Considerations for Operating
Pressure Vessels Below -20°F
INDUSTRIAL REFRIGERATION CONSORTIUM
RESEARCH & TECHNOLOGY FORUM
MAY 2-3, 2012
Rowe Bansch – RVS Corp.
ASME Section VIII Design
Standard Design Parameters for NH3
Refrigeration Pressure Vessels


Maximum Allowable Working Pressure (MAWP)
of 250 PSI
Minimum Design Metal Temperature (MDMT)
of -20°F
1
ASME Section VIII Design
Design for MDMT to -20°F is governed by
Paragraph UG-20

Impact testing is not required for carbon
steel pressure vessel materials up to 1” thick
at design temperatures as low as -20°F
Considerations for Operating Pressure Vessels Below -20°F
ASME Section VIII Design
Design for MDMT below -20°F is
governed by Section UCS

Fig. UCS-66 shall be used to establish impact
testing exemptions for listed carbon steels
Considerations for Operating Pressure Vessels Below -20°F
2
Considerations for Operating Pressure Vessels Below -20°F
ASME Section VIII Design
Figure UCS-66


Impact testing is required for a combination of
MDMT and material thickness below the curve
When required, Charpy V-notch impact tests
shall be made on weldments and all pressure
boundary materials
Considerations for Operating Pressure Vessels Below -20°F
3
Charpy Test Apparatus
NIST Charpy Verification Rig
University of Leeds
Charpy Test
The falling pendulum weight
strikes a 10 mm square
notched specimen
When struck, the specimen will
be broken or severely deform.
The pendulum’s travel distance
after striking the specimen is
measured on a gauge.
The difference between the
initial pendulum height and the
height after strike is related to
the energy absorbed by the
specimen
4
Charpy Impact Test Results
FATT - Fibrous (shear) Appearance Transition Temperature
Here is a technician performing a Charpy impact test
5
ASME Section VIII Design
Figure UCS-66


Standard materials (Curve B) are only exempt
to -20°F
SA516-70N, SA333 Gr6 (Curve C & D) are
exempt to -55°F
Considerations for Operating Pressure Vessels Below -20°F
ASME Section VIII Design
Figure UCS-66.1

When the coincident ratio (trE*/tn) is less than
one, Fig. UCS-66.1 provides a basis for a
further reduction in MDMT to as low as -55°F
Considerations for Operating Pressure Vessels Below -20°F
6
Considerations for Operating Pressure Vessels Below -20°F
ASME Section VIII Design
Paragraph UCS-66(b)(3)


When the coincident ratio (trE*/tn) is less
than or equal to 0.35, impact testing is not
required for MDMT as low as -155°F
This is primary basis for what we often refer
to as “dual stamping”
Considerations for Operating Pressure Vessels Below -20°F
7
ASME Section VIII Design
“Dual Stamping”


Primary Rating of 250 PSI
MAWP @ -20°F to 300°F
Secondary Rating of 87.5
PSI (35%) @ -60°F
Considerations for Operating Pressure Vessels Below -20°F
ASME Section VIII Design
“Dual Stamping”


Accepted practice for refrigeration vessels in
which the operating temperature is a function
of the pressure
Relief valve setting is based on the most
severe condition of coincident pressure and
temperature which is the 250 PSI @ -20 to
300°F
Considerations for Operating Pressure Vessels Below -20°F
8
ASME Section VIII Design
Recommendation


Specify “dual stamping” for all low side vessels
MAWP 250 PSI @ -20 to 300°F and 87.5 PSI
@ -60°F
Considerations for Operating Pressure Vessels Below -20°F
ASME Section VIII Design
Existing vessel operating below MDMT?



Often discovered during Mechanical Integrity
Inspection
Operating temperature has been lowered to
increase production or product quality
Original MDMT was not consistent with actual
operating temperature
Considerations for Operating Pressure Vessels Below -20°F
9
Reducing Original MDMT
National Board Inspection Code (NBIC)


Provides rules for inspection, repair and
alteration of vessels in service
Provides rules for decreasing the MDMT of
vessels in service
Reducing Original MDMT
NBIC Requirements


Revise calculations verifying the new MDMT
in accordance with the “R” Certificate
Holder’s Quality Control System
Re-ratings shall be in accordance with the
original Code Addenda to which the pressure
vessel was built
Considerations for Operating Pressure Vessels Below -20°F
10
Reducing Original MDMT
NBIC Requirements continued


Current inspection records verify the pressure
vessel is satisfactory for the proposed service
All NBIC requirements have been met to the
satisfaction of the Authorized Inspector and the
Jurisdiction at the location of the installation
Considerations for Operating Pressure Vessels Below -20°F
Reducing Original MDMT
Re-rating Documentation

“R” Certificate Holder Nameplate
 Specifying new MDMT and coincident pressure rating
 Attachment to pressure vessel witnessed by Authorized
Inspector
11
Considerations for Operating Pressure Vessels Below -20°F
Reducing Original MDMT
Re-rating Documentation

Form R-2 Report of Alteration
 Documenting new MDMT and coincident pressure
rating
 Design Certified by “R” Certificate Holder and
Authorized Inspector
 Certificate of Inspection by witnessing Authorized
Inspector
Considerations for Operating Pressure Vessels Below -20°F
12
Considerations for Operating Pressure Vessels Below -20°F
Reducing Original MDMT
Re-rating



Operating temperature complies with MDMT
Complete documentation for Mechanical
Integrity or Jurisdictional Inspection
Minimal cost compared to replacing pressure
vessel
Considerations for Operating Pressure Vessels Below -20°F
13
Considerations for Operating
Pressure Vessels Below -20°F
Questions?
14