BS 3423:1986 download free.Recommendations for Design of glass vacuum vessels ( including desiccators ) for laboratory use.
1 Scope
BS 3423 recommends minimum thicknesses and safety considerations for the design and construction of glass vessels for use under vacuum in a laboratory, including desiccators and filter flasks. Other configurations of desiccating vessels which are not for use under vacuum are outside the scope of BS 3423.
NOTE The titles of the publications referred to in BS 3423 are listed on the inside back cover.
2 Glass quality
The vessels should be free from defects that will weaken the glass, and substantially free from blisters, seed, striae, cord, etc. They should be well annealed.
3 Design features
Figure 1 and Figure 2 indicate by the letters A to H the features included in BS 3423 for vacuum vessels in common use.
Recommendations for these features are given in the clauses listed below.
Feature Clause
A Flat base with radiused or sharp
lower corners 5
B Lower and upper parts of side-wall 6
C Step to carry perforated plate 7
D Flange of body or lid 8
E Lid form 9
F Opening for vacuum 10
G Flange, ground finish 8
H Flange, grooved with elastomeric
insert 8
b) Base with sharp corners
r=l3mm,k1=5.25, te=8.Omm
NOTE 1 Desiccators having well-radiused corners (see Figure 3) are generally in accordance with the recommendations of 5.1.1, the base thickness being not less than O.05d1 those of smaller corner radius require a base thickness greater than O.5d1.
NOTE 2 Extra glass may be provided externally, as shown in Figure 5(a), to form a protective rim or to increase the stability of the vessel, or internally in the form of lugs to support a perforated plate, for example, as shown in Figure 5(b).
NOTE 3 When the side-wall makes an obtuse angle with the base, as shown in Figure 3 and Figure 4, the angle between the sidewall and the vertical axis of the vessel being y, an extra compressive stress is exerted about the corner, proportional to sec y; this can generally be ignored.
5.2 Semi-ellipsoidal base
A base of semi-ellipsoidal form should be designed as described in 92 for a semi-ellipsoidal lid.
6 Body thickness and shape (design
feature B)
6.1 Straight and stepped side-walls The side-wall of the vessel should be either stepped
or straight and the design principles are the same in each case. The body should be cylindrical or conical with a cone half-angle of less than 15°.
6.2 Part-spherical body
The form of body shown in Figure 6, having a curved sidewall terminating at an angle y to the axis of the vessel, should be designed as recommended in 9.3 for a part-spherical lid with an extra thickness allowance adjacent to the flange as given by equation 8 in Appendix A, or with the appropriate safe-handling minimum thickness given in Table 2.
7 Step thickness and radius of curvature (design feature C)
7.1 Recommendations
The internal corners of each step should be radiused as shown in Figure 7. The radii r1 and r2 should each be not less than te as shown in Figure 7.
NOTE To obtain a significant increase in strength, r1 and r2 need to be not less than 3 te
If the angle y in Figure 7 is less than 600 the step should be treated as a short conical section or swage and the thickness needed at any point should be not less than the value of te (in mm) given by the following equation.
8 Flange thickness and shape (design
feature D)
81 Recommendations
8.1.1 For a flange of the form shown in Figure 8(a), Figure 8(b) and Figure 8(c), the flange thickness tf (in mm) should be not less than the value given by equation 4(a) if d0 > d1 or not
k2 is the factor, equal to 2.25, for the flanges shown in Figure 8(a), Figure 8(b) and Figure 8(c);
d0 is the external diameter of the lid or body at the flange (in mm);
d1 is the internal diameter measured at the flange where the joint faces meet (in mm);
1 is the cantilever length (in mm);
[and p are as defined in Table 1.
NOTE 1 For the construction shown in Figure 8(a) and Figure 8(b) the width of flange face is equal to the cantilever length 1 plus the wall thickness t.
NOTE 2 Equation 4 indicates that it is an advantage to make the flange as narrow as is compatible with obtaining a good joint.
If the tangent at the flange to the curve described by the inside of the lid meets the flange at an angle yto the vertical axis as shown in Figure 8(b), the thickness tf calculated in equation 4 should be increased by the amount given in equation 8 to counter the radial stress introduced.
For the construction in which the flange terminates outside the line of the wall, the angle fi shown in Figure 8(d) should not exceed 130°.
9 Lid thickness and curvature (design
feature E)
9.1 General
Lids are subject to considerable stresses under vacuum and should be made to one of the forms which lend themselves to simple design calculation. Of these the best are hemispherical (see Figure 10), semi-ellipsoidal (see Figure 11), dished
(see Figure 12) and part-spherical (see Figure 9 and Figure 13).
NOTE 1 The term “dished” is the accepted description of the lid whose profile is formed by two radii and which has similar design characteristics to those of an ellipse.
For both the semi-ellipsoidal and dished forms the outer part of the curve should terminate perpendicular to the plane of the flange and the internal radii should be appropriately related to the internal diameter.
NOTE 2 The lid forms described in 9.2 are dealt with fully in BS 5500. However, it is found that when the equations in that standard are applied to desiccator lids, and the lids comply with the recommendations on curvature, the calculated thicknesses are less than those required for the safe-handling of glass fabrications. The relevant equations are therefore given for information in Appendix A but only the safe-handling thicknesses given in 9.2 are recommended.
9.2 Thickness
The thickness of hemispherical, semi-ellipsoidal, dished, part-spherical and conical lids designed in accordance with 9.3 should be not less than the appropriate value given in Table 6.
NOTE The values given in Table 6 are the recommended minimum values for safe-handing. Equations for calculating the minimum thickness of lids based on stress considerations alone
are given for information in Appendix A.
The thickness of lids that are not in accordance with the recommendations given in 9.3 should be not less than the value given by equation 1, with k1 = 5.25.
9.3 Curvature
For semi-ellipsoidal lids the ratio of the major axis to the minor axis of the ellipse formed by the lid in cross section should be not greater than 2.6: 1 (see Figure 11).
For dished lids the crown radius r1 should be not greater than d1 and the inside corner radius should be not less than 0.1d1 (see Figure 12).
For both the semi-ellipsoidal and dished forms the tangent to the curve described by the outer surface of the lid at the ground flange face should he at a right angle to the horizontal axis (see Figure 11 and Figure 12).