Fischer: Know your valve’s limitations 

Robert L. Fischer, P.E., is a physicist and electrical engineer who spent 25 years in chemical crops and refineries. Fischer is also a part-time faculty professor. He is the principal reliability marketing consultant for Fischer Technical Services. He may 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 should know your control valve’s limitations.
A shopper recently known as for help downsizing burners on a thermal oxidizer. Changes within the manufacturing process had resulted in an extreme quantity of warmth from the prevailing burners. All attempts to decrease temperatures had led to unstable flames, flameouts and shutdowns. The higher temperatures didn’t hurt the product however the burners have been guzzling 110 gallons of propane each hour. Given the high price of propane at that plant, there were, literally, hundreds of thousands of incentives to preserve vitality and reduce costs.
Figure 1. Operation of a cross linked air/gas ratio regulator supplying a nozzle combine burner system. The North American Combustion Practical Pointers book can 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 venture to retrofit smaller burners was being written. One of the plant’s engineers known as for a price estimate to vary burner controls. As we discussed their efforts to scale back gasoline utilization, we realized smaller burners may not be required to solve the problem.
Oxidizer temperature is basically decided by the place of a “combustion air” management valve. เกจวัดแรงดันออกซิเจนราคา how opening that valve increases strain within the combustion air piping. Higher pressure forces extra air through the burners. An “impulse line” transmits the air stress to a minimum of one aspect of a diaphragm in the “gas management valve” actuator. As air strain on the diaphragm increases, the diaphragm moves to open the valve.
The fuel valve is mechanically “slaved” to the combustion air being provided to the burner. Diaphragm spring pressure is adjusted to deliver the 10-to-1 air-to-gas ratio required for stable flame.
The plant was unable to take care of flame stability at considerably lower gas flows as a end result of there’s a restricted vary over which any given diaphragm spring actuator can provide accurate control of valve place. This usable management range is recognized as the “turndown ratio” of the valve.
In this case, the plant operators no longer wanted to totally open the gas valve. They needed finer decision of valve place with a lot lower combustion air flows. The diaphragm actuator needed to be able to crack open and then control the valve using considerably decrease pressures being delivered by the impulse line. Fortunately, changing the spring was all that was required to allow recalibration of the fuel valve actuator — utilizing the prevailing burners.
Dirty Harry would positively approve of this cost-effective change to the valve’s low-flow “limitations.” No capital venture. No burner replacements. No vital downtime. Only a few inexpensive components and minor rewiring have been required to save “a fistful of dollars.”

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