Specifying high-temperature hot-water boilers and systems

Learning Objectives

• Identify the codes and standards applicable to high-temperature hot-water systems.
• Identify industry-standard design practices associated with high-temperature hot-water systems.

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High-temperature hot-water (HTHW) plants are typically designed to operate at temperatures ranging from 350°F to 420°F. The system pressure must be at least 25 psig above the saturation pressure of the high-temperature water’s maximum temperature to prevent pump cavitation and flashing of superheated water to steam.

Section 1004 of the International Mechanical Code (IMC) describes the minimum allowable requirements for boilers. Depending on the design criteria, boilers must comply with Section I or Section IV of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC). Hot-water boilers designed and constructed to operate at temperatures below 250°F and pressures less than 160 psig need to comply with Section IV. Hot-water boilers designed and constructed to operate at temperatures above 250°F and pressures greater than 160 psig are considered high-temperature hot-water boilers and are required to comply with Section I.

As it relates to steam and per Section I of ASME, a power boiler is “a boiler in which steam or another vapor is generated at a pressure of more than 15 psig (100 kPa) for use external to itself,” and a heat-recovery steam generator (HRSG) is “a boiler that has as its principal source of thermal energy a hot gas stream having high-ramp rates and temperatures such as the exhaust of a gas turbine.” HRSGs are used to capture the heat from various exhaust gases and generate high-pressure steam, which in turn is used to power steam turbines. Also, superheaters, economizers, and other pressure parts connected directly to the boiler without intervening valves are considered as part of the scope of Section I.

Section 12 of the IMC describes the minimum allowable requirements for building hydronic piping systems that are a part of HVAC systems within a building. IMC Section 1201.3 allows the use of ASME B31.9-2007 (Building Services Piping) as an alternative compliance path to the requirements defined in Section 12. ASME B31.9 contains rules for the piping in industrial, institutional, commercial, public buildings, and multi-unit residences, which does not cover the range of sizes, pressures, and temperatures covered in B31.1-2018 (Power Piping). ASME B31.9 prescribes requirements for the design, materials, fabrication, installation, inspection, examination, and testing of piping systems for building services. It includes piping systems in the building or within the property limits. For campus systems and distributing HTHW between various buildings, the designer should consult guidelines and requirements stated in ASME B31.3-2016 (Process Piping).

The requirements for fuel-oil piping and storage are described in Section 13 of the IMC.

Typically, HTHW central heating plants are pressurized with an inert gas or steam. Figure 1 shows a schematic diagram of an HTHW central heating plant pressurized with an inert gas, typically nitrogen. In this setup, the expansion tank is on the suction side of the system pumps by means of an interconnecting balance line. No system water flows through the tank due to the action of the pumps. The point of connection of the balance line to the system’s return-water piping is known as “the point of no pressure change.” This phrase indicates that the total pressure value within the piping at that point remains the same whether the pumps are running or stopped. Compressed air should never be used to pressurize the system this is because it will be absorbed by the HTHW system and will cause pitting corrosion due to the formation of carbonic acid. Figure 2 shows a schematic diagram of an HTHW central heating plant pressurized with steam. In this setup, an external steam boiler is used to pressurize the system before firing the main HTHW generator(s). The external boiler provides a means for maintaining system pressures above saturation under all conditions of operation. Depending on the heating loads of the plant and the number of HTHW generators, a balance valve may be required to ensure that the minimum flow through each HTHW generator is achieved.

HTHW plants for government projects

For government projects, the Unified Facilities Criteria (UFC) document UFC 3-430-08N, Central Heating Plants, establishes the design procedures, codes, and standards that need to be followed in the design, installation, and operation of HTHW plants. The UFC system is prescribed by MIL-STD 3007 (Standard Practice for Unified Facilities Criteria and Unified Facilities Guide Specifications) and provides planning, design, construction, sustainment, restoration, and modernization criteria for military departments, defense agencies, and Department of Defense (DoD) field activities. The ASME standards described in this article are referenced in UFC 3-430-08N as mandatory compliance paths. For compressed air systems, UFC 3-430-08N references document DM-3.05, Compressed Air and Vacuum Systems, as a compliance path.

From a jurisdictional perspective, Section I of the ASME BPVS has administrative jurisdiction and technical responsibility for the construction of HTHW generators and only administrative jurisdiction on all boiler external piping (BEP) and joints. BEP is generally defined as the piping between the HTHW generator and the first block valve.

Requirements for burner management systems

The requirements for the burner management system (BMS) and associated emergency shutoff valve (ESOV) are described in NFPA 87: Standard for Fluid Heaters. NFPA 87 defines the requirements for:

• The location and construction of fluid heaters
• Heating systems
• Commissioning and operations
• Maintenance, inspection, and testing
• Heating system safety equipment and application.

It is standard practice to provide the HTHW generators with dual fuel burners. When more than one HTHW generator is designed, the designer should coordinate the design of the system with the requirements of NFPA 85: Boiler and Combustion Systems Hazards Code. Typically, the HTHW generators are located in an enclosed mechanical room—as such, the design of the make-up air systems to the burner becomes very important. This is because most of the burners cannot support a ducted outside-air inlet or a high negative suction pressure. In this scenario, a make-up air fan is required to deliver the make-up air to the burners; the outlets of the make-up air duct system should be located relatively close to the intake of the burners. An alternative to using a forced-air system to deliver make-up air to the burners is to use outside-air intake louvers and motorized dampers; this approach, although effective from a first-cost perspective, might create the risk of having the burners failing to start. As previously mentioned, the typical blower of a burner and the burner itself it is not well suited for high negative air pressure intakes, i.e., for “sucking” air through a louver. Further, the designer will need to consider the outside air temperature, as burner ignition and efficiency may be affected.

