Stainless steel – A good choice for the water industry

SUMMARY

Stainless steels offer lightweight, durable design solutions to the water industry. The resulting economic benefits are illustrated by examples from drinking water treatment and distribution, and wastewater treatment installations. The same characteristics also enable stainless steels to contribute to environmental benefits through lack of contamination of drinking water, reduced environmental impact of the water industry and recyclability. Together, these make stainless steel an ideal material for the water industry.

INTRODUCTION

Stainless steels offer:

• excellent corrosion resistance;
• tolerance of high flow rates;
• good strength and ductility;
• easy fabrication;
• ready availability in a wide range of forms;
• very low leaching levels into water;
• excellent durability;
• full recyclability.

Worldwide, the water industry is investing billions of dollars to meet increasing standards and increasing demand. This paper shows how the above characteristics of stainless steel can provide significant economic and environmental benefits to the water industry from the point of extraction, through the treatment plant and drinking water distribution, and finally in waste water treatment. In turn, this creates a large global market growth potential for stainless steel.

ECONOMIC BENEFITS

The excellent corrosion resistance of correctly applied stainless steels is the basic characteristic from which all the other advantages flow. There is no general corrosion, and the risk of localized corrosion can be minimized by material selection and good practice. As a result, there is no need for a corrosion allowance or any protective coating. This allows thin-walled stainless steel components to be used, which may be lighter and simpler to fabricate than equivalent components in traditional materials. Table 1 compares the weights and costs of equivalent spool pieces for 16 bar design pressure [1].

Relative weights and costs for equivalent runs of 16 bar 200 mm pipework (stainless steel = 100) [1]

The combination of thin wall and good ductility allows cheaper joining methods to be considered for pipe: tees can be pulled in the pipe wall and side branches welded on, see Figure 1; the ends of the pipes can be flared and joints made with loose backing flanges. Provided that due attention is paid to avoiding problems from galvanic corrosion, backing flanges do not even need to be made from stainless steel but can be made from coated carbon steel or aluminium alloy.

The combination of good strength and lightweight piping is also of benefit for water distribution systems in high rise buildings where the cost of the stainless option may be less than other materials even prior to installation. The lower weight may result in further advantages: for example cheaper transport, lifting, installation and supporting structures. Figure 1 shows a drinking water pipe, which was light enough to be installed easily by two people in a sports stadium [2]. In this particular case, a further factor influencing material choice was that the corrosion resistance of the stainless steel allowed the interval between periodic flushing of the system to be increased, so reducing operating costs for labour and wasted water.

Figure 1. 300 mm pipework being installed by two people [2].

Stainless steel systems do not require either a corrosion prevention system or modification of the water chemistry in order to prevent corrosion, so bringing further operating cost reductions.

The durability of stainless steel means components may still be in good condition and so have value at the end of the useful life of the plant. It may be possible to reuse the stainless steel items or refurbish them: it will certainly be possible to recycle them into new stainless steel.

Life Cycle Cost (LCC) Analysis

The economic benefits of choosing stainless steel can be revealed by an analysis of life cycle costs (LCC) - the total costs involved with installing, operating and maintaining a piece of plant over its complete life. These costs are all discounted back to present day values using a standard formula to enable a comparison to be made between different material choices. (Computer programs are available to make the analysis easier, e.g. from Euro Inox [3]). LCC analysis is often quite sensitive to the exact maintenance costs and these can be hard to obtain or predict. One case where there were good maintenance records for the carbon steel components was a municipal waste water treatment plant in Italy [4]. The analysis showed that a cost saving of 18% would have been possible over the 30 year design life if stainless steel had been used for screens, traveling bridges and railings throughout the plant. The responsible authority took note of this when designing a new plant: as a result, stainless steel components were used extensively. Increased reliability and reduced maintenance is particularly important where there is pressure to reduce the manning of plants. In some cases plant may need to operate unattended for long periods, for example in remote areas. Complete package treatment units may be particularly suitable for such applications [5]. Stainless steel has an important role in these units because of its durability.

In Tokyo, carbon steel bridges carrying drinking water have to be painted every 5-7 years. Analysis showed that whilst the installed cost of a stainless steel water bridge would be 10% more expensive, the saving in maintenance costs would result in an overall cost saving of 40% over 30 years. Today, all newly installed water bridges in Tokyo are in stainless steel, see Figure 3.

