Energy Conservation in Welding_Final_Avinash Abnave

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Energy Conservation in Welding

By

Avinash Abnave

Larsen & Toubro Limited,Heavy Engineering Division, Powai

1.0 Introduction

In the past few decades, Environmental Impact of industrial activities is one of the major concerns

in all countries. Apart from automobiles exhaust, a major cause for atmospheric pollution is

burning of coal, oil and gas in thermal power plants for generation of electricity. The hydel or

nuclear power generation is also cause for major environmental and safety concerns. So

conservation of energy in all industrial operations is an important objective for all industries. As a

responsible corporate citizen, Larsen & Toubro Limited had embarked on company-wide efforts

to conserve energy in all its operations.

Table –1. Welding related percentage of total energy costs

A recent survey in USA has brought out that about 20% of the energy consumed in heavy

engineering industries in United States is for welding related activities. ( see Table 1 ) Similardata is not available for India, but it is expected to be marginally higher than this. Also the cost of

energy is much higher in India compared to US and overall cost savings from energy conservation

will be of higher proportion. Hence optimization of energy consumption in welding and related

operations has received focused attention. A few examples of Energy saving in welding and

related activities are discussed in this paper with special emphasis on Welding Process selection,

Welding machines, Welding consumables and Pre-heat, Post-heat & PWHT.

2.0 Energy Efficiency of Welding Processes

Until recently, among the criteria for selection of welding process has been the need for better

quality of welds to maximise the performance of welds in service or higher productivity to bring

down the cost and cycle time for making the welds. The energy efficiency of the process as oneof the criteria for process selection is now receiving greater attention. There have been many



attempts to evaluate the energy efficiency of welding processes. Most of these consider only the

energy necessary to produce the weld. They do not reflect the total energy requirements of a

welding process accurately. So to compare the energy efficiency of each process, it is necessary to

consider energy consumption in total. A recent report published by TWI takes into consideration

Primary - the energy required for the heating the material to produce a satisfactory weld.

Secondary – the energy required for services and auxiliary equipment like:

Preheat / Postheat

Electrode Baking / Holding

Wire feed units, Motorised slides, Fume extractors

Job handling devices - Manipulators, Gantry, Robot

Weld preparation – Machining, Grinding

This Table-2 brings out the potential of friction stir welding as an energy efficient, environmental

friendly welding process

Table 2 - Comparison of the total energy consumption of welding processes

Process

Material

Thickness mm

Primary Energy

kJ

Secondary

Energy kJ

Total Energy

KJ

MMA

1 57 105 163

12 1420 1278 2698

50 17640 13035 30675

MIG / MAG

1 32 52 84

12 1280 277 1557

50 --- --- ---

GTAW

1 94 40 130

12 --- --- ---

50 --- --- ---

SAW

1 --- --- ---

12 1450 864 2314

50 16200 9870 26070

Laser

1 112 454 566

12 120 4268 4388

50 --- --- ---

EB (in vacuum)

1 --- --- ---

12 120 5283 5403

50 1000 7958 8958

Friction Stir

Welding

1 --- --- ---

12 1100 347 1447

50 3680 1495 5175

In submerged - arc welding by replacing the single filler wire by two smaller diameter wires

sharing the same current, the increase in current density in the wires can produce about 20%

increase in deposition rate, with corresponding energy savings. In MIG welding, use of smaller

diameter wires at the same operating current, can bring in similar increase in deposition rate and

corresponding energy saving.

3.0 Welding Power Source

A major share of energy consumption in welding goes to welding machines. The technology of

welding power source is advancing day by day and the journey started with huge generators has

reached to compact, energy efficient inverters.



In SMAW welding, usual arcing time achieved in a shift of 8 hrs is about 2.5 to 3 hrs. This means

idling time is around 5 to 5.5 hrs. The rectifiers draw current during idle time also and by using energy

saving kit, this consumption is brought down significantly. When machine is not in use, this energy

saving device enable to cut off the supply after 3 minutes. All rectifiers available in the production shop

are equipped with this energy saving kit. The Inverter power sources are very compact and occupy less

shop floor space, they are also energy efficient. Inverter has an average energy efficiency of 85 percent

and good power factor. For the same output welding current they utilize about 30 - 50% less primary

current than a rectifier or generator. During idling of the machine the power drawn is about 70% less

than a rectifier.

