Page 59 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

PIPING 2.9

Stainless steel is available in various wall thicknesses. Pipe is commonly available from
schedule 5 to 80, and tubing from 0.028 to 0.188.

Stainless Steel Finishes
For stainless steel piping used in pharmaceutical, food-processing, chemical, and electronics appli-
cations, the interior and exterior of the piping are often required to be finished as required by
FDA, EPA, USDA, or other applicable codes. Finishing the exterior makes it easier to keep clean.
Finishing the interior will prevent the adherence of any solids, increase corrosion protection, and
shorten pipe interior cleaning procedures. Finishing can be abrasive, electropolished, or both.
Abrasive finishes are mechanically produced by polishing and wearing away of the
surface. This is often specified by a particular size or “grit” of the abrasive used, such as a
220 grit. This signifies the size of the abrasive passing through a specific size mesh. The
larger the number, the finer the finish. Electropolishing is an electrochemical process using
an electrical current to deposit metal from an anode to a cathode. Electropolishing, which
is the opposite of electroplating, removes surface metal from microscopic high points faster
than from low points. The metal to be polished is the sacrificial anode. Surface ions of iron
are removed leaving a chromium-rich surface resulting in a smooth, corrosion-resistant
pipe interior. Another method used to specify standard sheet and pipe exterior finishes uses
numbers 1 to 8. An explanation of sheet finishes is given in Table 2.3. Table 2.4 gives the
grit equivalent of microinch measurement.

TABLE 2.3 Table of Standard Sheet Finishes

Number Description
Unpolished 1 A rough, dull finish that results from hot rolling to the specified
(as rolled) thickness, followed by annealing and descaling.
finishes
2D A dull finish that results from cold rolling, followed by annealing
and descaling, and perhaps a final light roll pass through unpolished
rolls. This finish is used where appearance is of no concern.
2B A bright cold-rolled finish resulting from the same process as that
for number 2D finish, except that the annealed and descaled sheet
receives a final light roll pass through polished rolls. This is the
general-purpose, cold-rolled finish that can be used as is or as a
preliminary step to polishing.

Polished 3 An intermediate polished surface obtained by finishing with a
finishes 100-grit abrasive. This finish is generally used where a
semifinished polish surface is required and usually receives
additional polishing during fabrication.
4 A polished surface obtained by finishing with a 120- to 150-grit
abrasive, following initial grinding with coarser abrasives. This is
a general-purpose bright finish with a visible “grain” that prevents
mirror-like reflection.
7 A highly reflective finish obtained by buffing finely ground
surfaces, but not to the extent of removing the scratch pattern.
8 A reflective surface obtained by polishing with successively finer
abrasives and buffing extensively until all scratch patterns from
preliminary grinding are removed.
Source: Data from Finishes for Stainless Steel. Publication SS201-683-14M-EB, American Iron and Steel
Institute.

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