Feedwater Heater Application of T-22 Tubing By Aziz Siman, Thermal Engineering International, Los Angeles, CA Joe Shelton, Southern Company Services, Birmingham, AL ABSTRACT ASME SA213 Type T-22 tubing has been utilized in boilers for the past 30 years. This material originated in Europe. and became a popular material for boilers. T-22 is a low alloy carbon steel with 2.25Cr 1Mo. United States utilities have considered this material as a viable option in feedwater heaters since the mid 1990s. INTRODUCTION Early industry feedwater heater tubing materials were Admiralty, copper and copper-nickel alloys. These material have high thermal conductivity and are fairly corrosion resistant. In the early 1960s, as the generating units grew in size and super-critical units were introduced in the industry, carbon steel and Mone y became popular materials for high pressure heater application. Copper deposits in boilers and turbine blades, along with Mone y tubing problems in replacement heaters forced the industry to turn to other feedwater heater tubing materials. The most popular materials were stainless steel and carbon steel. Carbon steel tubed feedwater heaters suffered premature failures due to tube inlet erosion, improper water chemistry such as, low ph level and high oxygen content, improper tubesheet overlay, and water level control problems which caused damage in the drain cooler inlet. Since the late1970s, stainless steel has been the most popular tubing material for both low and high pressure feedwater heaters. Carbon steel is still used for supercritical units as well as high chloride environments. Stainless steel is susceptible to SCC and chloride attack and has the lowest thermal conductivity of the common tubing materials. Most recently other types of stainless steel tubes such as ASME SA268-TP439, AL6XN, and Seacure have been suggested for feedwater heaters in high chloride environments. ASME SA213 T-22 is the most recent tubing material introduced in the industry. T-22 has been widely used in boilers for many years. An European equivalent of T-22 (10 Cr Mo 9 10) has been used successfully by European manufacturers for several years. In the early 1990s the first T-22 feedwater heaters were installed in US power plants. Table 1 is an estimated tubing utilization for feedwater heaters manufactured for US power plants. Table 2 is a summary of technical and commercial properties of different tubing material. Table 3 is the partial installation list of T-22 feedwater heaters in the US and Canada. EROSION-CORROSION OF CARBON STEEL TUBES Carbon steel tubed feedwater heaters suffer from three different groups of factors as suggest by Chexal and Horowitz. These are (1)- water chemistry related factors such as ph, oxygen content, and ph control agents, (2)- Hydrodynamic factors such as feedwater velocity and geometry of the flow path, and (3)- material composition, namely chromium, copper, and molybdenum content. Erosion-corrosion of carbon steel is controlled by oxide films stability. According to thermodynamic data 2 either Fe203 (hematite) or Fe304 (magnetite) will form on Fe in alkaline or neutral water with water temperature of 25 to 300 C (71 to 572 F). Several studies 3 have indicated that the addition of chromium and molybdenum to steel substantially lowers the erosion rate. According to Ducreux 3 the addition of as little as 0.25% Cr to steel lowers the erosion rate by several order of magnitude. ASME SA 213-T22 is a seamless low alloy carbon steel with 2.25% Cr and 1% Mo. It is a P number 5A material, which requires postweld heat treatment of tube to tubesheet joint.
T-22 TUBED FEEDWATER HEATERS The use of T-22 tubing in high pressure feedwater heaters is increasing yearly. Although 304 stainless steel is a highly reliable material, there are three main factors for choosing T-22 tubes over 304 stainless steel: 1. When space is a premium, T-22 feedwater heaters are smaller than stainless steel heaters. 2. When there is a chance that the feedwater can become contaminated with chlorides, T-22 is the better choice due to its high SCC resistance. 3. A T-22 heater is normally less expensive than a 304 stainless steel heater. Currently there are two major manufacturers of T- 22 tubing, Vallourec in France and Benteler in Germany. There are a few domestic companies which produce the tubes, however, they obtain the hollows from one of the European companies. Because the tubing must be obtained from Europe, the lead time for the tubes is several weeks longer than for stainless steel. Thus, the lead time for procuring a T-22 feedwater heater is longer than for a stainless steel feedwater heater. The common overlay for use with T-22 carbon steel tubes is Inconel. This should prevent tubesheet inlet end wear and is compatible with T-22 for welding. Normally tube inlet end inserts are utilized for carbon steel tubes to prevent inlet end erosion. These are not required for T-22 carbon steel due to the higher erosion resistance. The latest HEI Standards (6 d ' edition) does not address the velocity limit nor the minimum, wall thickness for T-22 carbon steel. T-22 is considered to have a velocity limit of 8 ft/secas is required for carbon steel. The technical committee of HEI is considering increasing this velocity limit to 8.5 Ft. /sec. T-22 is considered to ' have a 0.050" minimum wall thickness. as is required for carbon steel. CONCLUSION Utilities report that 304 stainless steel tubed feedwater heaters are getting upwards of 15 years of service with less than 1% failures. Part of this is the care taken by the utilities to properly operate the heaters and maintain good water chemistry. Another part is specification which requires a well designed feedwater heater. T-22 tubed feedwater heaters have been in service in the US for over 10 years with little or no problems. The heaters placed into service over the past five years have a near perfect record. European feedwater heaters with 10 Cr Mo 9 10 have been in service for over 15. years. Theoretically, when used as replacement feedwater heaters, T-22 heaters should last for at least 15 to 20 years as a minimum. How the existing T-22 tubed feedwater heaters perform over the next five to ten years will better tell how good a tube material T-22 is. As in the case of stainless steel, the key to optimum life of a T-22 feedwater heater is the operating conditions and the manufacturing/design specification. In conclusion, T-22 tubing should be adequate for most high pressure feedwater heater applications with design temperatures below 750 F. ACKNOWLEDGEMENTS The authors wish to acknowledge Mr. Mike Catapano of Powerfect, Mr. Ahmad Sanati of NRG El Segundo, and Mr. Sam Reynolds, consultant for their input to this paper. REFERENCES (1) V. K. Chexal, J. S. Horowitz, Flow Assisted Corrosion in Carbon Steel Piping, Parameters and Influences Paper presented at the Fourth Symposium on Environmental degradation of Materials in Nuclear Power Systems-Water Reactors, Jekyll Island, Georgia, August 1989. (2) J. A. Beavers, A. K. Agrawal, W... E. Berry, Corrosion Related Failures in Feedwater Heaters, EPRI CS-3184 Project 1265-11 July 1983. (3) J. Ducreux, Theoretical and Experimental Investigation of the Effect of Chemical Composition of Steels on Their Erosion- Corrosion Resistance, Paper 19 presented to the Specialists Meeting on "Corrosion-Erosion of Steels in High Temperature Water and Wet Steam", Les Renardieres, May 1982. (4) H. G. Heitmann, W. Kestner, Erosion-Corrosion in Water-Steam Cycles Causes and Countermeasures, VGB- Kraftwerkstechnik 62 No.3 P-180 March 1982. 2