Thanks for the update, but it is a pity that everybody lost so much time on this subject because you first stated that the exchanger had been cleaned.
Still the question remains how it is possible that an exchanger in gasoil service fouls so much that it can create a pressure drop of 16 bars.
Does the gasoil feed to the unit contain a lot of visbreaker gasoil, and/or delayed coker gasoil, and/or thermal cracker gasoil, and/or LCO from FCCU ?
If so you may need to put nitrogen blanketing on the gasoil storage tanks from which this unit is fed.