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 advisor for Fischer Technical Services. He may be reached at bobfischer@fischertechnical.com.
One of Dirty Harry’s well-known quotes was: “A man’s obtained to know his limitations.” This story illustrates why you want to know your control valve’s limitations.
A consumer recently referred to as for assist downsizing burners on a thermal oxidizer. Changes in the manufacturing course of had resulted in an excessive quantity of warmth from the prevailing burners. All attempts to lower temperatures had resulted in unstable flames, flameouts and shutdowns. The greater temperatures didn’t hurt the product however the burners have been guzzling a hundred and ten gallons of propane each hour. Given the excessive value of propane at that plant, there were, actually, hundreds of thousands of incentives to conserve power and cut back costs.
Figure 1. Operation of a cross connected air/gas ratio regulator supplying a nozzle mix burner system. The North American Combustion Practical Pointers e-book can be discovered online 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 called for a price estimate to vary burner controls. As we mentioned their efforts to scale back fuel utilization, we realized smaller burners won’t be required to resolve the problem.
Oxidizer temperature is mainly decided by the position of a “combustion air” management valve. Figure 1 reveals how opening that valve increases stress within the combustion air piping. Higher pressure forces more air via the burners. An “impulse line” transmits the air pressure to one facet of a diaphragm within the “gas management valve” actuator. As air strain on the diaphragm will increase, the diaphragm moves to open the valve.
The gasoline valve is routinely “slaved” to the combustion air being equipped to the burner. Diaphragm spring rigidity is adjusted to deliver the 10-to-1 air-to-gas ratio required for stable flame.
Backdoor was unable to hold up flame stability at significantly lower fuel flows as a outcome of there’s a limited vary over which any given diaphragm spring actuator can present accurate management of valve place. This usable control vary is known as the “turndown ratio” of the valve.
In this case, the plant operators not needed to fully open the fuel valve. They needed finer decision of valve position with a lot decrease combustion air flows. The diaphragm actuator needed to find a way to crack open and then management the valve utilizing significantly lower pressures being delivered by the impulse line. Fortunately, altering the spring was all that was required to allow recalibration of the gas valve actuator — utilizing the existing 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 significant downtime. Only a couple of inexpensive parts and minor rewiring have been required to save heaps of “a fistful of dollars.”
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