Fire Tube Boiler

By: DoYouknowIN | Views: 14064 | Date: 16-Nov-2011

A fire-tube boiler is a type of boiler in which hot gases from a fire pass through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.

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A fire-tube boiler is a type of boiler in which hot gases from a fire pass through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.

The fire-tube boiler developed as the third of the four major historical types of boilers: low-pressure tank or "haystack" boilers, flued boilers with one or two large flues, fire-tube boilers with many small tubes, and high-pressure water-tube boilers. Their advantage over flued boilers with a single large flue is that the many small tubes offer far greater heating surface area for the same overall boiler volume. The general construction is as a tank of water penetrated by tubes that carry the hot flue gases from the fire. The tank is usually cylindrical for the most part—being the strongest practical shape for a pressurized container—and this cylindrical tank may be either horizontal or vertical.

This type of boiler was used on virtually all steam locomotives in the horizontal "locomotive" form. This has a cylindrical barrel containing the fire tubes, but also has an extension at one end to house the "firebox". This firebox has an open base to provide a large grate area and often extends beyond the cylindrical barrel to form a rectangular or tapered enclosure. The horizontal fire-tube boiler is also typical of marine applications, using the Scotch boiler. Vertical boilers have also been built of the multiple fire-tube type, although these are comparatively rare: most vertical boilers were either flued, or with cross water-tubes.

In the locomotive-type boiler, fuel is burnt in a firebox to produce hot combustion gases. The firebox is surrounded by a cooling jacket of water connected to the long, cylindrical boiler shell. The hot gases are directed along a series of fire tubes, or flues, that penetrate the boiler and heat the water thereby generating saturated ("wet") steam. The steam rises to the highest point of the boiler, the steam dome, where it is collected. The dome is the site of the regulator that controls the exit of steam from the boiler.

In the locomotive boiler, the saturated steam is very often passed into a super heater, back through the larger flues at the top of the boiler, to dry the steam and heat it to superheated steam. The superheated steam is directed to the steam engine's cylinders or very rarely to a turbine to produce mechanical work. Exhaust gases are fed out through a chimney, and may be used to pre-heat the feed water to increase the efficiency of the boiler.

Draught for fire tube boilers, particularly in marine applications, is usually provided by a tall smokestack. In all steam locomotives since Stephenson's Rocket, additional draught is supplied by directing exhaust steam from the cylinders into the smokestack through a blast pipe, to provide a partial vacuum. Modern industrial boilers use fans to provide forced or induced draughting of the boiler.

Another major advance in the Rocket was large numbers of small-diameter fire tubes (a multi-tubular boiler) instead of a single large flue. This greatly increased the surface area for heat transfer, allowing steam to be produced at a much higher rate. Without this, steam locomotives could never have developed effectively as powerful prime movers.

Types of Fire Tube Boiler:
Fire tube boilers can be constructed in different configurations. The most common designs are:
1.    Cornish boiler: The earliest form of fire-tube boiler was Richard Trevithick's "high-pressure" Cornish boiler. This is a long horizontal cylinder with a single large flue containing the fire. The fire itself was on an iron grating placed across this flue, with a shallow ashpan beneath to collect the non-combustible residue. Although considered as low-pressure (perhaps 25 psi) today, the use of a cylindrical boiler shell permitted a higher pressure than the earlier "haystack" boilers of Newcomen's day. As the furnace relied on natural draught (air flow), a tall chimney was required at the far end of the flue to encourage a good supply of air (oxygen) to the fire. For efficiency, the boiler was commonly encased beneath by a brick-built chamber. Flue gases were routed through this, outside the iron boiler shell, after passing through the fire-tube and so to a chimney that was now placed at the front face of the boiler.

2.    Lancashire boiler: The Lancashire boiler is similar to the Cornish, but has two large flues containing the fires. It was the invention of William Fairbairn in 1844, from a theoretical consideration of the thermodynamics of more efficient boilers that led him to increase the furnace grate area relative to the volume of water. Later developments added Galloway tubes (after their inventor, patented in 1848), crosswise water tubes across the flue, thus increasing the heated surface area. As these are short tubes of large diameter and the boiler continues to use a relatively low pressure, this is still not considered to be a water-tube boiler. The tubes are tapered, simply to make their installation through the flue easier.

