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Demystifying Fluid Turbulence, Velocity and Flow Measurement

 One of the questions that come up time and time again with reference to flow meters is: “Do I really have to have all that straight pipe to make it work?” The best answer is often another question, “What kind of accuracy and repeatability are you expecting? The two are related.”

How and why they’re related makes for an interesting discussion which Dan Cychosz unpacks in his article Demystifying Fluid Turbulence, Velocity, and Flow Measurement, at automation.com. It’s all about what’s going on inside all those pipes. What we might visualize as a uniform flow is anything but. Encountering true laminar flow is pretty rare in the real world—things tend to be more turbulent, and this can reduce the effectiveness of a differential pressure (DP) flow meter.

Turbulent flow is encountered in all but the most viscous fluid flows. Turbulent flow resulting from higher flow velocities should not be confused with flow disturbances that add a velocity gradient to the flow. Flow disturbances are inherent in piping systems due to the need to change directions (elbows), control flow (valves) and take measurements (thermowells) among other things.

Turbulence causes all sorts of problems, but it is present virtually everywhere, so we really don’t think about it all that much until we encounter a situation calling for so-many pipe diameters worth of straight, smooth pipe upstream and downstream of a DP flow meter. Those requirements are there because the flow meter manufacturer has to achieve the promised performance, but can’t control the environment into which the flow meter will be installed.

A user wanting a flow meter buys it with the expectation that it can deliver the kind of performance outlined in the catalog. If the specifications promise accuracy of ±1 percent with a turn-down ratio of 10-to-1, it has to be able to deliver that performance, provided the user complies with reasonable installation requirements. Such requirements will likely call for a minimum length of straight, smooth pipe upstream and downstream from the primary element.

So, what the manufacturer is saying is: “For the unit to perform as we promise, we require at least some control over the working environment. If you expect the best performance, here’s how the piping must be arranged to minimize flow disruptions.” You can see that kind of thinking implemented in a product such as the Rosemount 3051SFP flow meter, which includes the straight pipe section as a built-in feature, but every application does not have the required space for this type of meter. Fortunately, there’s a solution at hand.

But what if the flow meter has to be installed in a location where that much straight pipe simply isn’t practical, say inside a skid unit where space is at a premium? The installer might be tempted to saw off the pipe and see what happens. A better solution is to change the nature of the primary element to minimize the effect of a flow disturbance. Replacing one large orifice bore with four smaller ones, which are called conditioning orifices, can cause the same pressure drop and deliver the same measuring precision, but without the same need for straight pipe length. Naturally, this comes at the cost of free passage which slightly increases the potential for clogging, but at least it offers another mechanism to solve a difficult application conundrum.

So a flow meter with a conditioning orifice, such as the Rosemount 3051SFC Compact flow meter, can make the difference in a tight installation. Solving these problems depends on having the right tools, and Emerson brings them to you.

You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Pressure and Flow Groups and other specialty areas for suggestions and answers.