A mud valve is like a plug in the bottom of a tank. AWE valves are always grade 316ss unless something more exotic is required. They can have rising or non-rising stems. This photograph is of a non-rising model. Note the non-metalic threaded nut to prevent corrosion >and galvanic action.
There is more than meets the eye to a mud valve and it is all to do with a bloke called Bernoulli. Take your garden hose, turn it on full with the nozzle on fine spray. Now point it to a flat surface. When you get to within a couple of millimetres the nozzle will be sucked down onto the flat surface. That’s Bernoulli forces at work. These are an important consideration in mud valve design.