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 school professor. He is the principal reliability consultant for Fischer Technical Services. He may be reached at
One of Dirty Harry’s well-known quotes was: “A man’s received to know his limitations.” This story illustrates why you want to know your control valve’s limitations.
A client lately known as for assist downsizing burners on a thermal oxidizer. Changes within the manufacturing course of had resulted in an excessive amount of warmth from the prevailing burners. All makes an attempt to decrease temperatures had ended in unstable flames, flameouts and shutdowns. The larger temperatures didn’t hurt the product however the burners have been guzzling 110 gallons of propane each hour. Given เกจวัดแรงดัน of propane at that plant, there have been, literally, hundreds of thousands of incentives to conserve energy and cut back costs.
Figure 1. Operation of a cross linked air/gas ratio regulator supplying a nozzle mix 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 known as for a value estimate to change burner controls. As we discussed their efforts to scale back gasoline utilization, we realized smaller burners might not be required to resolve the problem.
Oxidizer temperature is principally decided by the position of a “combustion air” management valve. Figure 1 shows how opening that valve will increase stress within the combustion air piping. Higher stress forces extra air by way of the burners. An “impulse line” transmits the air strain to 1 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 gasoline valve is routinely “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 secure flame.
The plant was unable to take care of flame stability at significantly lower gas flows as a result of there is a restricted range over which any given diaphragm spring actuator can present correct management of valve place. This usable management range is called the “turndown ratio” of the valve.
In this case, the plant operators no longer wanted to completely open the fuel valve. They needed finer resolution of valve position with much decrease combustion air flows. The diaphragm actuator wanted to have the ability to crack open and then management the valve using considerably decrease 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 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 important downtime. Only a couple of inexpensive components and minor rewiring had been required to save tons of “a fistful of dollars.”


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