3.    Scotch marine boiler: The Scotch marine boiler differs dramatically from its predecessors in using a large number of small-diameter tubes. This gives a far greater heating surface area for the volume and weight. The furnace remains a single large-diameter tube with the many small tubes arranged above it. They are connected together through a combustion chamber – an enclosed volume contained entirely within the boiler shell – so that the flow of flue gas through the fire tubes is from back to front. An enclosed smoke box covering the front of these tubes leads upwards to the chimney or funnel. Typical Scotch boilers had a pair of furnaces, larger ones had three. Above this size, such as for large steam ships, it was more usual to install multiple boilers

4.    Locomotive boiler: A locomotive boiler has three main components: a double-walled firebox; a horizontal, cylindrical "boiler barrel" containing a large number of small flue-tubes; and a smoke box with chimney, for the exhaust gases. The boiler barrel contains larger flue-tubes to carry the super heater elements, where present. Forced draught is provided in the locomotive boiler by injecting exhausted steam back into the exhaust via a blast pipe in the smoke box.
Locomotive-type boilers are also used in traction engines, steam rollers, portable engines and some other steam road vehicles. The inherent strength of the boiler means it is used as the basis for the vehicle: all the other components, including the wheels, are mounted on brackets attached to the boiler. It is rare to find super heaters designed into this type of boiler, and they are generally much smaller (and simpler) than railway locomotive types. The locomotive-type boiler is also a characteristic of the overtype steam wagon, the steam-powered fore-runner of the truck. In this case, however, heavy girder frames make up the load-bearing chassis of the vehicle, and the boiler is attached to this.

5.    Vertical Fire-Tube boiler: A vertical fire-tube boiler (VFT), colloquially known as the "vertical boiler", has a vertical cylindrical shell, containing several vertical flue tubes.

6.    Horizontal Return Tubular boiler: Horizontal Return Tubular boilers from the Staatsbad Bad Steben GmbH. Horizontal Return Tubular boiler (HRT) has a horizontal cylindrical shell, containing several horizontal flue tubes, with the fire located directly below the boiler's shell, usually within a brickwork setting.

1.    Water tubes: Fire-tube boilers sometimes have water-tubes as well, to increase the heating surface. A Cornish boiler may have several water-tubes across the diameter of the flue (this is common in steam launches). A locomotive boiler with a wide firebox may have arch tubes or thermic syphons. These increase the heating surface and give additional support to the brick arch.

2.    Reverse flame: In homage to the Lancashire design, modern shell boilers can come with a twin furnace design. A more recent development has been the reverse flame design where the burner fires into a blind furnace and the combustion gasses double back on themselves. This results in a more compact design and less pipe work.

3.    Package boiler: The term "package" boiler evolved in the early- to mid-20th century from the practice of delivering boiler units to site already fitted with insulation, electrical panels, valves and gauges. This was in contrast to earlier practice where little more than the pressure vessel was delivered and the ancillary components were fitted on-site.

Appurtenances, Settings, and Piping:
Section IV steam boilers must have at least one safety valve with a set pressure not to exceed 15 psi. The safety valve inlet must not be smaller than NPS ½ nor larger than NPS 4 ½ . Section IV hot water boilers must have at least one safety relief valve with a set pressure at or below the maximum allowable working pressure (MAWP) marked on the boiler. The safety relief valve inlet must not be smaller than NPS ¾ nor larger than NPS 4 ½ . The minimum relieving capacity of safety or safety relief valves on Section IV boilers must equal or exceed the maximum output of the boiler. More information on Section IV safety or safety relief valve requirements can be found in ASME Section IV, HG-400 and HG-701.

Section I boilers must have at least one safety or safety relief valve. If the boiler has more than 500 square feet of bare tube water heating surface, then it must have two or more safety or safety relief valves. One or more safety valves on a Section I steam boiler must have a set pressure at or below the MAWP of the boiler. If more than one valve is used, the highest set pressure cannot exceed MAWP by more than 3%. Additionally, the complete range of safety valve settings cannot exceed 10% of the highest set pressure. Safety relief valve settings on high temperature water boilers are permitted to exceed the 10% range referenced above. The minimum required relieving capacity of the safety or safety relief valves must not be less than the maximum designed output at the MAWP of the boiler as specified by the boiler manufacturer. Details concerning minimum required relieving capacities for organic fluid vaporizers can be found in ASME Section I, PVG-12. More information on Section I safety or safety relief valve requirements can be found in ASME Section I, PG-67 and PG-71.

