SOME TECHNICAL CONSIDERATIOHS OH THE USE OF TITANIUM CONDENSER TUBIHG IU PUBLIC UTILITY POWER GENETIATI01J TURBlliES C.F. Hanson Assistant Technical Sales Manager (Engineering) Imperial Metal Industries (Kynoch) Limited New Metals Division, Birnineham, England With pollution of rivers and estuaries in many parts of the world an unfortunate reality, performance of conventional condenser tube materials is falling short of full plant life reliability at an increasing num(ler of power station sites. The economic feasibility of using titanium for turbine conden­ sers has been proven in several installations in the United Kingdom, U. s. A. and Japa.'1 where cooling water conditions have led to prema­ ture failure of aluminium brass or cupro-nickel. Basic engineering parameters for titanium have been established and the well-documen­ ted coITosion-erosion resistance of the material in polluted saline cooling water substantiated on a practical basis. Attention of engineers is now directed tmrards those design parameters likely to lead to most effective use of titanium tube in new power stations. Design criteria discussed in this paper include heat transfer, which influences tube surfe..ce area and capital cost and galvanic corrosion, which affects choice of tube plate material and protec­ tive systems. In combination with other characteristics, these could lead to new concepts in design and fabrication of titanium condensers. 145 146 C. F. HANSON Corrosion and Erosion Proper~ies Commercially pure titanium is completely resistant to corrosion and erosion attack in sea-water at least up to 130°c.(1). 'l'he pro­ tective oxide film is maintained irrespective of oxygen, sulphide and ammonia content and under crevice conditions arising from scal­ ing, bio-fouling or local tube blockage. It is therefore resistant in polluted water causing deposit attack on copper base materials, pitting on stainless steels and all1E!onia stress corrosion on brQsses. Titanium is non-toxic to marine organisms, but remains unattacked under even severe bio-fouling. o. 5 p. p. m. chlorine injection eliminates such bio-fouling, the s;ystem no!'lllally employed on condenser installations being adequate.(2). Threshold velocity for erosion of titanium tube in clean sea­ water is 60 to 90 ft/second ( 1 ,3). In water of high sand content, threshold velocity is of the order 20 to 25 :ft/second with uninterrupted flow. At normal design velocities, 6 to 10 ft/second, impingement attack caused by local tube blockages or by air bubble release, which often afflicts copper base m~terials, is absent on titanium. The high in"tegri t;ir of titanium thus allows design of conden­ sers with much thinner tube wall than those normally specified with copper base materials. Modern titanium installations (1) use 0.028 and 0.020 in wall tubine and tubing as thin ns 0.012 in is under development in Japan (4,5). Additionally, the erosion and impineement resista~ce of titanium can allow increase in cooling water velocity in new con­ denser designs, with attendant benefit in :-educing surface area." It is normal practice to over-specify tube surface area for a given thermal duty to allow for subsequent plueeing of leaking tubes arising from corrosion or erosion damage. With titanium, this factor may be substantially reduced or eliminated. Ferrous sulphate can also be eliminated in systems re-tubed entirely in titanium but is not detrimental to titanium's resist­ ance in partially re-tubed condensers. The practice of passing spheres or brushes through conrlenser tubes to reduce the incidence of deposit attack is unnecessary with titanium, although it can be retained with high silt content cooling water as a means of improv­ ing heat transfer performance. Absence of copper, zinc and/or nickel corrosion products, particularly beneficial in once-through nuclear plant, can enable reduction in the size and cost of conden­ sate polishing plant. It is in fact the elimination of potentially hazardous chloride leakage into the system and the avoida~1ce of costly unscheduled TITANIUM CONDENSER TUBING IN POWER GENERATION TURBINES 147 outages that yields an attractive economic case in favour of ti taniu::n at power stations using polluted cooling l·1ater. Galvanic Corrosion Titanium is invariably the cathodic mel'.'lber of a bi-metal couple in a sea-water condenser. The use of compatible tube plate material, or effective protection of non-compatible tube plate material, is essential. Careful consideration of the galvanic cor1·osion hazard is even more important when nartia:: ly re-tubing . l·rit~1 titanium alongside existing copper base tubes. Naval Brass or 60:40 brass (r·1untz Netal) tube plate materials are cor•r:1only used ·with aluminiut:i brass or cupro-nickel tubing. Stainless steel clad tu.be plates are sometimes used in power gen­ eration condensers, and aluminium brass or aluminium bronze are