Most of the cases.. Custumer ask for expected boiler life & we tend to say approx 10 years..
How one arrive at this figure? I’ve asked this question so many time to may expert inthis field but not able to get any satisfactory answer.
Recently i went thru ASME VIII Division1 , where the Fatigue requirements are discussed.
If the boiler pressure variation is less than 20% then we need not to consider those variation in fluctuating load. Only boiler cold stat-up & shut down need to be considred while calculating the fluctuating cycle.
i.e for 12 bar(g) design pressure boiler, the maximum operating pressure is 90% of design pressure i,e 10.8 bar(g) 20% of pressure variation means , pressure below 8.6 bar(g) will be counted for fluctuating load.
also delta temperature is important while calculating the fluctuation temperature.
for Delta of 200 (Star-up condition) the equivalent cycle is 4 per star-up & for
delta of 100 (Shut-down condition) the equivalent cycle is 1 per shut down, i.e if boiler is closing 15 time per month, it 12 per year, the cycle it will go thru is (4+1) x 15 x 12 = 900 cycles.
Approx, 10000 cycle boiler can take by design, i.e boiler design life will be = 10000/900 = 11.11 year approx.