Safety or safety relief valves must be installed so the spindle is in a vertical position.
Each steam boiler must have:
•    A pressure gage with an internal siphon, a siphon in the gage piping, or equivalent protection (PG-60.6, HG-602).
•    A water level indicator (PG-60.1, HG-603).
Each Section IV steam boiler must have:
•    Two pressure controls (if the boiler is automatically fired); one is considered the operating control and the other is considered the high-limit control (Note: some jurisdictions require the high-limit control be equipped with a manual reset switch) (HG-605).
•    An automatic low-water fuel cutoff – if the boiler is automatically fired (Note: some jurisdictions require an additional low-water fuel cutoff with a manual reset switch) (HG-606).
Although not referenced in Section I, there should be some means of controlling pressure. This will vary with the size and complexity of the boiler.

Each Section I steam boiler with more than 500 square feet of water heating surface must have at least two feed water methods. If solid fuel, not in suspension, is used to fire the boiler or, if the furnace design can provide enough heat to damage the boiler after the fuel supply is stopped, the two feed water methods must be independent so as to prevent one method from being affected by the same interruption as the other method (PG-61). Using this type of fuel or furnace design does not lend itself well to relying upon a low-water fuel cutoff. This is the reason for requiring two means of supplying feed water.

If solid fuel in suspension, liquid or gaseous fuel, or heat from a turbine engine exhaust is used to fire a Section I steam boiler, one source of feed water supply is acceptable if the heat input can be shut off before the water level reaches its lowest permitted level. This scenario does work well with a low-water fuel cutoff. The inspector should not panic if the typical float-chamber type low-water fuel cutoff is not found on a large Section I boiler. The same results can be achieved with other styles of mechanisms or controls. It is better to simply ask the owner or owner's representative how the boiler is protected from low-water conditions and then tailor that part of the inspection around the method in use.

Each Section I high-temperature water boiler must have:
•    A pressure gage (PG-60.6).
•    A temperature gage (PG-60.6.4).
•    Although not referenced in Section I, there should be some means of controlling temperature. This will vary with the size and complexity of the boiler.
•    A means of adding water to the boiler while under pressure (PG-61.4). (There is no reference to a low-water fuel cutoff in Section I, but some installations may use such a device.)
Each Section IV hot water boiler must have:
•    A pressure or altitude gage (HG-611).
•    A thermometer (HG-612).
•    Two temperature controls (if the boiler is automatically fired); one is considered the operating control and the other is considered the high-limit control (Note: some jurisdictions require the high-limit control be equipped with a manual reset switch.) (HG-613).
•    An automatic low-water fuel cutoff – if the boiler is automatically fired and has a heat input greater than 400,000 Btu/hr. (Note: some jurisdictions require an additional low-water fuel cutoff with a manual reset switch.)(HG-614).
•    Provisions for thermal expansion (HG-709).
Clearances on the front, rear, sides, and top of all fire tube boilers for operation, maintenance, and inspection shall meet jurisdictional requirements. If no jurisdictional requirements exist, then the boiler manufacturer's requirements shall be met.
All fire tube boilers should be installed on foundations or supports suitable for the design and weight of the boiler and its contents. The foundation or support must also be unaffected by the heat of the operating boiler.
Section I boiler external piping is covered by PG-58 which references ASME B31.1.
Some jurisdictions may also regulate the piping which lies beyond the limits imposed by Section I.
Although most jurisdictions do not require inspection of the piping associated with a Section IV boiler, there are some installation requirements in Section IV the inspector should review. Please see HG-703 and HG-705.

Common Observations and Problems:
Fire tube boilers can come in different sizes and configurations; therefore, it is difficult to list a common set of problems.
Water leaks are always a possibility, especially with older boilers where corrosion may have been occurring for several years. The fire tubes will be the thinnest material in the entire boiler and if corrosion (either fireside or waterside)is very aggressive, they will show signs of leakage. This is easily detected if the inspector sees water in the furnace or any other fireside space.
Mud legs in locomotive or other firebox type boilers suffer from poor water circulation and many times will exhibit the most waterside corrosion compared to the rest of the boiler. The welded or threaded stays within the mud leg can also be thinned, sometimes to the point of separation.

