Reduction of Air In-Leakage and Flue Gas Bypassing in the Penthouse of Duke Power- Marshall Unit No. 4   Description of Authors: Mr. Ron E. Campbell Duke Power-Marshall Station Plant Manager Mr. Thomas J. Rush Cost Effective Maintenance, Inc. President UNIT DESCRIPTION... Marshall Unit No. 4 has been one of the most efficient electric power generating units in the USA for 25 years. This unit is a pulverized coal fired, Combustion Engineering boiler which provides 3,500 psi steam to a 700,000 kW tandem compound turbine. The boiler has two stages of reheat, and the rated steam temperatures are 1,007°F/1,000°F/1,000°F. The unit heat rate has averaged 8,912 BTU/kWh for the plant is thirty year life. The boiler side elevation is shown below in Figure No. 1.   Flue Gas Bypassing and Ash Accumulations Gradual deterioration of the original penthouse roof seals resulted in flue gas bypassing in the penthouse, and accumulations of flyash in the penthouse. The flyash deposition in the penthouse, and flue gas bypassing is depicted in Figure No. 2. The fluegas bypassing is a very small percentage of the total boiler flue gas flow, however, over the 8000 hours or so of operation between outages, the slow accumulation of settled flyash resulted in ash dunes of up to five feet in depth. The bulk volume of the flyash in the penthouse approached 100 tons. This ash remained hot, well after shutdown of the boiler, and was removed by vacuuming. Owing to the maze of tubing and headers in the penthouse, and considering the specific design of this boiler with two stages of reheat steam, there are more tube penetrations and headers in this penthouse than most boilers. Therefore, ash removal was difficult, time consuming and expensive. Air In-Leakage Flue gas bypassing and the consequent ash accumulations were the principal justifications for correcting the roof seal leakage. However, another factor is Duke Power is commitment to efficient power generation. Air in-leakage also resulted from the roof seal deterioration. This is depicted in Figure No. 3. The air in-leakage through the penthouse casing was deemed to be practically impossible to arrest at the penthouse boundary. This is because of the many penetrations of steam lines, vents, drains, and the expansion and contraction of the casing for each startup and shut down. These cycles, for this super critical boiler with 30 years of operation, have left numerous air leak paths through the penthouse casing. The sealing of any air in-leakage was deemed to be most practical by sealing the tube penetrations at the roof tubes. Air in-leakage will result in unit heat rate penalties. These penalties to heat rate are: 1.) Dry gas loss of heated excess air that provides no benefit to combustion 2.) I.D. fan horsepower to remove this additional tramp air 3.) If left to ever increasing quantities, air in-leakage will contribute to the excess oxygen measured at the oxygen analyzers, resulting in an oxygen starved furnace. This could lead to increased flyash carbon loss.   Reliability, Availability and Safety Eliminating the ash accumulations by roof sealing improvements also reduces tube repair time in the event of a tube failure in the penthouse, which produces lost revenue when the unit is off-line. Electrical production from this unit is vital to the Duke System. Additionally, there are personnel safety benefits of the maintenance personnel not having to contend with large accumulations of flyash that hold the heat for days. Description of Alternatives Considered and the ISOMEMBRANE® System During the overhaul outage of the spring of 1994, the penthouse seals were replaced with an advanced ceramic fiber sealing system. This process was selected after evaluation of traditional weld repairs and installation of updated metal seals. Based on past weld failures of repairs, a new design was deemed to be required. Basically, there were two alternatives. One was to remove the existing metal seals, refractory and insulation, and replace with an updated hermetically welded high crown design. The cost and outage time requirements eliminated this approach. The other alternative was to use the ISOMEMBRANE® system of sealing. This design uses advanced ceramic fibers, cement, and anchoring systems which were developed in Denmark. The system has been further refined and made cost effective by the USA licensee of this system, High Temperature Technologies, Inc. of Charlotte, North Carolina. This second method of repair/design upgrade was selected for both technical as well as time and cost considerations. From a technical standpoint, the areas to be sealed experience considerable expansion movement. This is a result of the size, pressure, and temperatures of this unit. The ISOMEMBRANE® system provides excellent sealing, without restricting expansion movement between the tubes. The ISOMEMBRANE® sealing system is shown on Figures 4, 5, and 6 below.   The ISOMEMBRANE® system requires careful surface preparation and cleaning. Then, studs are applied to headers and tubes. ISOMEMBRANE® is anchored to the proprietary studs and attachments. Use of ceramic fiber insulation specifically designed for this service is applied. Air and gas tight sealing is accomplished by application of high temperature adhesives, mortars, and cement. The adhesive used is rated to 2200°F with no volatility or other significant Haz Mat concerns. Scope of Repairs The seal system as described on Figures 4, 5, and 6 was applied to sections A-E, as shown on Figure No. 7. The installation was the result of extremely detailed coordination of all work groups, for time expediency. Due to the vital nature of this unit, a comprehensive coordination of all outage activities was imperative. The schedule of implementation can be shown in Figure No. 8.   Results After a year of operation, the penthouse was inspected and found to be lightly dusted with barely sufficient accumulation to show footprints. This is contrasted with previous five foot deep ash dunes. The savings in maintenance repairs are estimated at $65,000 for vacuuming that was not required, and $80,000 in maintenance personnel weld repairs. The history of repairs was such that vacuuming and weld repair costs were predictable and, before the ISOMEMBRANE® installation, always expected as recurring maintenance costs. The heat rate improvement of reducing air in-leakage is well known, and significant. The principal quantified savings are in reduced maintenance costs, more expedient repairs (reduced cool down time for repairs in the penthouse), and safety. The heat rate improvements, though not quantified yet, are expected to be significant. The success of the ISOMEMBRANE® on Unit No. 4 has resulted in similar plans for Unit No. 3 and other units in the Duke Power system. The authors wish to acknowledge the efforts of Roy Helm of Duke Power Marshall Plant, and all of the maintenance staff at Marshall Steam Plant. Also, Mr. Bill Turner of CEM, who was instrumental in developing and improving the field installation techniques.   Return to Top High Temperature Technologies, Inc. 2175 Dunavant Street ▪ Charlotte, NC 28203 ▪ 704-375-2111 ▪ www.isomembrane.co