Titanium Metals Corporation
CODEWELD® Titanium Tubing

APPLICATIONS AND DESIGN TRENDS

TITANIUM TUBING FOR SURFACE CONDENSERS
Titanium tubing in use by the power industry has proven to be the most reliable of all tubing materials. In a surface condenser environment, there is nothing to cause it to corrode or pit. It is immune to corrosion from chlorides or industrial pollution. It returns no harmful metal ions to the environment or to the condensate-feedwater stream. Titanium overcomes the problems of random tube failure caused by local conditions of inlet turbulence (inlet end corrosion), partial blockage by shells, mussels, debris, ammoninated condensate, and is highly resistant to erosion attack by steam impingement. It has consistently been shown that many of the problems in nuclear steam generators and fossil-fired boilers are a result of condenser inleakage; as are many of the unplanned outages.^(8)(9)

The history of titanium for as-welded condenser tubing began in 1959 when the first titanium was tested in condensers at both United Illuminating Company in Connecticut and at Public Service Electric and Gas Co. in New Jersey. Periodic examination of these tubes, which carried severely polluted estuary waters, showed zero corrosion. Other test locations followed in the U.S. and abroad. It was not until 1972, however, that the first condenser completely retubed with titanium went into service at Consolidated Edison's Arthur Kill station. Other companies quickly followed, and by 1994 there were over one hundred fifty power plant surface condensers, both nuclear and fossil, retubed completely with TIMET titanium tubes. Other countries have adopted a similar practice, and titanium tubes are being used in condensers in England, Ireland, France, Germany, Sweden, Belgium, Holland, Japan, India, Taiwan and Korea, to name just a few. It was not until 1977 that the first condenser designed for titanium tubes was put into service.

When retubing an existing condenser which has been designed for use with tubes other than titanium, there are six areas to consider. They are:

1. Support Plate Spacing
2. Tube Sheet Material
3. Effect of Reduced Weight
4. Effect of Reduced Water Velocity
5. Cathodic Protection System
6. Fouling

They are discussed more fully below.

1. SUPPORT PLATE SPACING:
Until the publication in 1978 of the Seventh Edition of HEI Standards for Steam Surface Condensers, there was no approved method for determining adequate support plate spacing to prevent tube vibration under extreme conditions. Consequently, when an operating power company wished to retube a condenser with a material other than the original, they had to return to the manufacturer to determine the required support plate spacing. It is the purpose of this section to provide guidelines for retubing condensers with titanium. The curves shown on Pages 27-29 were prepared by Heat Transfer Consultants, Inc., Hingham, MA, and give results comparable to those determined by the method outline in Section 6.2.4 of the HEI Standards for Steam Surface Condensers, Eighth Edition. They take into consideration 10% blockage of the steam exhaust area by piping or feedwater heaters and 10% moisture in the steam. No consideration is given to the conditions caused by the opening of massive steam dumps into the condensers. The spacing is based on exhaust steam only. The spacing for spans adjacent to the tube sheet may be increased by a factor of 1.18.

To use the curves, the exhaust steam loading in lbs. per hr. per sq. ft. of exhaust area must be determined. This is accomplished by using the total exhaust steam flow found on the condenser specification sheet and dividing it by the total turbine exhaust area. If there is more than one exhaust, this should be taken into consideration. Choose the curve with the exhaust steam loading in lbs. per hr. per sq. ft. of steam. Go vertically up the sheet to the desired tube gauge and read the required spacing on the left. For example, with 1" O.D. x .020" (25 BWG tubes) and steam loading of 4000 lbs. per hour per sq. ft. of exhaust area, the required spacing is 28.6". The space adjacent to the tube sheet could be 28.6 x 1.18 = 33.75".

2. TUBE SHEET MATERIAL:
Titanium is cathodic (more noble) to the copper alloys. Therefore, the copper alloy corrodes preferentially when coupled to titanium in an electrolyte such as seawater. If the existing copper alloy tube sheet is to be used, the metal should be protected by either a coating or an impressed current cathodic protection system or both. Consideration should be given to replacing the tube sheet with titanium. This effectively eliminates the galvanic couple (and therefore corrosion) and gives stronger tube joints. The water box can be either coated or be protected with an impressed current cathodic protection system, which would be smaller and lower in first cost and operation than one needed to protect the tube sheet also. An added advantage of titanium tube sheets is that the tube joints can be rolled and welded, if desired.

Either solid titanium or titanium explosively clad to steel may be used. If a solid plate is used, there has to be shell flange on the condenser since titanium cannot be welded to steel. Therefore, explosively clad tube sheets are used where there is no shell flange, allowing the steel backing to be welded to the steel shell.

A solid titanium plate is higher in initial cost than a clad plate. However, when a clad plate is used, welded tube joints are a must in order to prevent cooling water leakage around the thin cladding, forming a galvanic cell. This can result in disastrous corrosion. The added cost of welded tube joints far exceeds the extra cost of the solid plate.

3. REDUCED WEIGHT:
As noted above, titanium tubes are used with thinner walls than other materials. A 22 BWG tube in titanium weighs about 25% of that of an 18 BWG copper alloy tube. This change in weight should be analyzed to determine if the foundation supports are sufficiently strong to secure the decreased weight against the hydraulic load during operation. If the condenser is spring supported, the springs must be reset.

4. WATER VELOCITY
The thinner wall titanium usually results in a changed water velocity and reduced hydraulic pressure loss through the tubes. The effect of this reduction on the pump operating curve and electric motor drive should be considered. It will also have an effect on the heat transfer rate.

5. CATHODIC PROTECTION:
If there is an existing impressed current system on the condenser, the location of the anodes, reference cells, and operating parameters should be examined to ensure the system is compatible for operation with titanium tubes. The following are suggested guidelines:

1. Hydriding can take place at potentials greater than -0.75
volts SCE.
2. Do not exceed -.90 volts SCE.
3. System must have automatic potential control.
4. System must have good reference electrodes placed in
strategic positions.
5. Anodes should not be placed closer than 30" from the tube
sheet.

6. FOULING:
Experience has shown that in locations where chlorination of the cooling water can be done, bio-fouling is kept to a minimum. Unlike many other materials, titanium suffers no corrosion problems from attached marine life or from local high velocity caused by a partially-blocked tube. In locations where bio- fouling is severe and chlorination is not permitted, continuous cleaning systems using sponge rubber balls or nylon brushes can be used to restore the cleanliness factor to 1.0 or slightly less.

Condensers designed for titanium usually make use of a cleanliness factor of 0.90 as opposed to a cleanliness factor of 0.85 required for copper alloys. In many locations, the value for titanium is conservative. A test in a OTEC (Ocean Thermal Energy Conversion)^(10) heat exchanger alloy study showed a fouling resistance of .0005 BTU/hr ft^2 degrees F for untreated Gulf of Mexico water at a flow rate of 6 ft/sec for 90-100 days. Using HEI heat transfer rates for 70 degrees F water, this translates into a cleanliness of 96.7%. Other tests^(11) showed similar results.

MODULAR REPLACEMENT
A recent development in titanium condensers is module replacement rather than tube-by-tube replacement. In this concept, the condenser shell is completely gutted of tubes and support plates and prefabricated tube bundles or modules are slipped into place in the condenser shell. The advantages of this are:

- Reduced outage time

- New tube sheets and support plates allow use of thinner wall tubes resulting in higher heat transfer, lower cost and the elimination of galvanic corrosion.

- It allows redesign of the condenser to eliminate vibration and improve performance.

- The modules are shop fabricated and can be supplied with or without water boxes.

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