependent upon the particular material. If the application can employ a stainless steel probe, the advertised extra price for a non-round shape is modest, typically 8-10% more than a round probe. It is relatively uncomplicated to extrude non-round shapes from stainless steel, including the annular plenum chambers. However, if an application calls for an “exotic” material such as Monel, Hastelloy or Inconel, the price may balloon to 20-40% more than round tubes. While these materials are readily available in round-cross-section tubing, non-round shapes must be machined from bar stock to generate the outer profiles and drilled to create the plenum chambers.
Where intuition may fail
When it comes to flow meters, greater cost and greater flow metering do not necessarily go hand-in-hand. If you instinctively choose a more expensive option than an averaging Pitot tube without knowing precisely what you need, you may get more range and accuracy than you really require and an additional expense you certainly don’t want.
Dave Thomas is manager of pressure and flow products for Meriam Process Technologies, Cleveland, Ohio. E-mail him at firstname.lastname@example.org.