Resilient seated butterfly valve with interchangeable off-center and on-center discs

A butterfly valve comprising:

• (a) a body having a rotatable shaft aligned with a vertical center-line of the body;
• (b) a circumferential seal affixed to a circumferential body seat;
• (c) an on-center disc affixed to a shaft and an off-center disc affixed to a shaft, the on-center disc and the off-center disc interchangeable with one another;
• (d) the circumferential seal surface comprising (i) a section of a sphere, the center of which is located on the intersection of the vertical center-line extending through shaft and the horizontal center-line extending through the horizontal center of a disc or;
• (ii) an off-center seal region proximal to the inlet port;
• and (ii) an on-center seal region distal from the inlet port;
• (e) the on-center disc comprising (i) a spherical sealing edge, the center of which is located on the intersection of the vertical center-line extending through shaft and the horizontal center-line extending through the horizontal center of the disc;
• (ii) the spherical sealing edge configured for closure with the on-center seal region;
• and (iii) a front face of the on-center disc;
• and
• (f) the off-center disc comprising (i) a spherical sealing edge, the center of which is located on the intersection of the vertical center-line extending through shaft and the horizontal center-line extending through the horizontal center of the disc;
• (ii) the spherical sealing edge configured for closure with the off-center seal region;
• and (iii) an eccentric front face of the off-center disc, the front face of which extends beyond the front face of the on-center disc.