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 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. A further problem is to ensure proper mixing and complete curing of the coating. In refinery heat exchangers, there is practical evidence of effective use of zinc alloy sacrificial anode cathodic protection on aluminium brass and aluminium bronze tube plates in units tubed entirely in titanium. It is theoretically possible to use such a system on a large turbine condenser completely re-tubed in titanium; the main interest in this system is however in partial local area re-:tubing of existing units already equipped with zinc anodes. A particu1ar example is in re-tubing of the air extraction 150 C. F. HANSON zone where aluminium brass fails prematurely through ammonia stress corrosion on the shell side.

To demonstrate the effects of zinc anodes in a particular power station condenser design it is possible to calculate the relevant anode:cathode area relationship in the area where titanium is to be installed and by zero resistance ammeter techniques to measure the extent and direction of flow of current, thus simulating the actual service situation. Fig. 2 demonstrates the effect with and without ;i;inc alloy cathodic protection in a 600 NW conrlenser where four rows of 1 in diameter titanium tube are inserted in a 60:40 brass tube plate surrounding a central stainless steel air extraction pipe and surrounded by aluminium brass tubing.Without the zinc protection the Muntz Metal acts as an anode, all other metals in the couple being cathodic. Calculations show that the corrosion rate on the Nuntz Metal at the .level of current measured would be of the order of 0.0016 in per year in static sea-water and possibly as high as 0.004 in per year under flowing conditions. When a zinc alloy anode is inserted into the system, the current flow on 60:40 brass is reversed and it becomes cathodic. Corres­ ponding measurements including cupro-nickel together with titanium and aluminium brass tubing in the same tube plate confirm that zinc protects all these coupled materials. Provided, therefore, a zinc anode cathodic protection system is designed to be effective over the area of the tube plate in which titanium is installed and to remain effective by scheduled mainten­ ance, it would seem possible to install titanium tubes amongst cupro-nickel tubes or aluminium brass tubes or both in uncoated brass tube plates. In certain circumstances soft iron anodes are equally effective. Impressed current cathodic protection systems can also be used. Provided they are designed to have throwing power over the full tube plate surface, they will protect the tube plate and copper base tubes from galvanic attack from titanium as well as protecting the steel water box from the galvn~ic coupling with the tube plate. Tube-Tube Plate Joints Conventional techniques and equipment are used for roller expansion of titanium tubing. With tube at 1" diameter and 0.020" wall, optimum pull-out strength is of the order 0.5 - 0.6 tons. This increases with wall thickness to a level of 1.5 - 1.6 tons for tube at 0.048" wall thickness. The technology of fusion welding of titanium to itself for tube plate joints is well established. Welding of ,ioints on large condensers on site is however more of a problem and is expensive. A more recent development which has been applied for joining TITANIUM CONDENSER TUBING IN POWER GENERATION TURBINES 151

Fig. 2. Galvanic Current Measurements on Multi-coupled Metals Simulating Area Relationships on a 600J:.11'1 Turbine Condenser Test Conditions Synthetic Sea-water, Room Temperature, Areas calcu­ lated assuming throwing power down 1 in diameter tubes of 3 x diameter. Currents measured by Zero Resistance Ammeter. (a) 60:40 Brass Tube Plate. 4 rows Titanium tube surrounding central stainless steel air extraction i e surrounded b aluminium brass tubi no zinc anode in circuit. Half cell shown

Current 0.22mA 0.54mA 0.25mA Om A 0.075mA 0.003mA from Protected Corroding Corroding Protected solution Haterial Stainless Brass Brass Brass Brass Steel Relative 9sq cm 26sq cm 9.3sq c 1.35sq c 0.73sq cm ·, 5. 3sq cm ~ Z.R. A. Current

(b) As (a) with zinc alloy anode in circuit (Half cell shown)

Current 0.8mA 1.4m.A 1.1mA 0.2mA 0.51m.A 0.19mA ~ Protected Protected Protected Protected Protected Protected solution Material 60:40 60:40 Brass rass Brass Brass Relative 9sq cm cm ~ • ~- Current I' 4.2mA Corroding ' Zinc The remainder of ' --.~~~~~~~~~•Alloy the cell is • • •' symmetrical 0.5sq cm --

