Srdjan T. Mladenovic, MiroslavR.Radovanovic
University of Nis, Faculty of Mechanical Engineering, Nis, Serbia
,
Abstract:Operating costs of plasma cutting should form the basis for evaluating its profitability. Acceptable cut quality, increased traverse speed and lower cost per meter of the cut together with cheaper equipment and possibility of cutting various materials assure wider application of this procedure. Optimizing a plasma cutting operation based on operation cost is typically a trial-and-error process that is usually inspired in recommendations given by manufacturers of plasma cutting tools and consumables. The operating costs for plasma cutting are presented in this paper.
Key words: Advance machining, Plasma Cutting, Operating Cost
1Name of the author, title, company, address and e-mail.
2Name of the author, title, company, address and e-mail.
- INTRODUCTION
The plasma-arc process had its origin almost 70 years ago. In 1941 the U.S. defence industry was looking for better ways of joining light metal together for the war effort and, more specifically, for the production of airplanes. Out of this effort, a new welding process was born. An electric arc was used to melt the metal, and an inert gas shield around the arc and the pool of molten metal was used to displace the air, preventing the molten metal from picking up oxygen from the air. This new process "TIG" (Tungsten Inert Gas) seemed to be a perfect solution for the very specific requirement of high-quality welding.
By 1950, TIG had firmly established itself as a new welding method for high-quality welds on exotic materials. While doing further development work on the TIG process, scientists at Union Carbide's welding laboratory discovered that when they reduced the gas nozzle opening that directed the inert gas from the TIG torch electrode (cathode) to the work piece (anode), the properties of the open TIG arc could be greatly altered. The reduced nozzle opening constricted the electric arc and gas and increased its speed and its resistive heat. The arc temperature and voltage rose dramatically, and the momentum of the ionised and non-ionised gas removed the molten puddle due to the higher velocity. Instead of welding, the metal was cut by the plasma jet.
In Figure 1, both arcs are operating in argon at 200 amps. The plasma jet is only moderately constricted by the 3/16 inch (4.8 mm) diameter of the nozzle orifice, but it operates at twice the voltage and produces a much hotter plasma arc than the corresponding TIG arc. If the same current is forced through a nozzle with an even smaller opening, the temperature and voltage rise. At the same time, the higher kinetic energy of the gas leaving the nozzle ejects the molten metal, creating a cut.
Fig.1. TIG arc and plasma arc
- PLASMA CUTTING
Plasma cutting is a process that is used to cut steel and other metals of different thicknesses (or sometimes other materials) using a plasma torch. In this process, an inert gas (in some units, compressed air) is blown at high speed out of a nozzle; at the same time an electrical arc is formed through that gas from the nozzle to the surface being cut, turning some of that gas to plasma. The plasma is sufficiently hot to melt the metal being cut and moves sufficiently fast to blow molten metal away from the cut.
The characteristics of the plasma jet can be altered greatly by changing the gas type, gas flow rate, arc current, arc voltage and nozzle size. For example, if low gas flow rates are used, the plasma jet becomes a highly concentrated heat source ideal for welding. Conversely, if the gas flow rate is increased sufficiently, the velocity of the plasma jet is so great that it ejects molten metal created by the hot plasma arc and cuts through the workpiece.
Plasma cutting is an industrial process that is essentially controlled by the operators’ empirical mind-set, which is typically inspired in recommendations given by the manufacturers of the cutting torches that are to be used. Those recommendations, however, reflect the point of view of the manufacturers’ business, which includes not only selling the cutting torches but also the consumables. Yet, the manufacturers’ recommendations usually lead to solutions that are technically sound in terms of cutting quality, but do not necessarily correspond to the most cost-effective solutions on the user’s point of view.
As a result, the user customarily attempts to optimize the cutting operations by trial-and-error every time it is needed to setup the existing equipment for a new different task.
