Place celery, potatoes, onion and water in a saucepan. Add salt and curry powder.
Cover the saucepan and bring to a boil. Reduce heat to medium and simmer until vegetables are tender, about 20-25 minutes.
Blend vegetables in a blender. Return to saucepan, add butter and lemon juice. Cover and cook for 3-5 minutes. If it's too thick, add more water.