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. Fischer can also be a part-time college professor. He is the principal reliability advisor for Fischer Technical Services. He could also be reached at
One of Dirty Harry’s famous quotes was: “A man’s got to know his limitations.” This story illustrates why you should know your control valve’s limitations.
A client just lately referred to as for assist downsizing burners on a thermal oxidizer. Changes within the manufacturing process had resulted in too much warmth from the present burners. All attempts to decrease temperatures had led to unstable flames, flameouts and shutdowns. The higher temperatures didn’t harm the product however the burners were guzzling a hundred and ten gallons of propane every hour. Given the excessive price of propane at that plant, there have been, literally, millions of incentives to preserve energy and reduce 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 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 project to retrofit smaller burners was being written. One of the plant’s engineers called for a worth estimate to change burner controls. As we mentioned their efforts to reduce back fuel utilization, we realized smaller burners won’t be required to unravel the issue.
Oxidizer temperature is basically decided by the place of a “combustion air” control valve. Figure 1 shows how opening that valve will increase strain within the combustion air piping. Higher stress 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 strain on the diaphragm will increase, the diaphragm strikes to open the valve.
The fuel valve is routinely “slaved” to the combustion air being supplied to the burner. Diaphragm spring rigidity is adjusted to deliver the 10-to-1 air-to-gas ratio required for stable flame.
The plant was unable to hold up flame stability at considerably lower gasoline flows as a outcome of there’s a limited vary over which any given diaphragm spring actuator can present accurate management of valve position. This usable management vary is called the “turndown ratio” of the valve.
In this case, the plant operators not needed to completely open the gasoline valve. Cash needed finer decision of valve position with a lot decrease combustion air flows. The diaphragm actuator wanted to have the power to crack open after which management the valve using significantly decrease pressures being delivered by the impulse line. Fortunately, changing the spring was all that was required to permit recalibration of the gas valve actuator — using the existing burners.
Dirty Harry would definitely approve of this cost-effective change to the valve’s low-flow “limitations.” No capital project. No burner replacements. No significant downtime. Only a number of cheap components and minor rewiring were required to keep away from wasting “a fistful of dollars.”

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