In a mixing bowl, combine rhubarb, sugar, and flour. Let sit while you prepare the crust.
For the crust, mix flour and salt in a bowl. Cut in shortening until mixture resembles coarse crumbs. Add water, 1 tablespoon at a time, until dough forms a ball.
Divide dough in half. Roll out one half to fit a 9-inch pie dish. Transfer dough to pie dish.
Pour rhubarb mixture into crust. Dot with butter. Roll out remaining dough to cover pie. Place over filling, seal and flute edges. Cut slits in top crust to allow steam to escape.
Bake at 450°F (230°C) for 15 minutes. Reduce heat to 350°F (175°C) and bake for 40-45 minutes longer or until crust is golden and filling is bubbly.