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|  MIL-D-18641F(SH)
3.17.6 Orifice plate. One orifice plate with proper size orifice shall be furnished with the
distillation unit to limit the steam flow to the first-effect steam chest to that required for rated
capacity. Orifice plate shall be provided with a tab for pulling and shall be stamped with the orifice
size and manufacturer's part number.
3.17.7 Scale preventive compound. The piping system of the distilling unit shall be provided
with a connection for the injection of scale preventing compound, so located that the compound
will pass through the tubes of the feed heaters and all of the compound will pass into the first
effect evaporator shell with the feed.
3.17.8 Access openings. Access openings shall be provided for replacement, observation, and
cleaning (manually and chemically) of vapor separators. Any drain piping connected to the vapor
separator shall be cleanable through such access openings.
3.18 Salt water heater for types II and IV.
3.18.1 Salt water feed heater. The salt water feed heater shall be constructed for circulation
of the salt feedwater through the tubes. Baffling shall be provided to prevent direct impingement
of the entering steam on adjacent tubes (type II). Surface shall be provided to insure conformance
to the requirements of 3.13.
3.18.1.1 Tubes. The tubes of the salt water feed heater shall be 3/4 inch od number 18 BWG
(0.049 inch wall thickness) in accordance with MIL-T-15005. Tubes for feed heaters of class 2
distilling units shall be 90-10 imposition and for class 1 they shall be 70-30 composition. Tubes
shall be straight and shall be expanded into sheets at each end as specified in 3.14.13.1.1.
3.1$.1.2 Tube sheet tube holes. The holes for tubes in the tube sheets shall be reamed to
0.751 inch diameter with a plus tolerance of 0.005 inch. The holes for the inlet tube ends shall be
belled to approximately 7/8 inch diameter on a 1/2 inch radius at the outer face of the tube sheet-
Holes shall be provided with one centrally located groove 1/8 inch wide and 0.012 inch deep.
Edges of all holes which are not belled shall be rounded on a 1/16 inch radius at the faces of the
tube sheet.
3.18.1.3 Sheet thickness. The minimum thickness of tube sheets shall be 7/8 inch.
3.18.1.4 Tube spacing. The minimum center-to-center spacing of tubes shall be 15/16 inch.
3.18.1.5 Tube support plate. Tubes of 3/4 inch od shall be supported by tube support plates
so that the maximum span between tube sheets and support plates or between support plates will
not exceed 4 feet. The support plates shall be 9/16 inch minimum thickness. Holes for tubes shall
be drilled not more than 49/64 inch diameter and shall be rounded at each face of the tube
support to a 1/16 inch radius.
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