shown is usually provided on the exterior of the container 11 to monitor the magnitude of the air pressure therein typically the air pressure in the container 11 is increased to a relatively high level such as approximately fourteen p s i for example the primary pressure relief valve 15 provided on the standpipe 12 and secondary pressure relief valve 17 provided on the container 11 prevent the magnitude of such increased pressure from exceeding a predetermined maximum value when the magnitude of the air pressure within the container 11 reaches a desired value the pressure control valve 28 is then opened as a result some of the pressurized from the source 20 a flows through the pressure control valve 28 into the product discharge line 26 and out through the product outlet 27 consequently the magnitude of the air pressure within the container 11 can be maintained at or near a desired value when it is desired to unload material from the container 11 one of the individual product discharge valves 25 is opened while the other individual product discharge valves 25 are left closed as a result product contained within the associated discharge section 11 a of the container 11 can flow through the opened individual product discharge valve 25 into the product discharge line 26 the flow of product through the product discharge line 26 is facilitated by the flow of air from the air inlet 20 through the check valve 21 and the pressure.