Hot Cyclone Design Problems
Hirakud Power has gained much hard-earned hot cyclone boiler operating experience over the 15 years since Unit 1 commissioning. Most of the major operation and maintenance issues directly relate to the high-temperature, high-velocity recirculating bed material. What follows is a digest of the types of problems encountered plus a short description of the final repairs completed.
Warping and Erosion of Hot Cyclone Vortex Plates. Each of the two cyclones consist of 11 stainless steel plates inside the cyclone that act as anti-vortex plates — 10 are 1,865 x 635 x 12 mm, and the smaller one is 1,865 x 457 x 12 mm — held in place with special hardware. Unit 1 has experienced erosion of the plates, and several have actually fallen out of their mounting points. There’s also been bending; failure of the refractory in the cyclone neck, roof, and bull nose area; and burnout of the loop seal nozzle cap on a number of occasions.
To fix these problems, the plant staff added hard facing in a zigzag manner with LOMELT-201, a stabilized 18/9.5 Cr/Ni/Cb electrode, on each of the stainless steel plates, thereby improving the plate’s temperature range up to 1,100C. All the other accessory parts were welded with Inox-CW welding rod, good up to 1,000C. Additionally, each of the 12-mm vortex plates was upgraded to 16 mm and 310 series stainless, as was the loop seal bed nozzles and bed nozzle column pipes. An adequate expansion gap was added between each of the plates to eliminate the bending experienced earlier, and a 25-mm ceramic fiber blanket was inserted between the plate and refractory.
Erosion in Pressure Parts. Hirakud Power uses coal with 40% to 45% ash content; hence, the threat of boiler tube erosion is always present. Erosion was observed early in the life of Unit 1 boilers in the hanging economizer tubes; economizer, superheater, and convection tube return bends; and evaporative tubes on either side at the exit of the combustor and combustor water tubes, among other locations.
The plant staff developed a unique solution for the hanger tubes: add a three-element overlapping semicircular stainless steel shield. Baffles were also added to reduce the impact of flue gas, and stainless steel protective shields were added to the lead coils of each group. Unfortunately, experience showed that the erosion then just shifted to the next row of coils, so all of the exposed tube bends were protected with shields.
The economizer header outlet tubes (three rows) presented a tangential surface to the flow of erosive gases and also had to be protected with welded-on tube shields. In the economizer, a thin layer of metal spray was tried but was unsuccessful in reducing erosion. Subsequently, a thin layer of erosion-resistant refractory was applied with good results.
Erosion in Air Preheater Tubes. The initial approach used to control erosion in the air preheater tubes was the same as that used in the boiler pressure parts: weld on a metal shield to the top row of tubes in each bank. However, as the flow path between the side wall and tubes is small, the higher-velocity gases caused higher rates of erosion farther back in the air heater assembly. Next, the original tube materials were substituted with higher-corrosion-resistant ASTM A 423/95 CorTen steel with higher nickel content. Unfortunately, although erosion in the middle of the tubes was eliminated, the ends of each tube (next to the wall penetration) continued to experience excessive erosion. Repairs were made using stainless steel sleeves on the ends of the tubes, which didn’t significantly reduce unacceptable tube erosion; the repairs resulted in slower rates of erosion, but erosion all the same.
The air preheater erosion was more toward the rear side, approaching the electrostatic precipitator, than on the front side, facing the boiler. There seemed to be a serious non-uniformity present in the flow path velocities at the inlet of the air preheater. Subsequent pressure drop testing at different locations in the air preheater was completed and, along with the air preheater dimensional data, was entered into a computational fluid dynamics computer code for further evaluation. The simulation results indicated that without baffle plates (the original design), there is significant variation in flow path velocities with the mass flow in segments toward the rear of the air preheater of 1.5 kg/s, compared with 0.5 kg/s at the entrance of the air preheater. The simulation was accurately predicting what the technicians were seeing inside the heater during each overhaul.
A new inlet duct design (the inlet duct bends 120 degrees from the boiler to the air heater entrance) with three carefully designed baffles balanced the gas flow through the air preheater, eliminating accelerated erosion of the air heater tubes and any subsequent tube failures.
Email
Comments
Print
Reuse
