Chemical Engineering March 2019 - 34

Cover Story
pounds that can form acids (5-10 mpy)
* Inland locations exposed to a moderate
amount of precipitation or humidity, which
are considered moderately corrosive environments
(around 1-3 mpy)
* Dry rural environments, which usually have
very low corrosion rates (less than 1 mpy)
* Designs that trap water or moisture
in crevices that are more prone
to attack
FIGURE 4. Realtime radiography
(RTR) is performed on piping
to show the degree of CUI
damage that has occurred
codes or practices will be used to determine
and define that scope. These could include,
but are not limited to, the following standards
developed by the National Association
of Corrosion Engineers International (NACE;
Houston; www.nace.org) and the American
Petroleum Institute (API; Washington, D.C.;
www.api.org):
1. NACE SP0198 [3] details the variables to
consider. A key variable is the potential
source of water ingress, including: rainfall;
drift from cooling towers; condensate falling
from cold service equipment; steam
discharge; process liquids spillage; spray
from fire sprinklers; deluge systems and
washdowns; leaking heat tracing; groundwater;
and condensation on cold surfaces
after vapor-barrier damage
2. API 571 Section 4.3.2.3 [4] lists the following
critical factors:
FIGURE 5. RTR Images show
minimal OD corrosion (left)
and significant OD corrosion
(right)
* The physical location (industrial, marine or
rural); moisture (humidity), particularly designs
that trap moisture or when present
in a cooling tower mist; temperature; and
the presence of salts, sulfur compounds
and soil
* Marine environments, which can be very
corrosive (20 mpy), as are industrial environments
that contain acids or sulfur com*
Operating temperatures up to around
250ºF (121ºC) experience increased corrosion
rates. Above 250ºF, surfaces are
usually too dry for corrosion to occur except
under insulation (see Section 4.3.3 of
API 571)
With all this information, walkdowns can
commence. Site and NDE personnel will
benefit from meticulously planning around
the access points and techniques to be
used line by line and component by component.
Choosing how many locations per
line should have spot NDE is important.
Using modern digital technologies, the
walkdown information, the static equipment
information and drawings can all be
pre-loaded for NDE personnel to develop
reports with acceptance and rejection
comments as soon as the NDE is completed
in the field in real time.
General NDE selection
To minimize, if not eliminate, insulation removal,
realtime radiography (RTR) is a commonly
used pre-screening examination. After
the RTR results, digital or computed radiographic
(CR) testing is commonly performed
FIGURE 6. Computed radiography (CR) imagery is the
preferred method for visualizing CUI damage to smallerdiameter
pipes
34
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Chemical Engineering March 2019

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