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Boiler Tubes
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Economizer Tubes Steam Drum (Wrapper Sheet) Air in Tube Sheet Waterwall Tube Generating Tubes Downcomer Water Drum Superheater Tubes Screen Tubes Waterwall Header
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Classification of Tubes
Generating Tubes Screen Tubes Downcomers Superheater Tubes Water Wall Tubes (Roof Tubes and Floor Tubes) Support Tubes Risers Circulating Tubes
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Boiler Tubes Tubes Required for One Boiler Tube Type Number O.D.
Wall Thickness Screen tubes – Two Rows 30 each 2.0” 0.165” Generating Tubes – 19 Rows 60 each 1.25” 0.120” Refactory Retainer Waterwall Tubes 28 Rear Waterwall Tubes 45 Side Waterwall Feeders 19 Rear Waterwall Feeders Rear Waterwall Risers 29
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Tubes Required for One Boiler
Boiler Tubes Tubes Required for One Boiler Tube Type Number O.D. Wall Thickness Downcomers 8 4.0” 0.203” Superheater Support 2 3.0” Superheater – 1st Pass 54 1.25” 0.120” Superheater – 2nd Pass 51 Superheater – 3rd Pass Superheater – 4th Pass 45 Superheater – 5th Pass 42 Economizer Elements 72 2.0” 0.165”
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Generating Tubes Generating tubes make up the main tube bank of the boiler. They generate steam from the water within. These tubes are generally small in diameter in order that the boiler will have a high evaporation rate and a fast response.
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Screen Tubes Screen tubes are large diameter water tubes placed between the fires and the main generating tube bank, or between the fires and the superheater tubes. They protect these smaller tubes from the intense radiant heat. Usually consist of 2 to 4 rows of tubes
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Downcomers Downcomers supply the lower water drums and waterwall headers with water from the steam and water drum. Downcomers are the largest diameter tubes in the boiler. Downcomer are located away from the boiler heat source, possibly behind the generating tubes, between the inner and outer boiler casing or outside the boiler.
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Downcomers The design capacity of the downcomers depends on:
The total capacity on the generating tubes in weight of water evaporated per hour The desired circulation characteristics of the boiler
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Water Wall Tubes Water wall tubes are tubes installed along the walls, floor and roof of the furnace and either freely exposed, partially covered, or completely embedded in the refractory of the boiler. Water is supplied to the water wall tubes from the steam and water drum by downcomers and headers.
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Water Wall Tubes Steam generated in the water wall tubes is returned to the steam and water drum by means of water wall headers and risers or may be returned to the rear header in the straight tube boiler.
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Water Wall Tubes Advantages of Water Wall Tubes are:
Water wall tubes protect the refractory of the boiler The absorption of large amounts of heat from the furnace which permits higher combustion rates They increase the steam generating capacity of the boiler
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Water Wall Tubes The reduction of maintenance of the refractory by maintaining lower brickwork temperature. The equalization of furnace temperature
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Water Wall Tubes Types of water wall tubes
Bare tubes fully exposed in the furnace Partially studded and partially covered with refractory (Plastic Chrome Ore) Fully studded and completely covered with refractory.
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Water Wall Tubes The reason for the studding is to serve as an anchor for the refractory and to absorb heat from the refractory and transfer it by conduction in the tubes
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Support Tubes Support tubes are large diameter, thick walled water tubes used in vertical tube boilers for support of the upper steam and water drum. The support tubes may have hangers attached along their length for the purpose of giving support to the superheater tubes
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Risers Risers are large diameter tubes that return the steam and water mixture from the waterwalls back to the steam and water drum. A closed circuit comprising the steam and water drum, the downcomers, the water wall tubes, water wall headers and risers is thus formed
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Circulating Tubes Circulating tubes are large diameter found in straight sectional header boilers connecting the rear sectional headers with the steam and water drum. Their purpose is to return steam and water mixture that collects in the rear headers back to the steam and water drum
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Boiler Tubes What is the difference between a tube and pipe?
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Boiler Tubes Boiler tubes are designated by their outside diameter
Wall thickness is determined by boiler pressure they will be subjected to Boiler tube size will determine how quickly boiler can respond to change of load demands (surface area vs. volume)
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Boiler Tubes Boiler tubes are secured to the tube sheets by rolling or welding Leaking tubes are plugged or replaced
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Boiler Tubes The wall thickness is usually designated by BWG – Birmingham Wire Gauge. Tube are made out of seamless steel or seamless high temperature steel alloys depending on the location in the boiler.
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Boiler Tubes Boiler tube size will determine how quickly boiler can respond to change of load demands (surface area vs. volume) The tubes are long enough to allow the tube to extend a minimum of ¼” and not more than ½” beyond the end of the tube sheet. Pipe (1/8” to 12”) is designated by its nominal diameter.
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Boiler Tube Size The size of the tubes have a large bearing on the operating characteristics of the boiler. As the size of the tube used in construction of the boiler is reduced the ratio of heating surface are per unit volume of water becomes larger. Since more small sized tubes can be placed in the same space, more heating surface is obtained
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Boiler Tube Size Greater heating surface areas per unit volume of water and more tubes result in higher evaporation rates. High evaporation rates result in a boiler with a fast response to changes in load.
