JetAA
is it a batch neutraliser or continuous one?
Have you analysed the salts?

Have seen major problems on batch neutralisers but the continous one was operating very steady.
Have you checked the netralisation temperature, i.e. at the mixing point, perhaps that can give clue of the mechanism.

In general I do think some more details would be of value.
RH

Dear RogerH,
We have two sources of feed: one is napthenic caustic and another one is phenolic. For the naphthenic caustic the unit runs very smoothly. But when we swing to phenolic processing, piping gets clogged. We did flushing with water and found a lot of black solid material look like iron salts. We didnot analyse the salts, but I think almost of them should be sodium salts so they should be well dissolved in water.
Temperature may also the cause. Original design temperature is about 60oC, but now we're operating at 40oC only.
Because my plant is quite new, do you think the dirt inside pipeline due to bad flushing during commissioning stage is the concern?

JetAA
Doesnt sound like bad flushing if you repeatidly get the problem. If it were bad flushing one would expect longer and longer run lengths.
Concentration can have an effect. High NaOH concentrations can exceed the solubility and result in solids dependent on what the caustic contains.
One way be to dilute the feed since it seems to be easily solvable.
I guess if you can analyse the solids next time that will be the key to the solution.
Good luck