Considerations for Operating Pressure Vessels Below -20°F.pdf

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Considerations for Operating Pressure Vessels Below -20F

Rowe Bansch RVS Corp.

INDUSTRIAL REFRIGERATION CONSORTIUM

RESEARCH & TECHNOLOGY FORUM

MAY 2-3, 2012

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 -20F

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Design for MDMT to -20F 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 -20F



Design for MDMT below -20F is governed by Section UCS

Fig. UCS-66 shall be used to establish impact testing exemptions for listed carbon steels

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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

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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 pendulums 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

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Charpy Impact Test Results

FATT - Fibrous (shear) Appearance Transition Temperature

Here is a technician performing a Charpy impact test

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Figure UCS-66

Standard materials (Curve B) are only exempt to -20F

SA516-70N, SA333 Gr6 (Curve C & D) are exempt to -55F



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 -55F

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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 -155F

This is primary basis for what we often refer to as dual stamping

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Dual Stamping

Primary Rating of 250 PSI MAWP @ -20F to 300F

Secondary Rating of 87.5 PSI (35%) @ -60F



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 300F

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Recommendation

Specify dual stamping for all low side vessels

MAWP 250 PSI @ -20 to 300F and 87.5 PSI @ -60F



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

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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



NBIC Requirements

Revise calculations verifying the new MDMT in accordance with the R Certificate Holders Quality Control System

Re-ratings shall be in accordance with the original Code Addenda to which the pressure vessel was built

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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


Re-rating Documentation

R Certificate Holder Nameplate Specifying new MDMT and coincident pressure rating

Attachment to pressure vessel witnessed by Authorized Inspector

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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

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Re-rating

Operating temperature complies with MDMT

Complete documentation for Mechanical Integrity or Jurisdictional Inspection

Minimal cost compared to replacing pressure vessel

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Considerations for Operating Pressure Vessels Below -20°F.pdf

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