The butterfly valve has head loss characteristics of a full bore valve. The design is based on use of an engineered disc of the same dia. as the bore of the pipe arranged to pivot such that when it is across the bore is closes off the flow path. When turned through 90o the disc provides minimum resistance to the flow. The valve is a quarter turn valve. The main variations for this valve are the methods of sealing the perimeter of the disc in its closed position. The simplest variation is to use an elastomeric lined bore which is an interference fit on the disc. The other variations are based on offsetting the disc plane from the axis of rotation allow the disc to close against a circular face seal such that the fluid pressure increases the seal effect. The butterfly valve has been developing such that for many duties it now provides optimum solution for a leak tight on-off valve supplanting the gate valve. The butterfly valve can be engineered as a small valve of 40mm bore and can be made for extremely large sizes upto 1600mm bore.