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TECHNOLOGY REVIEW

Comparative Study for Stresses in Nozzle and Flange Welds Generated During Conventional Pressure Testing and Local Pressure Testing Using Bolted Devices

[+] Author and Article Information
Ayman M. Cheta

Pressure Equipment Integritychetaam@sbcglobal.net

Richard Brodzinski

Plant Integrityrichardbrodzinski@yahoo.com.sg

PREFIS is a proprietary software for evaluating cracks and cracklike flaws. It was developed by a joint industry project funded by a consortium of major petroleum companies.

Critical crack size is defined as the crack size (depth and length) where the crack is expected to start unzipping rapidly under specific loading conditions (see Fig. 2).

J. Pressure Vessel Technol 129(4), 775-780 (Nov 22, 2006) (6 pages) doi:10.1115/1.2767372 History: Received December 09, 2005; Revised November 22, 2006

Weld repairs and alterations of pressure vessels and piping built to ASME codes may require pressure testing to prove the integrity of the weld and/or design. Conventional hydrostatic pressure testing requires filling an entire vessel or piping system with water and pressurizing it to the test pressure. In recent years, several designs were developed to employ bolted devices to perform local pressure testing of flange-to-nozzle, flange-to-pipe, and nozzle-to-shell attachment welds. Due to the cost and equipment downtime associated with performing a full conventional pressure test and the desire to reduce repair costs, several petrochemical companies adopted the use of such devices. The purpose of this paper is to compare the stress values and stress distribution associated with conventional and local pressure testing techniques. The advantages and disadvantages of both approaches are discussed and the conclusions are supported by a practical example.

Copyright © 2007 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Conventional hydrotest

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Figure 2

Pressure test with 16in. welded cap

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Figure 3

Pressure test with 16in. bolted cap

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Figure 4

Conventional pressure test of pipe-to-flange weld

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Figure 5

Pressure test of pipe-to-flange weld using bolted plug

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Figure 6

Effect of restricting pipe dilation at the flange

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Figure 7

Longitudinal stress at Ao−Ai(N∕m2) (Problem 1, Cases 1 and 2)

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Figure 8

Membrane+bending stress in the shell due to axial thrust in the nozzle

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Figure 9

Membrane+bending stress at Bo−Bi(N∕m2) (Problem 1, Case 1)

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Figure 10

Pressure testing using DN 400 (NPS 16) welded cap (Problem 1, Case 2)

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Figure 11

Membrane+bending stress at Bo−Bi(N∕m2) (Problem 1, Case 2)

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Figure 12

Pressure testing using bolted cap (Problem 1, Case 3)

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Figure 13

Longitudinal stress at Ao−Ai(N∕m2) (Case 3)

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Figure 14

Membrane+bending stress (N∕m2) at Bo−Bi (Problem 1, Case 3)

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Figure 15

Limiting flaw size envelopes for Problem 1, Cases 1 and 2

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Figure 16

Conventional pressure testing

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Figure 17

Longitudinal stress at Ao−Ai(N∕m2) (Problem 2, Case 1)

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Figure 18

Pressure test using a bolted plug

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Figure 19

Longitudinal stress at Ao−Ai(N∕m2) (Problem 2, Case 2)

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Figure 20

Limiting flaw size envelopes

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Figure 21

External surface crack sizing (circumferential crack on a cylindrical shell)

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