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 be a part-time faculty professor. He is the principal reliability consultant for Fischer Technical Services. He could also be reached at
One of Dirty Harry’s well-known quotes was: “A man’s got to know his limitations.” This story illustrates why you need to know your control valve’s limitations.
A shopper just lately known as for assist downsizing burners on a thermal oxidizer. Changes in the manufacturing process had resulted in too much heat from the present burners. All makes an attempt to decrease temperatures had resulted in unstable flames, flameouts and shutdowns. The higher temperatures didn’t hurt the product however the burners had been guzzling one hundred ten gallons of propane each hour. Given the high value of propane at that plant, there were, literally, hundreds of thousands of incentives to preserve energy 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 e-book may 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 challenge 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 gasoline usage, we realized smaller burners may not be required to unravel the issue.
Oxidizer temperature is basically decided by the place of a “combustion air” control valve. Figure 1 exhibits how opening that valve will increase stress in the combustion air piping. Higher strain forces extra air through the burners. An “impulse line” transmits the air strain to one aspect of a diaphragm in the “gas management valve” actuator. As air stress on the diaphragm increases, the diaphragm strikes 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 ship the 10-to-1 air-to-gas ratio required for secure flame.
The plant was unable to keep up flame stability at considerably decrease fuel flows as a result of there’s a limited range over which any given diaphragm spring actuator can present correct control of valve place. This usable control range is called the “turndown ratio” of the valve.
In this case, the plant operators now not wanted to fully open the gas valve. They wanted finer resolution of valve position with much decrease combustion air flows. The diaphragm actuator wanted to be able to crack open after which control the valve using 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 — using the present 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 important downtime. Only เครื่องมือที่ใช้วัดความดันคือ and minor rewiring have been required to save tons of “a fistful of dollars.”

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