I have written about this hazard numerous times as it is the leading cause of flash fires within process vessels, but during a recent Combustible Dust project, I noticed that the 2017 edition of NFPA 654 has some new and specific requirements for this activity. I had always used NFPA 77 as my RAGAGEP for this activity, but now NFPA – 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids is a bit more specific in that it mentions the worker doing the addition(s) be donned in Flame Retardant Clothing meeting NFPA 2113…
9.3.7* Manual additions of solids through an open port or a manway into a vessel containing ammable atmospheres shall be permitted to be done in 50 lb (25 kg) batches or smaller, provided the requirements of 9.3.7.1 through 9.3.7.7 are satisfied:
(emphasis by me)