Fig.2.Plasma cuttingIn an industrial point of view, general contributions for the systematization of knowledge on the plasma cutting is an industrial process that is plasma cutting process appear to be out of question essentially controlled by the operators’ empirical since plasma torches and respective nozzles come in a mind-set, which is typically inspired in recommenda-wide range of sizes. Additionally, the topology of operations given by the manufacturers of the cutting complete plasma cutting systems varies from the torches that are to be used. Those recommendations, simple hand-held torches to complex CNC machines however, reflect the point of view of the manufacture-of different shapes and sizes.
There are several methods of plasma cutting. The well-known are: conventional plasma cutting, dual flow plasma cutting, air plasma cutting, oxygen plasma cutting, underwater plasma cutting and other.
In Table 1 are shown gas quality and pressure requirements formachine HyPerformance plasma HPR130.
In Table 2 are shown gas types and amperage of current for material types of plasma cutting.
In Table 3 is shown power supply machine.
Table 1.Gas quality and pressure requirements
Plasma gas /Quality
/Pressure
+/- 10%
/Flow rate
O2 Oxygen / 99.5% pureClean, dry, oil-free / 827 kPa /
8.3 bar / 4250 l/h
N2 Nitrogen / 99.9% pure
Clean, dry, oil-free / 827 kPa /
8.3 bar / 7080 l/h
Air / Clean, dry, oil-free / 827 kPa /
8.3 bar / 7080 l/h
H35 Argon-hydrogen / (H35 = 65% Argon, 35% Hydrogen) / 827 kPa /
8.3 bar / 4250 l/h
F5 Nitrogen-hydrogen / (F5 = 95% Nitrogen, 5% Hydrogen) / 827 kPa /
8.3 bar / 4250 l/h
Table 2. Gas types and amperage of current for material types
Mild steel /Stainless steel /
Aluminium / /
Gas types /
Plasma
/Shield
Cutting30 to 45 A / O2 / N2& F5 / Air / O2 / N2 / Air
Cutting
80 A / O2 / F5 / - / Air / N2 / -
Cutting
130 A / O2 / N2H35 / H35 Air / Air / N2 / N2 Air
Table 3. Power supply
General
Maximum OCV (U0) / 311 VDCMaximum output current (I2) / 130 Amps
Output voltage (U2) / 50 – 150 VDC
Duty cycle rating (X) / 100% @ 19.5 kW, 40°C
Ambient temperature/Duty cycle / Power supplies will operate between -10°C and +40°C
Power factor (cosϕ) / 0.88 @ 130 ADC output
Cooling / Forced air (Class F)
Insulation / Class H
Input power (input voltage (U1) X input current (I1)
200/208 VAC, 3-PH, 50-60 Hz, 62/58 Amps
400 VAC CE, 3-PH, 50-60 Hz, 32 Amps
1Name of the author, title, company, address and e-mail.
2Name of the author, title, company, address and e-mail.
- COST OF PLASMA CUTTING
How to calculate cost of operation and establish metrics for improvement?There are many costs associated with a mechanized plasma-cutting machine beyond the capital equipment purchase. There are general overhead costs, maintenance costs, service call charges, gas costs, consumable and torch costs, and electricity charges. The plasma-profiling machine is also likely to have a host of auxiliary equipment that may also be considered: material handling equipment, environmental control equipment, safety gear etc. The labor component for plasma cutting may include machine operators, helpers, maintenance personnel, secondary operation workers and others. The intent of this article is to review the most significant variables affecting annual cost of operation and to establish metrics for improvement.
In typical plasma cutting operations there are four major ongoing costs: cost of power, gas, cost of consumables, and cost of labor.
Productivity can be regarded as being the ratio between production speed and cost. For plasma cutting, productivity can be defined by an expression of the type
/ (1)Where VCis the traverse speed and CHis the cutting cost per unit time.
Both VCand CHdepend on several process variables and productivity can be improved either by increasing the traverse speed, or decreasing the cutting cost per unit time, or both.