not uncommon in the oil industry. Coating of tube plates is occasion­ ally specified, more often the steel 1mter boxes are protected in this manner. Sacrificial iron or zinc alloy anodes or platinised titanium impressed current cathodic protection systems are used, in some cases as an insurance to cover failure of coating$. There is ample evidence from U.K. experience that condenser tube plates in Naval Brass or Muntz Metal, otherwise unprotected, will suffer from accelerated corrosion attack around titanium tubes. This is supported by laboratory evidence. Fig. 1 shows that the accelerated rate of corrosion through coupling increases as the cathodic (titanium) area related to the anodic area gets larger. With 60:40 brass, corrosion is by dezincification. With naval and Admiralty Brass, corrosion is more general. Corrosion rates are higher in flowing sea-water. For example, from Fig. 1, Admiralty Brass at the extreme anode:cathode ratio of 1 :~9 (repre­ sentative of local failure of a tube plate coatine in the vicinity of a titanium tube) has a corrosion rate of 0.008 in per year in static sea-water, which increases to 0.020 in per year in flowing sea-water at 6 ft/second. Although with the brasses, tube plate thickness can incorporate a corrosion allowance, there remains the problem of preferential corrosion penetrating the brass at the tube-tube plate interface eventually causing a leakage path. Of the copper base materials, the most compatible with titanium is the high nickel content aluminium bronze (A.S.T.M. B171 Alloy E); in flowing sea-water the corrosion rate at 1 :30 anode:cathode ratio is 0.0004 in per year compared with 0.0003 in per year under static conditions. This alloy has recently been specified for a large titanium tubed turbine condenser to be installed in England during 1972. Nickel aluminium bronze (A.S.T.M. B171 Alloy D) has been used in two smaller industrial steam turbine condensers at the Coryton Refinery of Mobil Oil Company in the u. K. (1). After 3 years service there is no evidence of tube plate corrosion, as would be expected from laboratory data in Fig. 1- Fig. 1. Galvanic Corrosion of Titanium ~ Dissimilar Metal Couples at Different Area Ratios in Static Sea Water. ALUMINIUM BRONZE (ASTM Bl71 ALLOY E) UNCOUPLEp ~ STABILISED STAINLESS STEEL COUPLED ANODE: I I CAlHODE RA1101:0.1 60/40 BRASS ~1---LLLL:LI COUPLED ANODE: W:f§l CAlHODE RAllO 1: 10 BRONZE lA ST MB 171 ALLOY D) COUPLED ANODE: ™1 CAlHDDE RA110 I :30 ALUMINIUM BRASS (76 Cu -22Zn - 2 Al) MONEL (67 Ni - 31 Cu- I Fe-I Mn) 0·14 OH 0.10 0 0.02 0·04 0.06 0.08 0·10 0-12 OH 0·16 0·18 0-20 UN·COUPLED CORROSION RATE COUPLED CORROSION RATE OF DISSIMILAR METAL OF DISSIMILAR METAL IN SEA WATER IN SEA WATER AT mm /y~a r INDIC".ATED ANODE: CATHODE ::c AREA RATIOS > z mm/year Cl'l 0 z TITANIUM CONDENSER TUBING IN POWER GENERATION TURBINES 149 In these designs, the maximum anode:cathode ratio is less than 1 :10. Stainless steel clad tube plates should be compatible with titanium galvanically provided they are not subject to pitting attack. In high sulphide bearing waters, the probability of such attack is higher due to breakdown of passivity; it is because of this factor that stainless steel tubing is not employed in the United Kingdom in coastal or estuarine stations. Solid or clad titanium tube plates may eventually find appli­ cation in new condensers. Although more expensive, they could be used in high integrity nuclear plant where avoidance of chloride ingress into the core exchanger circni t is absolutely essential. On-site fabrication would be difficult, but, in thinner wall form and optimally designed for heat transfer efficiency at higher cooling water velocities, the 7Clfo reduction achievable in tube and tube plate weight compared with conventional tube materials could perhaps provide opportunity for modular construction in a fabrica­ ting shop under cfean conditions and transport of the completed module to site even for the largest turbines presently envisaged. Protective Systems As an alternative to selection of tube plate materials com­ patible with titanium, particularly in retubing where replacement would be unnecessarily expensive, it is possible to use coating or cathodic protection or both. Experience with rubber coating of brass tube plates and water boxes is variable. The primary hazard, when titanium is introduced into any system, is localised failure of the coating resulting in a very small anodic area. Either a perfect coating is required or a sacrificial anode or impressed current cathodic protection system must be used as an insurance. Coatings that have been applied on titanium tubed units include polysulphide rubber, coal tar pitch epoxy resin and metal oxide-filled epoxy resin. In each case scrupulous preparation of the tube plate surface is required to avoid dirt and grease and provide a key, the latter usually by shot blasting.