External Pressure
File: PVE-3473
Last Updated: May 18, 2011
By: LB
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External pressure (vacuum) calculations start off more complex than internal pressure calculations and once jackets or other sources of pressure are added the difficulty increases. The external pressure rating depends upon more variables and the failure mechanism is more difficult to understand. This article is only an introduction, but it also covers many of the areas of external pressure that we repeatedly have to explain. Common mistakes made with external pressure calculations are listed.
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Discontinuity Stresses in Flanged and Dished Heads
File: PVE-3101
Last Updated: Sept. 27, 2010
LB/BTV
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In ASME VIII-1 the design of spherical heads and cylindrical shells is based on formulas that calculate the actual stress in the component. The design is acceptable if the calculated stress is less than the allowable material stress.
Stress formulas are also found for flanged and dished (F&D) heads. However these formulas are based on experience and cannot be traced back to theoretical stress equations...
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Design Code used in Repair/Alteration/Used Vessels
File: N/A
Last Updated: Nov. 19, 2008
SM
When registering a used vessel or a repair/alteration, you must use the correct section, edition and addenda of the ASME Code.
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Loads on Flanges - The ASME Way
File: N/A
Last Updated: Sept. 9, 2008
LB
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ASME VIII-1 Appendix 2 provides a method of sizing flanges. The calculations use three loads - HT, HG & HD and two operating conditions - seating and operating. What are these loads, how are they calculated, and where are they applied to the flange?
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Origins of the ASME area replacement rules?
File: PVE-2461
Last Updated: Oct. 11, 2007
LB
The area replacement rules in the ASME code books have always interested me: You can cut a hole in a vessel as long as the nozzle attached to it replaces the lost area. How can this be a rational method of designing pressure vessels?
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Use of 44w in Pressure Vessels
File: N/A
Last Updated: Nov. 12, 2008
LB
Note: Only ASME can make code interpretations.
Use of Canadian Grade 44w in Pressure Vessels
More than a decade ago the then head review engineer of TSSA's pressure vessel division first told me that 44w was allowed in pressure vessels, pointing out the line in the IID code book showing CSA G20.41 38W. Today a TSSA field inspector is insisting that 44w is not acceptable for use in pressure boundaries on ASME VIII-1 pressure vessels. This memo is about a small vessel that was accidently made out of 44w materials.
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Weld Efficiencies
File: PVE
Last Updated: Dec. 2008
LB
Weld Efficiencies for ASME VIII-1 Vessels - Section UW-11(a)(5)(b)
Comment: Only ASME can make interpretations on the ASME VIII-1 Code
Introduction:
I have long struggled with the weld efficiencies presented in section UW of the ASME VIII-1 code. I have had more trouble with it than many other sections of the book combined. The ideas in this section are simple, but the ASME code written around it is anything but. Where ASME has not made the code readable, we must live with confused and diverging interpretations.
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Lethal Service - Quick Guide
File:PVE-3856
Date:June 23 2010
By:LB
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Requirements for lethal service are scattered through VIII-1, the code cases and the interpretations. ASME VIII-1 section UW-2 (2007 edition) has most of the requirements for lethal service. Two definitions of lethal service are provided in UW-2:
[A] vessel is to contain fluids of such a nature that a very small amount mixed or unmixed with air is dangerous to life when inhaled[.]
By "lethal substances" are meant poisonous gases or liquids of such a nature that a very small amount of the gas or of the vapour of the liquid mixed or unmixed with air is dangerous to-life when inhaled. For purposes of this Division, this class includes substances of this nature which are stored under pressure or may generate a pressure if stored in a closed vessel.
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It is up to the user to determine if a service is lethal. We at Pressure Vessel Engineering do not determine if the vessel service is lethal (but we might have opinions based on previous jobs).
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Unlisted Materials
File: File:PVE-4245
Last Updated: June 29 2010
By: LB
Background
The Canadian CRN registration system requires that all fittings used on a vessel or included in a registered piping system carry CRNs. To register the fittings, design validation based either or code calculations, finite element analysis or proof testing is required.
When a design is based on code listed materials, the code of construction provides allowable operating stress levels. If the design of the pressure containing item is simple, the regular code rules can be used and will supply a pass/fail judgement. If no code rules exist for a complex or unusual shapes, Finite Element Analysis (FEA) can provide the stresses which can be compared with the listed allowables for a pass/fail judgement.
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Flanged and Flued Expansion Joints
File:PVE-4305
Last Updated: June 22 2010
By: LB
ASME VIII-1 mandatory Appendix 5 provides guidelines for the design of flanged and flued expansion joints, but does not provide methods of calculating the stresses, fatigue life or spring rate. ASME Appendix 26 (and EJMA) provides rules to calculate these values, but the configuration of a flanged and flued expansion joint does not match that used in appendix 26 or EJMA.
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Hand Calculations - How To and How Not To - Rev 1
File: ASME/HandCalcs
Last Updated: July 19, 2010
LB
We at PVEng use hand calculations when we do not have a program or spreadsheet or when we want to create a spreadsheet and need verification. Hand calculations are respected more than programs for audits or code submission but in my opinion, they are much more likely to be wrong. The ideas presented here are designed to reduce the most common causes of hand calculation problems.
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Using the ASME VIII-1 Nozzle F Factor (UG-37)
File: N/A
Last Updated: Sept. 9, 2008
LB
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Summary:
The stresses around a nozzle located in a cylindrical shell are not the same in all directions. If a non-round nozzle is oriented in the correct direction, ASME allows us to take advantage of this.
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