Establishing Recommended Guidance for Local …...PWHT time and temperature parameters be consistent with the original Code of construction and in accordance with a written procedure

11

Establishing Recommended Guidance for Local Post Weld Heat Treatment Configurations Based on

Thermal-Mechanical Finite Element Analysis

ASME 2016 Pressure Vessels and Piping Conference July 17-21, 2016

Vancouver, British Columbia, Canada

Phillip E. Prueter, P.E.Brian Macejko

The Equity Engineering Group, Inc.

2

2 © 2016 E2G | The Equity Engineering Group, Inc. All rights reserved

INTRODUCTION • Post weld heat treatment (PWHT) is an effective way to minimize weld

residual stresses in pressure vessels and piping equipment. • PWHT is required for carbon steels above a Code-defined thickness

threshold and other low-alloy steels to mitigate the propensity for crack initiation and ultimately, brittle fracture, and is often employed to mitigate stress corrosion cracking due to environmental conditions.

• Performing local PWHT following component repairs or alterations is often more practical and cost effective than heat treating an entire vessel or a large portion of the pressure boundary.

• Spot or “bulls eye” configurations are often employed in industry to perform PWHT following local weld repairs to regions of the pressure boundary.

• Both the ASME Boiler and Pressure Vessel (B&PV) Code and the National Board Inspection Code (NBIC) permit the use of local PWHT around nozzles or other pressure boundary repairs or alterations.

• Additionally, Welding Research Council (WRC) Bulletin 452 offers detailed guidance relating to local PWHT and compares Code-based methodologies for implementing local PWHT on pressure retaining equipment.

3


BACKGROUND INFORMATION• While spot PWHT may be appropriate in certain cases, if the soak, heating,

and gradient control bands are not properly sized and positioned, it can lead to permanent vessel distortion or detrimental residual stresses. – These residual stresses can increase the likelihood of in-service crack initiation

and possible catastrophic failure due to unstable flaw propagation. • It is essential to properly engineer local or spot/bulls eye PWHT

configurations to ensure that distortion, cracking of adjacent welds, and severe residual stresses are avoided. – Often times, this may require advanced thermal-mechanical finite element

analysis (FEA) to simulate the local PWHT process and to predict the ensuing residual stress state of the repaired area.

• This study investigates several case studies of local PWHT configurations where advanced, three-dimensional FEA is used to simulate the thermal-mechanical response of the repaired region on a pressure vessel and to optimize the most ideal local PWHT arrangement. – Local plasticity and distortion are quantified using non-linear FEA.

• Commentary on ASME and NBIC Code-specified local PWHT requirements is rendered based on detailed non-linear FEA results, and recommended good practice for typical local PWHT configurations is provided.

4


CONSEQUENCES OF IMPROPER LOCAL PWHT

5


LOCAL PWHT TERMINOLOGY• Soak Band (SB): The region of metal that must

be heated to the recommended PWHT temperature. This typically includes the weld, heat affected zone (HAZ), and a portion of the base metal near the weld deposit.

• Heated Band (HB): The surface area over which a heat source is applied to achieve the required PWHT temperature in the soak band while also limiting temperature gradients to limit harmful stresses. This region physically consists of the soak band plus an additional area of base metal.

• Gradient Control Band (GCB): The surface area where insulation and possible heat sources are placed to limit potentially harmful metal temperature gradients (consists of soak band, heated band, and additional base metal).

From AWS D10.10:

6


DESIGN AND POST-CONSTRUCTION CODE GUIDANCE

• WRC Bulletin 452 • AWS D10.10/D10.10M:1999 (R2009)• NBIC: Part 3 - Repairs and Alterations• API 510: Pressure Vessel Inspection Code:

In-Service Inspection, Rating, Repair, and Alteration

• ASME Section VIII Division 1 • ASME Section VIII Division 2• ASME Section III Division 1 - Subsection NB• Australian Standard 1210: Pressure Vessels• British Standard PD 5500: Specification for

Unfired Fusion Welded Pressure Vessels• ASME PCC-2-2015: Repair of Pressure

Equipment and Piping

From NBIC:

7


SB, HB, AND GCB SIZING CODE GUIDANCE

• The table shown summarizes the recommended guidance from different design and post-construction Codes.

