A top-entry trunnion ball valve comprising: a housing formed having first and second fluid flow ports, a cavity through which fluid flows when the valve is open and a top opening through which the cavity is accessible; a ball housed in the cavity formed having a through hole for passage of fluid and rotatable between open and closed positions; a cartridge in the cavity sealed to the housing and comprising a seat sealed to the ball; wherein the cartridge and top opening are sized to enable the cartridge to be inserted into and sealed to the housing and removed from the cavity via the top opening.