Abstract: A main flow line and relief valve assembly comprising: a main line fitted with a standpipe; a relief valve fitted within the standpipe; and a sleeve inserted in the standpipe below the relief valve, with a distal end of the sleeve configured to prevent aero-acoustic resonances of the deep cavity formed by the relief valve and standpipe assembly. The aero-acoustic resonance is prevented by separating the vortex shedding frequency associated with the free shear layer convecting across the mouth of the deep cavity and the acoustic cavity frequencies of the deep cavity and by disrupting the free shear layer at the mouth of the deep cavity such that the vertical structure does not remain coherent and/or does not impact upon the downstream edge of the cavity mouth.