Proper ultrasonic flow measurement is not easy. There are many variables and factors that play a role. What matters should you take into account that are not in the specification sheets of ultrasonic flow meters? Independently we try to give some answers here.
Tip #1: Choose the control valve with care
We all recognise it: the enormous noise that a control valve can produce. The choice for a low-noise valve is then quickly made and more pleasant for everyone. But strikingly enough, a low-noise valve often causes more problems than a simple valve. This is because these control valves have been modified in such a way that the frequency spectrum has been shifted. They are therefore quieter in the audible frequency range, but make more noise in the ultrasonic frequency range. And let's just measure that. Fortunately, there is also a solution to the problem: in a good ultrasonic flowmeter, a slightly higher frequency can be chosen as the measurement frequency.
Tip #2: Custody transfer measurement using ultrasonic flowmeters
When searching for a suitable ultrasonic flowmeter you will discover that Custody Transfer flowmeters use a minimum of 4 channels. That's necessary, because they have to comply with the OIML-R117 standard (of course they also have to have a type approval). The reasons for this:
- Sensors monitor each other
- Better impression of the flow profile
- Improved accuracy
Sometimes custody transfer meters are used as a pair. If you carry out a redundant flow meter you not only have more reliability, but also the possibility to improve the accuracy even further. Even if an instrument is used in an Emergency ShutDown (ESD) system, a double flow meter is the norm. After all, no flow meter should be used for the safety system that is also used for process measurements. In order to meet this redundancy requirement, there are manufacturers that sell double flow meters.
Tip #3: Prevent sensor pockets from being flooded
Sometimes there is a chance that the cavity in the wall in which the sensor is placed is filled with fluid. With good flow meters it does not happen very often, because the pockets of the transducers are self-draining. But if the problem does occur, it can be solved by inserting the sensor head deeper into the tube. This prevents the sound from being transmitted directly through the wall. With good flow meters, the sensor itself is already sonic decoupled from the meter housing. The deeper insertion of the sensor does of course cause a disturbance of the flow profile, but in larger tube diameters this is usually not a significant problem.
Tip #4: Carbon dioxide attenuates acoustic signals
This tip can cause disappointment if you are firmly committed to use an ultrasonic flowmeter. Do you have a process gas to be measured with high concentrations of CO2 and a low process pressure? Then please do not use an ultrasonic flow meter. The CO2 is very dampening for acoustic signals, which results in unpredictable results, especially at low pressure. At higher pressure, the sound can move more easily in a medium and does not have to be a problem.
Tip #5: Control valve before or after the flow meter?
Noise producing parts in the vicinity of an ultrasonic flow meter can be disruptive. After all, an ultrasonic flowmeter depends on sending and receiving sound. Audible sound most often isn't a disturbancy because the frequency spectrum used by the flowmeter is inaudible.
Control valves and pressure regulators in particular are notorious for producing ultrasonic sound and should be taken into account in the design of the measuring line. And sometimes one is inclined to think: "The source of disturbance has no effect, because the source is located after the flow meter". Unfortunately, that's not true. The sound does not "flow" with the medium, it moves in all directions. And the best carrier for high frequency sound like ultrasound is a tube... The tip is: make sure there is enough distance between the flowmeter and the other components. A minimum distance of 10 tube diameters already solves a lot of problems.
Tip #6: Measuring liquid? Always use 3 signal paths
This tip should actually be one of the basic rules for ultrasonic measurement: use an ultrasonic flowmeter with at least 3 signal paths (so with 6 sensors) if your medium is a liquid. A minimum of 3 paths is needed to know with some certainty that your flow profile is symmetrical and developed. Let's explain. A developed flow profile is a laminar flow profile (with a Reynolds number <2300, the opposite is a turbulent flow profile). A laminar flow profile can occur in liquids (and sometimes in a gas in a thin pipe at low flow). A non-symmetrical flow profile is a problem that often occurs after a bend. With 3 measuring paths both the symmetry and the development of the flow profile can be clarified and settled. It is for this reason that calibrated meters have 4 paths. As said: gases always have a high Reynolds number, so the flow is always turbulent and this problem doesn't occur.
Tip #7: Prevent mechanical noise transmission
Determine how the meter run is attached to the surrounding installation. Occasionally, disturbing vibrations are coupled into the system through the attachment. In that case it is a consideration to mechanically decouple certain parts. Example: a meterrun was suspended from a construction with M8 threaded ends. Because threaded ends are thin, they easily produce harmonics that are disturbing. When these were disconnected with rubber, the problem was solved.
Tip #8: Indirect measurement method? Horizontal measurement
It does not happen very often, but if the gas to be measured has a fraction of a liquid, then it is possible that moisture at the bottom of the pipe is disruptive, in the case of an indirect measuring method. This changes the reflection point of the ultrasonic signal and thus the accuracy. Especially because it is not a constant phenomenon. The simplest solution is to place the sensors at an angle (actually rotate the measuring point). If the ultrasonic flowmeter is not yet purchased, it is better to choose a flowmeter with direct paths, because the sensors are by definition higher in the tube.
Tip #9: Small diameters: no room for sensors
This may seem like a killer, but still... The number of measuring paths depends on the pipe diameter. Because in small diameters of pipe there is limited space for the sensors. This obviously limits the number of possible measurement paths and the question whether ultrasonic is the right flow measurement technique for this application. Usually a multi-channel meter can be made up to about 2 inches.
Tip #10: Look at the response times of the complete measurement cycle
With most ultrasonic flow meters, the channels are read one after the other. This is necessary, because otherwise the signals from the different channels will be measured by the wrong sensors. Investigate whether the given response times in the specifications apply to a complete measurement cycle or to the reading out of a single sensor. This total response time can be important, for example in Emergency ShutDown systems. An ultrasonic flowmeter that digitally signs the packets sent can send and receive all channels simultaneously. This makes the total response time much shorter. When is that nice? When measuring flare gas, for example. Most of the time there is no flow. In the event of an incident, the plant has to get rid of pressure quickly, with a very high flow all of a sudden. Reports on emissions are then more accurate and he will steer any control loop more accurately.