For a given torch, the main process variables in plasma cutting are the amperage the current, the traverse speedVC, the pressure of the cutting gas, and the pressure of the protective gas. There are four major factors for the production cost in typical plasma cutting operations: electrical power, gases and torch consumables [1].
/ (2)Where CE, CP, CSand CMare respectively: the cost per unit time of the electrical power, the cost of the cutting gas (plasma), the cost of the compressed air that is used as protective gas (shield) and cost of the torch consumables. All the costs are expressed in €/h.
Cost of the electrical power can be defined by expression:
/ (3)Where cEis unit cost of electric energy(EUR/kWh), PEis electric power consumption (kW), PP is electric power of aggregate the plasma, PTis electric power of working table.
Cost of the cutting gas (plasma)and the cost of the compressed air that is used as protective gas (shield) can be defined by expressions:
/ (4)/ (5)
Where cPandcSis unit cost of plasma gas and shield gas(EUR/m3), QP is gas consumption(m3/h).
Cost of the torch consumablesCMto set a monthly or year monitoring of consumption, and later this value is translated into cost per time.
- OERATING COSTS
The major power consumer in a cutting machine is the DC power supply. Most of the energy consumed by the system is put directly to work on the material in a very hot energy-dense arc. To get a rough idea of the power consumption of plasma system is multiply the amperage output by the average operating voltage. To calculate kilowatts of input consumed, multiply by a power supply efficiency factor of around 85%. Examplean80A plasma system has an average operating voltage of about 100V. This means the power supply puts out 6.8 kW (8kVA x0.85 = 6.8 kW).
To arrive at daily or yearly power consumption multiply times the average up-time or arc-on time in a day. Arc-on time is the amount of time actually spent cutting over a given time interval. This can be measured by a pierce and arc-on time counter, or calculated from programming distances and speeds and daily throughput. Arc-on time will vary with material type and thickness, size of cut pieces, material handling, machine speed, torch height control speed, and many other factors. Most shops average about 55% actual arc-on time. That means in a given 8-hour shift only 4.4 hours are spent cutting. In the year we have 1144 hours arespent cutting (260 days).
Plasma systems use as plasma gas: oxygen, air, nitrogen, argon-hydrogen, and other gases.The consumption rate varies with the size of the plasma system and various operating conditions. Generally the operations manual will provide consumption rates in cubic meter per hour for a given nozzle size and operating pressure or flow tube setting. For example an80A oxygen plasma system consumes 2m3/hoursof oxygen when cutting. To find the cost of operation multiply the consumption rates of plasma gas by the arc-on time and cost of the gas, which is often measured in EUR per m3.The same system may use 8.5m3/hours of shield air. Shop air is generally considered free other than associated maintenance costs to keep it clean. But shield gases such as nitrogen O2, and mixes can be costly and should be calculated as above.
Consumable costs can be tracked on a weekly, monthly or yearly basis. These costs vary widely depending not only on the cost of the parts but on the performance and life of the parts, which is dependent on many factors. Consumable and plasma torch life varies with application, operating parameters, duration of cuts, number of pierces, operator skill etc. The best way to capture and begin to control consumable costs is to keep daily logs of parts life measured in number of pierces and arc hours. Over time, in a production environment, it is possible to closely track the number of pierces and the total arc-hours for a given set of parts on a given cutting job. If a plasma torch is operated and maintained correctly the annual cost of torches, gas swirling devices, shields, retaining caps and other parts should be low compared to the nozzle and electrode cost. But the reality in many shops is that overall consumable cost is 2 X the nozzle and electrode cost.
In Table 4 are shownconsumptionof the power, cutting and shield gas in m3per hour and consumable in EURper day.
