Fischer: Know your valve’s limitations 

Robert L. Fischer, P.E., is a physicist and electrical engineer who spent 25 years in chemical crops and refineries. Fischer can also be a part-time school professor. He is the principal reliability consultant for Fischer Technical Services. He could also be reached at
One of Dirty Harry’s famous quotes was: “A man’s obtained to know his limitations.” This story illustrates why you have to know your control valve’s limitations.
A consumer just lately known as for assist downsizing burners on a thermal oxidizer. Changes within the manufacturing process had resulted in too much warmth from the existing burners. All attempts to decrease temperatures had ended in unstable flames, flameouts and shutdowns. The higher temperatures didn’t hurt the product but the burners had been guzzling a hundred and ten gallons of propane each hour. Given the excessive cost of propane at that plant, there were, actually, hundreds of thousands of incentives to conserve energy and cut back prices.
Figure 1. Operation of a cross linked air/gas ratio regulator supplying a nozzle mix burner system. The North American Combustion Practical Pointers e-book can be discovered 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 worth estimate to alter burner controls. As we mentioned their efforts to reduce fuel utilization, we realized smaller burners might not be required to solve the problem.
Oxidizer temperature is mainly determined by the position of a “combustion air” control valve. Figure 1 reveals how opening that valve increases pressure within the combustion air piping. Higher strain forces more air through the burners. An “impulse line” transmits the air stress to one side of a diaphragm in the “gas control valve” actuator. As air strain on the diaphragm will increase, the diaphragm moves 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 deliver the 10-to-1 air-to-gas ratio required for stable flame.
The plant was unable to maintain flame stability at considerably lower 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 place. This usable control vary is named the “turndown ratio” of the valve.
In this case, the plant operators no longer wanted to fully open the gasoline valve. They needed finer decision of valve place with much decrease combustion air flows. The diaphragm actuator wanted to have the flexibility to crack open and then management the valve utilizing 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 prevailing burners.
Dirty Harry would definitely approve of this cost-effective change to the valve’s low-flow “limitations.” No capital challenge. No burner replacements. เครื่องวัดแรงดันเกจที่นิยมใช้ . Only a number of inexpensive parts and minor rewiring had been required to save “a fistful of dollars.”

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