Figure 2. Stainless steel water bridge, Japan.

The benefits of using stainless steel for drinking water distribution pipe work have long been recognized in Sweden. Some local authorities, e.g. Karlskoga, are now making exclusive use of it. The main advantages to them are its long, leak-free service life and its low weight, which makes for cheaper installation. Similar considerations have made the Tokyo authorities choose stainless steel for all the service pipes into buildings [6].

The first application in India of a stainless steel pipe to handle raw water took place as a result of a full comparison between cast iron and stainless steel. The advantages were reduced freight and installation costs because of the low weight as a result of reduced wall thickness; lower pumping costs because of the reduced friction as a result of the smooth bore and lack of corrosion; 50 year life where cast iron would require two replacements in that time plus many repairs for leaks. Even without taking all the operating cost savings into account, the life cycle costs of using 300 mm diameter Type 304 stainless steel were only 40% of those for cast iron for this 320 m long pipe. After three years of operation, the pumping efficiency improvements were being maintained and no maintenance had been needed on the pipeline [7].

Figure 3. Stainless steel pipe for raw water, Mettur, India [7].

Pipeline standards in the USA are now being amended to include stainless steel grades, which will remove another obstacle to their wider use in distribution networks. In some situations, the greater strength of the duplex grades may be particularly beneficial. The contractors for a sewage treatment works in the UK chose stainless steel for the odour control ducting as a result of their LCC analysis, Figures 4 and 5 [1]. The stainless steel was only slightly more expensive than the alternative because it did not need coating in situ after installation.

However, the real economic benefit was that it would not need expensive repairs after 15 years.

Figure 4. Stainless steel odour control ducting in a sewage treatment plant.

Figure 5. LCC analysis of alternative materials for ducting in Figure 4

The traveling distributors in a heavily-loaded sewage works in England needed continuing expensive maintenance because of corrosion of the carbon steel structures. A redesign was carried out, including the use of stainless steel for the structure, see Figure 6. After only two years operation, maintenance costs had been reduced by 98% and plant availability had increased - equivalent to a 25% increase in plant capacity [8].

Figure 6. Redesigned distributors in a sewage treatment works. The pyramidal support structure is in stainless steel.

In some jurisdictions, operating and maintenance cost predictions now have to be taken into account when the initial material selection decisions are being made [9]. This generally favours the durable stainless steel option. Nevertheless, each case must be considered separately, taking into account the particular project requirements - especially interest rates and the required payback period.

ENVIRONMENTAL BENEFITS

Today there is greater realization than ever of the need to minimize the impact of human activities, both on our environment and on human health. A supply of clean drinking water is vital to all communities and the resulting wastewater must not contaminate the environment when it is discharged. Materials used for the treatment, storage and distribution of drinking water must not introduce any contamination above the levels permitted by the relevant legislation. The stainless steel grades likely to be used in these applications have been tested in different countries. These tests have shown that the leaching of metallic elements is at level consistently below those allowed by the regulations. For example, the rig tests carried out as part of a European pre-normative research project [10] gave leaching values for chromium and nickel, which were less than 5% of those allowed by the European Drinking Water Directive [11]. This work is now forming the basis for tests, which will be used to assess the suitability of construction materials to be used in contact with drinking water under the European Approval Scheme, which is currently being developed.

In a separate approval study, following a change in their regulations, the UK Drinking Water Inspectorate (DWI) tested three grades of stainless steel (1.4307, 1.4404 and 1.4462). 24-hour stagnation tests in 54.5 mm bore pipes in three waters produced leaching levels of <1.0 µg/l Cr and <2.0 µg/l Ni[12,13]. The DWI Committee concluded that "...the use of products made from the specified stainless steel grades in contact with water for public supply would be unobjectionable on health grounds" [14]. These low leaching levels from new product have been shown to decline further during service [15].

A further benefit of the low levels of leaching from stainless steel water systems is that they do not introduce any contamination into the water, which might become a problem in the wastewater stream. When the wastewater is treated, metallic contamination will either stay in the discharge water stream or will accumulate in the sewage sludge. There are already some parts of Europe where restriction on the use of sewage sludge as an agricultural fertilizer is being discussed because of its metal content. Two possible sources are water distribution systems (including plumbing) and roofing. The use of stainless steel products in both these areas could ameliorate this situation.

