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Firetube Boilers

Print Date: 12/13/2017 1:26:20 AM

Description and Overview
Firetube boilers may be used in steam or hot water applications within the scope and service restrictions of ASME BPV Code Section I and ASME BPV Code Section IV. They may also be used to heat or vaporize liquids other than water, such as an organic or synthetic fluid.

 

Firetube boilers can be constructed in different configurations. The most common designs are:

Appurtenances, Settings, and Piping
ASME BPV Code 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 . ASME BPV Code 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 ASME BPV Code Section IV boilers must equal or exceed the maximum output of the boiler. More information on ASME BPV Code Section IV safety or safety relief valve requirements can be found in ASME BPV Code Section IV, HG-400 and HG-701.

ASME BPV Code 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 ASME BPV Code 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 BPV Code Section I, PVG-12. More information on ASME BPV Code Section I safety or safety relief valve requirements can be found in ASME BPV Code 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:

Each ASME BPV Code Section IV steam boiler must have:

Although not referenced in ASME BPV Code Section I, there should be some means of controlling pressure. This will vary with the size and complexity of the boiler.

Each ASME BPV Code Section I steam boiler with more than 500 square feet of water heating surface must have at least two feedwater 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 feedwater 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 feedwater.

If solid fuel in suspension, liquid or gaseous fuel, or heat from a turbine engine exhaust is used to fire an ASME BPV Code Section I steam boiler, one source of feedwater 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 ASME BPV Code 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 ASME BPV Code Section I high-temperature water boiler must have:

Each ASME BPV Code Section IV hot water boiler must have:

Clearances on the front, rear, sides, and top of all firetube 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 firetube 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.

ASME BPV Code 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 ASME BPV Code Section I.

Although most jurisdictions do not require inspection of the piping associated with an ASME BPV Code Section IV boiler, there are some installation requirements in ASME BPV Code Section IV the inspector should review. Please see HG-703 and HG-705.

Common Observations and Problems
Firetube 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 firetubes 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:

Internal

Internal inspections of firetube 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:

Miscellaneous Information
Additional information to aid inspections of firetube boilers can be found in the following publications and sources: