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 school professor. He is the principal reliability advisor for Fischer Technical Services. He may be reached at
One of Dirty Harry’s well-known quotes was: “A man’s obtained to know his limitations.” This story illustrates why you should know your management valve’s limitations.
A client just lately referred to as for help downsizing burners on a thermal oxidizer. Changes in the manufacturing course of had resulted in too much warmth from the prevailing burners. All makes an attempt to lower temperatures had led to unstable flames, flameouts and shutdowns. The greater temperatures didn’t harm the product however the burners had been guzzling 110 gallons of propane each hour. Given the excessive cost of propane at that plant, there were, literally, millions of incentives to preserve energy and reduce costs.
Figure 1. Operation of a cross related air/gas ratio regulator supplying a nozzle combine burner system. The North American Combustion Practical Pointers book may 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 known as for a value estimate to alter burner controls. As we discussed their efforts to scale back gas usage, we realized smaller burners might not be required to solve the problem.
Oxidizer temperature is principally decided by the position of a “combustion air” control valve. Figure 1 shows how opening that valve will increase pressure within the combustion air piping. Higher pressure forces more air by way of the burners. An “impulse line” transmits the air stress to a minimum of one facet of a diaphragm in the “gas control valve” actuator. As air pressure on the diaphragm will increase, the diaphragm strikes to open the valve.
The gas valve is mechanically “slaved” to the combustion air being supplied to the burner. Diaphragm spring pressure is adjusted to deliver the 10-to-1 air-to-gas ratio required for steady flame.
The plant was unable to keep up flame stability at significantly lower gasoline flows because there is a limited range over which any given diaphragm spring actuator can provide correct control of valve place. This usable control vary is known as the “turndown ratio” of the valve.
In this case, the plant operators no longer wanted to totally open the gas valve. They wanted finer resolution of valve place with much lower combustion air flows. The diaphragm actuator needed to have the ability to crack open after which management the valve utilizing considerably 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 — utilizing the prevailing burners.
Dirty Harry would definitely approve of this cost-effective change to the valve’s low-flow “limitations.” เกจวัดแรง . No burner replacements. No significant downtime. Only a quantity of cheap elements and minor rewiring have been required to save “a fistful of dollars.”

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