BS 12952-10:2021 pdf download – Water-tube boilers and auxiliary installations Part 10: Requirements for safety devices against excessive pressure
5.1.2 The minimum flow diameter of safety valves or main valves of POSV or CSPRS for boilers with a volume ≤ 10 litres shall be at least 6 mm and for volumes > 10 litres shall be at least 15 mm.
5.1.3 In the case of boilers without superheater(s) the safety valves or main valves of POSV or CSPRS shall be connected to the steam space. If the total required discharge capacity is q m and the steam space is simultaneously protected by n safety devices (n > 1) the discharge capacity of each of them should be at least q m /(n + 1).
5.1.4 In the case of once-through steam generators the safety valves or main valves of POSV or CSPRS shall be located at the steam outlet of the boiler.
5.1.5 In the case of natural and forced circulation boilers with superheaters which cannot be isolated the following shall apply:
a) Safety devices in accordance with
5.1.1 located at the superheater outlet shall be of sufficient capacity to prevent the allowable wall temperature of the superheater from being exceeded. The safety device at the superheater outlet shall be so arranged, that it is always open when the safety device on the saturated steam space opens, or alternatively, the heat source is immediately cut-out with no residual heat release allowed.
b) Safety devices in accordance with 5.1.1 a) rated for at least 75 % or in accordance with 5.1.1 b) or c) rated for at least 25 % of the required discharge capacity shall be located at the saturated steam space of the boiler. No safety devices need to be installed at the saturated steam space if the capacity of the safety device in accordance with 5.1.1 b) or c) located at the superheater outlet corresponds to the total steam quantity to be discharged. At least one pressure signal shall be transmitted to the control unit from the saturated steam space and at least one pressure signal shall be taken from the superheater outlet.
c) The superheater safety device shall be so set as to respond before the safety device on the saturated steam space.
5.1.6 A superheater that can be isolated shall be equipped with at least one safety valve or main valve of POSV or CSPRS at the superheater outlet. The safety valve or main valve of POSV or CSPRS shall be rated for at least 20 % of the required discharge capacity of the boiler. The safety valve or main valve of POSV or CSPRS located on the saturated steam space of the steam generator shall in this case be rated for the maximum continuous rating. If the superheater can be isolated precautions shall be taken to prevent overheating. Superheaters that can be isolated from the saturated steam space are not allowed for boilers firing fuels with considerable residual heat release.
5.1.7 Every reheater shall have at least one safety valve or main valve of POSV or CSPRS with a total discharge capacity of not less than the maximum steam flow for which the reheater is designed.
5.1.8 There shall be no shut off devices between the boiler and its protective safety devices or between the safety devices and their points of discharge.
5.1.9 Direct-loaded safety valves shall be installed in the vertical position. Safety devices shall be safeguarded against damaging external influences, e.g. the weather, which can impair the functional capability of the safety device. The transfer of vibrations onto the safety device shall be avoided. Safety devices where leaking fluid, e.g. due to open bonnet, can directly or indirectly endanger persons or the environment, shall be provided with suitable protective devices.BS 12952-10 pdf download.