Mechanical Integrity for Industrial
Ammonia Refrigeration for Vessels
Current Practices
• What do you currently do for pressure vessel MI?
▪ RTF? Ignorance is bliss?
▪ Drive-by visual inspection?
▪ Annual inspection by an “expert”?
▪ Wall thickness testing: annually, five years, other?
▪ Other?
▪ Do you treat uninsulated vessels
differently than insulated vessels?
Ammonia vessel failure risks/modes
▪ External Corrosion (general & pitting)
▪ Especially Corrosion Under Insulation (CUI)
▪ Physical Impact
▪ Failure of support(s)
▪ Stress Corrosion Cracking (SCC)
▪ Internal Erosion/Corrosion
▪ Weld Failure
What evidence do we have of failure modes?
Vessel MI scope
• Primary
▪ Vessel
▪ Coating system
▪ Vessel supports
▪ Sight glasses
▪ Associated piping, flanges...
▪ Secondary
▪ Insulation system
▪ jacket, vapor retarder, insulation
Guiding principles
▪ Primary MI risk to piping – external corrosion
▪ Emphasizes visual inspections to identify presence of active
external corrosion or indications of CUI
▪ Visual inspection findings
▪ No active corrosion
▪ Minor corrosion (or damaged vessel surface coating)
▪ Other than minor corrosion
▪ Take appropriate action once corrosion is found
▪ Strive to maintain vessels in a corrosion-free state
Summary of Recommendations
▪ Annual inspection (visual)
▪ Uninsulated vessels – inspect for:
▪ Active corrosion (on vessel itself or ancillary attached equipment)
▪ Deterioration of vessel coating system
▪ Insulated vessels – indications of:
▪ Damage to insulation system
▪ Moisture incursion
▪ Other signs of CUI
▪ Risk level based
Summary of Recommendations
▪ Annual inspection (visual)
▪ Support structure
▪ Corrosion or deterioration of vessel support
▪ Ensuring proper vessel anchoring
▪ Foundation deterioration
▪ Process safety information verification
▪ ASME Tag
▪ Proper vessel labeling
▪ Proper relief device installation
Summary of Recommendations
▪ Annual inspection (visual)
▪ Leak checking – walk-thru free and clear of ammonia smells
▪ VIp applied to all (100%) system vessels annually
▪ Oil pots, too!
Bansch, “Mechanical Integrity of Pressure Vessels”, IRC R&T Forum (2005).
Summary of Recommendations
▪ 5 yr Independent inspection – (visual, NDE)
▪ Independent qualified (per Section 5.3.3) inspector performs all
annual inspection requirements
▪ Staff from corporate or another facility
▪ Outside professional (e.g. NDE, Refrigeration)
▪ NDE requirements:
▪ Required if CML or CMZ previously established
▪ Risk level 1 or 2: wall thickness measured required
▪ Risk level 3, 4, or 5: NDE required only if corrosion present
Risk levels – Insulated Vessels
Vessel Type
Paint/Coating
Operating Temperature
Transfer vessels
Unpainted/uncoated
Normally operating below ambient dew point
temperature but above 10F (-12C) and
frequently cycling in temperature
Vessel‡
Unpainted/uncoated
Below ambient dew point temperature but above
10F (-12C)
Transfer vessels and oil pots
Painted/coated
Normally operating below ambient dew point
temperature but above 10F (-12C) and
periodically cycling in temperature
Risk
1
2
3
Vessel‡
Unpainted/uncoated
Consistently operating below 10
F (-12C)
Vessel‡
Unpainted/uncoated
Consistently operating below 0
F (-18C)
4
Vessel‡
Painted/coated
Consistently operating below 0
F (-18C)
5
‡ e.g. surge drum, suction accumulator, recirculator)
NDE and pressure vessels?
▪ No explicit requirement to establish initial CMLs during
routine inspections in IRC recommendations
▪ Why? We’ve found damage from installing CMLs
outweighs benefits derived
Definitions – CML and CMZ
▪ CML (Condition Monitoring Location):
A localized area, approximated as a point, where
repeated mechanical integrity inspections/tests
are conducted.
