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 is also a part-time college professor. He is the principal reliability marketing consultant 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 have to know your control valve’s limitations.
A shopper lately referred to as for help downsizing burners on a thermal oxidizer. Changes within the manufacturing process had resulted in too much heat from the existing burners. All attempts to lower temperatures had resulted in unstable flames, flameouts and shutdowns. The larger temperatures didn’t harm the product however the burners have been guzzling a hundred and ten gallons of propane each hour. Given the high price of propane at that plant, there were, literally, tens of millions of incentives to preserve power and scale back prices.
Figure 1. Operation of a cross linked air/gas ratio regulator supplying a nozzle combine burner system. The North American Combustion Practical Pointers guide could be discovered 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 worth estimate to vary burner controls. As we discussed their efforts to reduce gasoline usage, we realized smaller burners may not be required to unravel the problem.
Oxidizer temperature is mainly decided by the position of a “combustion air” management valve. Figure 1 reveals how opening that valve increases stress in the combustion air piping. Higher pressure forces more air by way of the burners. An “impulse line” transmits the air strain to one aspect of a diaphragm within the “gas management valve” actuator. As air stress on the diaphragm will increase, the diaphragm strikes to open the valve.
The gasoline valve is automatically “slaved” to the combustion air being provided to the burner. Diaphragm spring pressure is adjusted to ship the 10-to-1 air-to-gas ratio required for stable flame.
The plant was unable to keep up flame stability at considerably decrease fuel flows as a result of there’s a restricted range over which any given diaphragm spring actuator can present accurate control of valve place. Up-sell is identified as the “turndown ratio” of the valve.
In this case, the plant operators now not needed to fully open the gasoline valve. They wanted finer resolution of valve place with a lot lower combustion air flows. The diaphragm actuator wanted to find a way to crack open and then management the valve utilizing significantly lower pressures being delivered by the impulse line. Fortunately, changing the spring was all that was required to allow recalibration of the fuel valve actuator — using the present burners.
Dirty Harry would positively approve of this cost-effective change to the valve’s low-flow “limitations.” No capital project. No burner replacements. No significant downtime. Only a quantity of inexpensive elements and minor rewiring have been required to save lots of “a fistful of dollars.”

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