We are operating 4 drum DCU (let us say DCU-B) which was commissioned in 2011. For one year we did not face any problem with drum feed valve SP-6. But after one year we are finding coke formation in SP-6 valves at interval ranging from 1.5 month to 5 months. In another DCU (let us say DCU-A) that I worked in, we opened SP-6 after 4 years !! The differnece in valve size is 10″ in A and 16″ in B. But the steam purge line (MP steam) size is same, that is, 3/4″. Purge lines are always at MP steam temperature indicating that steam flow is there (except gland steam purge). Pressure of steam is also healthy.
Should the purge line size be according to feed line size? I have heard that these days SP-6 problems are being faced in all big size DCU.