Reg. 338 Boiler and Super Heater Tubes, Headers and Other Pressure Parts Tubes
Reg. 338 Boiler and Super Heater Tubes, Headers and Other Pressure Parts Tubes
Reg. 338 Boiler and Super Heater Tubes, Headers and Other Pressure Parts Tubes
338
BOILER AND SUPER HEATER TUBES, HEADERS AND OTHER PRESSURE PARTS TUBES
a) The working pressure of the tubes shall be determined by the following formula:-
2 f (T - C)
W. P. = Eqn.(87)
(D - T + C)
where,
T = minimum thickness of tubes, that is, nominal thickness less the permissible negative tolerance in mm. (inch)
C = 0.75 mm for working pressure up to and including 70 kg/cm².
or
(a) For integral economiser tubes, the maximum water temperature for which the part of the element is designed
plus 11°C (20°F).
(b) For furnace and boiler tubes, the saturation temperature corresponding to the working pressure plus 28°C
(50o F)
(c) For convection superheater tubes, the maximum steam temperature for which the part of the element is
designed plus 39oC (70oF).
(d) For radiant superheater tubes the designed maximum steam temperature plus 50 °C (90oF)
Permissible working stress for tubes:- For temperatures at or below 454 °C,
T. S. Et
or whichever is lower.
2.7 1.5
Sr
or Sc whichever is lower.
1.5
where,
Note:- In case Sc values are not available in Material Standard and such materials are known to have been used
in boilers in India or abroad, then for such material the allowable stress may be taken as the lower of
Et Sr
or
1.5 1.5
b) In no case, however, shall the thickness of tubes as supplied be less than those given in the table below (subject
to tolerances specified in Regulations 36(d), 43(d), 48(c), 53(c), 56A (IV) and 58(d).
TABLE
In the case of coiled tubes for boilers of the forced flow or forced circulation type, the minimum thickness of
tubes as supplied may, however, be as follows:-
Thinning and departure from circularity limits shall be demonstrated by one of the following methods.
(c) By measurement of 2% of the bends, including the first bend of each shift.
(ii) Thinning on tube bends. Thinning on tube bends shall comply with the following.
(a) At any location around the bend extrados, the reduction in thickness (in percent) below the calculated
minimum permissible design thickness of the straight tube, except where permitted by (b), shall not exceed:
__100___
4R + 2
D
where
R is the mean radius of the bend to the centre line of the tube (in mm).
(b) Bend thickness below the minimum value required under (a) shall be permitted in cases where the
manufacturer can demonstrate by bursting tests carried out on at least three bends that the strength of the bend is
not less than that of the straight tube.
(c) Where the amount of thinning at any location around the bend extrados on cold formed bends
exceeds 25% of the actual thickness of the tube on the straight the bends shall be suitably heat treated. The
actual thickness of the same plans as the line of the extrados at each and of the bend.
(iii) Departure from circularity in tube bends. The departure from circularity at the bend apex shall
not exceed 10% for bends performed in a single bending operation and 15% for bends which are hot pressed
after the primary, bending operation.
where
Dmax is the maximum outside diameter measured in the tube bend apex (mm),
Dmin is the minimum outside diameter measured at the same cross section as Dmax(mm)
d) For radiant superheater tubes, the maximum steam temperature for which the part of the element is designed
plus 50°C (90°F).
Reg. 337
BOILER AND SUPER HEATER TUBES, HEADERS AND OTHER PRESSURE PARTS TUBES
a) All tubes which are subject to internal pressure of water or steam shall be cold drawn or hot finished seamless or
electric-resistance welded in accordance with Regulation 36 & 63, Except as provided for in clause (b) below they shall
be without joint.
b) Tubes having an external diameter not exceeding 5 inches may be jointed and such joints may be flashed welded,
machine forge welded arc or gas welded. Tubes above 5 inches diameter may also be welded provided they are located
outside the furnace. Such welds must conform to the requirements of Chapter VIII.
c) Flash welding shall be undertaken on a welding machine of a type approved by the Inspecting Officer and the external
fin caused by welding shall be completely removed. The internal fin caused by welding shall also be removed subject to a
maximum height of 20 per cent of the wall thickness of the tube.
d) Arc welded butt joints shall be made by the metallic shielded arc process and post weld heat treated effectively except in
the following cases:
(i) in case of 0.5 Molybdenum steels if thickness does not exceed 13 mm and outside diameter does not exceed 127 mm.
(ii) In case of 1 Chromium ½ Molybdenum steel if thickness does not exceed 13 mm and outside diameter does not
exceed 127 mm and pre-heated to 125°C.
(iii) In case of 2-1/4 Chromium 1 Molybdenum steel, post weld heat treatment is not necessary under the following
conditions:
f) For design metal temperature over 454°C (850°F) material to be used shall be in accordance with Regulations 48, 53 and
56A.
g) The technique employed in all field welding of tubes shall be subject to the approval of the Chief Inspector of Boilers.
Where, however, material used is in conformity with the Code of the country of manufacture and it is covered by these
Regulations, the permissible stress figures specified in the Code at different temperatures may be accepted in lieu of
figures computed from the data required to the furnished under Regulations 338 and 340 in any of the following cases:-
(i) Where a certificate is furnished from the Inspecting Authority to the effect that the steel complies with requirements
of the grade steel (to be specified) and that the permissible stress for the working conditions as allowed for in the code
of the country of manufacture falls within the limits permissible under the Regulations.
(ii) Where the basis upon which these stresses have been arrived at is made available and such basis is not found to be
such as to give rise to stresses higher than those permissible under the Regulation.