A valve comprising: a body; and a flow control assembly inside the body, the flow control assembly including: a ball; a seat installed on the ball, the seat biased away from the ball by springs; a counterseat; wherein the seat is biased away from the ball and toward the counterseat by springs; wherein the ball is rotatable between an open position and a closed position to control flow through the body, the seat and the counterseat include mating surfaces that engage one another when the ball is in the closed position such that the seat seals against the counterseat within the body along the mating surfaces, the mating surface of the counterseat is a concave surface, the mating surface of the seat is a convex surface, and at least one of the mating surfaces has a toroidal contour; characterized in that the flow control assembly further comprises a cap configured to retain the seat on the ball within the body, the cap configured to rotate with the ball between the open position and the closed position and to contact the counterseat causing the counterseat to push the cap radially inward on the ball, driving the seat radially inward against the biasing springs and apart from the counterseat. a body; and a flow control assembly inside the body, the flow control assembly including: a ball; a seat installed on the ball, the seat biased away from the ball by springs; a counterseat; wherein the seat is biased away from the ball and toward the counterseat by springs; wherein the ball is rotatable between an open position and a closed position to control flow through the body, the seat and the counterseat include mating surfaces that engage one another when the ball is in the closed position such that the seat seals against the counterseat within the body along the mating surfaces, the mating surface of the counterseat is a concave surface, the mating surface of the seat is a convex surface, and at least one of the mating surfaces has a toroidal contour; characterized in that the flow control assembly further comprises a cap configured to retain the seat on the ball within the body, the cap configured to rotate with the ball between the open position and the closed position and to contact the counterseat causing the counterseat to push the cap radially inward on the ball, driving the seat radially inward against the biasing springs and apart from the counterseat. a ball; a seat installed on the ball, the seat biased away from the ball by springs; a counterseat; wherein the seat is biased away from the ball and toward the counterseat by springs; wherein the ball is rotatable between an open position and a closed position to control flow through the body, the seat and the counterseat include mating surfaces that engage one another when the ball is in the closed position such that the seat seals against the counterseat within the body along the mating surfaces, the mating surface of the counterseat is a concave surface, the mating surface of the seat is a convex surface, and at least one of the mating surfaces has a toroidal contour; characterized in that the flow control assembly further comprises a cap configured to retain the seat on the ball within the body, the cap configured to rotate with the ball between the open position and the closed position and to contact the counterseat causing the counterseat to push the cap radially inward on the ball, driving the seat radially inward against the biasing springs and apart from the counterseat.