An output buffer includes first and second input transistors, first and second output loads and a current source. The device comprises a flexible, rotatable drive shaft having an elongated proximal portion, an elongated distal portion, and an abrasive element mounted to the drive shaft between the elongated proximal and distal portions of the drive shaft and configured for rapid rotation together with the drive shaft. While the vessel is being filled with the particulate matter, the vessel also is subjected to a vibratory motion or at least one tamping motion, or the static electricity produced upon filling the vessel with the particulate matter is discharged. The subsurface safety valve further includes a hard seat positioned inside the tubular member, in which the hard seat defines a seating surface configured to receive a sealing surface defined on a bottom periphery portion of the curved flapper to form a sealing interface having a slope that varies along the sealing interface.