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Cast-Iron Boilers


Cast-iron boilers may be used in steam heating or hot water heating applications within the scope and service restrictions of ASME Section IV. Section IV service restrictions limit steam boilers to pressures not exceeding 15 psi and hot water boilers to pressures not exceeding 160 psi and/or temperatures not exceeding 250°F.

Cast-iron boilers can be constructed in different configurations. The three most common designs found in use are:

  • Vertical Sectional – made up of individual cast iron sections assembled so the sections resemble slices in a loaf of bread. This is probably the most common configuration.
  • Horizontal Sectional – made up of individual cast-iron sections assembled so the sections resemble a stack of pancakes.
  • One Piece – a single casting with no assembly joints. Another term used to describe this design is monobloc. This type of cast-iron boiler is usually small in size.

Sectional boilers are typically assembled with tapered connections called push nipples or elastomeric-type gaskets between the sections to seal the water-containing chambers. Another type of assembly uses external headers to connect the water containing chambers.

Cast-iron boilers can be found in almost any application where heating boilers are used. They are popular replacements for large welded steel boilers which may have been installed as the building was being constructed. Cast-iron sectional boilers can usually be installed in existing boiler rooms by moving the individual sections through doors or window openings. A very large boiler can be assembled in this manner without modifications to the building structure.

There will be two pieces of information missing from a cast-iron boiler nameplate: a National Board registration number and the year built. Cast-iron boilers are not registered with the National Board, and ASME Section IV makes no provisions for a year of construction to appear on the nameplate. Since most inspection forms ask for a year of construction, the inspector will have to estimate. If the boiler is original to the building, the age of the building would directly correspond to the age of the boiler. If the boiler is a replacement, the inspector will have to question the owner to determine its age.

Cast-iron boilers may be used in steam heating or hot water heating applications within the scope and service restrictions of ASME Section IV. Section IV service restrictions limit steam boilers to pressures not exceeding 15 psi and hot water boilers to pressures not exceeding 160 psi and/or temperatures not exceeding 250°F.

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 1/2 nor larger than NPS 4-1/2.

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 3/4 nor larger than NPS 4-1/2. The minimum relieving capacity of safety or safety relief valves must equal or exceed the maximum output of the boiler. Cast-iron boilers constructed since 1943 will have information on the nameplate indicating the minimum required safety or safety relief valve capacity. Cast-iron boilers constructed prior to 1943 may not have that information. In those circumstances, the inspector must estimate the maximum output of the boiler. Gas or oil burners generally have a rating plate or label containing the Btu output of the burner. A generally applied guideline for older boilers is to use 80% of the maximum burner output as the maximum boiler output. Boilers fired with solid fuel such as coal or wood will be extremely difficult to estimate, since there is no way for the inspector to calculate the cast-iron boiler heating surface. In those cases, the inspector should request the boiler owner/user perform an accumulation test in accordance with HG-512(a), or a maximum burned fuel evaluation in accordance with HG-512(b) and Appendix B. These procedures should only be used if the safety or safety relief valve capacity is in doubt.

Safety or safety relief valves must be installed so the spindle is in a vertical position.

More information on safety or safety relief valve requirements can be found in ASME Section IV, HG-400 and HG-701.

    Each steam boiler must have:

    • a pressure gage with an internal siphon or a siphon in the gage piping (HG-602);
    • a water gage glass, or for specific boilers, a Bull's-Eye type gage glass (HG-603);
    • 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). 

    Each 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 cast-iron 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 cast-iron boilers should be installed on foundations or supports suitable for the weight of the boiler and its contents. The foundation or support must also be unaffected by the heat of the operating boiler.

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.

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 1/2 nor larger than NPS 4-1/2.

Common Observations and Problems
Cast-iron boilers typically have a few inherent problems. The inspector should always look for water leaks at the connecting joints of sectional boilers. The inspector should request the removal of the sheet metal casing any time there is evidence of leakage and the leakage cannot be traced to an external source.

The most common problem associated with cast-iron boilers is cracking due to overheating or thermal shock. Overheating occurs when the boiler is allowed to operate with low-water conditions or poor circulation caused by sludge concentrated in the lower water passages of the boiler. Thermal shock can occur when a boiler is overheated and cold water is added in an attempt to raise the water level. Under those circumstances, cracking is usually the least that can happen. The worst that can happen is an explosion which shatters the cast-iron boiler into many pieces and cause destruction and injury.

Sectional cast-iron boilers use long rods, threaded on both ends, called draw bolts. It is not unusual for these draw bolts to appear loose when the boiler is cold. When the boiler is operating, the heat will cause the boiler to expand which tightens the draw bolts. A loose draw bolt on a hot boiler should be investigated by a competent cast-iron boiler service/repair company.

Cast-iron boilers typically have a few inherent problems. The inspector should always look for water leaks at the connecting joints of sectional boilers. The inspector should request the removal of the sheet metal casing any time there is evidence of leakage and the leakage cannot be traced to an external source.

Inspection

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 nameplate;
  • compare the safety or safety relief valve nameplate data (set pressure and relieving capacity) with the boiler nameplate to ensure the safety or safety relief valve is 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 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 thermometer reading on hot water boilers (if there is a reason to question the accuracy of the thermometer, it 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);
  • look closely for leaks at all pipe connections associated with the boiler;
  • look closely for leaks from the boiler and the joints between cast-iron sections;
  • instruct the owner or owner's representative to repair any leaks which may have been discovered (if a leak is detected as a result of a crack in the cast iron, there is no acceptable repair except replacement of the cracked section);
  • look for evidence of overheating (this may be difficult to detect on a cast-iron boiler; warped external sheet metal casings with scorched paint is usually a reliable indicator);
  • witness any pressure test required by the jurisdiction; and  
  • inspect the fuel-burning apparatus as required by the jurisdiction (for example, some jurisdictions mandate compliance with ASME CSD-1).  

Internal

Internal inspections of cast-iron boilers can prove to be difficult or almost impossible. Threaded plugs on the cast iron boiler could be removed, but the inspector will see very little past the immediate vicinity of the opening on many cast iron boiler designs. In addition, the threaded plugs are sometimes heavily corroded which virtually "welds" them to the cast iron. Removal of threaded plugs in this condition may damage the cast iron irreparably. Some boilers may have valves installed in the lowest threaded openings of the boiler to facilitate draining and/or flushing of the boiler. If valves are present, the inspector can ask for them to be opened briefly to observe the condition of the water. If no water is present when the valves are opened, this could be an indication the lowest portion of the boiler is filled with sludge. The inspector is advised to follow the jurisdiction's requirements for internal inspections of cast-iron boilers.

Float-type low-water fuel cutoffs and water-feeding devices (if applicable) must be disassembled so the inspector can observe the condition of the float, float mechanism, and the float chamber.

Threaded plugs in the piping connecting a water gage glass, water column, and low-water fuel cutoff to a steam boiler must be removed to allow inspection of the piping to ensure there is no blockage.

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

  • National Board Inspection Code
  • ASME Section IV
  • ASME Section VI
  • ASME CSD-1
  • Manufacturer's Installation, Operation, and Maintenance Documentation
  • Jurisdictional Laws, Rules, and Directives






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