Defect Analysis in Dish End and Nozzle Joint of Reactor Vessel · GMM Pfaudler Ltd. Elecon Engg. Co. Ltd. [email protected] [email protected] Abstract— The welding

Defect Analysis in Dish End and Nozzle Joint of Reactor Vessel

Prof. Purvi ChauhanDepartment of Production Engineering,

B.V.M. Engineering CollegeVallabh Vidhyanagar-388120, Gujarat, India

[email protected]

Dr. Amit TrivediDepartment of Production Engineering,


[email protected]

Prof. K. D. BhattDepartment of Production Engineering,


[email protected]

Mr. Hemant V. Suthar Mr. Yogesh R. Rana Planning Officer, Senior Engineer GMM Pfaudler Ltd. Elecon Engg. Co. Ltd. [email protected] [email protected]

Abstract—

The welding joints of chemical reactor vessel assume the significance as regards to protection of corrosion resistant glass lining and leakage of high pressure chemical species in and around the weld joint. The present study aims at understanding the influence of welding parameters of one such critical weld joint between dish end and nozzle of the reactor vessel. The influence of welding current and welding speed on welding defects is critically studied considering four independent cases. The two common defects observed in non destructive testing are porosity and slag inclusion. The study reveals the safe window of operating parameters for welding current and welding speed for the suggested welding procedure.

Key Words: Reactor Vessel, welding Defects, GTAW welding parameters, SMAW welding parameters

I. INTRODUCTION

A reactor vessel is a closed container designed to hold gases or liquids at a desired pressure and temperature for mixing or producing a desired chemical reaction. The main componentsof these reactor vessels are pan/mono-block, dish end and nozzle [1]. The assembly of nozzle to dish end is done bywelding operation which is very critical and it could lead to welding defects if process is not controlled as per welding procedure [2]. The joint geometry of the nozzle and dish end is single V- type butt joint. The major defects observed are porosity and slag inclusion. The significant welding parameters those are responsible for generating defects in welding are welding current, arc voltage, welding speed, gas flow rate and wind direction [3]. As current, voltage and speed govern the heat input predominantly they are considered for present analysis. The welding is carried out in an enclosure to restrain the influence of the wind. The tungsten electrode is properly

ground to avoid tungsten inclusion [4]. To find out the defects from the weld the non destructive testing methods namely Liquid Penetration Test (LPT) and Ultrasonic Test (UT) are deployed [5]. A case is presented to demonstrate the co-relationship of weld parameters with the weld defects.

II. BRIEF OVERVIEW OF REACTOR VESSEL

Figure - 1 Reactor vessel [6]The typical cross section of reactor vessel is shown in Figure-1.The basic components of the reactor vessel are pan/monoblock, top-dish, agitator, drive assembly and flush bottom

Side Bracket

Anchor Agitator

Bottom Outlet

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valve. The pan is used for holding the charge to be reacted, top dish has the nozzle for loading the charge, manhole cover for visual inspection and maintenance, agitator shaft hole and support is used for agitator drive assembly. The agitator is used for the homogeneous mixing of the reactive items in the pan. The drive assembly rotates the agitator and the flush bottom valve is for unloading the chemicals from reactor. The drive assembly contains gear box, electrical motor, muff coupling, mechanical seal as shown in Figure-1.

III. JOINT GEOMETRY OF DISH END WITH NOZZLE

Figure – 2 V- Joint geometry [6]

The typical joint geometry of the sample is shown in the Figure- 2[6]. The thickness for the plate is 16 mm and the included angle of the weld joint is 55°. The root face is 1.5 to 2 mm and the root gap suggested is 1.5 to 3 mm. In the reactor vessel the nozzle is joined to the dish end by single ‘V’ joint as shown in Figure - 3.

Figure - 3 V joint of dish end with nozzle [6]

The nozzle is joined to a swaged portion of the dish end. Here ‘V’ joint is used because it is difficult to weld the nozzle to the dish end from both the side as making a double ‘V’ joint is a difficult proposition. The ‘V’ joint of the surface is made by gas cutting torch and subsequently ground [6]. To ease weldingthe welding positioners are employed.The root pass is carried out by gas tungsten arc welding (GTAW) for sound initial pass and shielded metal arc welding (SMAW) is carried out for subsequent passes for faster and higher metal deposition.

IV. MAJOR DEFECTS IN THE WELDING

The major defects observed in the present case are porosity and slag inclusion [6-8].

(1) PorosityIt is a cavity type discontinuity formed by the gas entrapmentduring solidification. Porosity is formed in weld metal when dissolved gases present in the molten metal are entrapped due to their limited solubility limits at the room temperature [6-8]. The gases which may be present in the weld pool during welding include H2, O2, N2, CO, CO2, H2O, H2S, Ar, or He and H2, O2, N2 are considered soluble in molten weld pool to any significant extent. The hydrogen gas is considered to be the major cause of porosity in the welding of the materials such as low carbon steel and aluminum. The main reasons for porosity are high current and low travel speed, wind direction and improper shielding, unbaked electrode and improper initiation of re-striking electrode.

