Fischer: Know your valve’s limitations 

Robert L. Fischer, P.E., is a physicist and electrical engineer who spent 25 years in chemical plants and refineries. Fischer is also a part-time school professor. He is the principal reliability consultant for Fischer Technical Services. He may be reached at bobfischer@fischertechnical.com.
One of Dirty Harry’s famous quotes was: “A man’s obtained to know his limitations.” This story illustrates why you should know your control valve’s limitations.
A consumer lately referred to as for assist downsizing burners on a thermal oxidizer. Changes within the manufacturing process had resulted in too much warmth from the prevailing burners. All attempts to lower temperatures had resulted in unstable flames, flameouts and shutdowns. The greater temperatures didn’t harm the product but the burners were guzzling 110 gallons of propane each hour. Given the high value of propane at that plant, there were, actually, millions of incentives to preserve energy and cut back costs.
Figure 1. Operation of a cross related air/gas ratio regulator supplying a nozzle combine burner system. The North American Combustion Practical Pointers guide can be discovered 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 worth estimate to alter burner controls. As we discussed their efforts to cut back fuel utilization, we realized smaller burners won’t be required to unravel the issue.
Oxidizer temperature is mainly decided by the position of a “combustion air” control valve. Figure 1 reveals how opening that valve will increase strain in the combustion air piping. Higher pressure forces extra air through the burners. An “impulse line” transmits the air stress to 1 facet of a diaphragm in the “gas management valve” actuator. As air pressure on the diaphragm increases, the diaphragm moves to open the valve.
The fuel valve is mechanically “slaved” to the combustion air being supplied to the burner. Diaphragm spring rigidity is adjusted to ship the 10-to-1 air-to-gas ratio required for stable flame.
The plant was unable to hold up flame stability at significantly lower gasoline flows as a end result of there’s a limited vary over which any given diaphragm spring actuator can present accurate control of valve place. This usable control range is identified as the “turndown ratio” of the valve.
In this case, the plant operators not needed to fully open the gasoline valve. They needed finer resolution of valve place with a lot decrease combustion air flows. The diaphragm actuator wanted to have the ability to crack open after which control the valve using significantly decrease pressures being delivered by the impulse line. Fortunately, changing the spring was all that was required to permit recalibration of the fuel valve actuator — utilizing the prevailing burners.
เพชเชอร์เกจ would definitely approve of this cost-effective change to the valve’s low-flow “limitations.” No capital project. No burner replacements. No vital downtime. Only a few cheap elements and minor rewiring had been required to avoid wasting “a fistful of dollars.”
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