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Extra . Fischer, P.E., is a physicist and electrical engineer who spent 25 years in chemical plants and refineries. Fischer can additionally be a part-time school professor. He is the principal reliability marketing consultant for Fischer Technical Services. He may be reached at
One of Dirty Harry’s well-known quotes was: “A man’s received to know his limitations.” This story illustrates why you should know your control valve’s limitations.
A consumer recently known as for assist downsizing burners on a thermal oxidizer. Changes in the manufacturing process had resulted in too much warmth from the present burners. All attempts to lower temperatures had resulted in unstable flames, flameouts and shutdowns. The greater temperatures didn’t hurt the product but the burners were guzzling a hundred and ten gallons of propane every hour. Given the high price of propane at that plant, there were, actually, hundreds of thousands of incentives to preserve vitality and cut back prices.
Figure 1. Operation of a cross related air/gas ratio regulator supplying a nozzle combine burner system. The North American Combustion Practical Pointers book can be found online at Fives North American Combustion, Inc. 4455 East 71st Street, Cleveland, OH 44015. Image courtesy of Fives North American Combustion, Inc.
A capital venture to retrofit smaller burners was being written. One of the plant’s engineers referred to as for a value estimate to vary burner controls. As we mentioned their efforts to scale back gasoline utilization, we realized smaller burners won’t be required to solve the problem.
Oxidizer temperature is principally determined by the position of a “combustion air” management valve. Worldwide how opening that valve increases stress in the combustion air piping. Higher pressure forces extra air via the burners. An “impulse line” transmits the air strain to a minimum of one side of a diaphragm within the “gas control valve” actuator. As air pressure on the diaphragm increases, the diaphragm strikes to open the valve.
The gasoline valve is mechanically “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 secure flame.
The plant was unable to take care of flame stability at significantly lower gasoline flows as a end result of there’s a restricted vary over which any given diaphragm spring actuator can present accurate management of valve position. This usable control vary is named the “turndown ratio” of the valve.
In this case, the plant operators now not needed to fully open the gas valve. They needed finer resolution of valve place with much decrease combustion air flows. The diaphragm actuator wanted to have the power to crack open and then control the valve utilizing considerably lower pressures being delivered by the impulse line. Fortunately, changing the spring was all that was required to permit recalibration of the gas valve actuator — using the prevailing burners.
Dirty Harry would definitely approve of this cost-effective change to the valve’s low-flow “limitations.” No capital venture. No burner replacements. No vital downtime. Only a few inexpensive parts and minor rewiring had been required to avoid wasting “a fistful of dollars.”

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