Is there any mechanism or tool that helps in cleaning (accessing) gate pocket from Fe debris?

Will the 3E butterfly with horizontal stem provide better sealing in such condition?

A full answer will require more details from your side, but since you are lacking response let me try anyway:

A gatevalve allowing Fe-debris to be catched in pockets under normal conditions indicates not the best valve construction suitable or selected under the present conditions.

Cleaning might be difficult, and best solution could (as you suggest as alternative) be to change the conditions/ equipment.

Is pigging possible and will this help?

Change to gate-valve construction without pockets?

Change to other type of valve (same building length and flanges), double eccentric BFL possible?

State of pipeline or fluid: changing or cleaning to avoid Fe-debris into pipeline possible?

A butterfly  valve with the shaft horizontal is less likely to be affected by accumulating debris than a gate valve.  Just be sure the seat pocket in the butterfly valve does not contain cavities in which the seat flexes.  Otherwise, the debris can collect within the seat cavity, preventing the seat from flexing and preventing the valve from closing.  Some triple offset butterfly valves with the flexible element on the disc may be a good choice depending upon the process requirements.  As always, you don't get something for nothing.  Every type of valve has its strengths and weaknesses.  Just be sure you are not solving one problem but creating another.  You must best match the process conditions to the valve performance expectations.

Some butterfly valves come equipped with "steam jacket" that keeps body hot in the critical areas, avoiding the fast cool of powders under pressure (being compacted into a "solid").
Back to gate valves, have you tried to "crack open" say 4% of opening stroke and allow the valve to self-clean in seat pocket area (this can be done using the vortex caused by throttling flow through wedge gate valve) obviously, the longer it has been compressing the powder while valve closes, the more difficult it becomes to "flush seat".

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