A method of preventing blockage of circulating bed material in a circulating fluidized bed reactor arrangement, the method comprising steps of: - defining a reactor load at a digital control system of the circulating fluidized bed reactor arrangement, - feeding fuel and combustion gas at a predefined rate based on the reactor load into a furnace of the circulating fluidized bed reactor arrangement, - combusting the fuel with the combustion gas in the furnace and emanating flue gases and solid particles from the furnace into a flue gas channel, - separating solid particles from the flue gases in a particle separator arranged in the flue gas channel and circulating a flow of solid particles from the particle separator via a return leg back into the furnace, and - collecting a continuously flowing bed (52, 54) of the solid particles in a gas lock in the return leg, - measuring gas lock bed pressure values within the continuously flowing bed of the solid particles, characterized in that the method comprises the steps of - forming a gas lock bed height indication signal on the basis of measured gas lock bed pressure values, wherein a definition stage of the method comprises the steps of - changing the reactor load defined at the digital control system sequentially to multiple load values, - defining and storing to the digital control system a range of normal gas lock bed height indication signals, formed in normal solid particles circulation flow conditions, as a function of the reactor load, and - defining and storing to the digital control system a reactor load dependent alarm criterion, the fulfillment of which comprises that a current gas lock bed height indication signal is outside the range of normal gas lock bed height indication signals for the prevailing reactor load, for indicating deviation in the flow of solid particles being circulated, and a stage of using the method comprises the steps of - comparing at predetermined intervals a current gas lock bed height indication signal with the reactor load dependent alarm criterion, and - decreasing the reactor load in case the current gas lock bed height indication signal fulfils the reactor load dependent alarm criterion, for preventing blockage of the gas lock. - defining a reactor load at a digital control system of the circulating fluidized bed reactor arrangement, - feeding fuel and combustion gas at a predefined rate based on the reactor load into a furnace of the circulating fluidized bed reactor arrangement, - combusting the fuel with the combustion gas in the furnace and emanating flue gases and solid particles from the furnace into a flue gas channel, - separating solid particles from the flue gases in a particle separator arranged in the flue gas channel and circulating a flow of solid particles from the particle separator via a return leg back into the furnace, and - collecting a continuously flowing bed (52, 54) of the solid particles in a gas lock in the return leg, - measuring gas lock bed pressure values within the continuously flowing bed of the solid particles, characterized in that the method comprises the steps of - forming a gas lock bed height indication signal on the basis of measured gas lock bed pressure values, - forming a gas lock bed height indication signal on the basis of measured gas lock bed pressure values, wherein a definition stage of the method comprises the steps of - changing the reactor load defined at the digital control system sequentially to multiple load values, - defining and storing to the digital control system a range of normal gas lock bed height indication signals, formed in normal solid particles circulation flow conditions, as a function of the reactor load, and - defining and storing to the digital control system a reactor load dependent alarm criterion, the fulfillment of which comprises that a current gas lock bed height indication signal is outside the range of normal gas lock bed height indication signals for the prevailing reactor load, for indicating deviation in the flow of solid particles being circulated, - changing the reactor load defined at the digital control system sequentially to multiple load values, - defining and storing to the digital control system a range of normal gas lock bed height indication signals, formed in normal solid particles circulation flow conditions, as a function of the reactor load, and - defining and storing to the digital control system a reactor load dependent alarm criterion, the fulfillment of which comprises that a current gas lock bed height indication signal is outside the range of normal gas lock bed height indication signals for the prevailing reactor load, for indicating deviation in the flow of solid particles being circulated, and a stage of using the method comprises the steps of - comparing at predetermined intervals a current gas lock bed height indication signal with the reactor load dependent alarm criterion, and - decreasing the reactor load in case the current gas lock bed height indication signal fulfils the reactor load dependent alarm criterion, for preventing blockage of the gas lock. - comparing at predetermined intervals a current gas lock bed height indication signal with the reactor load dependent alarm criterion, and - decreasing the reactor load in case the current gas lock bed height indication signal fulfils the reactor load dependent alarm criterion, for preventing blockage of the gas lock.