• Note: Recommended HB temperature gradient is to achieve no less than 50% of the soak band temperature at edge of heated band per WRC 452 guidance.

• In general, specific guidance and justification for bulls eye PWHT configurations is not explicitly provided.

8


DESIGN AND POST-CONSTRUCTION CODE GUIDANCE• ASME Section VIII Division 1 and Division 2 state that: spot or bulls eye

local heating is permitted provided that other measures (based upon sufficiently similar documented experience or evaluation) are taken that consider the effect of thermal gradients, all significant structural discontinuities (such as nozzles, attachments, head-to-shell junctions), and any mechanical loads which may be present during PWHT.

• API 510 states that: Local PWHT may be substituted for 360° banding provided that the application is reviewed, and a procedure is developed by an engineer experienced in the appropriate engineering specialties.

• ASME PCC-2 references WRC Bulletin 452 and API 510 and states that: evaluation of spot or bulls eye areas usually require the use of FEA performed by an engineer experienced in the evaluation of heat treating procedures.

• NBIC doesn't explicitly identify the need for evaluation of local PWHT configurations, although it does reference AWS D10.10 and state that: local PWHT time and temperature parameters be consistent with the original Code of construction and in accordance with a written procedure acceptable to the Inspector and when required, by the jurisdiction.

9


JUSTIFICATION FOR BULLS EYE LAYOUTS• What is the general recommendation?

– It is crucial to evaluate bulls eye configurations with FEA, particularly near structural discontinuities.

– Often times the appropriate engineering justification for a local PWHT is not established by either the owner-user, the PWHT contractor, or a third party.

– Appropriate simulation of local PWHT becomes even more important for layouts near other structural discontinuities such as head-to-shell junctions, stiffening ring-to-shell junctions, and support saddle-to-shell junctions.

– FEA permits analysts to determine if undesirable residual stresses or distortion of the pressure boundary are likely for a given layout and also enables optimization of the PWHT procedure.

– Even if visible distortion is not present following PWHT, this does not rule out the possibility of detrimental residual stresses being induced from the procedure.

10


LOCAL PWHT NEAR A HEAD-TO-SHELL JUNCTION• Bulls eye PWHT of a nozzle-to-shell junction near a 2:1 semi-elliptical head (the

nozzle weld was subjected to field repairs during a turnaround). • PWHT is required due to service environment (not required due to thickness). • 3D, steady-state thermal -mechanical FEA is employed. • The carbon steel vessel is roughly 54 inches in diameter with a cylindrical shell

and head thickness of 7/16 inches. • Bulls eye and full circumferential local PWHT configurations are compared.• Bulls eye configuration:

– SB extends 1/2 inch beyond the toe of the nozzle fillet weld.– HB extends 𝟐𝟐 𝑹𝑹𝑹𝑹 in either direction beyond the SB. – GCB is represented by the surrounding insulated material. – Temperature gradient across the HB reflects less than a 50 percent drop from the SB

temperature. – The temperature profiles are idealized and assumed to be circular (in practice, the layout of the

heating coils is generally square or rectangular). • Full circumferential configuration:

– SB is assumed to equal a width of 6 times the shell thickness on each side of the fillet weld. – HB extends 𝟐𝟐 𝑹𝑹𝑹𝑹 beyond the SB in both directions. – The reminder of the region near the nozzle is assumed to be insulated (GCB).– Target soak temperature = 1175°F for both simulations.

11


ELEVATED TEMPERATURE MATERIAL PROPERTIES

• Elevated temperature yield and tensile properties are taken from ASTM DS11 which provides tensile data for carbon steels at temperatures above Code specified values: – Data up to 1200°F is

utilized in this case.• The yield and tensile

trends employed in the FEA for SA-516-70 carbon steel are plotted.

• Temperature dependent elastic-plastic materials properties are utilized in the simulations.

