Display omitted •Gas leakage from riser to bubbling fluidized bed via a loop seal is investigated.•Detailed description of the pressure situation in a CLC facility.•Tracer gas measurements with CO2 at hot conditions.•Evaluation of gas distributions from loop seals.•Operation and plant design proposals for minimization of gas leakage. Chemical Looping Combustion (CLC) is a technology to generate a pure stream of CO2 during the combustion of fossil or renewable fuels for power generation. This is carried out in a system of interconnected fluidized bed reactors, at least one so-called air reactor and a fuel reactor. Gas leakages are a major problem in those CLC systems as undesired gases in the exhaust can make the later sequestration more expensive and might lead to further gas cleaning. Therefore, the off-gas of the fuel reactor, where the fuel is mainly converted, should be as pure as possible. In those interconnected fluidized beds, the loop seals or so-called siphons should prevent the gas leakage from one side to the other. The present work focuses on the pressure conditions inside an experimental 25 kWth pilot scale fluidized bed reactor system for CLC to minimize gas leakage between the single reactors. The system consists of a riser air reactor and a two-stage bubbling fluidized bed fuel reactor system. By using CO2 as tracer gas at different positions, the gas distribution from the two loop seals through the system as well as the leakage flow from the air reactor via the cyclone and loop seal to the fuel reactor is analyzed. From a detailed experimental and sensitivity analysis of the system behavior in different operation conditions, measures to minimize gas leakage are developed. A key factor for the understanding of the gas leakages in the system was derived from pressure drop analysis of 23 pressure ports. Each process unit is examined and design criteria for such systems of interconnected fluidized beds are derived.