It is time to can green beans, a vegetable that must be pressure canned for safety. So you may be asked why a pressure cooker or a pressure saucepot cannot be used as a pressure canner. To be classified as a pressure canner, a pressure cooking vessel must hold at least four, upright, closed quart jars. That is because smaller pressure cookers or pressure saucepans heat too quickly and cool down too slowly to provide adequate heat for a canning process. Both heating and cooling down times are included in calculating the recommended process for killing the organism that causes botulism.
We recommend using a vessel that is designated for pressure canning. Pressure canners with a capacity of 16 quarts typically hold seven quart jars or eight or more pint jars. Pressure canners with a capacity of 22-23 quarts typically hold seven quart jars or as many as eighteen double stacked pints. Three manufacturers, Presto, Mirro, and All-American, make pressure canners in the USA.
For more information see HGIC 3025: Choose the Right Canner for Home Canning.