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7/23/2019 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
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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.
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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
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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
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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