A wafer butterfly valve is designed to clamp between two flanges in your pipe work. Most wafer butterfly valves fit the majority of flange standards (click to see the different standards) out there (ours certainly do). The seat in the butterfly valve is what seals the valve in the line as the two faces of the flanges clamp against this and hold the valve in place.
Our wafer butterfly valves have a guide that comes off the top and in some sizes the bottom of the valve. These are exactly that, a guide for your bolts to go through. They do not form part of the clamping process.
Below is a short video showing some wafer butterfly valves clamped between flanges on a water truck. They are only 80mm valves but the principal is the same for all sizes.