4.1.3 Material selection - part 2
Over the past twenty years, work has progressed in the development of titanium for use as a condenser tube material. Its major attribute is a tenacious oxide film, which rapidly repassivates if ruptured under normal condenser operating conditions.
Jet impingement tests on both welded and seamless titanium tubing at water velocities of 10 m/s for 60 days have shown the material to be immune to attack. Under these conditions, aluminium-brass and cupro-nickel tubing failed rapidly. Titanium tubing manufacturers (Imperial Metal Industries Ltd.) quote impingement attack threshold values of 20 m/s in clear seawater and 6 m/s in water with a high sand content. Thus, even at localised tube blockages where water velocities can rise to 6 m/s, the tubing retains its resistance to impingement attack, whereas this velocity would be beyond the reliable range of the copper-based materials (Fig 4.8).
The use of titanium tubing in condensers designed for conventional copper-based tubing materials, however, poses a problem of incompatibility between tube/tubeplate materials.
Couples of titanium and naval brass, and titanium and iron/steel have junction potentials of -288 mV and - 660 mV respectively, in relation to a saturated calomel electrode. This creates the potential problem of anodic attack of the tubeplate material which could lead to penetration of the tubeplate within the design life of 30 years. Where naval brass tubeplates have to be used, such as in retubing situations, the practice has been to coat the tubeplate and waterbox internals with a multilayer neoprene rubber, thus effectively insulating against galvanic action.
For new construction, tubeplates manufactured from aluminium bronzes ASTM-B171 Alloy D, and ASTM-B171 Alloy E, have couple potentials with titanium tubes of -215 mV (SCE) and -133 mV (SCE), respectively. The corrosion rates of aluminium bronze tubeplates are therefore low enough to allow their use without any protective coating. The waterboxes, however, are still protected with suitable coatings as in a retubing situation.
In-service trials have been carried out at a number of power stations, listed in Table 4.3, under various operating conditions ranging from the particularly aggressive sand-laden waters of Bristol Channel power stations to the polluted environment experienced by Thameside power stations.
Following the satisfactory performance of the titanium tubes in these trials, two condensers were retubed with titanium; one with seamless tubing and one with seam-welded tubing. Results from these trials confirmed that the tube installation procedures and subsequent service has been satisfactory.
Fifteen years ago, titanium was considered extremely expensive and unlikely to become economic.
The cost constraint on the use of titanium tubing can be minimised by a reduction in the required tube wall thickness. This, coupled with savings expected from leak-free operation, establishes the use of titanium as an economic material for power stations where aggressive attack is either experienced or predicted.
As a result of the favourable in-service trials and increasing economic competitiveness, titanium tubing is now used in all new power stations where impingement attack of the condenser is considered possible. On-site rig tests, which are carried out at new power station sites prior to the ordering of titanium, confirm that in all cases the corrosion resistance is good, and that no operating problems are envisaged.