Abstract: In an engine cooling system wherein supply pipes 40, 50 supply cooling fluid (e.g.: air) to the turbine 20 from a compressor bleed 10, upon failure of one of the supply pipes 50 the backflow of cooling fluid from the turbine to the point of failure may be stemmed by use of a vortex amplifier 140, 160 connected into each supply pipe 40, 50, the tangential ports of the vortex amplifiers being interconnected by a balance pipe 180. This allows the total minimum bleed flow rate of cooling fluid supplied to the turbine from the compressor bleed to be reduced.