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Piping Line Blanks, Spacers, Spectacle Blinds (Figure-8s), and Restricting Orifices

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This file calculates design thicknesses, allowable pressures, dimensions, and weights of line blanks, spacers, spectacle blinds (figure-8's), and restriction orifices per ASME B31.3-2018, para. 304.5, Eq. (15).

Data are applied from ASME B31.3, ASME B16.48, ASME B16.5-2017, and ASME B16.47-1999.

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(PDF file) Sample Raised Face Spec Sheet

(PDF file) Sample Ring Joint Spec Sheet

Flange faces may be plain, raised face, or ring joint.

The pressure/temperature rating of a line blank (or line blind) should be the same as for the flange joint.

A spacer is required to have its thickness the same as the line blank or restricting orifice with which it is meant to be exchanged.

A spectacle blind (or figure-8) selection should consider space required and weight. Alternatively, Blind and spacer are normally chosen for sizes above NPS 12. Materials which are prone to rust should be evaluated for possibility they can accept suitable coating.

A restricting orifice is made like a spacer, except it has one or more holes which restrict flow. This item should show on the P&ID as "FE" (Flow Element) or "RO" (Restricting Orifice).

The restricting orifice should normally be made with thickness same as that required for a line blank. This is recommended in case the orifice is small enough to cause a high pressure drop or in case the orifice becomes plugged. An orifice does not have to be centered in the disk.

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