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Reg. 340

Rectangular Headers Symmetrical in Form:-

a) The working pressure shall not exceed the smaller of the value obtained from the following formula:-

(i) WP = c f (t - c1 )² Eqn. 88(a)



(ii) WP = 6.25 t² f E Eqn. 88(b)

where,

W. P .= working pressure.
t = thickness.
b = internal breadth between the supporting sides of the header.
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.

b)

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 Eqn.(89)
where,

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) and 88(b).

c)

The permissible stress at working metal temperature shall be as given in the table below:

TABLE
PERMISSIBLE STRESS AT WORKING METAL TEMPERATURE FOR RECTANGULAR SECTION HEATERS

Working Metal Temperature in degrees
Cast steel 44 - 55 kg/mm²
(28 to 35 tons/sq. in.)
Carbon and Alloy Steels
C F Kg./cm² lbs./sq.in.  
288 550 703 10,000 The lower value obtained in each case at the specified temperature calculated on the following basis:-
316 600 633 9,000
343 650 562

8,000

For temperatures at or below
454 ºC (850 ºF)
371 700

532

7,560

399 750

505

7,200

T. S. or Et
3.5 1.6
427 800

475

6,750

For temperature above 454ºC (850 ºF)
454 850

443

6,300

Sr or Sc
1.6
482 900

309

4,400
 

where,

T. S .= Minimum Tensile strength of the material at room temperature.
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.

d) 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).
e) 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.
f)

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 Eqn.(89A)
d² K
where

WP = Working Pressure.
f = Permissible stress for the material at the working metal temperature.
t = thickness of the plate at the weakest part.
d = the least width between the walls of the rectangular header.
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.

 
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