Table 4. Consumption of power, cutting and shield gas and consumable
Factors / Mild Steel / Stainless Steel / Alumi-niumPower (current 80A) / kWh / 8.2 / 7.5 / 10.2
Cutting gas
(plasma gas) / O2 / 2 / - / -
N2 / - / 0.8 / -
F5 / - / 0.4 / -
H35 / - / 0.3 / 1.5
Protective gas
(shield gas) / O2 / 0.5 / - / -
N2 / - / 4.5 / 2
Air / 8 / - / 6
Consumable / EUR / 10 / 10 / 10
In Table 5are shownoperating costs for one month if cutting machine works 25 days (110 hours). A price of 0.105 EUR/kWh was used for the electricity costs. The following gas prices were taken as the basis for calculating the plasma gas costs: N2–1.5 EUR/m3, O2 - 1.5 EUR/m3, F5 - 2 EUR/m3and H35 - 2 EUR/m3. Air is generally considered free, but consumption of air compressor during compression to 6-8 bar is about 2 kWh.
Table 5. Operating costs for one month
Factors / Mild Steel / Stainlees Steel / Alumi-niumPower / 95 / 86.5 / 118
Cutting gas (plasma) / O2 / 330 / - / -
N2 / - / 132 / -
F5 / - / 88 / -
H35 / - / 66 / 330
Shield gas / O2 / 82.5 / - / -
N2 / - / 990 / 330
Air / 23 / - / 20
Consumable / 250 / 250 / 250
Overall / EUR / 780.5 / 1612.5 / 1048
The table shows that the operating costs of a plasma cutting are big, and it is therefore necessary to optimize the machine and several recommendations are given in the conclusion.
- CONCLUSION
Plasma cutting is a process that is used to cut steel and other metals.Here are some recommendations for optimizing plasma cutting machine to lower cost of operation and increase productivity:
1) Maximize up-time on the machine. A cutting machine should be cutting. Preventative maintenance is essential to prevent costly downtime for repairs. Material handling solutions such as multiple cutting beds, overhead cranes, and plate handlers can minimize manual loading and offloading and keep the operator focused on the cutting process. Motion matters as well: If the torch height controls or machine traverse speed is slow the machine spends more time positioning the torch than cutting metal.
2) Minimize secondary operations: Controlling costs of secondary options is achieved by optimizing cut quality. To do this requires not only a well-maintained machine but also a well-trained operator. The highly skilled operator produces more cut pieces, of higher quality, with less scrap material and less rework down the line. Getting good cut quality from the plasma arc cutting process requires careful control over process parameters and attention to detail.
3) Control consumable costs: Controlling consumable costs, like controlling cut quality is part equipment and part operator. A good operator will get the most out of a set of parts and prevent catastrophic failures.
ACKNOWLEDGEMENT
This paper is part of project TR35034 The research of modern non-conventional technologies application in manufacturing companies with the aim of increase efficiency of use, product quality, reduce of costs and save energy and materials,funded by the Ministry of Education and Science of Republic of Serbia.
REFERENCES
[1]COOK D., (2000),Cost of Operation in Mechanized Plasma Cutting, Magazine "Welding Design & Fabrication", Ohio, USA.
[2]FERREIRA P., MELOI., GONÇALVES-COELHO A., MOURÃO A., (2009),Plasma cutting optimization by using the response surface methodology Extending the C-K design theory, The annals of “DUNĂREA DE JOS”, University of Galaţi, Galaţi, Romania, pp 213-218
[3]Radovanović M.,(2005), Plasma Cutting of Metals, 7th International Conference on Accomplishments of Electrical and Mechanical Industries - DEMI 2005, University of Banjaluka, Faculty of Mechanical Engineering, Banjaluka, Bosna and Hercegovina, pp.165-170
[4]Radovanović M.,(2005), Determining of Cutting Data by Plasma Cutting, Seventh International Scientific Conference "Smolyan-2005", University of Plovdiv "Paissiy Hilendarski", Technical College-Smolyan, Smolyan, Bulgaria, pp. 235-239
[5]Radovanović M.,(2004),Comparison of abrasive water jet cutting and plasma cutting, International scientific conference UNITECH'04, Technical University of Gabrovo, Gabrovo, Bulgaria, pp. II-137-II-142