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 can also be a part-time faculty professor. He is the principal reliability 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 have to know your control valve’s limitations.
A shopper just lately called for assist downsizing burners on a thermal oxidizer. Changes in the manufacturing process had resulted in too much warmth from the present burners. All attempts to lower temperatures had ended in unstable flames, flameouts and shutdowns. The larger temperatures didn’t hurt the product but the burners were guzzling 110 gallons of propane every hour. Given the high value of propane at that plant, there have been, literally, tens of millions of incentives to preserve vitality and cut back prices.
Figure 1. Operation of a cross connected air/gas ratio regulator supplying a nozzle mix burner system. The North American Combustion Practical Pointers book may be found online at Fives North American Combustion, Inc. 4455 East 71st Street, Cleveland, OH 44015. Image courtesy of Fives North American Combustion, Inc.
A capital venture 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 mentioned their efforts to cut back gas usage, we realized smaller burners may not be required to solve the problem.
Oxidizer temperature is principally 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 stress forces more air by way of the burners. An “impulse line” transmits the air pressure to at least one side of a diaphragm within the “gas control 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 equipped to the burner. Diaphragm spring rigidity is adjusted to ship the 10-to-1 air-to-gas ratio required for steady flame.
The plant was unable to take care of flame stability at considerably lower gasoline flows because there is a limited range over which any given diaphragm spring actuator can present correct management of valve position. This usable control range is called the “turndown ratio” of the valve.
In this case, the plant operators now not needed to completely open the gasoline valve. เกจ์วัดแรงดันแก๊ส wanted finer resolution of valve place with much lower combustion air flows. The diaphragm actuator wanted to be able to crack open and then management the valve utilizing considerably decrease 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 venture. No burner replacements. No important downtime. Only a few cheap elements and minor rewiring were required to keep away from wasting “a fistful of dollars.”

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