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 school professor. He is the principal reliability consultant for Fischer Technical Services. He may be reached at bobfischer@fischertechnical.com.
One of Dirty Harry’s well-known quotes was: “A man’s obtained to know his limitations.” This story illustrates why you should know your control valve’s limitations.
A shopper just lately called for help downsizing burners on a thermal oxidizer. Changes within the manufacturing process had resulted in an excessive amount of warmth from the prevailing burners. All makes an attempt to decrease temperatures had led to unstable flames, flameouts and shutdowns. The larger temperatures didn’t harm the product but the burners have been guzzling one hundred ten gallons of propane every hour. Given the high value of propane at that plant, there have been, actually, tens of millions of incentives to conserve vitality 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 may be discovered on-line at https://online.flippingbook.com/view/852569. 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 called for a worth estimate to alter burner controls. As we discussed their efforts to scale back fuel usage, we realized smaller burners might not be required to solve the issue.
Oxidizer temperature is basically determined by the place of a “combustion air” management valve. Figure 1 exhibits how opening that valve increases stress within the combustion air piping. Higher pressure forces more air by way of the burners. An “impulse line” transmits the air strain 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 supplied to the burner. pressure gauge หน้าปัด 4 นิ้ว is adjusted to ship the 10-to-1 air-to-gas ratio required for steady flame.
The plant was unable to take care of flame stability at significantly lower gas flows as a outcome of there’s a limited vary over which any given diaphragm spring actuator can present accurate management of valve place. This usable control vary is known as the “turndown ratio” of the valve.
In this case, the plant operators no longer wanted to totally open the gas valve. They wanted finer resolution of valve place with much lower combustion air flows. The diaphragm actuator wanted to have the power to crack open and then control 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 fuel 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 important downtime. Only a couple of cheap parts and minor rewiring had been required to save “a fistful of dollars.”
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