Po = maximum operating pressure
T = average operating temperature
Vs = volume of system, not including tanks
HG-709.3 Hot Water Supply Systems. If a system is equipped with a check valve or pressure reducing valve in the cold water inlet line, consideration should be given to the installation of an airtight expansion tank or other suitable air cushion. Otherwise, due to the thermal expansion of the water, the safety relief valve may lift periodically. If an expansion tank is provided, it shall be constructed in accordance with Section VIII, Division 1 or Section X. Except for prepressurized tanks, which should be installed on the cold water side, provisions shall be made for draining the tank without emptying the system. See Fig. HLW-809.1 for a typical acceptable installation.
HG-710 STOP VALVES
HG-710.1 For Single Steam Boilers. When a stop valve is used in the supply pipe connection of a single steam boiler, there shall be one used in the return pipe connection.
HG-710.2 For Single Hot Water Heating Boilers
(a) Stop valves shall be located at an accessible point in the supply and return pipe connections as near the boiler nozzle as is convenient and practicable, of a single hot water heating boiler installation to permit draining the boiler without emptying the system.
(b) When the boiler is located above the system and can be drained without draining the system, stop valves may be eliminated.
HG-710.3 For Multiple Boiler Installations. A stop valve shall be used in each supply and return pipe connection of two or more boilers connected to a common system. See Figs. HG-703.1 and HG-703.2.
HG-710.4 Type of Stop Valve(s)
(a) All valves or cocks shall conform with the applicable portions of HF-203 and may be ferrous or nonferrous.
(b) The minimum pressure rating of all valves or cocks shall be at least equal to the pressure stamped upon the boiler, and the temperature rating of such valves or cocks, including all internal components, shall be not less than 250°F (120
°C).
(c) Valves or cocks shall be flanged, threaded, or have ends suitable for welding or brazing.
(d) All valves or cocks with stems or spindles shall have adjustable pressure type packing glands and, in addition, all plug type cocks shall be equipped with a guard or gland. The plug or other operating mechanism shall be distinctly marked in line with the passage to indicate whether it is opened or closed.
(e) All valves or cocks shall have tight closure when under boiler hydrostatic test pressure.
HG-710.5 Identification of Stop Valves by Tags. When stop valves are used, they shall be properly designated substantially as follows by tags of metal or other durable material fastened to them:
Supply Valve - Number ( )
Do Not Close Without Also Closing Return Valve -
Number ( )
Return Valve - Number ( )
Do Not Close Without Also Closing Supply Valve -
Number ( )
HG-715 BOTTOM BLOWOFF AND DRAIN VALVES
(a)
Bottom Blowoff Valve. Each steam boiler shall have a bottom blowoff connection fitted with a valve or cock connected to the lowest water space practicable with a minimum size as shown in Table HG-715. The discharge piping shall be full size to the point of discharge.
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See PDF for table 
(b) Boilers having a capacity of 25 gal (95 l) or less are exempt from the above requirements, except that they must have an NPS ¾ (DN 20) minimum drain valve.
(c) Drain Valve. Each steam or hot water boiler shall have one or more drain connections, fitted with valves or cocks. These shall be connected at the lowest practicable point on the boiler, or to the lowest point on piping connected to the boiler, at the lowest practicable point on the boiler. The minimum size of the drain piping, valves, and cocks shall be NPS ¾ (DN 20). The discharge piping shall be full size to the point of discharge. When the blowoff connection is located at the lowest water containing space, a separate drain connection is not required.
(d) Minimum Pressure Rating. The minimum pressure rating of valves and cocks used for blowoff or drain purposes shall be at least equal to the pressure stamped on the boiler but in no case less than 30 psi (200 kPa). The temperature rating of such valves and cocks shall not be less than 250°F (120°C).
HG-716 MODULAR BOILERS
(a) Individual Modules
(1) The individual modules shall comply with all the requirements of Part HG, except as specified in HG-607, HG-615, and this paragraph. The individual modules shall be limited to a maximum input of 400,000 Btuh (gas), 3 gal/hr (1 l/hr) (oil), or 115 kW (electricity).
(2) Each module of a steam heating boiler shall be equipped with
(a) safety valve, see HG-701
(b) blowoff valve, see HG-715 (a)
(c) drain valve, see HG-715 (c)
(3) Each module of a modular hot water heating boiler shall be equipped with
(a) safety relief valve, see HG-701
(b) drain valve, see HG-715 (c)
(b) Assembled Modular Boilers
(1) The individual modules shall be manifolded together at the job-site without any intervening valves. The header or manifold piping is field piping and is exempt from Article 2, Part HG, HF, HB, or HC.
