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Abstract:
Controlling material flow by mass is not a new concept, but application to our industries is recent. Learn how these highly accurate meters work and how to apply them in your production environment.
Detail: [By: Mark Drukenbrod]
Because of the current penchant to totally automate systems, flow meters (especially mass flow meters) are becoming increasingly popular. There appears to be an information gap about how they do their job, and what they require to read the flow of your material in your mill. In this column, we will take a look at the coriolis flow meter, which is by far the most accurate and (not strangely) the most expensive.
As we all know, the most effective way to regulate flow through a mill is on a mass flow basis, taking into account the density of the product that you are processing. But how is this done without the actual weighing of a volume of the product flowing through the mill? Enter the coriolis flow meter, an esoteric little device which can accomplish this task in real time based on a physical phenomenon observed by a French physicist (named Gustave Coriolus) in about 1815. Now I'm not sure he defined this effect by observing water going down a bathtub drain, but the swirling that we see (clockwise in the northern hemisphere, counter clockwise in the southern) is due to the coriolis effect. To demonstrate, find a merry-go-round. Stand at the outside edge of it while it is rotating in a clockwise direction. Now walk toward the center of the carousel in a straight line. You will notice a force pushing you counterclockwise. It feels like somebody is trying to hold you back by grabbing your left shoulder. If you then pass the center of the carousel and proceed to the other side, you will notice a force pushing on your left shoulder, apparently trying to speed you up. This is the coriolis force, and the same thing happens to a fluid in a pipe. And that's only half of it. The more dense you are (well, maybe we should say massive), the greater the force is. Not only that, but the faster you traverse the carousel, the greater the force. In addition, all of these forces are proportional and relational -- the stuff great measurements are made of. This force is exerted on all matter traveling in an arc around a center point, no matter how small the arc.
Now imagine yourself a fluid. You are moving down a pipe that is shaped like an arc. Your shoulder will be pressing on one side of the tube as you follow the arc. This will cause the tubing that you are flowing through to deflect. Measure that deflection, and you get a number that is proportional to your mass and speed. It's as simple as that. Gulp! Simple?! How do we take this measurement...certainly not with a ruler or even a linear velocity detection transducer (LVDT), because the deflection is far too small to measure. To solve the problem, we cleverly use an energy source to vibrate the tube. We then put a vibration sensor on each end of the tube to measure the vibration. When there is no flow through the tube, the relative phase difference between the two vibration sensors is fixed, or unity. The result of the fluid flowing through the tube will show up as a phase difference between the two vibration sensors, since the fluid itself is adding a force component because of its deflection of the tube. Who thinks of this stuff? Well, other physicists. But there's how it works and why. Now, as a reminder of how old I am, I can remember when these instruments first came out. I was working at a different mill manufacturer, and it was proposed by one of their big customers that they integrate one of these devices into a mill control system. I remember that the meter itself was a Neptune, one of the innovators of the field, now absorbed by another company. Boy, were there problems. They all had to do with the state of the manufacturing art of the day. The meter was case-referenced. In other words, the vibrations were measured with respect to the case of the instrument. You can see how, just possibly, you would get false readings if the case itself was forced to vibrate by some outside force. The problem was caused by merely mounting this meter to a mill power frame that housed a 50 or 100 HP electric motor. No matter how vibration free the drive of the mill was, this arrangement caused just too much vibration for these original meters. The original meters could not distinguish between vibration transmitted to the meter by outside forces (such as pumps and valves) and vibration or deflection caused by the material in the tube. The final solution was to move the meters off the power frame and bolt them onto concrete pylons.
Within what seemed only days, the manufacturers replaced these early single tube meters with dual tube meters in which the two tubes were vibrating in opposite phase. The actual measurement was made from one tube relative to the other. This meant that numerically and electrically, any case-induced vibration was cancelled out. While this is an elegant and effective fix from an instrument designer's point of view, it necessitated several extra pieces of hardware, the most detrimental of which was the flow divider used to split the material flow into the two tubes. It caused the meters to be more difficult to clean and provided a perfect surface for bag bits and other foreign material to congregate and plug. Dual tube meters are still the most popular today and they are very accurate and trouble-free...just make sure your product is dirt-free by placing a strainer between the premix tank and the meter. These meters can be mounted almost anywhere and under almost any conditions of vibration.
In an effort to find a way to make a relative measurement instrument without using two tubes, some manufacturers went to a racetrack or "U" design. In this design, a single tube is bent into a modified "U" shape, and the measurement is made relative to the two sections of the same tube, or the two sides of the "U." This design got rid of the problem of the flow divider, but was prone to have a pressure drop that was too high for most milling systems. In an effort to eliminate this problem, these manufacturers made the measuring tube larger in diameter. Another benefit of this design is that if you mount it upside down, it is self-draining. This point seems small until you have to clean one of these units, then it becomes a serious fault.
I had the pleasure of using a meter just recently that was of yet another new design, called "floating reference". Here, fluid flows through a single tube and a second tube is used only as a reference. Through a neat bit of design work, the reference tube is mechanically isolated from the case, and also from the measurement tube, to which it is mounted. This setup allows a straighter single tube without a case reference.
Remember, whichever of these units you might choose, that proper premix preparation and proper mounting of the instrument are both very important to the overall accuracy of the system. Volumetric flow meters are critical of where they are placed in a system. In other words, there must be a straight run of pipe on either side of the meter. Coriolis mass flow-meters do not share this limitation, and will read well just about anywhere you have space to mount them. They should be mounted where external vibrations and pipe stresses (caused by operating pumps and valves) are minimal. Where you need to use two meters in the same run of pipe, care must be taken to keep them as far apart as possible. This is to minimize what is known as meter crosstalk, which is where a fluid-borne artifact of one meter's vibration is detected by the next. Some manufacturers suggest coupling the last meter via flexible tubing in these cases, while others tune both meters to a different vibration frequency, thereby isolating each meter.
Your premix has much to do with the proper operation of the meters. It needs to be well-dispersed and free of anything that might clog the meters, such as bits of string, polyethylene film and paper. These normal premix components can be removed as mentioned before, by a simple strainer basket in line between the premix tank and the meter. The premix should be as air-free as possible, also. The air bubbles in the premix tend to damp the tube vibrations, giving false readings. Also, since we almost never add the same amount of air each time we premix, you might find that the very meter that you purchased to narrow your SPC plot will contribute to widening it instead.
One thing is sure, however. The days of trusting your constant displacement type pump to meter flow through your mill system are all but over and good riddance. As pumps, they're great..as meters, they leave a lot to be desired. Be sure to choose your meter wisely. Inform your selected vendor of the range of densities you will be running in your system, how and where you want to mount the meter. Then take his advice, get yourself hooked up with a PLC and a VFD for your pump, and start controlling by mass flow.
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