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 college professor. He is the principal reliability consultant for Fischer Technical Services. He could also be reached at bobfischer@fischertechnical.com.
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.
Off the record referred to as for assist downsizing burners on a thermal oxidizer. Changes in the manufacturing process had resulted in too much warmth from the existing burners. All makes an attempt to decrease temperatures had led to unstable flames, flameouts and shutdowns. The greater temperatures didn’t harm the product however the burners were guzzling one hundred ten gallons of propane every hour. Given the excessive cost of propane at that plant, there were, literally, tens of millions of incentives to conserve vitality and reduce costs.
Figure 1. Operation of a cross related air/gas ratio regulator supplying a nozzle mix burner system. The North American Combustion Practical Pointers e-book could be found on-line at https://online.flippingbook.com/view/852569. 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 gas utilization, we realized smaller burners might not be required to resolve the issue.
Oxidizer temperature is principally decided by the place of a “combustion air” management valve. Figure 1 shows how opening that valve will increase stress within the combustion air piping. Higher stress forces more air through the burners. An “impulse line” transmits the air stress to 1 facet of a diaphragm within the “gas control valve” actuator. As air strain on the diaphragm will increase, the diaphragm strikes to open the valve.
The gasoline valve is automatically “slaved” to the combustion air being supplied to the burner. Diaphragm spring tension is adjusted to ship the 10-to-1 air-to-gas ratio required for stable flame.
The plant was unable to keep up flame stability at considerably lower gas flows as a result of there is a limited range over which any given diaphragm spring actuator can provide accurate control of valve position. This usable management range is named the “turndown ratio” of the valve.
In this case, the plant operators not wanted to fully open the gasoline valve. They needed finer resolution of valve position with much lower combustion air flows. The diaphragm actuator needed to have the flexibility to crack open after which management the valve using significantly lower 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 prevailing burners.
Dirty Harry would undoubtedly approve of this cost-effective change to the valve’s low-flow “limitations.” No capital challenge. No burner replacements. No significant downtime. Only a quantity of cheap parts and minor rewiring had been required to keep away from wasting “a fistful of dollars.”
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