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|  MIL-D-18641F(SH)
tubes and 3/4 inch od tubes, to a diameter at the outer face of the tube sheet of 1/2 inch for 3/8
inch od tubes, 3/4 inch for 5/8 inch od tubes, and 7/8 inch for 3/4 inch od tubes.
3.14.15 Tube sheet thickness. The minimum acceptable tube sheet thickness shall be 3/4 inch
for 5/8 inch od tubes and 5/8 inch for 3/8 inch od tubes. (For 3/4 inch od tubes see 3.18.1.3.)
3.14.16 Tube spacing. The minimum acceptable tube spacing shall be 13/16 inch, center-to-
center for 5/8 inch od tubes and 17/32 inch for 3/8 inch od tubes (For 3/4 inch od tubes see
3.18.1.4.)
3.14.17 Support plates. Tubes of 3/8 inch and 5/8 inch od shall be supported by tube support
plates so that the maximum span between tube sheet and support plates or between support plates
will not exceed 2 feet and 3 feet respectively. If the construction is such that the cross-flow baffles
drilled for the tubes are installed between the tube sheet and support plates and between support
plates, the thickness of the support plates and baffles shall be not less than 1/4 inch. If such cross-
flow baffles are not installed, the support plates shall be not less than 9/16 inch thick. Holes for
tubes in support plates shall be drilled not more than 1/64 inch diameter larger than the nominal
outside tube diameter. Holes for tubes shall be rounded on each face of the support plate to a
1/16 inch radius. Holes in cross-flow baffles shall have sharp edges removed (For 3/4 inch od tubes
see 3.18.1.5.)
3.14.18 Shell expansion. Provision shall be made for expansion and contraction of the shell
of shell and tube type heat exchanger components of distillation units. Acceptable provisions for
shell expansion and contraction are:
a. One support shall be constructed to flex and the other support shall be rigid, both
drilled as specified in 3.14.6.2
b. Both supports shall be rigid. One support shall be provided with slotted holes for the
foundation bolts. A bushing shall be provided around the foundation bolt, so
dimensioned that the bolt head is prevented from binding the support when the nut
is tightened. Clearance between a shoulder on the bushing and a machined or spot-
faced surface surrounding the bolt hole shall not exceed 0.005 inch, in order to
minimize impact on the foundation bolt under high impact shock load. Total clearances
of the inside diameter of the bushing over the nominal bolt diameter and the width of
the slotted hole in the support over the od of the bushing shall not exceed those
specified in 3.14.6.2 The other support shall be drilled as specified in 3.14.6.2
c. When U-bent tubes are used, an expansion joint shall be provided in the shell between
two rigid supports.
d. When one support constructed to flex is used, the rigid support shall be constructed
to withstand the entire high impact shock load in the direction longitudinal of the heat
exchanger. When a shell expansion joint is heated between two rigid supports, each
support shall be constructed to withstand the high impact shock load equivalent to the
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