Description and Overview
Firetube boilers may be used in steam or hot water applications within the scope and service restrictions of ASME Section I and 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:
- Horizontal Return Tubular (HRT) – this is an old and very simple design which is still being manufactured. The boiler consists of a shell, a tube sheet on each end of the shell, and tubes connecting the two tube sheets. The boiler is mounted above a steel or masonry furnace. The products of combustion leave the furnace and are directed through the tubes at one end of the boiler. After passing through the tubes, the products of combustion exit the opposite end of the boiler and are directed to the stack or chimney.
- Firebox – this type of boiler includes locomotive boilers as well as what some have called firetube firebox boilers. The products of combustion pass through a locomotive boiler one time giving it a general classification of a one-pass boiler. Some firetube firebox boilers may be two-pass or three-pass depending upon the arrangement of baffles and tubes. The common characteristic of firebox boilers is that the furnace is at least partially contained within the boiler and is water cooled for a large portion of its surface area. Multi-pass firebox boilers are very common and many older examples will be found in places such as schools where they are used to heat water or produce steam for space heating applications.
- Scotch – this type of boiler is commonly referred to as a scotch marine boiler. In a scotch boiler, the furnace is a large diameter tube, within the boiler, surrounded by water. Some older, large scotch boilers had two or three furnaces but modern boilers typically have only one. Scotch boilers may be two-pass, three-pass, or four-pass depending upon the arrangement of baffles and tubes. Four passes are generally recognized as the practical maximum when balancing economic heat transfer and condensation induced corrosion. Each pass through the boiler transfers heat from the products of combustion to the water in the boiler. After a number of passes, it becomes more difficult to economically extract heat from the cooling products of combustion. Additionally, if the products of combustion are cooled too much, the combustion gases will condense which can cause corrosion. A further subclassification of scotch boilers describes the end closure opposite the burner end of the boiler. A wet-back means the end closure is water cooled and a dry-back means the end closure is not water cooled and relies on fire brick, refractory, or a combination of both to prevent the end closure from overheating.
- Vertical – this type of boiler is a one-pass boiler with the furnace at the bottom and tubes running between the lower and upper tube sheets. The furnace can be enclosed on its sides with a water cooled jacket or it may be made up of masonry. The top tube sheet in a steam boiler can be above or below the water line. When it is above, it is called a dry-top and when it is below, it is called a wet-top. A vertical boiler has a small "footprint" and can be installed in boiler rooms with limited space. Vertical boilers are very popular in the dry cleaning industry.
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 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 a 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 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 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.
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.
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:
- 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 feedwater 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 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:
- 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.
Additional information to aid inspections of firetube boilers can be found in the following publications and sources:
- National Board Inspection Code
- ASME Section I
- ASME Section IV
- ASME Section VI
- ASME Section VII
- ASME CSD-1
- NFPA 85
- Manufacturer's Installation, Operation, and Maintenance Documentation
- Jurisdictional Laws, Rules, and Directives