The novelty of an ultrasonic flowmeter is the ability to measure flow by clamping or handholding it on the outside of a pipe. Pretty amazing! In fact, because of this magical flow monitoring method of operation, some people think it works on every application. Well, boys and girls, this is still a flow meter. And like all flow meters, they have some basic requirements. One such requirement for the optimum performance of a clamp-on ultrasonic transit time flow meter is that it needs some straight run of pipe. So, let’s review the liquid’s flow hydraulic requirements.
The perfect application would have a symmetrical flow profile. The illustration on the right looks similar to a top-down view of a motorboat going down a river. The flow is fastest at the center and, due to friction, slowest where the wake is cascaded out towards the sides. In fact, this illustration is a pipe cutaway with the liquid flowing from left to right. The blunt or elliptical shapes constitute Reynolds numbers where they represent turbulent and laminar flow rates.
One common denominator is that, due to friction, the fastest flow is located in the center of the pipe and slowest near the pipe wall. This type of symmetrical flow normally occurs when the application has 10 pipe diameters after an elbow and 5 pipe diameters before the next obstruction (for traditionally pumped applications which usually have a flow rate of @7 ft/sec). If you were to install your flow meter sensors with this symmetrical flow profile, the ultrasonic signal would traverse the pipe and take an average cross section of the velocities to calculate the optimum flow performance.
The opposite of a symmetrical flow profile is referred to as an asymmetrical flow profile. Some people refer to it as a turbulent flow application. In this illustration on the left, the flow travels up a pipe and through an elbow. The distance on the inside of the elbow is shorter than the outside of the elbow. Hence, why the flow tends to tumble from the discharge. If you are to install your flow sensors after this elbow, you will typically get a fault condition or no indication of flow.
The ultrasonic transit time flow meter uses a math equation to determine the correct transducer spacing. Once you program your flow meter with the pipe’s OD, ID, material, and type of liquid, it will calculate the correct transducer spacing to use; taking account the 10 + 5 pipe diameters of straight run of pipe (for a symmetrical flow profile).
The transducers transmit signals back and forth utilizing an average mean velocity to calculate the volumetric flow. Under these conditions, your flow meter will offer optimum accuracies and performance. Although this type of installation works properly in the laboratory, it may not be the case in the field.
Should you not have the required straight run of pipe, it could affect your success of monitoring an application. In this case, having a short run of pipe could cause interrupted flow patterns from an inline obstacle and the trajectory of the ultrasonic signals can miss the receiving transducer. If this is marginal, it might require prospecting the transducer spacing to establish the signal. Once this (not in the manual) procedure is used, you may not obtain the optimum accuracy of your flow meter.
In applications where there is turbulence, short run of pipe, or large amounts of suspended solids/aeration, the transducer signal may get blocked from being received. This would result in a fault or, better known as, signal error indication. This would indicate that you have no flow data to read.
So, how does one address this run of pipe situation in a typical field installation? The rule of thumb is to try to search out the best possible location with the required straight run of pipe. If not available, then use the 2/3rds rule: When you have limited straight run of pipe, attempt to clamp your transducers downstream 2/3rds of the available straight run of pipe and install your transducers. If that does not work, then you will need to prospect your transducers for a signal connection.
Today’s ultrasonic flow meters’ signal strengths are more of a “Yes” or “No.” So, there is no longer a need to monitor the % or signal strength bars. If you get a signal, run with it.
As I indicated earlier, just like every other flow meter technology, clamp-on ultrasonic flow meters require a set of specifications. And if you can’t meet the specifications, the flow meter did not fail, the application was not suitable for it. Should you get in your vehicle and drive hundreds of miles to create a flow survey and you get a broken application… Don’t bother trying it! There is no point in showing your customer a flow meter that doesn’t work. It destroys your credibility and the confidence in the technology. If the customer really wants to monitor this application, then they will bring out the backhoe to find you some straight run of pipe!
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