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 can additionally be a part-time college professor. He is the principal reliability advisor for Fischer Technical Services. He may be reached at
One of Dirty Harry’s famous quotes was: “A man’s received to know his limitations.” This story illustrates why you have to know your control valve’s limitations.
A shopper just lately called for assist downsizing burners on a thermal oxidizer. Changes in the manufacturing course of had resulted in an excessive amount of heat from the prevailing burners. All makes an attempt to decrease temperatures had led to unstable flames, flameouts and shutdowns. The larger temperatures didn’t harm the product but the burners were guzzling one hundred ten gallons of propane every hour. Given the high cost of propane at that plant, there were, literally, hundreds of thousands of incentives to conserve vitality and reduce costs.
Figure 1. Operation of a cross linked air/gas ratio regulator supplying a nozzle mix burner system. The North American Combustion Practical Pointers guide could be found 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 called for a worth estimate to vary burner controls. As we discussed their efforts to scale back gas utilization, we realized smaller burners won’t be required to resolve the issue.
Oxidizer temperature is mainly determined by the place of a “combustion air” control valve. Figure 1 reveals how opening that valve increases pressure within the combustion air piping. Higher strain forces extra air by way of the burners. An “impulse line” transmits the air strain to a minimum of one facet of a diaphragm within the “gas control valve” actuator. As air strain on the diaphragm will increase, the diaphragm moves to open the valve.
The gas valve is mechanically “slaved” to the combustion air being equipped 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 maintain flame stability at significantly decrease gas flows because there is a limited range over which any given diaphragm spring actuator can present accurate management of valve place. This usable control range is called the “turndown ratio” of the valve.
In this case, the plant operators now not needed to totally open the fuel valve. They wanted finer decision of valve position with a lot lower combustion air flows. The diaphragm actuator needed to be able to crack open after which control the valve utilizing significantly decrease pressures being delivered by the impulse line. Fortunately, altering the spring was all that was required to allow recalibration of the gasoline 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 challenge. No burner replacements. No vital downtime. Only เครื่องมือความดัน of cheap parts and minor rewiring were required to save “a fistful of dollars.”

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