Water is a precious commodity and it is important to minimize wastage through leaks in the distribution system. In many countries the distribution network is old and in need of repair. However, this can be both expensive and disruptive when the main pipes run under the streets of a busy city. Trenchless relining of such mains has been developed for such a situation. Rather than dig up a complete street, it is only necessary to excavate chambers into which short lengths of pipe can be inserted, welded onto the preceding length and then pushed along the original pipe with hydraulic jacks. It has been possible to push lengths up to 1000 m - even following slight curves - see Figure 7 [15]. This technique was nominated for a European sustainable development award in 2000.

Figure 7. Installation of stainless steel pipe lining using trenchless technology, Turin.

Thin walls, long service life and high expectation of complete recycling at end-of-life mean that stainless steels can reduce the material intensity of the water industry. In other words, more of the needs of society can be met for longer times with less material. This will help to make our society more sustainable.

REALISING THE BENEFITS

It must be understood that "Stainless Steels", under some circumstances, can corrode. It has been a common experience in many countries that it is necessary to pay careful attention to grade selection, fabrication, installation and operation of stainless steels when they are used in the water industry. These requirements are well documented in publications [1,17-19] and in the Operational Guidelines and Code of Practice [20] produced as part of the recent approval exercise with the UK Drinking Water Inspectorate. The most important aspects are:

• choose the correct grade for the chloride content of the water;
• avoid crevices when possible by good design;
• follow good fabrication practices, particularly removing weld heat tint;
  
• drain promptly after hydrotesting.

The national stainless steel development associations like the Indian Stainless Steel Development Association are now taking these messages to the water industry and its supply chain around the world. Heeding these messages will enable all involved to realise the economic and environmental benefits of applying stainless steels in the expansion of this vital global industry.

ACKNOWLEDGEMENT

The author is grateful to colleagues in The Nickel Institute and the Indian Stainless Steel Development Association stainless steel industry for help in the preparation of this paper.

REFERENCES

• Applications for Stainless Steel in the Water Industry, IGN 4-25-02, 1999, WRc.
• Nickel, Vol 16, No 4, June 2001, pp 8-9.
  
• Life Cycle Costing program, Euro Inox.
• Fassina L. and Powell C.A., Stainless Steel in Wastewater Purification Plants in Italy and Abroad - Life Cycle Cost as the Basis for Contract Specifications, in The Contribution of Stainless Steel for Utility Companies, June 2001, Centro Inox.
• Steel Package Water and Waste Water Treatment Units, Steel Construction Institute, 2000.
• Nickel, Vol 15, No 2, Dec 1999, p. 12.
• Stainless India Vol. 7, No. 2, p. 5.
• Nickel, Vol 15, No 2, Dec 1999.
• US Government Accounting Standards Board Statement 34, June 1999.
• Co-normative Research on Test Methods for Materials in Contact with Drinking water, MAT1-CT94-0058, Centre de Recherche et de Contrôle des Eaux, Paris, 1999.
• Drinking Water Directive, 98/83/EC.
• UK Drinking Water Inspectorate, private communication.
• Lewus M.O. et al, Review of Metal Release from Ferritic, Austenitic and Duplex Stainless Steel Grades, Exposed to Potable Water and Related Environments, 4th Stainless Steel and Market Congress, Paris, 2002.
• UK Drinking Water Inspectorate Regulation 25 Letter 7/2001.
• Powell C. A. and Strassberg W., Stainless Steel for Potable Water Service, 2nd European Stainless Steel Congress, Dusseldorf, VDEh, 1996.
• Nickel, Vol 15, No 2, Dec 1999.
• Stainless Steel for Potable Water Treatment Plants, NiDI No. 10087, 1999.
• Guidelines for the Use of Stainless Steels in Municipal Waste Water Treatment Plants, NiDI No 10076, 1995.
• Microbiologically Influenced Corrosion of Stainless Steel by Water used for Cooling and Hydrostatic testing, NiDI No 10085, 1998.
• Operational Guidelines and Code of Practice for Stainless Steel Products in Drinking Water Supply, British Stainless Steel Association, 2002.

* General Manager, Nickel Institute (NI)
Executive Director, Indian Stainless Steel Development Association (ISSDA)