The ASME Section Committee B31.1-Power Piping has been assigned technical responsibility on all BEPs. Applicable ASME B31.1 editions and addenda are referenced in ASME BPVC, Section I, page 58.3. The ASME Section Committee B31.1 also has total administrative jurisdiction and technical responsibility on the construction and installation of all non-boiler external piping and joints (NBEPs); this includes piping systems carrying steam, water, oil, gas, and air. ASME B31.1 provides specific requirements for the following:

• BEP (steam, feedwater, blowoff, blowdown, and drains
• NBEP blowoff and blowdown piping
• Instrument, control, and sample piping
• Spray-type desuperheater piping
• Pressure-reducing valves
• Pressure-relief piping
• Piping for flammable or combustible liquids
• Piping for flammable gases and toxic fluids
• Piping for corrosive fluids
• Temporary piping
• Steam trap piping
• Pump suction and discharge piping
• District heating and steam distribution systems.

Section 120 of ASME B31.1 describes the minimum requirements for loads on pipe-supporting elements. Per this section, “The broad terms ‘supporting elements’ or ‘supports’ as used herein shall encompass the entire range of the various methods of carrying the weight of pipelines, insulation, and the fluid carried.” Table 121.5 of B31.1 shows the suggested steel-pipe support spacing based on the fluid that the piping system is carrying. Thermal expansion of the piping system and its impact on the pump and boiler connections will also need to be considered. In some instances, and depending on the diameter and location of the pipes, the building structure will need to be designed and/or reinforced as needed to accommodate the installation of thrust blocks and anchors.

From a seismic perspective, and if the HTWH plant or any of its associated systems located in a structure that has a seismic design category C, D, E or F, Section 1705.12.6 of the International Building Code (IBC) requires special inspections in accordance with Section 13.2.3 of American Society of Civil Engineers (ASCE) 7: Minimum Design Loads and Associated Criteria for Buildings and Other Structures. Further, and as it relates to structures assigned a seismic design category B, C, D, E or F, Section 1705.13.2 of IBC references Section 13.2.1 of ASCE 7 as having the technical responsibility for the design and installation of architectural, mechanical, and electrical components, supports, and attachments. Section 13.3 of ASCE 7 defines the calculation methodology that the design professional must follow when sizing and selecting various seismic supports and restraints for mechanical systems. Section 13.6 establishes the minimum requirements that mechanical and electrical components and their supports must meet; the requirements for piping systems are defined in Section 13.6.8.

In some instances, and depending on the fluid type that the piping system is carrying, the requirements stated in standard ASME B31.3 may be more stringent than the requirements stated in ASME B31.1. One must note that the requirements stated in ASME B31.1 are applicable to piping typically found in electric power-generating stations, industrial and institutional plants, geothermal heating systems, and central and district heating and cooling systems, while the requirements stated in ASME B31.3 are applicable to piping typically found in petroleum refineries; chemical, pharmaceutical, textile, paper, semiconductor, and cryogenic plants; and related processing plants and terminals.

ASME B31.1 has eight mandatory appendices and six nonmandatory appendices. The mandatory appendices are as follows:

• Appendix A—Allowable stress tables.
• Appendix B—Thermal-expansion data.
• Appendix C—Moduli of elasticity.
• Appendix D—Flexibility and stress-intensification factors.
• Appendix F—Referenced standards.
• Appendix G—Nomenclature.
• Appendix H—Preparation of technical inquiries.
• Appendix J—Quality control requirements for boilers’ external piping.

The nonmandatory requirements are as follows:

• Appendix II—Rules for the design of safety valve installations.
• Appendix III—Rules for nonmetallic piping and piping lined with nonmetals.
• Appendix IV—Corrosion control for ASME B31.1 power piping systems.
• Appendix V—Recommended practice for operation, maintenance, and modification of power piping systems.
• Appendix VI—Approval of new materials.
• Appendix VII—Procedures for the design of restrained underground piping.

As shown in Figure 1, it is recommended to use a variable-speed pumping system. Each pump will be sized for 50% of the total plant load, with two pumps operating and one pump on standby to provide the facility with a N+1 pumping capacity. The sizing and selection of the pumps is dependent on the design differential temperature between the supply and return. A minimum temperature differential should be 100°F. HTHW central heating plants require high-performance pumps that can operate continuously at the design temperatures and pressures. At minimum, such pumps should be provided with mechanical seals with a heat exchanger for cooling the water to cool the face temperature of the seals to 140°F  maximum, alloy steel impellers (minimum of 11% chromium), stainless steel shafts, sleeves, and pump trim, oil-lubricated ball or roller bearings, and water-cooled stuffing box seals.

The design and specifications of the steel pipe flanges and flanged fittings must be based on the HTHW system design temperature and associated pressure and in accordance with the temperature-pressure ratings of ASME B16.5.

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