(2) Slag inclusionSlag inclusion is formed due to entrapment of oxides or non metallic solid material in the weld deposited between the weld metal and base metal. Due to their low specific gravity, the slag normally floats over the molten metal unless it is restrained [6-8]. Because of the stirring action of the arc the slag may be forced down below the molten metal and the high viscosity of the weld metal, rapid solidification at a low temperature may prevent the release slag inclusion which usually appears as a linear discontinuity or interrupted bonding. The main reasons for the slag inclusion are faster cooling rates, improper cleaning and improper bead geometry [6-8].

V. EXPERIMENTAL DETAILS

The joint is prepared as per the weld joint detail given in Figure- 2. The influence of current, speed and voltage arepresently studied on formation of defects [6].

1) Influence of the currentThe welding current is varied for root pass weld by GTAW in the range of 130-150 amps with 10 amp increments per step. The first pass of weld by SMAW is varied from 160-180 ampswith 10 amp increments. All the subsequent pass of welding is carried out in the range of 200-240 amps with increment of 10 amps each. As per standard welding procedure [2-6] & design of experiments carried out, the parametric details of experiment are shown in Table 1 and constant parameters are shown in Table 2.



Table 1: Variation of Welding Current for four cases

Table 2: The Constant parameters maintained for four cases [6]

Voltage (volts)

Speed (mm/min)

GTAW(root pass)

23 110

SMAW (Ø 4.0 mm)first pass

26 110

SMAW (Ø 5.0 mm)subsequent pass

26 185

Case 1: A liquid penetration test for the given case showed nodefect. The result of ultrasonic test is shown in Figure- 4 where x-axis represents the depth of specimen and the y axisrepresents the intensity level of the sound signal in dB. The first peak represents the back wall echo from the top of the surface. The intermittent peak at 5.8 mm from the bottom of the surface shows a flaw. The curve shown in the Figure- 4 is a limiting line crossing which the defect can be considered critical and as the peak is crossing the defect it is considered critical. The distance of defect is located at 100 mm from the right side which is introduced due to the wind flow of the environment. The wind flow has caused the rupture of shielding gas and caused air entrapment in the molten metal.

Figure 4 Ultrasonic test for case I [6]

Case 2: The liquid penetration test for the given case showed no defect ensuring the absence of surface defect. In ultrasonic test the defect was seen at 15.0 mm from the bottom of the surface. The defect was located at a distance of 150 mm and 230 mm respectively from the right side of the test piece. The defect at the 150 mm is introduced due to striking and re-striking of the electrode as the re-striking of electrode was noticed on examination. As the arc is blown off, air entrapment in the molten metal has caused the defect. The defect at the distance of the 230 mm is attributed to improper slag removal of earlier weld passes. In both the flaws observed in the ultrasonic test the echo signals are crossing the limiting curve and are defect of severity.

Case 3: In liquid penetration test for the given case no defect is seen ensuring the absence of surface defect. In ultrasonic test,the intermittent peak is observed at 5.8 mm from the top surface and another flaw is located at 4.8 mm respectively. Thedefects on test piece from the right side are respectively at 423mm and 322 mm. The defect at 423 mm showed porosity due to improper baking of the welding electrode. The defect at the distance of 322 mm is attributed to turbulence in the molten pool causing an air entrapment.

Case 4: In liquid penetration test for the given case no defect was seen ensuring the absence of surface defect. In ultrasonic test the intermittent peak is observed at 12.0 mm from thebottom and another flaw at 14.8 mm from the bottom. The distance of defect is from the right side of the test piece and is located at 175 mm and 280 mm respectively. The defect located at 175 mm is introduced due to higher current of the previous subsequent pass of the SMAW process. The bead geometry of the weld is not smooth so craters and under cuts are generated. These geometric defects not being ground correctly at the undercut, the flux has trapped in that under cut space. The defect at the 280 mm is attributed to faster cooling rate.

2) Influence of the speedWelding speed is the next parameter studied to understand the influence of it on welding defects. Welding speed is varied keeping welding current, gas-flow rate and arc voltage asconstant.The parametric details of experiment are shown in Table 3 and constant parameters are shown in Table 4 respectively. As per welding procedure [2, 6] and design of experiments carried out in the present case the welding speed is varied for root pass weld by GTAW in the range of 56 to 98 mm/min. The first pass of weld by SMAW is varied from 64 to 112 mm/min. All the subsequent pass of welding is carried out in the range of 130 to 172 mm/min.

