Fischer: Know your valve’s limitations 

Robert L. Fischer, P.E., is a physicist and electrical engineer who spent 25 years in chemical vegetation and refineries. Fischer can be a part-time faculty professor. He is the principal reliability 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 have to know your control valve’s limitations.
A client recently called for help downsizing burners on a thermal oxidizer. Changes in the manufacturing course of had resulted in an excessive quantity of warmth from the prevailing burners. pressure gauge makes an attempt to lower temperatures had ended in unstable flames, flameouts and shutdowns. The greater temperatures didn’t hurt the product but the burners had been guzzling one hundred ten gallons of propane each hour. Given the excessive price of propane at that plant, there were, actually, hundreds of thousands of incentives to conserve energy and reduce prices.
Figure 1. Operation of a cross connected 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 challenge to retrofit smaller burners was being written. One of the plant’s engineers known as for a worth estimate to change burner controls. As we discussed their efforts to scale back gas utilization, we realized smaller burners may not be required to solve the issue.
Oxidizer temperature is mainly determined by the place of a “combustion air” control valve. Figure 1 exhibits how opening that valve will increase strain within the combustion air piping. Higher strain forces more air through the burners. An “impulse line” transmits the air pressure to one aspect of a diaphragm within the “gas control valve” actuator. As air pressure on the diaphragm will increase, the diaphragm strikes to open the valve.
The fuel valve is routinely “slaved” to the combustion air being provided to the burner. Diaphragm spring rigidity is adjusted to deliver the 10-to-1 air-to-gas ratio required for secure flame.
The plant was unable to maintain flame stability at significantly lower gas flows as a end result of there is a restricted vary over which any given diaphragm spring actuator can provide correct control of valve place. This usable control vary is called the “turndown ratio” of the valve.
In this case, the plant operators not wanted to completely open the fuel valve. They needed finer decision of valve place with a lot lower combustion air flows. The diaphragm actuator wanted to have the ability to crack open and then management the valve using significantly decrease pressures being delivered by the impulse line. Fortunately, changing the spring was all that was required to allow 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. No important downtime. Only a few inexpensive components and minor rewiring were required to save “a fistful of dollars.”


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