Put the lima beans in a kettle with the water. Bring to a full boil and boil for 2 minutes. Remove from heat and allow them to stand, covered, for 1 1/2 hours. Then add the ham hocks, onions, and celery. Season with brown sugar, mustard, and salt. Cover and simmer for 2 hours. Remove meat from bones, keeping it in large pieces. Skim excess fat from top of sauce. Return meat to kettle.