OSHA’s PRCS standard defines the terms Blinding and Blanking in the same definition, but there is actually a difference between these practices. Here is OSHA’s definition:
Blanking or blinding means the absolute closure of a pipe, line, or duct by the fastening of a solid plate (such as a spectacle blind or a skillet blind) that completely covers the bore and that is capable of withstanding the maximum pressure of the pipe, line, or duct with no leakage beyond the plate.
Here are better definitions that explain the major difference in these practices:
A blank is designed as a flat plate BETWEEN TWO FLANGES, typically inside the flange bolt pattern (ASME B31.3 Paragraph 304.5.3). The blank must be sized for the line’s full design pressure (maximum non-shock pressure rating).
A blind is designed as a bolted flat plate, which can be used to TERMINATE A PIPE LINE (ASME B31.3 paragraph 304.5.2b). When a blind is required, use the geometry specified in ANSI/ASME B16.5.
I like ANSI’s definition better:
Blinding/Blanking. Inserting a solid barrier across the open end of a pipe or in between two flanges leading into or out of the confined space and securing the barrier in such a way as to prevent leakage of material into the confined space.
