How to Calculate Blind Flange Thickness for Hydrostatic Testing?

It is sometimes necessary to insert blind flanges or plates into piping systems when hydrostatically testing the pipes to prevent damage to equipment of a lower pressure rating or when isolating the plant before maintenance or entry. 

The thickness of a blind flange used depends upon the pressure and the pipe diameter and can be estimated from the equation given in ASME B31.3,

Tm = dg (3P/16SE)0.5 + C


T= minimum thickness (mm)
P = maximum test pressure (N/mm2)
SE = allowable stress (N/mm2)
dg = internal diameter of gasket used with slip plate (mm)
C = corrosion allowance* (mm)
*(only applicable if the line is spaded for a long period)

For a  typical stock plate, the allowable stresses are,

Carbon Steel (BS 4360:43A) - 165 N/mm2
Stainless Steel (BS 1449:316S16) - 173 N/mm2

The effect of this difference is negligible, and the lower figure should be taken to cover both cases.

It is important to note that blind plates must not have significant defects such as center punch marks.

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