Source: Imperial Metal Industries (Kynoch) Limited, Birmingham, England. 152 C. F. HANSON titanium to stainless steel tube plates on large chemicaJ. plant callandrias is the technique of explosion welding. Fig. 3 illus­ trates the principle of this process which could be applied to the manufacture of large condensers with solid titanium or titanium clad steel tube plates. Heat Transfer

Heat transfer performance of thinner wall titanium tubing under steam condensing conditions plays a major part in determining the relative economics of titanium copper base tube materials in new condenser designs. Three factors are relevant, of which the most significant is the ability to increase cooling water flow rates to the optimum level, related to pumping costs and turbine vacuv.m requirements, without attendant erosion problems. Secondary factors influencing heat transfer performance are the use of thinner wall thickness tubing, although this is somewhat marginal in prac­ tice, and the lower fouling factors associa.ted with titanium sur­ faces under steam condensing conditions. Table 1 shows a series of heat transfer measurements on titanium compared with other condenser tube materials under different conditions. In the United Kingdom, heat transfer measurements have been conducted on titanium tubing in a 30 JilW condenser operating in relatively clean sea-water. Table 2 compares heat transfer per­ formance of a 31% nickel-2% iron-2% manganese cupro-nickel and titanium, all tubes being 1" O.D. x 0.048" wall thickness. Although titanium has an inferior transfer coefficient in the clean condition, after a 14 month trial the position is reversed. Fig. 4 shows the correction factor related to waterside velocity that may be applied in calculating the surface area requirements of titanium condenser tube in clean sea-water. It is evident that the.re is substantial scope for reducing surface area in designing new titanium tubed condensers. By increasing flow rate, area can be reduced by up to 15% compared with a condenser designed for 70:30 cupro-nickel and by up to f11/, compared with a unit in aluminium brass. A further reduction of 5 to 6% is possible allowing for titanium's lower fouling factor in cooling water of relatively low solids content. For example, usin,:; the appropriate correction factors applied to B. E. A.N. A. or Heat Exch.anger Institute Codes, the overall heat transfer coeffi­ cient for 111 diameter x 0.048" 72:30 cupro-nickel tube at a flow rate of 6.5 ft/sec is 550 BTU£ft °F Hr. For titanium at 8 ft/sec, the coefficient is 620 BTU/ft2°F Hr (13% increase) and at 9 ft/sec the coeffici~nt is 690 BTU/ft °F Hr (25% increase). -I Fig. 3 Explosion Welding of Tube Plate Joints Fig. 4 Correction Factor to be Applied to Overall ~ Heat Transfer Coefficient for 70:3Q Cupro­ z TUBE DETONATOR c Nickel Determined by B.E.A.N.A. or H.E.I. ~ Codes to Give Titanium Overall Coefficient () 0 in Clean Sea Water. z 0 Before welding 1. 14 mz Cl> m

"'-I 1. 12 c ""'0 Cl> f..c z 0 G') ------INSERT POSITIONING LEGS­ +> () 1.10 z COLLISION POINT t & "ti 0 s:: 0 ::Em •r-1 1.08 +> () "'G') Q) m During welding f..c z f..c m 0 1.06 0 )> "'-I 0 •r-1 z !il 1. 04 -I Q) c A "'Cl> mz 1. 02 Cl>

1. 0 Completed weld 3 4 5 6 7 8 t Flow Rate ft/sec TUBE Courtesy - Central Electricity Gene~ating Board Table 1 - Heat Transfer Coefficient of Titanium Tubing comparea with Conventional Condenser Tube Naterials - Effect of Flm·r Rate and 'dall Thickness under Steam Condensing Conditions

Heat Transfe2 Coefficient NateriHl Tube Dimension BTU/rt-°F Hr. Source Varicition with Flow Rate 6 ft/sec 8 ft/sec 10 ft/sec

