The primary basis for titanium's expanded application in the refinery and chemical processing industries has been its superior resistance to the aggressive, mildly reducing, oxidizing, and/or chloride containing environments found in the chemical processing industry and to its ability to handle refinery process streams containing hydrogen sulfide, chlorides, dilute hydrochloric acid, ammonia, and hydrogen. In conditions under which crevice corrosion might result, use of TIMETAL Code-12 (Grade 12) or TIMETAL 50A Pd (Grade 7) can eliminate the problem. Both alloys also provide improved resistance to reducing acid conditions, and TIMETAL Code-12 offers increased strength particularly as the temperature is increased (see Table 7).

REFINERY SERVICE
In this application, the most commonly used forms of titanium are tube and plate for shell and tube heat exchangers. The selection of titanium in many refineries has been based on its practical immunity to corrosion from cooling waters of all types and its excellent resistance to refinery process stream corrodents. See Table 11.

Solid titanium or titanium-clad tube sheets are preferred in order to eliminate galvanic corrosion of a tube sheet of an anodic material, such as a copper alloy or carbon steel. Water boxes should be coated for protection. If a Ti-clad sheet is used, the tube joints should be welded, but if a solid tube sheet is used, roller expanded tube joints serve well. A third type of tube sheet involving a loose liner is sometimes used. In this design, a thin plate (3/16"-1/4") of titanium is drilled with the tube sheet pattern. The O.D. of the plate extends outward to be held in place by the water box flange. This is an economical form of construction, but necessitates welding the tube ends to the liner.

The use of thinner wall titanium tube helps compensate for its lower thermal conductivity when compared to copper alloys. However, in many refinery applications, the resistance to heat transfer of the tube wall is such a small part of the overall heat transfer rate that the changeover to titanium is not detrimental. In many cases, because the tube walls remain smooth and tend to avoid buildup of deposits or fouling films, the overall heat transfer rate improves with time as compared to copper alloys. Titanium's resistance to erosion and erosion- corrosion permits designing with high fluid flow rates (up to 80 ft/sec if pressure drop considerations make it practical). High velocity flow rates mean higher heat transfer rates and lower fouling factors. Experience has shown that titanium exchangers handling seawater can be designed with a fouling factor of .0005.^(12)

Although titanium alloys are nontoxic to bio-growth in natural waters, design and cleaning strategies can effectively avoid bio- fouling or deposit formation on heat transfer surfaces. In addition, periodic chlorination and/or use of automatic sponge rubber ball or nylon brush cleaning systems have been very successful. Titanium tubing with enhanced I.D. and O.D. surfaces is also available. See Section IV-H. (Page 25)

Since the modulus of elasticity of titanium is lower than that of some of the copper alloys that it replaces, and, because it is usually used in thinner walls due to its high corrosion resistance, titanium tubed heat exchangers require shorter support plate or baffle spacings. Table 12 shows the approximate span reduction required for existing exchangers designed for other materials and being tubed with titanium. If the existing exchanger using copper alloy tubes of the gage shown operated without vibration damage, then reducing the spacing by the amount shown will give a comparable deflection of the titanium tubes.

CHEMICAL PROCESSING

The chemical process industry uses a great deal of titanium in sheet and plate form in addition to tubes and plate used in heat exchangers.

Vessels fabricated of titanium have been used for a variety of applications (see Table 13). Construction can be solid titanium, titanium-clad steel or steel vessels with titanium linings. Lined vessels or equipment can consist of loose or mechanically fastened liners and may not be suitable for rapidly changing or low pressure conditions. Loose linings are generally positioned using gasketed flanges, whereas fastened liners frequently utilize titanium bolts to achieve liner attachments.

A totally fastened liner seal is obtained by TIG fillet welding titanium bolts to the liner and using a transition piece from the liner edge to the steel wall with welded explosively-clad Ti/steel straps. This technique is often the most cost effective approach for new construction or for the retrofitting of vessels and ducting operating at low to moderate pressures and temperatures.

