(D) Safety Valves
(D.1) Safety valves, relief valves, and safety relief valves shall conform to the requirements of PG-67, PG-68, PG-69, PG-70, PG-71, PG-72, and PG-73 of Section I of the ASME Boiler and Pressure Vessel Code.
122.2 Blowoff and Blowdown Piping in Nonboiler External Piping
Blowoff and blowdown piping systems shall be, where possible, self-draining and without pockets. If unavoidable, valved drains at low points shall allow system draining prior to operation. In order to minimize pipeline shock during the operation of blowoff systems, 3D pipe bends (minimum) should be used in preference to elbows, and wye or lateral fittings should be used in preference to tee connections.
(A) From Boilers
(A.1) Blowoff piping, located between the valves described in para. 122.1.4(A) and the blowoff tank or other point where the pressure is reduced approximately to atmospheric pressure and cannot be increased by closing a downstream valve, shall be designed for the appropriate pressure in accordance with Table 122.2. The provisions of paras. 122.1.4(A.3) and 122.1.7 shall apply. The size of non-BEP blowoff header to the safe point of discharge shall not be smaller than the largest connected BEP blowoff terminal [see para. 122.1.4(A.4)].
(A.2) Blowdown piping, in which the pressure cannot be increased by closing a downstream valve, shall be designed for the appropriate pressure and temperature in accordance with Table 122.2. The provisions of para. 122.1.4(B.3) shall apply. The size of non-BEP blowdown piping between the shutoff valve described in para. 122.1.4(B) and the flow control valve shall not be smaller than the BEP boiler shutoff valve [see para. 122.1.4(B.4)] unless engineering calculations confirm that the design flow rate can be achieved with a smaller piping size without flashing the blowdown prior to the flow control valve.
(A.3) When the design pressure of Table 122.2 can be exceeded due to closing of a downstream valve, calculated pressure drop, or other means, the entire blowoff or blowdown piping system shall be designed in accordance with paras. 122.1.4(A) and 122.1.7 for blowoff and para. 122.1.4(B) for blowdown piping.
(A.4) Non-BEP blowdown piping downstream of the flow control valve shall not be smaller - and preferably will be larger - than the connection on the boiler [see para. 122.1.4(B.4)].
(B) From Pressure Vessels Other Than Boilers
(B.1) The design pressure and temperature of the blowoff piping from the pressure vessel to and including the blowoff valve(s) shall not be less than the vessel MAWP and corresponding design temperature.
122.3 Instrument, Control, and Sampling Piping
(A) The requirements of this Code, as supplemented by para. 122.3, shall apply to the design of instrument, control, and sampling piping for safe and proper operation of the piping itself.
(B) The term “Instrument Piping” shall apply to all valves, fittings, tubing, and piping used to connect instruments to main piping or to other instruments or apparatus or to measuring equipment as used within the classification of para. 100.1.
(C) The term “Control Piping” shall apply to all valves, fittings, tubing, and piping used to interconnect pneumatically or hydraulically operated control apparatus, also classified in accordance with para. 100.1, as well as to signal transmission systems used to interconnect instrument transmitters and receivers.
(D) The term “Sampling Piping” shall apply to all valves, fittings, tubing, and piping used for the collection of samples, such as steam, water, oil, gas, and chemicals.
(E) Paragraph 122.3 does not apply to tubing used in permanently closed systems, such as fluid-filled temperature responsive devices, or the temperature responsive devices themselves.
(F) Paragraph 122.3 does not apply to the devices, apparatus, measuring, sampling, signaling, transmitting, controlling, receiving, or collecting instruments to which the piping is connected.
122.3.1 Materials and Design. The materials utilized for valves, fittings, tubing, and piping shall meet the particular conditions of service and the requirements of the applicable specifications listed under general paras. 105, 106, 107, and 108 with allowable stresses in accordance with the Allowable Stress Tables in Appendix A.
The materials for pressure retention components used for piping specialties such as meters, traps, and strainers in flammable, combustible, or toxic fluid systems shall in addition conform to the requirements of paras. 122.7 and 122.8.
122.5 Pressure-Reducing Valves
122.5.1 General. Where pressure-reducing valves are used, one or more relief devices or safety valves shall be provided on the low pressure side of the system. Otherwise, the piping and equipment on the low pressure side of the system shall be designed to withstand the upstream design pressure. The relief or safety devices shall be located adjoining or as close as practicable to the reducing valve. The combined relieving capacity provided shall be such that the design pressure of the low pressure system will not be exceeded if the reducing valve fails open.
122.5.2 Bypass Valves. Hand controlled bypass valves having a capacity no greater than the reducing valve may be installed around pressure reducing valves if the downstream piping is protected by relief valves as required in para. 122.5.1 or if the design pressure of the downstream piping system and equipment is at least as high as the upstream pressure.
122.5.3 Design of Valves and Relief Devices. Pressure reducing and bypass valves, and relief devices, shall be designed for inlet pressure and temperature conditions. Safety and relief valves shall be in accordance with the requirements of para. 107.8 of this Code.
122.6 Pressure Relief Piping
Pressure relief piping within the scope of this Code shall be supported to sustain reaction forces, and shall conform to the requirements of paras. 122.6.1 and 122.6.2.
122.6.1 Piping to Pressure-Relieving Safety Devices
(A) There shall be no intervening stop valve(s) between piping being protected and the protective device(s).
(B) Diverter or changeover valves designed to allow servicing of redundant protective devices without system depressurization may be installed between the piping to be protected and the required protective devices under the following conditions:
(B.1) Diverter or changeover valves are prohibited on boiler external piping or reheat piping.
(B.2) One hundred percent (100%) of the required relieving capacity shall be continuously available any time the system is in service.
(B.3) Positive position indicators shall be provided on diverter or changeover valves.
(B.4) Positive locking mechanisms and seals shall be provided on diverter or changeover valves to preclude unauthorized or accidental operation.
(B.5) Diverter or changeover valves shall be designed for the most severe conditions of pressure, temperature, and loading to which they are exposed, and shall be in accordance with para. 107.
(B.6) Provision shall be made to safely bleed off the pressure between the isolated protective device and the diverter or changeover valve.
122.6.2 Discharge Piping From Pressure-Relieving Safety Devices
(A) There shall be no intervening stop valve between the protective device or devices and the point of discharge.
(B) When discharging directly to the atmosphere, discharge shall not impinge on other piping or equipment and shall be directed away from platforms and other areas used by personnel.
(C) It is recommended that individual discharge lines be used, but if two or more reliefs are combined, the discharge piping shall be designed with sufficient flow area to prevent blowout of steam or other fluids. Sectional areas of a discharge pipe shall not be less than the full area of the valve outlets discharging thereinto and the discharge pipe shall be as short and straight as possible and so arranged as to avoid undue stresses on the valve or valves.
(D) Discharge lines from pressure-relieving safety devices within the scope of this Code shall be designed to facilitate drainage.
(E) When the umbrella or drip pan type of connection is used, the discharge piping shall be so designed as to prevent binding due to expansion movements.
(F) Drainage shall be provided to remove water collected above the safety valve seat.
(G) Carbon steel materials listed in Appendix A may be used for discharge piping which is subjected to temperatures above 800°F (427°C) only during operation of pressure relieving safety devices provided that
(G.1) the duration of pressure relieving safety device operation is self-limiting
(G.2) the piping discharges directly to atmosphere
(G.3) the allowable stresses for carbon steel materials at temperatures above 800°F (427°C) shall be taken from Section II, Part D, Table 1A for materials applicable to Section I and Section VIII, Division 1 of the ASME Boiler and Pressure Vessel Code.
