|
SP0178-2007
<br />3.2.2 Rivets shall not be used.
<br />3.2.3 The use of internal bolted connections should
<br />be avoided to the fullest extent possible.
<br />3.2.4 Continuous lap -welded joints may be used but
<br />are not preferred. For sheet lining material, this type of
<br />construction may not be acceptable.
<br />3.3 Connections
<br />3.3.1 All connections to the tank or vessel shall be
<br />flanged.
<br />3.3.2 Threaded connections should not be used in
<br />tanks and vessels operating in corrosive environments
<br />(see Figure A4, Appendix A). However, if threaded
<br />connections cannot be avoided in corrosive
<br />environments, these parts shall be fabricated of
<br />corrosion -resistant materials, or constructed as shown
<br />in Figure Al 0, Appendix A.
<br />3.3.2.1 CAUTION: Dissimilar metal (galvanic)
<br />corrosion occurs when, for example, an alloy is
<br />used to replace the steel bottom of a tank, or in a
<br />similar circumstance when alloy appurtenances
<br />must be part of the construction of a vessel. If a
<br />lining is then applied to the steel and part of the
<br />alloy (usually 150 to 610 mm [5.9 to 24 in.]), any
<br />discontinuity in the lining exposes a small anode
<br />surface. Once corrosion starts, it progresses
<br />rapidly because of the large exposed alloy
<br />cathodic area to the much smaller anodic area.
<br />Without the lining, galvanic corrosion causes the
<br />steel to corrode at the weld area, but at a much
<br />slower rate. The recommended practice is to
<br />apply the lining to all of the alloy as well as the
<br />steel, thereby eliminating the possible occurrence
<br />of a large -cathode -to -small -anode surface.
<br />3.3.3 Nozzle connections to be lined shall be as short
<br />as possible and be a minimum of 50 mm (2 in.) in
<br />diameter (see Figure A4, Appendix A). Connections
<br />less than 50 mm (2 in.) in diameter shall be suitably
<br />attached through a reducing flange (see Figure A10,
<br />Appendix A). When trowel -applied thick -film linings are
<br />required, additional nozzle inside diameter shall be
<br />allowed for lining thickness.
<br />3.4 Appurtenances Inside the Tank or Vessel
<br />3.4.1 The standard practices in Sections 3, 4, and 5
<br />shall apply to any item to be installed inside a tank or
<br />vessel that is to be lined. Such appurtenances include,
<br />but are not limited to, agitators, anti -swirl baffles, outlet
<br />connections, gauging devices, vortex breakers, and
<br />internal piping.
<br />3.4.2 If appurtenances inside the tank or vessel,
<br />including nuts and bolts, cannot be lined, they shall be
<br />made of corrosion -resistant materials. (CAUTION: See
<br />Paragraph 3.3.2.1.)
<br />3.4.3 If bolted connections are necessary and cannot
<br />be made of corrosion -resistant materials, the mating
<br />surfaces shall be lined before assembly. Gaskets shall
<br />be used on mating surfaces and the sealing surfaces of
<br />nuts and bolts to protect the lining.
<br />3.4.4 Dissimilar metals shall be electrically isolated
<br />from the steel tank or vessel surface whenever
<br />possible. Where dissimilar metals are used, selection
<br />shall be such that the galvanic effect is minimized.
<br />Other corrosion mitigation methods may be required
<br />(see Figure A8, Appendix A).
<br />3.4.5 Heating elements shall be offset from the tank
<br />or vessel surface to provide access for surface
<br />preparation, application, inspection, and cleaning.
<br />Elements shall be positioned so as not to damage the
<br />lining system.
<br />3.5 Structural Reinforcement Members
<br />3.5.1 Structural support members should be installed
<br />on the exterior of the tank or vessel. However, if such
<br />members are installed internally, they shall be
<br />fabricated of simple shapes such as smooth, round
<br />bars or pipe for ease of applying the lining material.
<br />3.5.2 The use of internal flanged connections,
<br />stiffening rings, reinforcement pads, angles, channels,
<br />I -beams, and other complex shapes should be
<br />avoided. If they must be installed internally, these
<br />members shall be fully welded and welds and sharp
<br />edges ground to a radius of at least 3.2 mm (0.13 in.)
<br />or as agreed between the tank or vessel fabricator,
<br />tank or vessel owner, and lining applicator (see Figures
<br />Al and A6, Appendix A).
<br />3.6 Heat Sinks
<br />3.6.1 Heated, forced curing of lining systems is often
<br />preferred if not specifically required. During tank or
<br />vessel design and fabrication, especially with field -
<br />erected units, consideration must be given to avoiding
<br />or minimizing heat sink areas. Such areas might
<br />include opposite saddles or support lugs, flat bottoms
<br />on foundations, and stiffening rings.
<br />3.6.2 These situations may be addressed either by
<br />tank or vessel design or by construction or insulation of
<br />the foundation or supports. Another possible solution
<br />is the use of temporary constructions, such as false
<br />floors or temporary shelters, to achieve uniform heating
<br />and curing.
<br />2 NACE International
<br />
|