Currently, water bath canning is not recommended for vegetables, unless they are pickled. The added vinegar raised the PH level and allows foods to be water bath canned where they wouldn't normally be able to. However, if you are canning green beans in just water and salt, they need to be pressure canned. Many people say "my grandmother did it for 50 years and we are all fine" - and I agree. Feel free to use your judgement when canning - however, pressure canning is currently considered the only "safe" way to can green beans.