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 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 famous quotes was: “A man’s obtained to know his limitations.” This story illustrates why you have to know your management valve’s limitations.
A client lately referred to as for help downsizing burners on a thermal oxidizer. Changes within the manufacturing process had resulted in an excessive quantity of heat from the present burners. All attempts to decrease temperatures had ended in unstable flames, flameouts and shutdowns. The higher temperatures didn’t harm the product but the burners have been guzzling a hundred and ten gallons of propane every hour. Given the excessive cost of propane at that plant, there have been, literally, millions of incentives to preserve vitality and reduce costs.
Figure 1. Operation of a cross linked air/gas ratio regulator supplying a nozzle combine burner system. The North American Combustion Practical Pointers e-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 called for a worth estimate to alter burner controls. As we discussed their efforts to reduce fuel utilization, we realized smaller burners might not be required to unravel the problem.
In record time is principally determined by the position of a “combustion air” management valve. Figure 1 shows how opening that valve increases strain in the combustion air piping. Higher pressure forces more air via the burners. An “impulse line” transmits the air pressure to 1 aspect of a diaphragm in the “gas management valve” actuator. As air stress on the diaphragm increases, the diaphragm moves to open the valve.
The gas valve is automatically “slaved” to the combustion air being equipped 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 maintain up flame stability at significantly decrease fuel flows as a outcome of there’s a limited vary over which any given diaphragm spring actuator can provide correct management of valve position. This usable control range is named the “turndown ratio” of the valve.
In this case, the plant operators now not needed to totally open the gas valve. They needed finer decision of valve position with much lower combustion air flows. The diaphragm actuator wanted to find a way to crack open after which control the valve using significantly lower pressures being delivered by the impulse line. Fortunately, changing the spring was all that was required to permit recalibration of the gasoline 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 challenge. No burner replacements. No significant downtime. Only a few inexpensive components and minor rewiring were required to keep away from wasting “a fistful of dollars.”

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