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 can also be a part-time faculty 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 need to know your control valve’s limitations.
A shopper just lately known 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 makes an attempt to lower temperatures had resulted in unstable flames, flameouts and shutdowns. The greater temperatures didn’t hurt the product but the burners had been guzzling 110 gallons of propane every hour. Given the high cost of propane at that plant, there have been, actually, millions of incentives to conserve power and scale back costs.
Figure 1. Operation of a cross connected air/gas ratio regulator supplying a nozzle combine burner system. The North American Combustion Practical Pointers book could be discovered 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 referred to as for a worth estimate to change burner controls. As Proven mentioned their efforts to reduce gasoline usage, we realized smaller burners might not be required to solve the issue.
Oxidizer temperature is mainly decided by the position of a “combustion air” control valve. Figure 1 shows how opening that valve will increase pressure in the combustion air piping. Higher stress forces more air via the burners. An “impulse line” transmits the air stress to one facet of a diaphragm in the “gas management valve” actuator. As air pressure on the diaphragm increases, the diaphragm moves to open the valve.
The gasoline valve is automatically “slaved” to the combustion air being equipped to the burner. Diaphragm spring pressure 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 decrease gasoline flows because there’s a limited range over which any given diaphragm spring actuator can present correct control of valve place. This usable control range is recognized as the “turndown ratio” of the valve.
In Restricted , the plant operators not wanted to totally open the fuel valve. They wanted finer resolution of valve position with much decrease combustion air flows. The diaphragm actuator needed to have the ability 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 gas 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 vital downtime. Only a number of cheap elements and minor rewiring were required to keep away from wasting “a fistful of dollars.”

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