Page 60 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

2.10 CHAPTER TWO

TABLE 2.4 Grit to Microinch Conversion

Grit Microinch (µ in)
120 45–48
150 30–33
180 20–23
240 15–18
320 9–11

Finishes are often indicated as Ra (arithmetic mean roughness average, or roughness
average) and Rq, the equivalent of RMS (root mean square). Both of these are measured
in microinches and denote the smoothness of the surface. The smaller the number, the
finer the finish. The Ra reading is approximately 87.5 percent of the Rq (RMS) reading.
Other methods of expressing smoothness are centerline average (CLA) and arithmetic
average (AA).

Passivation
When the interior surface of stainless steel piping is required to have a very low rate of
corrosion, it must be made passive. This is accomplished by the formation of a thin surface
film that acts as a barrier to corrosion. The surface film (or passive film) is made thicker
by exposing it to oxidizing and chelant solutions or electropolishing. Oxidizing solutions
frequently used are nitric acid, ammonium persulfate, hydrogen peroxide, and citric acid.
Chelants are nontoxic organic acids or their viable salts and nontoxic synergizing agents.
ASTM A 380 suggests other passivation chemical combinations that have proven successful.

CORRUGATED STEEL PIPE

Corrugated steel pipe is available from 6 in (125 mm) to 96 in (2.66 m). It is fabricated
from flat steel that has been rolled into various shapes and impressed with grooves around
the circumference of the pipe, generally described as circular arcs connected by tangents.
Corrugations are measured by pitch (the dimension from crest to crest at right angles to the
corrugations) and depth.
Longitudinal pipe seams are riveted, welded, bolted, or have helical lock seams. Joints
are generally steel bands with a gasket under the sleeve and tightened by bolts inserted in
an integral angle. Fittings are usually made from straight piping and shop fabricated into
the desired shape.
Corrugated steel pipe shall conform to different AASHTO standards, depending on the
actual pipe material specification.

OTHER METALLIC PIPING MATERIALS

The chemical compositions of various metal piping materials selected for special corrosion-
resistant characteristics are given in Table 2.5.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com)
Copyright © 2009 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.