Liar, Liar, Pants on Fire
Someone once commented that people will accept preposterously incorrect data as the absolute truth, as long as it's on a computer printout -- association with a supposedly impartial machine seems to enhance the credibility of the data.
The same thing happens with process heating equipment controls and instrumentation. You'll accept readings and data without questioning, all because the source is some inanimate object. Truth is, some of these cold, objective devices can be the most shameless liars you'll ever encounter.
Lie No. 1: Double Decimal Deception. My first encounter with heat processing equipment was reviewing and approving aerospace temperature certification tests on ovens. An array of thermocouples inside the work chamber would have to agree within +/-15 or 25 degrees of setpoint, depending on the operating temperature. These were the "Bad Old Analog Days," when state-of-the-art was a multi-pen strip chart recorder. You didn't have to see too many charts to realize the ink line was up to 5 degrees wide. Believe me, that smoothed the path to many a certification.
With the arrival of digital recorders capable of readouts to two decimal places, the certification process got much tougher. I recall a bunch of us laboring 16 straight hours to get a badly needed furnace to certify. Toward the end, we were less than 0.1 degree out of range, but because the recorder read out to hundredths of a degree, we had to round the temperature up to the next degree, and that kept us from going home. Ironically, the thermocouples were not calibrated that accurately, so the readings could have been in error anyway.
Lie No. 2: Pressure Gauge Prevarication. Another time, I was setting up the burner system on a oven that had many small burners fed from common air-gas manifolds. According to the manifold pressure gauge readings, the combustion system was coasting along at low fire. That was a surprise -- the gas valve was shut and the combustion air blower wasn't running. The problem? Dud gauges. Due to age or abuse, they no longer read correctly, and they were showing manifold pressures of 2 or 3" w.c. without anything flowing through the pipe.
Lie No. 3: Fooled on Flows. This one has happens a lot. The oven control panel has a meter labeled "% Firing Rate." Some will look at it and say, "The burner's firing at 40 percent," or whatever number they see on that meter. Wrong. On most systems, the meter gets its cue from a feedback slidewire on the valve control motor, so all it tells you is slidewire position.
But doesn't that indicate firing rate? Usually not. There are a lot of ways the linkage between the drive motor and valve can be set up, so 40% on the motor doesn't necessarily equate to 40% opening of the valve. Worse yet, many air and gas control valves don't have a linear flow vs. position curve (figure 1). Even if you could get the valve and control motor rotations perfectly matched, flow rate would be about 70% at a meter reading of 40%.
Cutting Through the Lies and DeceptionTemperature recorders, gauges and control valves are inanimate objects, so you have to assume they mean you no ill. They just can't tell the difference between the truth and a lie. But how do you keep from being conned?
First, know what they are and are not capable of. If you need accuracy of a degree or less from a temperature measuring system, make sure the sensor is capable of that accuracy and that your controller/recorder is properly calibrated. If they're not, there's no point in beating yourself up over a deviation of half a degree. For all you know, the temperature actually might be correct.
Second, do not rely solely on pressure gauges for checking air and gas pressures. Gauges are delicate devices, and just one pressure spike or a careless bump can knock them out of calibration. Manometers are a better way to go. At the very least, periodically recalibrate your gauges against a manometer.
Third, if you want to know what a burner firing rate is, forget about that little meter on the panel. Take some air and gas pressure readings, preferably with a manometer.
Fool me once, shame on you. Fool me twice, shame on me.