Establishing CMLs on pressure vessels is not required as long
as prompt action is taken to arrest areas of active corrosion is
found during annual or independent inspections
Definitions – CML and CMZ
▪ CMZ (Condition Monitoring Zone):
Not a fixed point but a wider area or region of
a vessel identified for periodic mechanical
integrity assessment.
A
B
C
D
A CMZ could encompass an area where corrosion is
discovered. Often a CMZ is associated with areas of a vessel
that contain liquid-only, vapor/liquid, or vapor-only.
Horizontal recirculator (typ)
CMZs
Horizontal recirculator (typ)
CMLs
5
1
4
ASME
Notes
3
2
1
Required when internal
erosion is a risk
2
Potential external corrosion
risk when liquid temp is > 10°F
3
Potential external corrosion
risk when liquid temp is > 10°F
4
Optional
5
External corrosion risk
elevated at nozzle connections
(vessel shown with insulation removed)
When corrosion is discovered - classify
▪ Minor corrosion
▪ Corrosion does not cover entire
circumference or zone of a vessel and
▪ Absence of pitting corrosion
▪ Other than minor corrosion
▪ Corrosion covering entire circumference or zone of a vessel or
▪ Presence of pitting corrosion
Quiz – which is “minor corrosion”
(a)
(b)
Quiz – which is “minor corrosion”
(a)
minor corrosion
(b)
Assumed to be other than minor
due to presence of elastomeric insulation
Corrective action required
▪ Minor corrosion
1. Oxidized portions of the vessel or support structure must be
a) Cleaned
b) Corrosion arrested
c) Surface coating restored
2. Reinsulate (if applicable)
3. Return to service
4. Document
If corrective action is not taken,
▪ Condition monitoring zones (CMZs) are
established in locations coincident with
minor corrosion region or area
▪ Follow-up inspections conducted annually
at the CMZ
▪ Did the corrosion progress to “other than
minor corrosion?”
▪ When other than minor corrosion is found,
proceed according to required action
Corrective Action Required
▪ Other than minor corrosion
1. Suitable NDT technology must be used to measure vessel wall
thickness
2. Vessel wall must be greater than tmin (head and shell)
3. If fit for continued service,
a) Restore surface coating
b) Reinsulate (if applicable)
c) Return to service
d) Document
4. If not fit for continued service, schedule to replace promptly
Fitness for continued service?
Fitness for continued service?
Shell tmin
Head tmin
Minimum head thickness
after head forming
Reminder on your new vessels
▪ IIAR 2-2008 (Addendum A) – new requirements for
manufacture of pressure vessels
▪ 9.1.4 Heads of pressure vessels shall be hot formed or stress
relieved after cold-forming.
NOTE: It is recommended that high-side vessels receive
post-weld heat treatment per Appendix J (Informative).
▪ 9.1.6 In applications where vessels are subject to external
corrosion, the vessels shall be designed and specified with a
minimum of 1/16" [1.6 mm] corrosion allowance.
Uninsulated Pressure Vessel Inspection Summary
Conduct Visual
Inspection
External Corrosion
Found? (on vessel or
Document successful inspection and
restoration (if applicable)
No
support?)
Yes
Classify active corrosion
Extensive surface
coverage or pitting?
Arrest corrosion
& restore?
No:
(Minor Corrosion)
Yes
No
Re-inspect at next
interval
Yes: (Other than Minor Corrosion)
Evaluate whether vessel
is fit for continued service
Fit for service?
Schedule vessel for prompt
replacement & document
No
Arrest corrosion &
restore?
Yes
Yes
No
Establish CMLs/CMZs and reassess
annually
Insulated Pressure Vessel Inspection Summary
Conduct Visual
Inspection
Document successful inspection and
restoration (if applicable)
No
Insulation damage
found?
Yes
Insulation
damage
extensive?
Identify Vessel Risk Level
(See Table 2)
Risk level 1-2
Within 12
months, inspect
vessel surface
Risk level 3
Re-inspect at next
interval
Within 60
months, inspect
vessel surface
Fit for service?
Extensive surface
coverage or pitting?
Repair Insulation
Damage
Yes
Risk level 4-5
Within 36
months, inspect
vessel surface
No
No
Yes
Yes
No
Arrest active
corrosion & restore?
Yes
No
Establish CMLs/CMZs and reassess
annually
Schedule vessel for prompt
replacement & document
Questions?
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