Robert Applegate feels compelled to stay current with process control technology – especially when it may have a direct impact on controlling acid rain and other pollutants. Twenty weeks out of his year are devoted to a high-pressure boiler-emission monitoring system at a northeastern steel mill, where he is a consultant.

At the mill, critical readings about system operations are taken from field instruments connected via 4–20mA loops to a central distributed-control system or DCS. Additional DCS loops send control information to the emission monitoring system, enabling the mill operator to verify continued compliance with EPA-mandated requirements. The DCS data, also used for proper/safe boiler operation, is not available for troubleshooting/calibration while operating the boiler.

It’s In The Chemistry
What role do low-current loops play in efficiently operating the mill and limiting pollutant levels to abide with the EPA-mandated standard? The mill contains five major boilers whose outputs feed into stacks. The emission-monitoring system extracts continuous stack samples, measuring the nitrous oxide and oxygen concentration levels.

The idea, he says, is that by controlling the admission of NOx and sulfur dioxide, or SO2, mills can lower the concentration of nitric acid and sulfuric acid, or H2SO4 (the result of SO2 combined with water). “That’s a major step toward reducing acid rain,” says Mr Applegate.

“This is where 4–20mA technology comes in and why it’s critical to derive what we call ‘emission factor.’ We look at all the fuels that flow into the boiler, the monitoring instruments that report the fuel flow rates. Then we take those rates, combine them with the amount of heat present in that fuel, and generate the heat input. Now we have a heat input to the boiler and a steam flow out of the boiler, and we have the concentration in the stack. From those three measurements, we develop the emission factor.”

The system determines how much NOx went up the stack in that set of circumstances. Here Mr Applegate points to the critical need for ‘noninvasive’ testing. “The instruments reporting fuel flows are also used to control the boiler, and they have to be calibrated. I can’t go out there and say ‘I’ll just disconnect the oil flow meter and measure the flow.’ If I did that, the boiler would trip.”

No More Shutdowns
At one time, he says, the only way to troubleshoot those instruments was to shut the boilers down – which meant waiting for once- or twice-a year planned outages. “That meant there were long periods when I couldn’t verify what a process control instrument was actually doing – unless I had a tool like a milliamp process clamp meter. Now I can go out there and measure the circuit loop between the instrument and the DCS, and say ‘Alright, I’m at 55 percent.’ Now I look at what’s coming to the NOx system and verify that it’s also 55 percent. You can compare the input side to the output side without disconnecting anything.”

Dual readings such as milliamps and percent of scale, he notes, give the technician another way to track critical indicators. For example: “A reading of 4mA equates to zero percent of scale, and a reading of 20mA equates to 100 percent,” he says. “In my case, 55 percent would give me a reading of 12.8mA.”

And how do units of flow equate to currents and percentages? “When you set everything up, you pick a span number. You say the span is going to be full-scale. If it’s an oil flow meter, it might be zero to 30 gallons per minute. At 30gpm, you’d be generating 20mA, and you’d be looking for 100 percent at the DCS level.”

He notes that, in the ‘DCS world,’ readings of all kinds are converted from absolute readings of current between 4mA and 20mA, to a percent of some factor. Working with percentages is often easier than keeping current values in mind, while achieving the same results.

“We’re using the instrument for dual purposes: To monitor the boiler and ensure it’s safe for continuous operation, and to compare the amount of heat created by the boiler with the gas concentration in the stack. Those numbers calculate the emission factor, which tells us how much NOx the system is generating.”

All Loops Are Created Equal
Applegate points out an inherent advantage of 4–20mA technology. “At the mill, a regulator device sources that current. Within limits, it doesn’t care whether that loop is 10 feet or 1,000 feet long. The current is the same everywhere in the loop. The longer it is, the higher the resistance.

But, because it’s regulating current, the system just increases the voltage in order to drive the current to the level it wants to be. It’s a low-impedance circuit that isn’t impacted by noise in the surrounding area. That’s because the current is high enough that any noise generated in the proximity of the circuit doesn’t compare with the strength of that low-impedance, relatively high-current circuit.”

In a sense, 4–20mA technology is a great equaliser. And that is perhaps why the technology is still used in new industrial systems, even as newer bus technologies have been introduced.