The present invention relates to a method to operate safely a catalytic fixed bed reactor operated in upward-flow configuration by preventing fluidization of the catalyst fixed bed present in said reactor; it relates also to a method to operate safely a furnace suitable for performing endothermic reactions containing a plurality of catalytic fixed bed reactors operated in upward-flow configuration by preventing fluidization of the catalyst fixed bed, and to a method to debottleneck safely a catalytic fixed bed reactor involving a gas flowing in upflow direction where the method includes increasing the reactant gas flow in the reactor while preventing the risk of fluidization of the catalyst bed present in the reactor by preventing fluidization of the catalyst fixed bed; the prevention of the fluidization of the catalyst fixed bed comprising a step of control of a pressure drop margin remaining before fluidization of the catalytic bed.