E-Zine November 2016
We often talk about how to use a flow control product correctly; how to install it the way that it will work the best; how to decide which flow element or controller to select. We don’t always talk about what happens when you think you’ve done everything right, and the flowmeter or control system doesn’t work.
There are obvious things to do first. Check your installation. Check your wiring. Check your application against what you told the vendor you had. Be sure you understand exactly what the symptoms are. All these things should be done, perhaps more than once, or by more than one person, before you call the vendor. You may find that the problem is that you neglected to mention a critical parameter to the vendor. For example, excessive sanding of a water well will seriously affect the flowmeter installed on the discharge of that well. Replacing the flowmeter internals repeatedly is not the answer to the problem. Replacing the well screens, or installing a de-sander, is the answer. Often, people forget to mention things like sand. Another thing flowmeter users often forget to look for, or may not even know how to look for, is the presence of entrained gas or air in the line. This also tends to wreak havoc with flowmeters --- and it isn’t the flowmeter’s fault.
Then, it is time to call in the vendor. If you’ve cultivated a professional relationship of respect and trust with the local salesperson, it is well worth starting there. Many salespeople in the automation industry are either engineers or former technicians and operators, and have street-wise intelligence about many different applications. Of course, some are just “suits.” You can often find a good local vendor who will come out and physically go over the application with you. With the vendor rep, isolate and investigate each piece of the system individually, and then move to the system as a whole. This approach diagnoses well over 80 percent of problems with sensors and control systems. Most of the problems boil down to a flowmeter that isn’t working right, or an application problem that is creating the poor performance.
Once you know which it is, an application problem or a hardware/software problem, you can attack and solve it.
Click here to read “The Vendor Relationship (Part 2)”
From Flow Control (February 2002)
Fact or Myth: Flowmeter Straight Run 'Rule of Thumb'
By David W. Spitzer
E-Zine November 2016
Have you ever heard the “rule of thumb” to install flowmeters with 10 diameters of straight run upstream of the flowmeter and 5 diameters of straight run downstream of the flowmeter? Is this a fact or a myth? I suggest that there is a bit of both. Let me explain.
In general, flowmeter manufacturers test their flowmeters downstream of different upstream piping configurations where the upstream fittings tend to distort the velocity profile. Common configurations include installing the flowmeter downstream of a reducer, downstream of an elbow, downstream of two elbows, downstream of two elbows out of plane, and the like. The flowmeter can be installed at different distances downstream of the fittings to determine how much straight run is required to sufficiently attenuate the velocity profile distortion so as to not affect the flow measurement.
For example, a flowmeter located 2 diameters of straight run downstream of an elbow may exhibit significant measurement error while the same flowmeter located 8 diameters downstream of the same elbow may measure accurately. In this example, the flowmeter would measure accurately when installed with 10 diameters of upstream straight run (per the “rule of thumb”).
However the same flowmeter may require 30 diameters of upstream straight run to measure accurately downstream of two elbows out of plane --- a configuration that tends to generate more velocity profile distortion. Installing the flowmeter 10 diameters downstream of two elbows out of plane (per the “rule of thumb”) would cause the flowmeter to not measure accurately.
On the other hand, some flowmeters measure accurately with no upstream straight run --- regardless of the upstream piping configuration.
Therefore, each flowmeter has its own unique straight run requirements for various upstream piping configurations. Flowmeters should be installed in accordance with these unique requirements for that flowmeter and not a “rule of thumb”.
This article originally appeared in Flow Control magazine.
Quiz Corner: How Do You Adjust a Pulsed Magnetic Flowmeter with a 4-20 mA Analog Output?
By David W. Spitzer
E-Zine November 2016
Which of the following adjustments are applicable to a pulsed magnetic flowmeter with a 4-20 mA analog output?
A. A. Zero and full scale flow
B. Zero and full scale flow; 4 mA and 20 mA
C. Zero flow; 4 mA and 20 mA
D. Full scale flow; 4 mA and 20 mA
Commentary
The analog output zero and span (4 mA and 20 mA) circuitry needs to be adjusted or trimmed to ensure accuracy so Answer A is not correct.
Pulsed magnetic flowmeters measure the induced voltage on the electrodes when the coil is turned on and then when it is turned off. The flowmeter measures flow signal plus noise when the coil is on. Noise is measured when the coil is off. The flow signal can be calculated by subtracting the two (flow signal plus noise minus noise). Therefore the noise cancels itself out at zero flow so there is no zero adjustment per se. Answer B and Answer C are not correct.
The span does need to be adjusted or trimmed so Answer D is correct.
On a side note, when pulsed magnetic flowmeters were first introduced, two engineers spent an entire morning (unsuccessfully) searching for the zero adjustment.
Additional Complicating Factors
Other flowmeter technologies that can exhibit a well-defined zero that cannot be adjusted include differential pressure, turbine and vortex shedding flow technologies.
From Flow Control (November 2015)
ISSN 1538-5280
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