Rectangular Headers Symmetrical in Form:-
||The working pressure shall
not exceed the smaller of the value obtained from the
|(i) WP =
f (t - c1 )²
|(ii) WP =
t² f E
W. P .= working pressure.
t = thickness.
b = internal breadth between the supporting sides of
c = 3.413 for wrought steel and 2.926 for steel castings.
Where the sides are corrugated or otherwise reinforced
by substantial supports so that the length of the portion
between the corrugations or supports does not exceed
b, shall be taken as 5.82 for wrought steel and 5.12
for steel castings.
f = permissible stress at working metal temperature
as given in the table under clause C.
c1 = 0.08cm.(0.03in.)
w = internal width of the header between the tube plate
and the opposite side.
E = efficiency of ligaments between the holes.
If the faces of the headers are machined locally at
the tube holes or hand holes the thickness at the part
may be as much as 4.8 mm. (3/16 in.) Less than given
by the above equation; but irrespective of the thickness
obtained by the use of the formulae, the thickness of
the headers at the tube holes shall be not less than:
|t = ( C x d + C1 )1/2
t = thickness.
d = diameter of the hole.
C = 0.2235 mm. 0.0088 in
C1 = 6.4 mm. (1/4 in.)
In no case shall the thickness be less than 8 mm. (5/16
in.) except that in small patches not exceeding 322
mm2 (one-half sq. in.) in area, the thickness may be
50 per cent of the thickness used in the equations 88(a)
The permissible stress at working metal
temperature shall be as given in the table below:
PERMISSIBLE STRESS AT WORKING METAL TEMPERATURE FOR
RECTANGULAR SECTION HEATERS
|Working Metal Temperature
|Cast steel 44 - 55 kg/mm²
(28 to 35 tons/sq. in.)
|Carbon and Alloy Steels
||The lower value obtained
in each case at the specified temperature calculated
on the following basis:-
|For temperatures at or
454 ºC (850 ºF)
|For temperature above
454ºC (850 ºF)
T. S .= Minimum Tensile strength of the material at
Et = Yield point (0.2% proof stress) at the working
metal temperature t.
Sr = the average stress at the temperature t
to produce rupture in 100,000 hours and in no case more
than 1.33 times the lowest stress to produce the rupture
in 100,000 hours.
Sc. = The average stress at the temperature t
to produce an elongation of 1% (creep) in 100,000 hours.
||The working metal temperature
shall be taken as:-
For saturated steam and water drums and headers the saturation
temperature corresponding to the working pressure of the
boiler plus 28 ºC (50 ºF).
For superheater headers the designed maximum steam temperature
for the header plus 28 ºC (50 ºF).
||Where headers are adequately
protected from the gases of combustion or swept by such
gases in the third or subsequent pass of a boiler, the
working metal temperature shall be taken as the saturation
or designed maximum steam temperature as defined above,
whichever applies. A covering of refractory or insulating
material which may be liable to become dislodged shall
not be deemed adequate protection.
End Attachments.-The ends of rectangular headers may
be formed integral with the header or attached by welding.
The working pressure for the flat ends shall be calculated
by the following formula:-
|W. P. =
||f ( t - C )²
WP = Working Pressure.
f = Permissible stress for the material at the working
t = thickness of the plate at the weakest part.
d = the least width between the walls of the rectangular
C = 1 mm (0.04).
K = 0.32 for ends integral with or flanged and butt
welded to the header.
= 0.40 for ends directly strength welded to the header
in an approved manner.