The following empirical relationship identifies two factors that can be modified in boiler convective passes to reduce Fly Ash Erosion (FAE): Particle Loading and local gas Velocity.
E = C x M” x Vn Eq. 1.
E - Erosion Rate (mils/hr)
C - Correlation constant
M” - Particle Flux (lb/hr-sq.ft.)
V - Particle Impact Velocity (ft/sec)
n - Velocity Exponent
This equation indicates that FAE rate is proportional to particle loading and to the local gas velocity raised to a power - typically in the range of 2.3 - 3.5. This relationship indicates the importance of velocity control for the reduction of FAE.
To estimate the change in Relative Erosion Rate* for a change in particle (ash) loading and impact velocity, the following relationship can be used:
E1/E2 = (M”1/M”2) x (V1/V2)^n Eq. 2
Referring to the Relative Erosion Rate plot below, for an area where the solids loading and velocity are 20 percent above average levels, then the relative erosion rate would be:
E1/Eavg = (1.2/1.0) x (1.2/1.0)^2.4 = 1.86
Note that a 20% increase in ash loading increases FAE rate proportionately, while a 20% increase in gas velocity increases the erosion rate by 55%. The combined effect is an 86% increase in FAE rate over locations at average flow conditions (see plot below). It’s not unusual for some areas to have velocities 1.5 - 2.0 times average conditions!
* Relative Erosion rate is referenced to the design erosion rate established by the manufacturer. Typically design values provide a 20 to 30 year life expectancy before pressure parts are worn down to allowed minimum wall thickness. This procedure can be used to calculate life expectancy for local conditions that are not at average 'design' values.