Use of Cold Air Velocity Test (CAVT) to Locate Erosion Prone Zones in Pulverized Coal Fired Utility Boiler
3, World Congress on Engineering, (July 2011)Abstract
Thermal power plants in India are facing the
problem of forced outages due to unexpected boiler tube
failures. Fly Ash Erosion (FAE) is one of the prime reasons for
these failures. The main cause of this FAE is the localized
increase in the velocity of flue gases which in turn increases the
velocity of fly ash particles. The rate and extent of FAE
depends on various parameters such as particle velocity, angle
of impact, particle composition, shape, size & sharpness factor,
temperature of particle, surface temperature of tube,
population density of particles, coal quality, combustion
efficiency, erosive resistance of the tube surface including
compositional & temperature variations. Particle velocity is the
most important parameter as the rate of erosive loss is
proportional to the velocity raised to an exponent that ranges
between two and four. Particle velocity is driven by the local
flow velocity at any particular boiler location. It is difficult to
physically measure velocities of flue gases inside the boiler
when it is in operation. However, it is required to know the
velocity flow field in various zones so that its effect on the
various failure mechanisms can be predicted. The primary tool
to combat Fly Ash Erosion is flow modification in conjunction
with a cold air velocity test before and after modification. The
cold air velocity test is performed to predict the velocities in
the respective zones of the boiler. A comprehensive EPRI
Guideline has been published which provides a step-by-step
procedure of CAVT. In this paper authors have used cold air
velocity technique (CAVT) to determine local velocity profiles
across various pressure parts. The use of CAVT to identify
regions of excessive velocity, followed by the installation of
diffusion and distribution screens, may provide utilities with
the most optimum solution to the problem. The authors have
tried to predict the Cold Air Velocity. The actual geometry of
the flue gas path of the 210 MW boiler is created using Pro-E
and meshed using GAMBIT is imported in FLUENT for
analysis. The inlet and outlet are given pressure boundary
conditions. The k-epsilon realizable is used as turbulence
model. The results obtained are then compared with the
experimental data of CAVT to validate the model. For
comparing the results, the points where the actual CAVT is
performed are replicated in the simulated model. Cold Air
Velocity Test is successfully simulated using FLUENT. The
results of the simulation are in good agreement with the
experimental results of CAVT with error of order of ±23 %.