TUBESHEET MATERIALS/GALVANIC CONSIDERATIONS
Selection of a tubesheet material for use with titanium tubes in a heat exchanger depends on several factors. Tubesheets can be solid titanium, explosively clad titanium on steel, loose lined titanium on steel, or a dissimilar metal.

Solid tubesheets are used primarily in all-titanium units. Design follows conventional practice. Either explosively clad tubesheets or loose liners are used with steel or other dissimilar metal shells. Loose liners offer economic use of material and are easy to maintain and fabricate.

When explosively clad or loose lined tubesheets are used, the tubes should be seal welded to prevent minor tubeside leakage from reaching the steel, causing undetectable corrosion. Seal welding is not required if solid tubesheets are used unless crevice corrosion at the tube-to-tubesheet joints is a possibility or leakage cannot be tolerated.

If titanium tubes are to be inserted into dissimilar metal tubesheets, as is often the case in retubing jobs, the possibility of galvanic corrosion must be considered. In actual practice, titanium tubes have been used with a variety of tubesheet materials as indicated in Table 9 for oil refinery service. Generally, titanium is more noble than most of the commonly used tubesheet materials. Galvanic corrosion might, therefore, be expected on the dissimilar metal but not on titanium. For sea water service, metals close to titanium in the galvanic series are recommended if titanium or titanium-clad tubesheets are not used. Aluminum bronze (CDA Alloy) or nickel/copper Alloy 400 have proven to be acceptable. Sufficient strength to resist deformation of ligaments during roller expansion of titanium tubes should be present in the material of choice.

If galvanic corrosion appears likely in a given situation, some protective measures can be taken. Other measures include insulation of the titanium from contact with dissimilar metals, proper use of cathodic protection or sacrificial zinc anodes, and use of coatings. Epoxy coating have been used on some tubesheets tubed with titanium in power plants. However, little experience is available from the chemical process industries, presumably because of the more severe conditions encountered. Some favorable experience is reported for polysulfide rubber and filled resin coatings in preventing galvanic corrosion on tubesheets in oil refinery environments overseas.

Use of an all titanium tube bundle will eliminate the possibility of premature removal of a bundle from service due to tubesheet galvanic corrosion failure. If seal welding of tubes to tubesheets is required, solid titanium or titanium-clad tubesheets must be used.

GASKET MATERIALS
A variety of gasket materials have been used in titanium equipment. A key consideration in selecting a gasket material is the environment to which it will be exposed. Manufacturers of gasket materials should be consulted for recommended materials to withstands the conditions of temperature and corrosives being considered.

From the titanium viewpoint, prevention of crevices is important. Materials which give elastically, rather than creep, will seal tightly, thereby minimizing crevices. The rubbers--such as natural or butyl--have given good results and are to be preferred over non-yielding materials. Teflon, with its tendency to creep, requires heavy flanges to maintain tight joints.

DESIGN FOR WELDING ACCESS AND DISTORTION
When designing equipment to be fabricated of titanium, consideration must be given to providing proper access during welding. Designs must provide space for manipulation of the torches and trailing shields, which are necessary equipment for welding titanium, particularly where nozzles or attachments are close to large flanges.

Full access to the root side of welds is also desirable for titanium equipment. This stems from the necessity for inert gas shielding and inspection of titanium welds. If a weld joint cannot be reached, both inspection and repair are made more difficult.

Titanium tends to shrink and distort more than steel during welding. The low thermal conductivity of titanium results in high metal temperatures and, consequently, loss of strength. Titanium tack welds, weld metal deposits and abutting edges soften and move more than would be expected for steel. Measures need to be taken to maintain joint alignment. In addition, designs should employ balanced (two sided) welds wherever possible. Tee joints with full penetration groove and fillet on the same side of the joint should be avoided if possible.