12


LOCAL PWHT CONFIGURATIONSGradient Control Band (insulated)Heated BandSoak Band

Gradient Control Band (insulated)Heated BandSoak Band

13


TEMPERATURE DISTRIBUTIONS DURING PWHT HOLD

• Bulls eye configuration/temperatures conform to NBIC guidance.

14


STRESSES DURING PWHT HOLD

• For the bulls eye configuration, equivalent stresses are on the order of 24 ksinear the head-to-shell weld location during PWHT.

• This poses a risk for crack initiation at this weld location during PWHT. The restraint of the head is evident when examining the non-symmetric stress distribution relative to the nozzle.

15


STRESSES AFTER PWHT (AMBIENT TEMPERATURES)• Residual stresses (after

PWHT) and equivalent plastic strains are plotted.

• For the full circumferential configuration, residual stresses and plastic strains are essentially zero. – This is mainly attributed to

the removal of restraint from colder adjacent material in the circumferential direction.

• For the bulls eye configuration, residual stresses are yield-level near the nozzle-to-shell junction. – There is significant

circumferential restraint from colder adjacent base metal.

16


LOCAL PWHT THAT RESULTED IN DISTORTION• Bulls eye PWHT of a nozzle-to-shell

junction (to accommodate a new LWN nozzle addition) remote from any major structural discontinuities.

• PWHT is required due to service environment (not required due to thickness).

• 3D, steady-state thermal-mechanical FEA is employed.

• Carbon steel vessel roughly 180 inches in diameter with a cylindrical shell thickness of 3/8 inches.

• Actual field PWHT configuration resulted in permanent plastic deformation.

– 3/4-7/8 of an inch maximum inward-to-maximum outward displacement measured (using straight edge).

– The goal is to reproduce the observed distortion in the FEA and compare results to more ideal PWHT configurations.

17


LOCAL PWHT CONFIGURATIONS• Multiple PWHT Configurations are Compared as follows:

– Target soak temperature = 1150°F for all simulations.– Temperature gradient across the HB reflects approximately 50 percent drop from the SB temperature.

• Case 1: Actual PWHT Configuration Employed in Field (resulting in permanent distortion of the shell):

– WRC 452 minimum limits for SB, HB, and GCB are extended by a factor of 2 in longitudinal/axial direction

– This configuration was inadvertently employed in the field.

• Case 2: WRC 452 minimum limits for nozzles in spherical shells:– SB extends roughly 6 times the shell plate thickness from the outside edge of the nozzle in both

directions– HB extends roughly 𝟐𝟐 𝑹𝑹𝑹𝑹 in either direction beyond the SB– GCB extends roughly 𝟐𝟐 𝑹𝑹𝑹𝑹 in either direction beyond the HB

• Case 3: Modified WRC 452 limits (2:1 circumferential extension):– WRC 452 minimum limits for SB, HB, and GCB are extended by a factor of 2 in circumferential direction

• Case 4: Full Circumferential Band:– SB extends roughly 6 times the shell plate thickness from the outside edge of the nozzle– HB extends roughly 𝟐𝟐 𝑹𝑹𝑹𝑹 beyond the SB– GCB extends roughly 𝟐𝟐 𝑹𝑹𝑹𝑹 beyond the HB

18


TEMPERATURE DISTRIBUTIONS DURING PWHT HOLD

19


RESIDUAL STRESS AFTER PWHT

20


PREDICTED PERMANENT DISTORTION AFTER PWHT

No residual

distortion

21


FULL CIRCUMFERENTIAL BAND OPTIMIZATION

• The full circumferential band configuration results in yield-level residual stresses near the nozzle-to-shell junction, even though SB and HB widths and temperature gradients conform to generally accepted guidance offered in numerous Codes and Standards.

• A FEA simulation with an ideal or optimized full circumferential band layout is carried out (dimensions given below).

• SB extends 7 inches beyond the toe of the nozzle weld in each direction• HB extends roughly 21 inches beyond the SB in each direction • GCB extends roughly 28 inches beyond the HB in each direction

Little-to-no residual

stress

22


COMPARISON OF RESIDUAL STRESSES

23


SUMMARY OF SIMULATION RESULTS• Elastic-plastic FEA predicts

displacements (1/2 - 5/8 of an inch) that reasonably agree with measured values for the actual configuration.