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Boiler Tube Size Calculate the surface area and volume of Boiler tube which is 4 inches in diameter and 1 inch long.
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Boiler Tube Size Surface area equals tube circumference X tube length.
Area = ΠD*L Area = 3.14*4*1 Area = in2
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Boiler Tube Size Volume equals cross section area times length.
Volume = (Π/4)*D2*L Volume = * (4*4) * 1 Volume = in3
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Boiler Tube Size Ratio of surface area to volume contained
Surface Area/Volume Surface Area/Volume = / Surface Area/Volume = 1 to 1 ratio
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Boiler Tube Size Calculate the surface area and volume of Boiler tube which is 2 inches in diameter and 1 inch long.
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Boiler Tube Size Surface area equals tube circumference X tube length.
Area = ΠD*L Area = 3.14*2*1 Area = in2
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Boiler Tube Size Volume equals cross section area times length.
Volume = (Π/4)*D2*L Volume = * (2*2) * 1 Volume = in3
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Boiler Tube Size Ratio of surface area to volume contained
Surface Area/Volume Surface Area/Volume = /3.1416 Surface Area/Volume = 2 to 1 ratio
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Boiler Tube Size The 2” tube has twice the surface area to volume then the 4” tube. So the 2” tube will have a evaporation rate twice that of the 4” tube.
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Boiler Tube Size Calculate the velocity of the water thru the same 2” and 4 inch tubes for a 600 psia boiler. ṁ = ρVA ṁ = Mass Flow (lbs/hr) ρ = Density (lbs/ft3) V = Velocity (ft/sec) A = Area (ft2)
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Boiler Tube Size ṁ = ρVA ṁ = VA/νspec νspec = Specific Volume
ṁ = 60,000 lbs/hr From Steam Tables νf =
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Boiler Tube Size νf = ft3/lbm Area of the 2” tube is
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Boiler Tube Size νf = 0.02013 ft3/lbm Area of the 2” tube is A = ΠD2/4
A = ft2
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Boiler Tube Size ṁ = VA/νf
60,000 lbs/hr = V * ft2 / ft3/lbm
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Boiler Tube Size Velocity = ft/sec
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Boiler Tube Size Calculate the velocity of the water thru the 4 inch tube for a 600 psia boiler. ṁ = ρVA A = ΠD2/4 A = 3.14*((4/12)2)/4 A = ft2
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Boiler Tube Size ṁ = VA/νf
60,000 lbs/hr = V * ft2 / ft3/lbm
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Boiler Tube Size Velocity = 3.85 ft/sec
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Boiler Tubes Boiler Tubes are secured to the drums or headers by rolling or welding.
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Boiler Tubes The drum or header which the tube is going to rolled must be clean a free of any chemicals, dirt or any other solids. The holes must clean and be as perfectly circular as possible. The ends of the tubes should extend beyond the drum or header by ¼ to 3/8 inches.
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Boiler Tubes The tubes are usually held in place by collars on either end of the tubes on the outside of the drums or headers
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Boiler Tubes Once the tube is securely held in place and expanding or rolling tool is place inside the tube and expands diameter of tube against the metal of the drum or header. When the tube is expanded and is secured the portion of the tube which is extending out in the drum or header is them flared or “belled” with a belling tube.
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Boiler Tubes This aids in reducing fluid friction as water enters or leaves the tubes. When all the tubes have been rolled the boiler is closed and filled with water. The water pressure is pumped up to 1.5 times the design pressure (Mean Allowable Working Pressure – MAWP). If any tubes leak the are then rerolled and retested.
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Boiler Tubes
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Boiler Tubes
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Boiler Tubes
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Boiler Tubes Tubes the are secured in the boiler by welding must be done by a certified welder with a certified welding procedure which must be approved by the U.S.C.G. and the American Bureau of Shipping (ABS). After all the tubes have been welded in place the boiler will require a 1.5 times MAWP hydro test.
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Boiler Tubes The Coast Guard or ABS my require that some or all the welds be X-Rayed. If there are any leaks, the weld will have to be ground out and then rewelded. The boiler will have to be retested. If you have to do any welding repairs on the boiler, you are required to notify the Coast Guard at your first port of call.
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Leaking Boiler Tubes If it is determined that you have a leaking boiler tube and the boiler tube can not be replaced. The boiler tube will have to plugged. You will have to determine which tube is leaking. This is usually done by filling the boiler with water and pressurizing it.
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Leaking Boiler Tubes Once you have determined which boiler tube is leaking you have to gain access to it on the waterside of the boiler. The tube is then plugged at both ends with metal plugs that are part of the spare parts for the boiler. You must ensure that the tube you are plugging has a hole in it.
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Leaking Boiler Tubes If you do not hole the tube its is possible that the air in the tube will expand while the boiler is being brought up to pressure and blow out the plugs.
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Leaking Boiler Tubes
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