About Central Boilers Board •Contents Chapter Wise • Contents Regulation WiseIndian Boilers Act 1923

Reg. 276, Reg. 277

General

 The thickness T specified in equation 74 of Regulation 278 is the thickness of the end plate after manufacture and is applicable over the whole area of the end plate up to the point where, for ends of partially spherical shape, the dishing radius joins the corner radius; from this point a gradual thinning is permissible up to maximum of 10 per cent of the thickness T at the point where the corner radius joins the straight portion of flanged end plate. A similar gradual thinning is permissible for end plates of semi-ellipsoidal shape. This permissible reduction in thickness applies also to the flange for the manhole opening. In no case, however, when the end plate is but welded to the drum shell, shall the thickness of the edge of the flange for connection to the shell be less than the thickness of an unpierced seamless shell of the same diameter and material, determined by equation 72.

Reg. 277

Dished End with Opening

a) Heads with openings:-
 i) Holes cut in the heads (manholes or tube holes) should be round or elliptical. ii) Small size openings complying with conditions of sub-regulation (c) do not require any additional thickness of head or reinforcement of opening. iii) Large size openings require an increase in thickness of the head, according to sub-regulation (b) or a reinforcement of the opening according to sub-regulation (d).
b) Shape factor K for heads with large unreinforced openings:- The shape factor K to be used in calculating the thickness of heads varies depending on the height of the head H and on the dimensions of the largest opening. The value of K is indicated in Fig. 23D, by means of curves, each of which corresponds to a value of the ratio.-
 d  --------- Ö D T

where,

d = diameter of the largest opening in the head (in the case of an elliptical opening, the larger diameter of the ellipse);
D = outside diameter of the head; and
T = minimum thickness after dishing.

In addition, the following conditions shall be satisfied:

 T < 0.1 D

 d < 0.5 D

The same condition and the same curves apply for openings simply cut in the plate of the head as well as to those which are flanged inwards. In the latter case, the radius r’ of the flanging (see Fig. 23C) should not be smaller than 25 mm (1 inch). The thickness of the flanged portion may be smaller than the calculated thickness T.

Unreinforced openings and flanged-in openings in dished heads shall be arranged so that the distance from the edge of the head is not less than as shown in Fig. 23E.

In all cases the width of the band separating two adjacent openings should, in projection, be at least equal to the diameter of the smallest opening as shown in Fig. 23E.
c) Small openings which do not require any reinforcement:-

Figure 23D indicates that for a given ratio of H/d, the shape factor K of a plain head corresponds to a certain values of the ratio,

 d (DT)1/2

Piercing of holes with a diameter smaller than or equal to that which corresponds to this value can thus be made without any reinforcement in a head, the thickness of which is equal to the minimum required for a plain head.

The position of those openings shall however comply with the conditions stated in sub-regulation (b).

d) Reinforcement of large openings:-Large holes may be cut without increasing the thickness of the head, provided they are sufficiently reinforced.

The reinforcement may consist either of a welded nozzle or one or two reinforcing welded plates, or of a combination of these two procedures.

To determine the strength of the construction, one shall proceed as follows (see Fig. 23F). Let the imaginary diameter d’ of the opening be given by the formula

 d' = d - A T

where,

d = actual diameter of the opening in the head (outside diameter of the nozzle);
T = minimum thickness of the head; and
A = effective cross-section of reinforcement.

This effective area A corresponds, within the Limits given below, with the actual cross-section of the reinforcing plates and with the cross-section of the nozzle decreased by the cross-section necessary to support the pressure, for those parts of the nozzle situated outside the internal surface of the head.

The shape factor K can then be read corresponding to the ratio

 d (DT)1/2

The maximum limits for the are of reinforcement that should be taken into account are

(a) L1 = (2 RT)1/2. for the width of a reinforcing plate;
(b) L2 = (d Tt)1/2 for the length of a nozzle.

where,

R = internal radius of the spherical part of the head (or for elliptical heads the internal radius of curvature of the meridian at the centre of the opening)
Tt = actual thickness of the nozzle,
L1 and L2 = dimensions given in Fig. 23F.

The position of the outer edges of the reinforcement shall comply with the provisions of sub-regulation (b) and Fig. 23E.

In cases where the allowable stress of a part of the reinforcement is lower than that of the head, the corresponding area A should be multiplied by the ratio:

allowable stress of the reinforcement material
--------------------------------------------------------------------------------------
allowable stress of the head material

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