Re: [PipingDesign] Generic Leak Test Procedure

From: <Gopalan.Iyer@ril.com>
Date: Thu Jun 03 2004 - 05:25:00 EDT

I would like to share our plant procedure that may be of some help to you. Also I am furnishing some of our thoughts regarding hot taps for your reference.

Procedure:

First, the branch location is identified to meet the process objectives and avoid interference with any other part.

  Spectroscopic Techniques
  Pipeline metallurgy and cooling rates experienced by the   pipe surface due to gas flow are critical factors for   choosing an in-service pipeline welding procedure.   Traditionally, lab methods are used to analyze carrier   pipe chemistry. Filing samples from the pipeline are   obtained using the burring technique. Samples are then   sent to the laboratory for analysis. If a sample is   inadvertently contaminated or the CE value is too high,   then a different location had to be sought and new samples   collected and analyzed.

The external wall of the pipe is visually inspected to verify the location of all weld seams and any attachment that might interfere with the design.

Weld seams in the vicinity of the hot tap are MT inspected to check for cracks.

The pipe wall is checked ultrasonically to determine actual thickness and the presence of any laminations or subsurface defects that could affect the strength of the hot tap nozzle.

The branch assembly along with flange is then welded to the shell and pneumatically pressure tested.

A pneumatic test is to be used in conditions where the pipe temperature is above the boiling point of water that could be used for a hydrostatic test.

The reinforcing pad is then installed in 2 parts, or inserted in one part prior to welding of branch, MT inspected, and pneumatically pressure tested.

The hot tap valve is then installed and the complete assembly pressure tested.

The shell opening is cut using a hot tap machine, the hot tap cutter retracted, and the valve closed.

Since welding on the hot tap nozzle results in localized heating of the pipe, consideration must be given to dissipating the heat to avoid combustion of the fluid inside. If the wall is thin, burn through of the shell may have to be considered.

Welding on in-service pipelines requires weld procedure development and qualification, as well as a highly trained workforce to ensure integrity of welds when pipelines are operating at full pressure and under full flow conditions. Low hydrogen welding procedures are used exclusively to perform all welding associated with hot tapping, which greatly reduces the potential for delayed Hydrogen Induced Cracking (HIC). Low hydrogen welding procedures also substantially reduce the risk of burn-through.

  During actual welding, the branch to carrier pipe fillet   welds are examined using the dry magnetic powder process   following the root pass, hot pass and cap. Subsequent to   the completion of the weld, a wet contrast magnetic   particle inspection is carried out 12 hours after to check   for HIC indications.

  Hot tap coupons are also evaluated to confirm the quality   of the hot weld. Typically, an “L or 7” shaped weld   pattern is deposited on the coupon, immediately after   attaching the branch connection, following the same hot   welding procedure. Hardness survey data gathered from this   replicated branch weld evaluates and further validates the   welding parameters chosen. This critical evaluation   further ensures the permanence and integrity of each   assembly.

The most serious concern with any alteration of an existing pipeline is the risk of a failure due to brittle fracture. This concern is addressed in API 653 which limits hot tapping to only steels of known acceptable toughness or steels of unknown toughness that have a minimum pipe metal temperature at or above the exemption curve given in API 653.

This curve shows that the safe minimum pipe metal temperature at the time of hot tap increases with increasing pipe thickness. While this will prevent brittle fracture at the time of the hot tap, consideration must be given to the minimum pipe metal temperature subsequent to the hot tap if it can be lower in order to ensure that the pipe will remain safe from the risk of brittle fracture.

Hope this is in line with the query, you have raised.

Regards,
K. Gopalan,
Central Engineering services,
Reliance Petrochemicals,
India.

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                                        [PipingDesign] Generic Leak Test
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I'm working on a 16" lateral gas pipeline where the section will be hot-tapped. My client will be conducting the leak test on the section connecting the live line to the new section before coming into a regulating station. Does anybody have a generic procedures about this so that i could refer to?

Thanx in advance.


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