Benefits

Overview

A supply of clean drinking water is vital to all communities and the resulting wastewater must not contaminate the environment when it is discharged. In recent years, the use of stainless steel in water and wastewater treatment plants and for plumbing systems has increased in many countries. Type 304L and 316L grades of stainless steel are the standard materials of construction, and duplex and super austenitic alloys are considered for more arduous service. They are used to provide:

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

Slide presentation: Global Applications for Stainless Steel in the Water Industry by C.P.Cutler. BSSA Conference 2008

Design and Economic Advantages

Stainless steel provides a material with extremely low corrosion rates in the handling of waters. With excellent corrosion-erosion characteristics in high flow rates of water (up to about 40m/s), stainless steel can easily handle changes of cross-section, high aeration, pumping turbulence, and high velocities. Stainless steel systems require no corrosion allowance or coatings and can be designed using thin walls. Water chemistry control is not required to prevent corrosion attack, although bactericide treatments will still be necessary as for all potable water streams. Also, as higher flow velocities can be accommodated, smaller cross-sectional piping sizes can be used for the same mass flow rate than would be permissible with conventional materials. Reduced size, low weights and ease of fabrication can also be a significant advantage in installations where space and handling capabilities are restricted, which may be the case in upgrading a plant or for packaged treatment units.

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, 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 ductile cast iron, coated carbon steel or aluminium alloy. Lower weight may result in further advantages: for example cheaper transport, lifting, installation and supporting structures. Thus by making the most of the properties of stainless steel, their perception as an expensive option on first cost is not always correct and benefits can very often be realized through long service life and justified by life cycle cost analysis. Software to assist with carrying out Life Cycle Costing analysis of stainless steel is available on the EuroInox; website, www.euro-inox.org

Articles, Publications and Life Cycle Costing Studies

Environmental Advantages

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 gave leaching values for chromium and nickel which were less than 5% of those allowed by the European Drinking Water Directive 98/83/EC. 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, 304L, 316L and alloy 2205. 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.

Initial low leaching levels from new product have been shown to decline further during service as in the results of a 3.5 year study of both the cold and hot water systems in a Scottish hospital in Table I.

Table 1: Leaching levels in Scottish hospital study, µg/l.

*Mo levels were all <2.0

Days
in Use

Cold Water

 Hot Water

304

316

304

316

Ni

Cr

Ni

Cr

Ni

Cr

0

1.7

0.2

0.6

<0.1

3.8

0.2

1

1.1

0.2

1.9

0.4

4.5

<0.1

2

1.3

0.2

1.9

<0.1

4.3

0.3

3

1.4 

<0.1

1.3

<0.1

5.5

0.4

4

1.7

0.3

1.6

0.4

5.7

0.9

11

1.5

<0.1

6.1

0.2

9.3

0.9

18

<0.5

<0.1

1.1

0.5

11.1

0.3

25

1.0

0.3

0.7

<0.1

15.4

<0.1

32

1.1

<0.1

2.1

<0.1

14.0

<0.1

180

1.0

<2.0

<0.5

<2.0

2.8

<2.0

1250

0.6

<0.5

<0.5

<0.5

1.2

<0.5

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 fertiliser is being discussed because of its metal content. Two possible sources are from residential and commercial plumbing and metallic roofing(water run-off). The use of stainless steel products in both these areas avoid this situation.

Recycling

Stainless steels used in water plants and systems are virtually 100% recyclable and therefore have a useful scrap value. Click here for more information about recycling of stainless steels.