Solid titanium construction is generally more cost effective than clad construction when vessel wall thicknesses are below approximately (25-38mm) 1"-1 1/2". For pressure vessels and piping, maximum design allowables (in tension) for titanium alloys covered under the ASME Pressure Vessel Code are indicated as a function of temperature in Table 7. TIMETAL 50A Pd is qualified for the same values shown for TIMETAL 50A. TIMETAL Code-12 is attractive as a pressure vessel and piping material, particularly at temperatures above 150 degrees C (300 degrees F) where its design allowables are roughly double those of the TIMETAL 50A. These alloys are ASME Code approved up to 316 degrees C (600 degrees F) above which creep becomes limiting. In any case, design temperatures should be less than approximately 430 degrees C (806 degrees F) to avoid excessive oxidation and oxygen embrittlement in continuous service.

Titanium-clad steel construction is favored at high pressures where very heavy walled vessels are required. Clad vessel construction involves the fillet-welded batten strap method using titanium sheet inlays, described elsewhere.^(13)

Titanium piping is available in seamless or seam-welded forms. The seamless form tends to become more cost-effective as tube wall thicknesses increase and diameters decrease. Titanium piping connections can be achieved via various gasketed flange joints, threaded joints, or butt-welded joints (for titanium to titanium joints only). Almost all common gasket materials are compatible with titanium, including virgin fluorocarbon materials such as Teflon. When environments can decompose fluorine or chlorine-containing gasket materials, selection of more compatible gasket materials is required. Where crevice corrosion in gasket to flange metal joints is of concern, the use of TIMETAL Code-12 or to 50A Pd flanges or flange faces is advisable. Dielectric (insulating) pipe flange or coupling connections are also available if dissimilar metal galvanic couples need to be avoided.

For threaded joints, loose-fitting, coarse threaded shoulder- locking joints are preferred over conventional or API pipe thread joints to limit galling or seizing problems. Titanium pipe or bolt threads should be coated with anti-seizing lubricants such as molydisulfide or graphite, or wrapped with Teflon tape. Other anti-seizing surface treatments for threaded or bearing surfaces are also effective including plasma-sprayed titanium dioxide, tungsten carbide, and other hard metal oxide/carbide coatings; surface nitriding; and lubricating ion-plated surface treatment.

CLEANING TITANIUM HEAT EXCHANGERS AND VESSELS
Cleaning solutions for titanium must maintain passivity and avoid hydrogen uptake by the titanium, while being compatible with dissimilar metal equipment components. This can be a challenge, since proper inhibition of titanium in reducing acid media requires addition of oxidizing species, which may be harmful to steel or copper alloys. In addition, the filming amine (absorption) type of inhibitors commonly used for steels are not effective on titanium.

Proper inhibition is most critical when reducing acids, such as HCI, H2SO4, sulfamic or oxalic acids, are used to remove scaling. Common, inexpensive but potent inhibitors for titanium include specific multivalent transition ions (as chlorides, sulfates, or oxides), which extend titanium's passivity into hotter, more concentrated reducing acids. These include ferric (Fe^+3), cupric (Cu^+2), chromate (Cr^+6), permanganate (Mn^+6), molybdate (Mo^+6), or vanadate (V^+5) ion species. As a general rule, a concentration of at least 500 ppm of these metal ions should be used with acid concentrations up to 10 wt.% at temperatures higher than 75 degrees C (167 degrees F).

Nitric acid does not require inhibition and can be safely used over the full concentration range to temperatures of 95 degrees C (203 degrees F). Caustic media may be used to concentrations of 10 wt.%, but should be inhibited with 1% sodium nitrate, chlorate, or hypochlorite additions if solution temperatures exceed 40 degrees C (105 degrees F). Since rapid titanium attack may occur, hydroflouric acid cleaning solutions are not to be used for scale removal.

If the presence of galled or smeared surface iron is suspected, it should be removed from titanium equipment to prevent possible pitting or hydrogen uptake in hot brine service. This surface iron contamination can be removed with a light (5 min) pickle in a near ambient temperature 35 vol. % HNO, 5 vol. % HF solution. This procedure will remove less than 0.03 mm from the titanium surface. A thorough water flush should immediately follow pickling to prevent excessive equipment attack.