Instrumentation: Trust but Verify
By Clarence Richard, Clarence Richard Co.
Verification is an important step in any process. It’s the law when it comes to OSHA lockout and confined space procedures. We do it when crossing the street. We do it when we look at our restaurant receipt. I do it when talking with people. I do it when looking at your instrumentation. When I can’t verify it, I put it in my “to be verified (TBV)” column.
I was working on a large 100 MBTU gas burner. I was asked to come in after two other technicians were not able to stabilize the burner performance. We trusted in initial burner settings the manufacturer recommended and we verified its performance with a combustion analyzer. The initial settings were changed. The operator, manager, and process engineer had trouble believing the setting. The burner was performing well, and they felt my adjustments were wrong. Running their continuous mix asphalt mix plant at 200 tn/hr usually would indicate a 30% firing rate. The new firing rate was only 20%. Their energy bill was now cut by a third. They verified by asking, “Why is the material being heated to a temperature that required 2/3 the throttle required before?” Good question. In a nutshell, oxygen (O2) should run between 11% and 14% with CO under 250 ppm. Previous burner technicians adjusted the O2 to 17%. We only need enough O2 for complete combustion. Standard practice is to introduce just enough extra excess air (EA) to insure complete combustion. 17% O2 has much more air going through the process than slowing down of the air flow at 14% O2. We are heating volumes of air pouring out the stack at 200°. Furthermore, all of that cold excess air is cooling the aggregate being heated and dried.
Switching gears, continuous weighing is a constant quality control challenge. The important thing is reliable feedback. The volumetric method of weighing (monitoring the speed of the auger, rotary valve, belt, etc.) is not always adequate. Sometimes belt scales, impact plates, and mass flow meters are not as failproof as the application requires. Continuous weigh devices--within limits--do a good job and, in some applications, an excellent job. Depending on the product being weighed, gravimetric rate readings should be viewed with skepticism. If material sticks to the component resting on the load cell, the rate indication will increase without the rate really increasing. That’s like thinking you are driving at 70 mph but actually doing 60 mph.
When we have problems--whether it is an issue with product hanging on our load cell component, the scale needs maintenance, or we are assuming volumetrically--we find neither method is working for us. In this case, cross checking one against the other can raise red flags or comfort us that the process is being measured and controlled to specifications. Volumetric measuring does not work when your bin runs empty or when bridging occurs. It may not work well enough if your product density changes such as with pulverized recycled asphalt shingles.
When volumetric alone does not get the job done, the combination of cross checking gravimetric with volumetric causes many red flags as your process is continuously running. Compare a volumetric indicator with a gravimetric indicator. When calibrated, they should read nearly the same. When the readings are not nearly the same, taking actions like cleaning the plate, filling the bin, clearing a rock from a belt scale, or stopping the process may be required.
Clarence Richard is controls engineer, EZ- Flo Scales and Controls, Minnetonka, MN. For more information, call 952-939-6000 or visit www.ez-flo.us.