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 is also a part-time faculty 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 received to know his limitations.” This story illustrates why you need to know your control valve’s limitations.
A consumer recently called for help downsizing burners on a thermal oxidizer. Changes within the manufacturing course of had resulted in too much warmth from the present burners. All makes an attempt to decrease temperatures had resulted in unstable flames, flameouts and shutdowns. The larger temperatures didn’t hurt the product however the burners have been guzzling one hundred ten gallons of propane every hour. Given the excessive price of propane at that plant, there were, actually, millions of incentives to preserve energy and cut back costs.
Figure 1. Operation of a cross connected air/gas ratio regulator supplying a nozzle mix burner system. The North American Combustion Practical Pointers book could 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 value estimate to change burner controls. As we mentioned their efforts to reduce gas utilization, we realized smaller burners may not be required to solve the problem.
Oxidizer temperature is principally decided by the place of a “combustion air” management valve. Figure 1 reveals how opening that valve increases stress within the combustion air piping. Higher strain forces extra air by way of the burners. An “impulse line” transmits the air strain to one side of a diaphragm in the “gas management valve” actuator. As air stress on the diaphragm will increase, the diaphragm strikes to open the valve.
The gasoline valve is routinely “slaved” to the combustion air being supplied to the burner. diaphragm seal 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 significantly lower gas flows as a end result of there is a restricted range over which any given diaphragm spring actuator can present accurate control of valve position. This usable control vary is called the “turndown ratio” of the valve.
In this case, the plant operators now not needed to fully open the gasoline valve. They needed finer decision of valve position with a lot decrease combustion air flows. The diaphragm actuator wanted to have the ability to crack open and then management the valve using considerably lower pressures being delivered by the impulse line. Fortunately, altering the spring was all that was required to permit recalibration of the gasoline valve actuator — utilizing the existing burners.
Dirty Harry would undoubtedly approve of this cost-effective change to the valve’s low-flow “limitations.” No capital challenge. No burner replacements. No significant downtime. Only a few cheap parts and minor rewiring were required to save “a fistful of dollars.”


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