(2) The assembled modular steam heating boiler shall also be equipped with
(a) feedwater connection, see HG-705 (a)
(b) return pipe connection, see HG-703.2
(3) The assembled modular hot water heating boiler shall also be equipped with
(a) makeup water connection, see HG-705 (b)
(b) provision for thermal expansion, see HG-709
(c) stop valves, see HG-710.2
HG-720 SETTING
Boilers of wrought materials of the wet-bottom type having an external width of over 36 in. (900 mm) shall have not less than 12 in. (300 mm) between the bottom of the boiler and the floorline, with access for inspection. When the width is 36 in. (900 mm) or less, the distance between the bottom of the boiler and the floorline shall be not less than 6 in. (150 mm), except that, when any part of the wet bottom is not farther from an outer edge than 12 in. (300 mm), this distance shall be not less than 4 in. (100 mm).
HG-725 METHODS OF SUPPORT
HG-725.1 Loadings
(a) The design and attachment of lugs, hangers, saddles, and other supports shall take into account the stresses due to hydrostatic head in determining the minimum thicknesses required. Additional stresses imposed by effects other than working pressure or static head, which increase the average stress by more than 10% of the allowable working stress, shall also be taken into account. These effects include the weight of the component and its contents, and the method of support.
(b) In applying the requirements of (a) above, localized stresses due to concentrated support loads, temperature changes, and restraint against dilation of the boiler due to pressure shall be provided for. Lugs, hangers, brackets, saddles, and pads shall conform satisfactorily to the shape of the shell or surface to which they are attached or are in contact.
HG-725.2 Boilers Over 72 in. (1 800 mm) in Diameter. A horizontal-return tubular boiler over 72 in. (1 800 mm) in diameter shall be supported from steel hangers by the outside-suspension type of setting, independent of the furnace wall. The hangers shall be so designed that the load is properly distributed.
HG-725.3 Boilers Over 54 in. (1 400 mm) up to 72 in. (1 800 mm) in Diameter. A horizontal-return tubular boiler over 54 in. (1 400 mm) and up to and including 72 in. (1 800 mm) in diameter shall be supported by the outside-suspension type of setting, or at four points by not less than eight steel brackets set in pairs, the brackets of each pair to be spaced not over 2 in. (50 mm) apart and the load to be equalized between them. [See Fig. HG-725(a).]
HG-725.4 Boilers up to 54 in. (1 400 mm) in Diameter. A horizontal-return tubular boiler up to and including 54 in. (1 400 mm) in diameter shall be supported by the outside-suspension type of setting, or by not less than two steel brackets on each side.
HG-725.5 Supporting Members. If the boiler is supported by structural steel work, the steel supporting members shall be so located or insulated that the heat from the furnace can not impair their strength.
HG-725.6 Lugs or Hangers. Lugs, hangers, or brackets made of materials in accordance with the Code requirements may be attached by fusion welding provided they are attached by fillet welds along the entire periphery or contact edges. Figure HG-725(b) illustrates an acceptable design of hanger bracket with the additional requirement that the center pin be located at the vertical center line over the center of the welded contact surface. The bracket plates shall be spaced at least 2½
in. (64 mm) apart, but this dimension shall be increased if necessary to permit access for the welding operation. The stresses computed by dividing the total load on each lug, hanger, or bracket, by the minimum cross-sectional area of the weld shall not exceed 2800 psi (19 MPa). Where it is impractical to attach lugs, hangers, or brackets by welding, studs with not less than 10 threads/in. (approx. 4 threads/cm) may be used. In computing the shearing stresses, the root area at the bottom of the thread shall be used. The shearing and crushing stresses on studs shall not exceed 8% of the strength given in Table HF-300.1 for bolting materials.
HG-725.7 Settings. Boilers of wrought materials of the wet-bottom type having an external width of over 36 in. (900 mm) shall be supported so as to have a minimum clearance of 12 in. (300 mm), between the bottom of the boiler and the floor, to facilitate inspection. When the width is 36 in. (900 mm) or less, the clearance between the bottom of the boiler and the floorline shall be not less than 6 in. (150 mm), except when any part of the wet bottom is not farther from the outer edge than 12 in. (300 mm). This clearance shall be not less than 4 in. (100 mm). Boiler insulation, saddles, or other supports shall be arranged so that inspection openings are readily accessible.
