Our 21,000 gallon tanks are equipped with 4 inch butterfly valves on the bottom of the front and back of the tanks, we also have a third 4 inch valve on the front of the tank about 2' off the ground to allow for decanting fluids. These valves are all 4 inch female NPTF. There is also a 3 inch male NPTF fill pipe in the front, which leads to the top of the tank. The 8,400 gallon tanks have the same inlet and outlets except the front valves are at floor level. The double-wall tank has only one 4 inch butterfly valve in the front.