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 college professor. He is the principal reliability marketing consultant for Fischer Technical Services. He could additionally be reached at
One of Dirty Harry’s well-known quotes was: “A man’s obtained to know his limitations.” This story illustrates why you need to know your control valve’s limitations.
A client recently known as for assist downsizing burners on a thermal oxidizer. Changes within the manufacturing process had resulted in too much heat from the present burners. All attempts to decrease temperatures had resulted in unstable flames, flameouts and shutdowns. The larger temperatures didn’t hurt the product however the burners were guzzling 110 gallons of propane every hour. Given the excessive value of propane at that plant, there have been, literally, tens of millions of incentives to conserve power and reduce prices.
Figure 1. Operation of a cross linked air/gas ratio regulator supplying a nozzle mix burner system. The North American Combustion Practical Pointers e-book can be discovered online at 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 referred to as for a price estimate to alter burner controls. As we discussed their efforts to scale back fuel usage, we realized smaller burners may not be required to unravel the problem.
Oxidizer temperature is mainly decided by the position of a “combustion air” management valve. Figure 1 shows how opening that valve will increase pressure in the combustion air piping. Higher pressure forces extra air by way of the burners. An “impulse line” transmits the air stress to at least one facet of a diaphragm in the “gas management valve” actuator. As air stress on the diaphragm will increase, the diaphragm strikes to open the valve.
The fuel valve is automatically “slaved” to the combustion air being equipped to the burner. Diaphragm spring tension is adjusted to ship the 10-to-1 air-to-gas ratio required for secure flame.
The plant was unable to maintain up flame stability at considerably decrease gas flows as a outcome of there is a limited vary over which any given diaphragm spring actuator can present accurate control of valve place. เกจวัดแรงดันแก๊ส is called the “turndown ratio” of the valve.
In this case, the plant operators no longer wanted to completely open the gasoline valve. They needed finer resolution of valve position with a lot lower combustion air flows. The diaphragm actuator needed to be able to crack open after which management the valve using considerably 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 prevailing burners.
Dirty Harry would definitely 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 parts and minor rewiring have been required to save “a fistful of dollars.”

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