Current in ampere Defect location in

mmProcess GTAW SMAW

Case ROOTpass

1st

passFilling passes

Case 1 130 160 200 5.8 mm from bottom (UT)

Case 2 140 170 220 15.0 from bottom & 5 mm from top (UT)

Case 3 140 170 230 5.8 mm & 4.8 mm from top(UT)

Case 4 150 180 240 12.0 mm & 14.8 from bottom(UT)



Table 3 Variation of Welding speeds for five cases [6]

Welding speed in mm/min

Defect location in mm

Process GTAW SMAWCase ROOT

pass1st

passFilling pass

Case 1 56 64 130 15 mm,13.8 mm &13.4 mm from topsurface (UT)

Case 2 70 75 135 110 mm & 150 mm from right (LPT)

Case 3 80 95 157 400 mm & 450 mm from right (LPT)

Case 4 90 105 164 125 mm from right(LPT)

Case 5 98 112 172 75 mm from right(LPT)

Table -4 The constant parameters maintained for five cases [6]

Current (ampere)

Voltage (volt)

GTAW(root pass)

150 23

SMAW (Ø 4.0 mm)first pass

180 25

SMAW (Ø 5.0 mm)subsequent pass

205 25

Case 1: The result of this test piece shows a few defects in the dye penetration test on the surface of the test piece. The result of ultrasonic test has shown a defect from left side located at 15 mm, 13.8 mm and 13.4 mm from the top of the plate. The distances of defects in length of test piece from the right side of the test piece are 100 mm, 325 mm and 450 mm respectively. The defect at the distance of 100 mm is introduced due to striking and re-striking of electrode. The length of arc not being maintained has resulted into entrapment of gases that has found its way through the ruptured shielded zone. The defect at the 325 mm is introduced due to higher wind flow that was noticed at the operational level. The defect at 450 mm is introduced due to improper cleaning of the surface after welding. The weldjoint was not properly cleaned and the cut cross section revealsthe slag locked in undercut formed due to higher heat input (low welding speed).

Case 2: The result of this test piece shows hair line crack on the surface in the dye penetration test. The distance of the defects along the length of a test piece located from the right side of the test piece are respectively 110 mm and 150 mm. The defect at the distance of 110 mm is introduced due to improper grinding as grinding particles were impregnated. The defect at the distance of 150 mm is introduced due to uncontrolled arc length, as the process is manual. The weld joint tested by the

ultrasonic testing has shown no defect. It is thus eminent that the welding speed selected for the case has not contributed to welding defects.

Case 3: The result of this test piece has shown minor defect inthe dye penetration test on the surface of the test piece. The distances of defects in length of test piece from the right side is located at 400 mm and 450 mm respectively. The defects at the distances of 400 mm and 450 mm are introduced due to the re-striking of the GTAW. The present welding speed selected for the case also has not contributed to any welding defects.

Case 4: The result of this test piece shows minor defect in the dye penetration test on the surface of the test piece. The distance of defect in length of test piece from the right side is at 125 mm. The improper grinding of the weld surface has resulted into impregnation of grinding wheel particles as observed in the weld cut cross section. Thus the present welding speed selected for the case has not caused any welding defects.

Case 5: The result of this test piece shows minor defect in the dye penetration test on the surface of the test piece. The distance of defect in length of test piece is from the right side at75 mm. The defect in dye penetrant test at the distance of 75 mm is introduced due to improper cleaning. While testing the same weld joint by the ultrasonic testing no defect is found in the test. This clearly indicates that the welding speed selected for the given case has no influence on welding defects.

VI. RESULT AND CONCLUSION

The experimental investigations carried out to understand the influence of welding parameters on weld defects are summarized below:

Variation in Welding Current The welding current specified in case II (140 amps for

GTAW, 140 to 150 for 4.00 mm diameter of SMAW electrode and 190 amps to 210 amps for 5.00 mm diameter of SMAW electrode) is recommended for quality weld.

Variation in Weld Speed The defects found in ultrasonic testing at 13.0 mm to 15 mm

from the top surface of test piece at lower welding speed in case I ( 56 mm/min for GTAW, 64 mm/min for the 4.0 mm diameter and 130 mm/min for 5.0 mm diameter electrode for SMAW).

The welding speed considered in case II to V could notshow weld defect except surface cracks and hence speed adopted in case II to V are recommended for quality welds.

The higher welding current causes higher heat input and thus this results in undercuts and defects. Alsi it is concluded that the lower welding speed should not be used as it has caused higher heat input and this has resulted into undercut and a probability of slag retention.



VII. REFERENCES

[1] Boiler and Pressure Vessel Design Code: 2007, ASME Section VIII, Division 1, ASME Publication, 2007. [2] Welding and Brazing Qualification, ASME Section IX: 2007, ASME Publication, 2007. [3] Dr. R.S. Parmar, Welding Process and Technology, Khanna Publisher, New Delhi, 2008.[4] Non Destructive Examination, ASME Section V: 2007, ASME Publication, 2007.[5] Baldevraj, C. V. Subramanian, T. Jaykumar, Non-Destructive Testing of Weld, Woodhead Publishing, March 2000.[6] Dissertation report on “Defect Analysis in Dish End and Nozzle Joint of Glass Lined Reactor”, Sardar Patel University, Y. R. Rana, H.V. Suthar, 2008. [7] V. M. Radhakrishan, Welding technology and design, New age international, New Delhi.[8] John P. Stewart, The Welder’s Hand book, Reston Pub Co. 1981.



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Defect Analysis in Dish End and Nozzle Joint of Reactor Vessel · GMM Pfaudler Ltd. Elecon Engg. Co. Ltd. [email protected] [email protected] Abstract— The welding