23376514-10-procedure-for-pwht.pdf

7/27/2019 23376514-10-PROCEDURE-FOR-PWHT.pdf

1/5

LARSEN & TOUBRO LIMITEDPAGE NO.- 1 OF 5

DATE:-03.11.07

MSQ (EPCC-1) UPGADATION

PROJECT REV.NO.- 0

PROCEDURE FOR POST WELD HEAT

TREATMENT

PROCEDURE NO.-LT/MSQ/P/PWHT/010

CONTENTS:

1.0 PURPOSE

2.0 REFERENCE DOCUMENT

3.0 PWHT

4.0 EQUIPMENTS MATERIALS

5.0 INSPECTION

6.0 SAFETY

APPROVED BY L & T REVIEWED / APPROVED BY JACOBS


2/5


DATE:-03.11.07


PROJECT REV.NO.- 0


TREATMENT


1.0 PURPOSE:

This procedure provides information; method and control for Post Weld Heat

Treatment (PWHT) of AG/UG piping at IOCL Panipat Refinery MSQ Project.

2.0 REFERENCE DOCUMENT:

ASME B 31.3 (Latest revision)

ABS Spec. SW-231 Approved WPS

Approved Welding specification charts

3.0 POST WELD HEAT TREATMENT (PWHT)

3.1 The Heat Treatment is performed by electrical resistance heating coils or by induction.

Heating elements of NI-Cr wire of suitable gauge, insulated with porcelain beads, shall

be wrapped all round the weld portion and heated electrically. The method is by

locally heating a circumferential band including the entire weld and adjacent area of

base metal.

3.2 Heating elements are wrapped with an overlap of 75mm either side of weld with

sufficient mineral wool to shield the heat. Heat is controlled by varying the electrical

energy manually and / or using timer control. Width of the heating band shall be six

times the wall thickness of thickest part for girth butt welds.

3.3 Width of the insulation shall be 12 times the thickness of thickest part.

3.4 Care shall be taken so that exposed pipe surface temperature shall not exceed 50% of

the peak temperature or 400 C whichever is minimum.

3.5 Weld joints shall be free from grease, oil, etc. prior to PWHT.

3.6 Before PWHT, Line No. & Joint No., dia, type of joint, Recorder No., Calibration

date, Starting time and date etc. shall be written on the SR chart, duly signed and

sealed by L&T QC Engineer. After completion of PWHT , recorders shall be opened

presence of L&T QC Engineer and charts verified with joints for PWHT.

3.7 PWHT shall be carried out at a temperature range as given below.



3/5


DATE:-03.11.07


PROJECT REV.NO.- 0


TREATMENT


Description To be Followed at site Requirement as per ASME B 31.3

Heating Rate 150 10 200 C / Hr Max.

Cooling Rate 150 10 200 C / Hr Max.

Holding Temperature 620 10 593 C to 649 C

Holding Time1 hr / 25mm

(Minimum 1hr)1 hr / 25mm (Minimum 1hr)

3.8 HEATING AND COOLING RATES

3.8.1 Heating and cooling rate shall be as per 3.7 above.

3.8.2 Whenever not specified, the heating rate above 300 C, and cooling rate after soaking

up to 300 C , shall be followed.

3.8.3 Controlled heating shall be started from 300 C and controlled cooling shall be up to

300 C

3.8.4 Corrective action should be adopted during power interruption and shall be decided on

case to case basis.

3.8.5 HOLDING TIME

3.8.6 Holding / Soaking time shall be as per 3.7 above.

3.8.7 The following guidelines shall be used to determine the thickness and subsequent

selection of the soaking time of PWHT.

For butt welds, the thickness shall be the thickness of the material at the weld.

For fillet welds, the thickness shall be the throat thickness.

For branch welds, the thickness shall be governed as per relevant clauses of B

31.3 For combination of different welds in a component, the maximum in the

definition given above shall govern.

3.9 HARDNESS TEST

3.9.1 Maximum hardness value of welds after PWHT shall be as per specification.



4/5


DATE:-03.11.07


PROJECT REV.NO.- 0


TREATMENT


3.9.2 Hardness testing shall be carried out using Portable Telebrinell / Poldi / or any PMC

approved harness tester.

3.9.3 Whenever hardness value is more than 200 BHN, re-PWHT shall be carried out as per

this procedure.

3.9.4 Hardness Value after PWHT shall be recorded for each weld in the approved format.

LT/PNCP/AU/PWHT-HARD/006

3.9.5 One hardness test per weld shall be performed on weld metal and Heat affected zone.

3.10 NUMBER OF THERMOCOUPLES

SIZE OF PIPENO OF

THERMOCOUPLES

POSITION OF

THERMOCOUPLES

Up to 4 dia One Bottom

Above 4 8 dia Two Top and bottom (180 apart)

Above 8 24 dia Three 120 apart

Above 24 dia Four 90 apart

4.0 EQUIPEMENTS / MATERIALS

4.1 TEMPERATURE RECORDERS

The recorders shall be six-pointer suitable for 1000C & 1200C range, having

continuous recording chart having copper lead with colour dots. Each recording

instrument will give information and immediate indication of a thermocouple open

circuit condition. Calibration certificate for each recorder shall be available for review.

Revalidation of calibration shall be carried out at site once in every 6 months.

4.2 TEMPERATURE CONTROL SYSTEMThe temperature control system shall guarantee a post weld heat treatment within the

limits of the specification.

4.3 THERMOCOUPLES EXTENSION CABLE

Copper constantan compensating cables shall be used. The Copper lead (+ve) shall be

connected to the Chromel Conductor (non-magnetic) and the Nickel Lead (-ve) shall

be connected to the Alumel Conductor (magnetic)



5/5


DATE:-03.11.07


PROJECT REV.NO.- 0


TREATMENT


4.4 THERMAL INSULATION

The insulation consists of mineral wool, wrapped around heating elements. The

insulation shall extend a minimum distance of 150mm beyond the outer edges of

heated band. The total insulation covering will be such as to provide minimum heat

loss.

4.5 THERCOUPLE

All thermocouples shall be of the Chromel Vs. Alumel type, type K . thermocouples

shall be fixed using Capacitor discharge Method. Calibration reports of thermocouples

shall be submitted for review prior to start of work.

5.0 INSPECTION & DOCEMENTATION

Documents shall be maintained in format No. LT/MSQ/PWHT-HARD/024

6.0 SAFETY

All safety precautions shall be taken as per safety requirements of the project. Proper

permit shall be obtained as required, prior to start of job.
