Throughout the UK, leading metal fabricators are investing in on-site nitrogen generation systems to ensure the quality of their laser-cutting operations, to increase output, and make substantial cost savings when set against conventional liquid storage and delivered bottled gas supplies.
As is well known throughout the industry, laser-cutting metal relies on maintaining a reliable supply of nitrogen as an assist gas, a blanket to prevent burning during the process. The purity of the gas is an essential factor in preventing oxidation of laser-cut edges when providing a high-quality finish.
However, besides these technical process considerations, many production companies are realising the equally important issue of how they source their nitrogen supplies. A comparison between on-site nitrogen generation and that of delivered nitrogen bottles, or liquid gas from large cryo plants is highly recommended. Indeed, metal fabricators must consider the fuel costs and carbon emissions associated with truck deliveries; potential wastage from on-site liquid storage; equipment rental fees, administration expense, and, not least, safety concerns. As a result, some companies are beginning to realise that on-site generation of nitrogen represents a more sustainable, controllable, convenient, and cost-effective solution.
Four Dee Project
One such organisation which has made the changeover investment is Four Dee (Northern Ireland) Ltd, a steel fabrications company based in Magherafelt. Laser cutting has become an integral part of its business. Now running five lasers in total, the company has invested heavily over the years in the latest technology.
Nitrogen has become an essential utility for Four Dee. The purchase of a 10kW fibre laser, as part of the company’s 2019 expansion plan, underscored evidence that inflated gas prices were a significant contributor to year on year operational increases.
As Alan Donnelly, Project Manager, Four Dee NI explains: “We were cutting a lot of mild steel. So, when it came to nitrogen supplies, it was clear we needed to be able to control the quantity and quality required, be independent of bulk deliveries, and reduce our costs. On-site generation provided the solution. Why buy nitrogen when you can generate and store your own?"
The decision was made to install a compact, high-pressure nitrogen production system comprising a nitrogen generator, a full-feature VSD+ rotary screw compressor, air and nitrogen receivers, plus a booster, dryers and filters, all integrated into one pre-commissioned skid unit. The system’s 300 bar output of 99.99 per cent pure nitrogen, at the rate of 38 Nm3/h is stored in high-pressure bottles with a 16-cylinder rack.
Four Dee NI now “‘produce what we need when we need it”, benefitting from nitrogen control and production speed increases with the prospect of payback on the installation within two years.
Cost comparison: Onsite generation vs. delivered nitrogen
The cost of delivered liquid nitrogen can range from 10p per Nm3 to 30p per Nm3. For bottles, that can be at 60p to £3 per Nm3. In stark contrast, the product cost of generating nitrogen on-site can be as little as 3p per Nm3, averaging at 4.5p, and costing no more than 9p per Nm3 depending on purity, flow, and pressure requirements.
To illustrate the potential cost-savings benefit of on-site nitrogen production in laser applications here are some comparison figures for a typical single fibre laser site with high uptime.
For example, take a 4kw fibre laser with automation running 18 hours per day, five days a week, 48 weeks a year with up to 70% laser uptime (high due to automation) and cutting a variety of materials but typically 4-6 mm stainless steel. That operation would use an average of 118,500 Nm3 per year in liquid or bottled nitrogen.
Based on 30p per Nm3, plus delivery charges to the site of £2160 and equipment rental of £5000, the fabricator is looking at an annual cost for liquid nitrogen of £42,710 per annum. The yearly equivalent in bottle banks, requiring 550 banks at £150 per bank, amounts to £82,500.
An equivalent solution in the form of a HPN2 (high pressure nitrogen skid) - running 4000 hours - would suit this requirement. Based on a 5-year hire purchase agreement at the current rate, and including all equipment, service and associated energy costs, the annual figure would be in the region of £ £32,500 p.a. For customers in this example, this equates to saving between £10,000 and £50,000 per year for the first 5 years, then, when the HP finance is paid off, the saving becomes even larger - circa £25,000 to £65,000 per year.
For typical users without a very high uptime (one shift and no automation) the equipment would be at suitably lower cost, energy costs would be more than halved, and service costs would be reduced also.
There are some important practical issues to take on board in assessing the benefits of on-site nitrogen production compared with conventional sourcing.
1) Environmental and safety issues: On-site generation means minimised environmental impact of process operations and increased safety for employees involved in its procedures. Furthermore, the handling of high-pressure bottles and liquid nitrogen at -196 degrees C can be eliminated.
2) Eliminating product losses: Power consumption and product loss are significant factors in the supply of liquid nitrogen. Primarily, this is seen as boil-off losses from the tank.
Delivered nitrogen, stored on-site in a cooled tank before use loses some of its volume to venting. This amounts on average to 0.3-3% of its volume each day, dependent on ambient temperatures and consumption demand. Also, if bottled gas is the supply method used, a certain volume of gas, dependent on bottle size and final pressure, will be returned to the supplier following every single use. This loss can constitute up to 10-15% of the delivered gas.
3) Site location: Suppliers’ delivery schedules may not always coincide with production demand peaks. Furthermore, the remote location of some operating sites makes a reliable, continuous, independent supply a necessity to maintain process operations.
In summary, reduced running costs, increased production outputs and fast return on investment are the principal factors influencing metal fabrication companies to consider on-site generation solutions when reviewing purchase decisions involving nitrogen supply.