Abstract: A security cap to be attached to a union nut of a piping union where the union nut is provided with at least one radially outwardly extending post where the security cap includes: a cylinder of friable material of selected wall thickness defining an internal chamber of selected diameter at least as great as the diameter of the union nut having an opening defined by an open end of the cylinder and adapted to receive the union nut where cover means are provided to cover the end of the cylinder opposite the opening; and elongate slot means cut in the cylinder sidewall means where the slot means extend from the open end of the cylinder and at an oblique angle relative to a line drawn parallel to the longitudinal axis of the cylinder where the slot is adapted to receive the union nut post to retain the union nut within the chamber defined by the cylinder; and means to retain the union nut within the chamber.