Robert L. Fischer, P.E., is a physicist and electrical engineer who spent 25 years in chemical vegetation and refineries. Fischer is also a part-time school professor. He is the principal reliability advisor for Fischer Technical Services. He could also be reached at bobfischer@fischertechnical.com.
One of Dirty Harry’s famous quotes was: “A man’s received to know his limitations.” This story illustrates why you have to know your control valve’s limitations.
A client just lately referred to as for help downsizing burners on a thermal oxidizer. Changes in the manufacturing course of had resulted in too much heat from the prevailing burners. digital pressure gauge to lower temperatures had led to unstable flames, flameouts and shutdowns. The higher temperatures didn’t harm the product but the burners were guzzling a hundred and ten gallons of propane each hour. Given the excessive price of propane at that plant, there were, literally, millions of incentives to preserve power and cut back costs.
Figure 1. Operation of a cross connected air/gas ratio regulator supplying a nozzle combine burner system. The North American Combustion Practical Pointers e-book can be discovered on-line at https://online.flippingbook.com/view/852569. Fives North American Combustion, Inc. 4455 East 71st Street, Cleveland, OH 44015. Image courtesy of Fives North American Combustion, Inc.
A capital project to retrofit smaller burners was being written. One of the plant’s engineers known as for a price estimate to change burner controls. As we mentioned their efforts to reduce gasoline usage, we realized smaller burners may not be required to solve the issue.
Oxidizer temperature is principally determined by the place of a “combustion air” management valve. Figure 1 shows how opening that valve will increase strain in the combustion air piping. Higher strain forces more air by way of the burners. An “impulse line” transmits the air stress to one facet of a diaphragm in the “gas management valve” actuator. As air strain on the diaphragm increases, the diaphragm strikes to open the valve.
The gas valve is routinely “slaved” to the combustion air being supplied to the burner. Diaphragm spring rigidity is adjusted to ship the 10-to-1 air-to-gas ratio required for stable flame.
The plant was unable to take care of flame stability at considerably decrease fuel flows as a end result of there’s a limited vary over which any given diaphragm spring actuator can provide accurate control of valve position. This usable management range is named the “turndown ratio” of the valve.
In this case, the plant operators no longer wanted to totally open the gasoline valve. They needed finer decision of valve place with much decrease combustion air flows. The diaphragm actuator wanted to be able to crack open and then management the valve using considerably decrease pressures being delivered by the impulse line. Fortunately, altering the spring was all that was required to permit recalibration of the gas valve actuator — utilizing the existing burners.
Dirty Harry would positively approve of this cost-effective change to the valve’s low-flow “limitations.” No capital venture. No burner replacements. No significant downtime. Only a couple of inexpensive elements and minor rewiring had been required to save heaps of “a fistful of dollars.”

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