Scotch boilers sometimes have poor water circulation between the bottom of the furnace tube and the bottom of the boiler shell. In addition, it may be worse at different locations along the length of the furnace tube. When inspecting this area, the inspector should look for accumulations of sludge or sediment within the entire length of the boiler shell. If one area is clean, it must never be assumed the other areas will be clean. The top of the furnace tube (waterside) can also be a location for sludge or sediment to collect. Any sludge or sediment build-up which rests against the furnace tube can adversely affect its ability to transfer heat to the surrounding water. This can cause the furnace to overheat and, in some cases, the furnace will collapse.
If there is poor or no water treatment, sediment can accumulate enough to plug the spaces between the tubes in extreme cases. Just as with the furnace, this condition can lead to overheated and damaged tubes.
The fireside of the tubes can also be subject to scale and deposit build-up when the boiler is fired with oil or solid fuel. This adversely affects boiler efficiency and can cause the tubes to overheat.
Tube ends that are projecting beyond the tube sheet more than the Code allows can overheat and crack. If the tubes are attached to the tube sheet by welding, cracks in the tube ends can propagate to the tube sheet, and possibly run into the ligaments between the tubes. A tube sheet can easily be damaged beyond repair with cracks of this nature, and it can start with a fraction of an inch in excess tube projection. Please see PFT-12.2, HG-360.2, and HW-713.

 External – while in operation
Upon entering the boiler room, the inspector should perform a general assessment of the boiler, piping, controls, fuel system, and combustion air supply.

The inspector should then:
•    Review the current operating certificate (if one was issued in the past) and compare the information to the associated boiler and its stamping or nameplate.
•    Compare the safety or safety relief valve(s) nameplate data (set pressure and relieving capacity) with the boiler stamping or nameplate to ensure the safety or safety relief valve(s) is(are) adequate for this installation.
•    Inspect the safety or safety relief valve operation as described in the National Board Inspector Guide for Pressure Relief Devices.
•    Inspect the low-water fuel cutoff and water feeding device (if applicable) as described in the National Board Inspector Guide for Water Level Controls and Devices.
•    Inspect the feed water supply system (if applicable) to ensure it meets Code and jurisdictional requirements.
•    Inspect the pressure or temperature controls as described in the National Board Inspector Guide for Operating Controls.
•    Check the pressure or altitude gage reading (if there is a reason to question the accuracy of the gage, it should be replaced or recalibrated).
•    Check the temperature gage reading on Section I high-temperature water boilers, or the thermometer reading on Section IV hot water boilers. (If there is a reason to question the accuracy of either, they should be replaced or recalibrated.)
•    Check the water gage glass to ensure it provides a clear indication of the water level in a steam boiler. (Please see the National Board Inspector Guide for Water Level Controls and Devices.)
•    If a steam boiler has a MAWP over 400 psi, ensure that any remote water level indicators are functioning and indicate the same water level as the gage glass (PG-60.1.1).
•    Look closely for leaks at all pipe connections associated with the boiler.
•    Look closely for leaks originating from under the boiler casing and insulation and instruct the owner or owner's representative to remove the casing and insulation as necessary to pinpoint any leaks.
•    Look for evidence of overheating.
•    Witness any pressure test required by the jurisdiction.
•    Inspect the fuel-burning apparatus as required by the jurisdiction.

Internal inspections of fire tube boilers can range from looking into inspection openings with a mirror and flashlight to actually crawling inside when the boiler and access openings are large enough. Any time the inspector's head enters the fireside or waterside of the boiler, the atmosphere must first be checked for oxygen content and the presence of flammable, explosive, or hazardous gases. The inspector must comply with all applicable confined space entry rules and procedures.

The inspector should:
•    Look in all inspection openings to check for scale, sludge, and sediment and, instruct the owner or owner's representative to remove any build-up which prevents a thorough inspection.
•    Look for corrosion, overheating, bulges or blisters, and cracks.
•    Look at the steam/water line area for evidence of corrosion and oxygen pitting on steam boilers.
•    Investigate any appearance of water in the fireside spaces.
•    Check all stays with telltale holes for evidence of leakage through the hole which would indicate a broken stay.
•    All stays should be examined to determine if they are sound and able to support the stayed area.
•    Check for cracks in the tube ends and tube sheet ligaments.
•    Look through the tubes to check for obstructions and sagging of the tubes.
•    Ensure that refractory and/or fire brick is properly placed and secure.
•    Look for flame impingement on any surfaces exposed to the direct flame.
•    Ensure any supporting structure or foundation for the boiler is in good condition.
•    Examine the interior and operating mechanism of float-type low-water fuel cutoffs and water-feeding devices.
•    Ensure that all piping and connections for low-water cutoffs, water columns, and gage glasses are free of obstructions.

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