Table-3 Sample Calculation for Energy Consumption for Rectifier & Inverter

Parameter** Rectifier Inverter

Capacity 400 Amps 300 Amps

Input KW (Arcing) 11.2 5.6

Input KW (Idling) 1.4 0.28

Power consumed KWH (Arcing) 28 14

Power consumed KWH (Idling) 7.7 1.54

Total KWH Consumed 35.7 15.5

Cost of running / Day (at Rs.5.29/Unit) 189 82

* 2.5 Hrs arcing is considered per 8 Hrs shift

**Arcing data taken at 200 Amps output.

Saving is Rs.107 per Inverter per Shift

4.0 Welding Consumables

The welding electrodes are required to be stored in temperature and humidity controlled stores.

We have a hot room for storage of welding consumables with a capacity for 20000 Kg. Hot room

is equipped with four de-humidifier to control the temperature and humidity. Power consumptionfor the hot room per year is about rupees one lakh. Further the electrodes are baked in a oven at

temperature ranging from 150oC to 400

oC depending on the type of electrode and they are held

in holding ovens at 100oC till they are issued. By changing over to procurement of low hydrogen

electrodes in vacuum-sealed packets, the need for hot room storage capacity is reduced and

baking and holding ovens utilization is considerably brought down. This has brought about

considerable savings in power consumption.

Table 4 -Procurement of electrodes in Vacuum-Sealed packets:

2005 2006 2007 (Planned)

1,500 kg 16,500 kg 35,000 kg

Table 5 -Typical data for Power savings by use of Vacuum Sealed Packets:

Power Consumed for RatingPower

Consumption/daySaving Rs

Baking Oven 4.5 KW 40 units343

Holding Oven 2.0 KW 25 units

Backing considered as 300ºC for 2 hrs followed by holding



5.0 Pre-heating, Post-Heating & PWHT:

Fabrication of high wall thickness pressure vessels demand high preheats to be maintained

throughout the welding operations followed by post heating before the weld can be cooled to

room temperature. The preheating and post heating are often done by gas burners using LPG. The

LPG fuel was replaced by Piped Natural Gas, which is about 15% cheaper and safer to handle.

Further several initiatives were taken to control the consumption of Fuel gas.

- Burner efficiency improvement to save gas

- Providing Insulation / hoods to retain heat at the joint

- Infrared sensor to monitor preheat temperature and PID control to regulate fuel consumption.

5.1 Burner efficiency improvement to save gas

Low velocity candle type burner

(Rs 75/Hr)

ST.80 Spot burner

(Rs. 90/Hr )

5.2 Providing Insulation / hoods to retain heat at the joint

Typical savings on one job.

Number of seams welded -14

Welding cycle time 5-6 days

Total gas cost without hood, without insulation per seam Rs. 88,000

Savings in gas cost due to application of hood and insulation per seam Rs. 20,000

Total saving in gas consumption in one job Rs 2,80,000

Cost of hood and Insulation : Rs. 65,000

Net Savings: Rs. 2,15,000

Hood to retain heat at the joint Dished End and Nozzle

covered with Insulation

Forged nozzle Pre-heating



5.3 Infrared sensor to monitor preheat temperature and PID control

The control of preheat temperature is monitored by an infrared sensor device ahead of the pointof welding. The PID controller regulates the gas supply to the burners depending on thetemperature. A temperature recorder on the PID controller record the preheat temperature and is

available as a quality record.

PID controller with gas inlet

connectionsInfra red sensor

Infra red sensor measure

preheat temperature

6.0 Post weld Heat treatment

Maximum use of PNG is in furnaces for PWHT. All furnaces were changed from refractory

lining to Ceramic Fibre lining several years back. Recently an energy audit of PWHT furnaces

was carried out with the help of Indian Institute of Technology, Bombay and some suggestions for

improvement of burner arrangement and operation of burners with optimum air-fuel ratio for low

fuel consumption are implemented.

7.0 Conclusion

As a result of many planned initiatives across all areas of operation, including the above

initiatives in welding and related areas, L&T could achieve considerable reduction in Electricity

and Gas consumption over the years.

In recognition of the above institutionalized systems and procedures for efficient utilization and

conservation of energy, the Heavy Engineering Division of L&T was awarded the First Prize

in General category in National Energy Conservation Awards-2005, conferred by the Ministry ofPower.

274.55

117.25

250.13

117.1

210.97

64.5

0

50

100

150

200

250

300

( R s .

i n

L a k

h s )

2002-03 2003-04 2004-05

Electrical PNG

Energy Conservation in Welding_Final_Avinash Abnave

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