1. Why Grade 7 for Condenser Tubes
Grade 7 titanium is a commercially pure grade alloyed with palladium, normally 0.12 to 0.25 percent, which gives it the highest resistance to crevice corrosion among the standard titanium grades. In a condenser, the tube-to-tube sheet joints form crevices that can attack less resistant materials. Grade 7 also resists pitting, stress corrosion and erosion in seawater and polluted cooling water, which makes it the material of choice for the most demanding power plant condensers.
2. The Seamless Manufacturing Route
Seamless tubes are produced by extruding a pierced billet into a hollow and reducing it by cold pilgering and drawing with intermediate annealing. The seamless construction gives a uniform wall and a smooth bore with no weld zone, which is valued in condenser service. The final passes set the wall tolerance and the surface quality, and the tube is straightened and cut to length. Each heat is traced from the ingot to the finished tube.
3. Polishing and Surface Finish
After annealing, the tube is pickled to remove oxide and then polished to a smooth, bright surface. Polished surfaces have low roughness values, commonly specified in the range of Ra 0.4 to 0.8 µm, which reduces the adhesion of deposits and biofouling. The smooth surface also improves the heat transfer and makes the tubes easier to clean during outages. The surface finish is measured and documented for each lot.
4. Testing of the Finished Tubes
Each tube is tested to the requirements of ASTM B338 and ASME SB338. Tensile testing verifies the strength and ductility, and the flattening and flaring tests prove that the tube can be rolled into the tube sheet without cracking. Eddy current examination to ASTM E426 detects surface and near-surface defects along the full length, and hydrostatic or pneumatic pressure testing confirms leak tightness. The dimensional and surface checks complete the programme.
5. Power Plant Applications
Grade 7 polished condenser tubes are used in thermal power plants cooled by seawater or aggressive river water, in desalination plants and in process heat exchangers. The palladium addition gives protection in crevices and under deposits, and the polished surface keeps the tubes clean in service. Operating experience shows long life with low tube loss, which keeps the plant available and reduces maintenance cost.
6. Handling and Installation
The polished surface is protected during handling: tubes are stored with capped ends, lifted with clean slings, and never dragged across rough surfaces. During installation, the tubes are rolled into the tube sheet with controlled expansion, and the bundle is cleaned before commissioning. The surface finish is preserved by clean tools and careful handling, and the tubes are protected from contamination until the unit is closed.
Polished surfaces are commonly specified in the range of Ra 0.4 to 0.8 µm.
Seamless construction gives a uniform wall with no weld zone.
Eddy current, flattening, flaring and pressure tests are standard.
Frequently Asked Questions
Q: Why is the palladium addition important?
Palladium suppresses crevice corrosion in hot chlorides, which is the main risk at the tube-to-tube sheet joints of a condenser.
Q: What is the benefit of a polished surface?
A polished surface resists fouling, improves heat transfer and is easier to clean, which keeps the condenser efficient in service.
Q: Are seamless tubes better than welded for condensers?
Seamless tubes have a uniform wall and no weld zone, which is preferred in critical service, while welded tubes are also accepted by the standard.
Q: What tests are done on condenser tubes?
Tensile, flattening, flaring, eddy current and hydrostatic or pneumatic pressure tests, plus dimensional and surface checks.
Q: In which plants are these tubes used?
In power plant condensers cooled by seawater or aggressive water, desalination plants and process heat exchangers.
