A vapor compression system of the type having in serial refrigerant flow relationship a compressor (17,18), a heat rejection heat exchanger, an expansion device and an evaporator, comprising: a flash tank economizer disposed in serial flow relationship between the heat rejection heat exchanger and the expansion device said flash tank economizer including: a flash tank; a first flow control device disposed between the heat rejection heat exchange and said flash tank; an economizer vapor line to fluidly interconnect said flash tank to a mid-stage of the compressor; and a second flow control device disposed in said economizer vapor line; characterised by a third flow control device fluidly interconnecting said economizer vapor line to an inlet of said compressor (17,18) such that during periods in which the pressure in said flash tank equals or exceeds the critical pressure of the refrigerant, said second and third flow control devices (28,43) may be opened to thereby drain refrigerant from said flash tank to thereby reduce the pressure to a subcritical condition; a fourth flow control device disposed within the economizer vapor line at a point between the compressor mid-stage and the point in which the third flow control device is fluidly connected to said economizer vapor line such that during the refrigerant draining process, the fourth control device can be closed in order to prevent the refrigerant from entering the compressor mid-stage; and a controller to control said second flow control device to prevent flow in said economizer line when pressure in said flash tank equals or exceeds the critical pressure of the refrigerant or when a mid-stage pressure of the compressor is greater than the pressure in the flash tank. a flash tank economizer disposed in serial flow relationship between the heat rejection heat exchanger and the expansion device said flash tank economizer including: a flash tank; a first flow control device disposed between the heat rejection heat exchange and said flash tank; an economizer vapor line to fluidly interconnect said flash tank to a mid-stage of the compressor; and a second flow control device disposed in said economizer vapor line; a flash tank; a first flow control device disposed between the heat rejection heat exchange and said flash tank; an economizer vapor line to fluidly interconnect said flash tank to a mid-stage of the compressor; and a second flow control device disposed in said economizer vapor line; characterised by a third flow control device fluidly interconnecting said economizer vapor line to an inlet of said compressor (17,18) such that during periods in which the pressure in said flash tank equals or exceeds the critical pressure of the refrigerant, said second and third flow control devices (28,43) may be opened to thereby drain refrigerant from said flash tank to thereby reduce the pressure to a subcritical condition; a fourth flow control device disposed within the economizer vapor line at a point between the compressor mid-stage and the point in which the third flow control device is fluidly connected to said economizer vapor line such that during the refrigerant draining process, the fourth control device can be closed in order to prevent the refrigerant from entering the compressor mid-stage; and a controller to control said second flow control device to prevent flow in said economizer line when pressure in said flash tank equals or exceeds the critical pressure of the refrigerant or when a mid-stage pressure of the compressor is greater than the pressure in the flash tank.