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Communication Feature | June 2015

Cold air velocity test for power plant boilers

To study the problem and evolve a solution for the erosion of tubes due to fly ash associated with high velocity in the second pass of a boiler, a flow study of boiler is required. To get flow profile within the second pass, CAVT is carried out. The flow profile thus obtained is studied and flow corrections applied, which helps in reducing localised tube erosion due to fly ash and hence eliminating tube failures.

CAVT procedure
This test is conducted during plant shutdown when about 80 per cent of MCR flow is maintained to determine the areas/locations at which velocities are higher than the respective plane average velocity. It is carried out at different pre-selected planes and locations right from screen tube to economizer and the readings taken are witnessed by the client´s representatives. Instrument used for the test is a hot wire anemometer.

The readings are then tabulated in regular formats, studied and analysed to locate high velocity spots. The required data of coal, tube bundles´ details and spacing, temperature and pressure profiles, and boiler drawings etc. are to be provided by the client prior to the test. Based on the analysis, suggestions are forwarded for installation of various types of screens along with structures at different locations. The suggested scheme of screens and its structural supports are discussed with clients and then finalized.

  • Normal duration of CAVT is 8 hours.
  • The screens are then manufactured, supplied and installed at site.
  • Post-installation (of screens) CAVT is carried out and results compared with pre-installation CAVT to verify the effectiveness of screens.

Benefits of CAVT

  • Reduction in high-localized velocities.
  • Reduction in tube erosion due to fly ash.
  • Reduction in boiler tube failures.
  • Improvement in heat transfer.
  • Reduction in air heater outlet temperature.
  • Overall improved flow profile enhances down stream equipments´ efficiency.

Article by: Arun K Sheth, Director, Mechwell Industries Ltd.

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