The Alberta Boiler Safety Association released a warning following a deadly ball valve failure that resulted in the death of a worker:
“A threaded NPS 2 forging brass ball valve, with a marked steam working pressure of 150 psig, failed while in service as a high pressure steam boiler blow-off valve. The boiler was designed to operate at a maximum allowable pressure of 125 psi. The failure occurred when the body of the valve, which was of two-piece construction, came apart, resulting in the release of high temperature water and steam from the boiler. In this incident, a worker in the vicinity of the boiler received serious burns and later died in hospital.
The preliminary findings of ABSAs investigation into the incident indicate the forging brass material specification of the valve was not permissible for a boiler external piping application. Alberta regulations require that boiler blow-off valves and the piping that joins the valves and other fittings to the boiler must be designed and constructed in compliance with the requirements of the ASME B31.1 Power Piping Code for boiler external piping. The material specification of the valve body design was identified as ASTM B283 C37700, forging brass. While this material is permitted by ASME B31.1 for use in some applications, its use in boiler external piping is specifically prohibited (see Note 1 of Table A-6, ASME B31.1). All power boiler owners and equipment suppliers are asked to review the material specifications of valves and other fittings used in power boiler service and to replace immediately any inappropriate ones when they are found.”
ASME Section I is a part of the Boiler and Pressure Vessel Code (BPVC) published by the American Society of Mechanical Engineers (ASME). Section I provides requirements for the design, fabrication, inspection, testing, and certification of power boilers. This section covers various types of boilers, including fire-tube boilers, water-tube boilers, electric boilers, etc. It also provides guidelines for the installation, operation, and maintenance of these boilers.
ASME Section I depicts “code boundaries” for different boiler types and systems. For a Section I firetube boiler, the code boundaries for instrumentation blowoffs terminate after the first blowoff valve. For the bottom blowdown, the code boundary extends through the second blowdown valve. Any valves, pipes, or fittings within this scope must conform to the design requirements in ASME Section I. ASME Section I – 2021 refers the designer to ASME B31.1 Power Piping – 2020 for construction rules for materials, design, fabrication, installation, and testing of the boiler external piping (BEP). ASME B31.1 has complete technical responsibility for the design requirements of the relevant components within the BEP boundaries, including the valve that failed in this incident.
Refer to ASME Section 1 – 2021 Figures PG 58.2-1 through PG-58.2-6 for a complete depiction of ASME Section I code jurisdiction piping.
ASME B31.1 specifies that the minimum pressure and temperature rating for all valves and fittings in the BEP code boundary shall be equal to the pressure and temperature specified for the connected piping on the side that has the higher pressure, except that in no case shall the pressure be less than 100 PSIG.
Blowoff and Blowdown Piping:
ASME B31.1 defines Blowoff and Blowdown Piping as piping connected to a boiler and provided with valves or cocks through which the water in the boiler may be blown out under pressure. Drain piping and piping connected to water columns, gage glasses, etc. are not considered “blowoff” piping in this code and thus are not limited to the same rules. For piping classed as blowoff or blowdown by ASME B31.1, when the internal design pressure exceeds 100 PSIG, all pipe shall be of steel construction per Paragraph 122.1.4. Galvanized pipe and fittings are not permitted for blowoff piping. However, if the internal design pressure does not exceed 100 PSIG, nonferrous pipe may be used and the fittings may be bronze, cast iron, malleable iron, ductile iron, or steel. Although allowed in some use cases, consideration should be taken when using nonferrous alloys or austenitic stainless-steel components as they may be particularly sensitive to corrosion in certain aqueous environments.
*Nonferrous refers to alloys that do not contain iron in their composition
Within the code boundary for blowdown piping and in the case of the design pressure exceeding 100 PSIG, the fittings are to be steel construction and Schedule 80 or better.
Fittings and Valves:
ASME B31.1 designates that at an internal design pressure not exceeding 250 PSIG, the blowoff valves shall be of bronze, cast iron, ductile iron, or steel construction. If the selected valve is constructed of cast iron, it shall conform to the requirements of the applicable ASME standard for Class 250. If the selected valve is constructed of bronze, ductile iron, or steel, the valve shall meet the design requirements of each applicable code.
For an internal design pressure exceeding 250 PSIG, the valves material shall be steel and at a minimum have a class rating of 300.
Care must be taken selecting a valve material for use in Boiler External Piping, BEP. Although ASME B31.1 permits the use of non-steel valves in BEP, not every material classification or grade is permitted. Some materials have size limitations or temperature limitations that, depending on the system design pressure, may or may not be installed within the BEP code boundary. The designer must refer to the applicable ASME B31.1 code year to qualify the use of a specific valve material.