Title: Boiler Efficiency vs. Steam Quality- The Challenge of Creating Quality Steam Using Existing Boiler Efficiencies
Abstract: A boiler works under pressure and it is not pos sible to see what is happening inside of it. The terms wet and are every day idioms in the steam industry. yet very few people have ever seen these phenomena and the actual move ment inside a boiler has remained highly specula tive. This paper and support test video of actual boil er operations will illustrate the effects steam quality vs. boiler efficiency during different boiler and steam system demands. There are four different operating situations that effect the steam quality. Each of the following situa tion will be described in detail using visual aids and supporting literature: Case I: On/Off Feedwater Control: Wide swings in the level of the boiler can result in unneces sary low alarms and shut downs. Case II: Reduced Operating Pressure: By run ning a boiler at a lower pressure, the boiling action within the boiler becomes much more violent caus ing to be carried over in to the steam system. Case ill: A Demand of 15% over Capacity: Over loading a boiler will cause excessive amounts of to be carried along with the steam into the system. Case IV: TDS Control: Without proper control of IDS within the boiler carry-over of into the steam system will occur causing damage to equip ment and/or waterhammer. BACKGROUND Boiler efficiency measures how much combus tion energy is converted into steam energy: steam quality measures how much liquid is present in the steam produced. A major benefit of using steam as a heat transfer media is the large amount of heat released when it condenses into water. With a latent heat of vaporiza tion (or condensation) as high as 1000 BTU per pound, it takes very little steam to carry a large Spirax Sarco, Inc. Allentown, PA amount of energy. Other advantages include the s non-toxic and non-flammable characteristics of steam plus its ability to deliver heat at a constant, controlled temperature. Stearn can also be deliver to users with conventional piping and valve .equip ment that is inexpensive, readily available. requires little maintenance, and has a long service life. Compared to other heat delivery and distribution systems, steam is less expensive to operate and is 100% recyclable. But in spite of these advantages, many steam users experience system safety problems, prematur equipment failures, and poor steam system efficien cy. Specific problems can include frequent boiler shut downs from low level; damaged steam pipe and valves due to hammer. vibration, co rosion and erosion; reduced capacity of steam heaters; and overloaded steam traps. These proble s are most frequently caused by low stearn quality, often called wet or carry-over. Steam quality is a measure of the amount of Ii uid contaminating the steam. (i.e. steam at 100% quality contains no liquid and appears as a 100% clear gas; steam at 90% quality contains 90% by weight of stearn and 10% by weight of in the form of a fog, cloud or droplets.) Wate droplets entrained in high velocity steam can be as abrasive as sand particles. They can erode pipe fit tings and rapidly eat away at valve seats. And if a puddle of is allowed to accumulate in steam pipes, it will eventually be picked up by the high velocity stearn, accelerated to near steam velocity, and crash into elbows, tees, and valves, causing ero sion, vibration and water harnmer.This ha mer will gradually and sometimes catastrophical Iy loosen pipe fittings and supports. Since steam is produced by the rapid boiling o· in high heat flux boilers, it can entrain as it escapes from the surface. This entrain ment, while damaging to the steam system, is inde pendent of boiler efficiency. Basically, both high and low efficiency boiler operation can produce or not produceexcessive entrainment. While entrainment cannot be completely prevented, it can
Publication Year: 1998
Publication Date: 1998-04-01
Language: en
Type: article
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Cited By Count: 2
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