1. What Grade 7 Titanium Is
Grade 7 titanium is a commercially pure titanium grade alloyed with a small addition of palladium, normally in the range of 0.12 to 0.25 percent. The palladium addition is small but has a strong effect on electrochemistry: it shifts the corrosion potential in the noble direction and suppresses crevice corrosion and stress corrosion cracking in hot chloride solutions and reducing acids. As a result Grade 7 is regarded as one of the most corrosion-resistant titanium grades available in tube form.
2. Why Power Plants Use Titanium Condenser Tubes
Power plant condensers cool exhaust steam with water drawn from rivers, lakes or the sea. The cooling water can contain chlorides, suspended solids, biofouling organisms and pollutants that attack conventional tube materials. Titanium condenser tubes resist pitting, crevice corrosion and erosion, and they tolerate ammonia and chloride excursions that would damage copper-nickel or stainless steel tubes. The thin walls made possible by titanium also give excellent heat transfer, keeping the condenser clean and efficient over decades of service.
3. Standards and Tube Dimensions
Grade 7 condenser tubes are supplied to ASTM B338 and ASME SB338, and to GB/T 3625 for projects executed to Chinese standards. Condenser tubes are commonly produced in outside diameters from about 15 to 32 mm with wall thicknesses from 0.5 to 1.2 mm, although other sizes are available on request. The chemical composition, including the palladium content and the interstitial elements oxygen, nitrogen, carbon and hydrogen, is controlled to the limits of the specification and reported on the certificate.
4. Manufacturing and Surface Condition
Seamless Grade 7 tubes are produced from extruded hollows that are cold pilgered and drawn to final size with intermediate annealing. Welded tubes are formed from strip and seam welded by gas tungsten arc welding under inert gas shielding, then cold worked and annealed. The finished tubes are straightened, cut, cleaned and supplied with a bright or pickled surface. Each tube is subjected to eddy current examination to ASTM E426, hydrostatic testing, and the flattening and flaring tests required by ASTM B338.
5. Performance and Failure Avoidance
Grade 7 tubes are rolled or welded into the tube sheets, and the rolled joints are tested to prove leak tightness. The high resistance of the alloy to crevice attack means that the crevices at the tube sheet joints do not become corrosion sites, which is the main reason for the long life of titanium condensers. Operating experience shows that titanium tubes can remain in service for the full design life of the plant with minimal tube loss, and cleaning by brushing or water jetting does not damage the surface.
6. Selecting the Right Grade
Grade 2 titanium is adequate for clean fresh water service, while Grade 7 is preferred when the water is aggressive, when crevices are unavoidable, or when the unit must tolerate long shutdowns with stagnant water. Grade 12, alloyed with molybdenum and nickel, is an alternative where slightly higher strength is needed. The final selection is made on the basis of water chemistry, temperature, and the fouling and cleaning regime of the plant.
Palladium in the range of 0.12 to 0.25 percent suppresses crevice corrosion.
Tubes are supplied to ASTM B338, ASME SB338 and GB/T 3625.
Eddy current, hydrostatic, flattening and flaring tests are standard.
Frequently Asked Questions
Q: Why is palladium added to Grade 7 titanium?
Palladium promotes passivity of the titanium surface and dramatically improves resistance to crevice corrosion and stress corrosion in hot chlorides and reducing acids.
Q: Can Grade 7 tubes be used in seawater?
Yes, Grade 7 is one of the best materials for seawater service and is widely used in coastal power plants with once-through seawater cooling.
Q: What tests are done on condenser tubes?
Eddy current examination, hydrostatic or pneumatic pressure testing, and the flattening and flaring tests of ASTM B338, together with tensile and chemical verification.
Q: How are the tubes fixed in the tube sheet?
Tubes are normally expanded into the tube sheet by rolling, and in high-pressure units the joints may be seal welded as well.
Q: What is the difference between Grade 2 and Grade 7?
Both are commercially pure base grades, but Grade 7 contains palladium, which gives superior crevice corrosion resistance at only a small cost premium.
Q: Do titanium tubes foul in service?
Fouling is generally low because the smooth surface and corrosion resistance discourage adhesion, and the tubes can be cleaned by standard brushing methods without damage.
