Fischer: Know your valve’s limitations 

Robert L. Fischer, P.E., is a physicist and electrical engineer who spent 25 years in chemical plants and refineries. Double is also a part-time college professor. He is the principal reliability consultant for Fischer Technical Services. He could additionally be reached at
One of Dirty Harry’s well-known quotes was: “A man’s got to know his limitations.” This story illustrates why you have to know your management valve’s limitations.
A client lately referred to as for assist downsizing burners on a thermal oxidizer. Changes in the manufacturing process had resulted in an extreme quantity of heat from the existing burners. All makes an attempt to decrease temperatures had resulted in unstable flames, flameouts and shutdowns. The greater temperatures didn’t harm the product however the burners had been guzzling a hundred and ten gallons of propane each hour. Given the high price of propane at that plant, there were, actually, tens of millions of incentives to conserve vitality and scale back 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 may be found on-line at Fives North American Combustion, Inc. 4455 East 71st Street, Cleveland, OH 44015. Image courtesy of Fives North American Combustion, Inc.
A capital project to retrofit smaller burners was being written. One of the plant’s engineers called for a value estimate to alter burner controls. As we discussed their efforts to reduce gasoline utilization, we realized smaller burners might not be required to solve the issue.
Oxidizer temperature is principally determined by the place of a “combustion air” management valve. Figure 1 shows how opening that valve will increase stress within the combustion air piping. Higher pressure forces more air through the burners. An “impulse line” transmits the air pressure to at least one side of a diaphragm in the “gas control valve” actuator. As air stress on the diaphragm increases, the diaphragm strikes 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 steady flame.
The plant was unable to keep up flame stability at considerably decrease gasoline flows as a end result of there is a limited vary over which any given diaphragm spring actuator can present correct control of valve position. This usable management vary is known as the “turndown ratio” of the valve.
In this case, the plant operators now not wanted to completely open the fuel valve. They needed finer resolution of valve place with much lower combustion air flows. The diaphragm actuator wanted to be able to crack open after which management the valve using significantly decrease pressures being delivered by the impulse line. Fortunately, altering the spring was all that was required to permit recalibration of the gasoline valve actuator — using the existing burners.
Dirty Harry would undoubtedly approve of this cost-effective change to the valve’s low-flow “limitations.” No capital project. No burner replacements. No significant downtime. Only a couple of cheap parts and minor rewiring had been required to save “a fistful of dollars.”

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