The ornamental design for an automatic drainvalve, as shown and described.
FIG. 1 is a perspective view of an automatic drain valve showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The portions of FIGS. 1-7 shown in a coarse dot pattern and outlined in dashed lines are included for the purpose of illustrating portions of the automatic drain valve and form no part of the claimed design.