..!JI Titanium 'l OD x 0.036" 575 680 770 Heat Exchanger ...."5__i1 ;} OD x 0.020" 655 760 870 Institute Code 2.J1 70:30 Cupro-Nickel 4 OD x 0.036" 625 735 835 .211 90:10 Cupro-Nickel 4 OD x·0.036" 685 805 915 Clean Sea-water Titanium 0.018" uall 770 880 980 U. s. Naval Engineering Admiralty Brass 0.049" wall 790 910 1010 Experimental Station, 70:30 Cupro-Nicl:el 0.049" wall 630 730 795 Annapolis, Hd. U.S.A. 90 :10 Cupro-Nickel 0.025" wall 770 850 940 Clean Sea-water

Table 2 - Heat Transfer Coefficients for Titanium and 70 :30 Cupro-Nickel 1" x 0.048" tube in a 30 rnv condenser cooled by Clean Sea-water

Overall Coefficient, BTU/ftcup Hr. 0 Sea-water Veloci tJ.• corrected to 60 F Mean ~·rater Temperature ft/sec Titanium 70:30 Cupro-Nickel CleM Condition Fouled Condition Clean Condition Fouled Condition 6 592 558 605 528 7 638 608 654 577 8 683 655 695 625 ::i: > 9 722 700 735 674 z en 0 Courtesy - Oentral Electricity Generating Board, U.K. z TITANIUM CONDENSER TUBING IN POWER GENERATION TURBINES 155

Vibration and Fatigue Characteristics

Particular attention must be paid to titanium's vibration properties when installing a condenser because of lower modulus and use in thinner wall form. It can be calculated that the natural frequency of 0.028 in wall titanium tubing is equivalent to that of 0.036 in wall 70/30 cupro-nickel, Admiralty Brass and stainless steel tube. Most existing condensers, however, use tube at 0.048in wall. Calculations are therefore necessary for each individual case and it may be essential that support plate spacings are altered or even additional support plates added to reduce the span. This is even more important for a shell-side condition which might give rise to flow induced vibration and possible tube clashing. In a major U.K. installation of 0.036 in wall titanium tube, it was found necessary to reduce the maximum support plate spacing from 61 in for 0.048 in wall cupro-nickel tube to 37 in for the titanium tube. Under these conditions natural frequency resonant vibration on titanium occurred at 78 - 80 c.p.s. Measurements on the installed tube nest showed a maximum amplitude on the titanium tube nest of 0.006 in at 10 c.p.s. compared with 0.016 in at 30 c.p.s. with a cupro-nickel tubed nest. In some re-tubing installations it is possible to use partial mid-span support plates in local regions of high vibration to eliminate tube clashing. In new condensers, the support plate spacing can be calculated from the outset. Titanium has a high resistance to damage in support plate holes. Its high fatigue limit is not diminished either in sea­ water or under steam conditions. In the many installations of titanium tube in the power generation, and chemical industries, there has been no evidence of fret !;ing attack on tube surfaces at support plates in mild steel. Conclusion Careful attention to design detail can minimise the extra cost of initial ins talla ti on of titanium condenser tubing. i'lhen costs of replacing conventional materials and of unscheduled outage for repair or plugging of leaking tubes are taken into account, titanium is economically attractive at most sites operating with polluted cooling water, and will certainly be increasingly used in the future. 156 C. F. HANSON

References

1. Titanium Information Bulletin -·"Titanium Heat Exchangers for Service in Sea Water, Brine and Other Natural Aqueous Environments", Im erial Metal Industries och Limited, New Metals Division, February 1972 revised issue

2. Forster, B., "Titanium Heat Exchangers" Naval Materials Symposium, Defence Sales Organisation, ~linistry of Defence, London, November 1970, 7.1 - 7.6 3. Schlain, D., "Corrosion Properties of Titanium and Its Alloys", Bulletin 610, Bureau of Mines, U.S. Department of the Interior, 1964, 8 - 10

4. Kusamichi, H., Ki taoka, K., and Yasuda, N., "Thin :-ra11 Titanium Tubes - their Performance and Possibilities", Kobe Titanium, 1970, 1 - 8 5. Several Contributors, "Titanium and Zirconium", Volume 19, No. 9, Sept. 1971.