– This validates the computational techniques employed.

• The local bulls eye and the modified (2:1) bulls eye also produce residual plastic deformation.

• In general, increasing the circumferential extent of a local bulls eye configuration is known to mitigate induced residual stresses.

– In this case, extending the layout by a factor of 2 in the circumferential direction appears to be insufficient.

• The full-band layout is the best of the 4 cases (no distortion), but local (yield-level) residual stresses are present at the nozzle junction.

• Even full circumferential bands (that meet generally accepted guidance) can result in ineffective PWHT with detrimental residual stresses.

Residual Plastic Distortion for the Actual Configuration

(Deformation Scale Factor = 1)

• Typical recommended SB and HB widths that are a function of shell thickness may not always be appropriate, particularly for thin-walled components, or large thickness differences between the nozzle and shell.

24


GENERAL LOCAL PWHT RECOMMENDATIONS• It is recommended to always evaluate spot or bulls eye configurations using a

FEA-based approach prior to implementation, particularly for layouts near structural discontinuities.

– Even if visible distortion is not present following PWHT, this does not rule out the possibility of detrimental residual stresses being induced.

• Full circumferential PWHT configurations represent the preferred form of local PWHT (they also need to be properly engineered).

– Depending on vessel and nozzle geometry and thickness, additional analysis may be needed to size circumferential bands (generally accepted Code-based guidance is not always sufficient).

– For cases where this is not feasible, a circular bulls eye is not recommended for cylinders. – For bulls eyes, increasing the circumferential extend of the soak band relative to the axial extent is

sometimes an effective way to minimize residual stresses and reduce the restraint of the colder adjacent material, but analysis should be performed to confirm this and to optimize the layout.

• Permit unrestrained thermal expansion of the vessel or column. – This typically involves removing structural attachments, piping, and permitting at least one

support saddle to freely move for horizontal vessels. Failure to do this can result in distortion or cracking of attachment welds.

• All pressure boundary attachments near repair welds such as lugs, supports, or nozzles should be heated to the desired soak temperature to minimize differential thermal expansion.

25


GENERAL LOCAL PWHT RECOMMENDATIONS (CONT.)• Consider all mechanical loads, including weight, prior to any PWHT.

– For columns, self weight can cause buckling of the shell due to axial dead loads. For local heating near the support skirt, a buckling check for the skirt should be performed as well.

– For horizontal vessels, stresses near the mid-span should be checked, and for PWHT near the middle of the vessel, added support may be required to prevent permanent shell bowing.

• Maintain approximately constant temperatures through discontinuities such as nozzles, reinforcing pads, or attachments that are in the SB, HB, or GCB regions.

– This often means extending heating elements to encompass adjacent components. Failure to achieve this has been known to cause cracking of attachment welds.

• While performing FEA-based simulations of local PWHT layouts during plant turnarounds and outages may be difficult due to timing constraints, an improper local PWHT that may cause distortion or elevated residual stresses:

– This can ultimately result in more costly equipment downtime to accommodate additional fitness-for-service evaluations to address inadvertent damage or component repairs.

• ASME/NBIC bulls eye configurations are not inherently effective in achieving the desired outcomes of PWHT. In fact, they can induce severe residual stresses if not properly engineered.

26

Corporate Headquarters20600 Chagrin Boulevard, Suite 1200Shaker Heights, OH 44122

Satellite OfficesHouston, TXVictoria, TXAlberta, Canada

216.283.9519www.EquityEng.com

Corporate Headquarters20600 Chagrin Boulevard, Suite 1200Shaker Heights, OH 44122

Satellite OfficesHouston, TXVictoria, TXAlberta, Canada

216.283.9519www.EquityEng.com

Phillip E. Prueter, P.E.Principal Engineer

The Equity Engineering Group, Inc.Shaker Heights, Ohio [email protected]

Documents

Establishing Recommended Guidance for Local …...PWHT time and temperature parameters be consistent with the original Code of construction and in accordance with a written procedure