Process for the preparation of a polyolefin in at least two slurry loop reactors comprising a first loop reactor connected in series with a second loop reactor, comprising the steps of: - introducing one or more olefin reactants, diluents and polymerization catalyst into said first loop reactor, and while circulating said olefin reactants, diluents and polymerization catalyst in said first loop reactor; - polymerizing said one or more olefin reactants to produce a polyolefin slurry comprising liquid diluent and solid olefin polymer particles; - withdrawing polyolefin slurry comprising solid olefin polymer particles and diluent from said first reactor and introducing the withdrawn particles into the second loop reactor, by means of one or more settling legs provided on said first reactor, wherein each settling leg has an inlet connected to the first reactor and an outlet connected to the second reactor by means of a transfer line, - characterized in that at least one settling leg is continuously open allowing continuous transfer of solid olefin polymer particles from said first loop reactor to said second loop reactor, and in that the process further comprises the step of controlling the continuous transfer of solid olefin polymer particles from said first loop reactor to said second loop reactor by at least one continuously open settling leg, wherein the step of controlling the continuous transfer of solid olefin polymer particles from said first loop reactor to said second loop reactor by at least one continuously open settling leg comprises the step of controlling the number of continuously open settling legs and/or the step of controlling the flow rate of said at least one continuously open settling leg. - introducing one or more olefin reactants, diluents and polymerization catalyst into said first loop reactor, and while circulating said olefin reactants, diluents and polymerization catalyst in said first loop reactor; - polymerizing said one or more olefin reactants to produce a polyolefin slurry comprising liquid diluent and solid olefin polymer particles; - withdrawing polyolefin slurry comprising solid olefin polymer particles and diluent from said first reactor and introducing the withdrawn particles into the second loop reactor, by means of one or more settling legs provided on said first reactor, wherein each settling leg has an inlet connected to the first reactor and an outlet connected to the second reactor by means of a transfer line, - characterized in that at least one settling leg is continuously open allowing continuous transfer of solid olefin polymer particles from said first loop reactor to said second loop reactor, and in that the process further comprises the step of controlling the continuous transfer of solid olefin polymer particles from said first loop reactor to said second loop reactor by at least one continuously open settling leg, wherein the step of controlling the continuous transfer of solid olefin polymer particles from said first loop reactor to said second loop reactor by at least one continuously open settling leg comprises the step of controlling the number of continuously open settling legs and/or the step of controlling the flow rate of said at least one continuously open settling leg.