This plumbing section primarily focuses on hot and cold drinking water systems. Similar considerations also apply when stainless steel is used for cooling water, compressed air and fire protection systems.
- Advantages of stainless steel in plumbing systems
- Overview of stainless steel plumbing systems
- Case histories metal leaching into water
- Regulations and approvals links and references
Advantages of Stainless Steel Plumbing Systems
- General corrosion rates in drinking waters are negligible and a corrosion allowance is unnecessary
- Can be left visible to the eye if required, blending well with modern architecture and interior design
- Excellent corrosion-erosion resistance even at water flow rates over 30m/s. Can easily handle changes of cross-section, sharp changes in direction and turbulence downstream of pumps and valves
- Smaller pipe sizes can be used for the same mass flow rate than would be permissible with conventional materials
- Minimal frictional loss over time and no compensation in pumping pressures
- Strong yet ductile. Designers can take account of higher strength properties to further influence diameters and wall thickness
- Not easily damaged by use in public areas
- A variety of joining techniques
- Galvanically compatible with copper alloy fittings
- Extremely low levels of metal leaching
- 100% recyclable
Overview of Stainless Steel Plumbing Systems
Stainless steel plumbing particularly in large commercial or municipal buildings has seen a growth market since the early 1990s. Germany, Japan, Italy and Switzerland are established users and with increasing restrictions on leaching from materials in drinking waters in Europe, Japan, China and USA, stainless steel is becoming increasingly recognized as a solution to this problem. There has been commercial development of several types of connection, e.g. pressfittings, which present a quick and reliable means of joining and allow savings on installation costs. Fashionable taps and faucets are also available in stainless steel.
In cold waters, Type 304 grades perform well in waters containing up to 200 ppm chlorides, while Type 316 grades are suitable up to 1000 ppm chlorides. For hot water systems, it is advisable to use more conservative limits of, say, 50 ppm chlorides for Type 304 grades and 250 ppm chlorides for Type 316 grades. Stainless steel is resistant to the continuous chlorine disinfection levels normally used in drinking water. Short term shock sterilisation, such as 25-50 ppm free chlorine for up to 24 hours, is also acceptable provided the system is flushed through thoroughly afterwards.
Although type 304 and 316 grades of stainless steel can be susceptible to chloride stress corrosion cracking at temperatures above 50°C when chlorides are allowed to concentrate on the metal surface, the body of experience suggests that it is very unlikely to occur inside piping using potable waters with chlorides less than 250ppm at temperatures used in domestic systems. To preserve the outside of the pipework, chloride contamination should be minimized. If insulation is needed on stainless steel, it should always have a low chloride content (<0.05%). where insulation is exposed to external wet chlorides, it should be kept dry (e.g. by plastic casing), or a barrier of aluminium foil >0.06mm thick with a 50mm overlap, or coating applied between the insulation and the stainless steel.
Stainless steels are galvanically more noble in waters than steel, galvanized steel and cast iron and should be electrically insulated from any such attachments to avoid corrosion of the less noble metals at the interface. They are slightly more noble than copper, but in practice copper base fittings have performed well in stainless pipe assemblies. Stainless steel or copper hot water cylinders can be used with stainless steel piping systems.
Various controlled studies have served to give confidence in the use of stainless steel plumbing systems. Two examples are:
A 10 year Japanese study covering tubing trials in 2 large buildings, a hotel and a supermarket, examined lengths of type 304 stainless pipework in waters ranging from pH 7.1-8.8, 20mg/l chlorides, residual chlorine levels of 0.1-0.9mg/l and 28-50°C. Connections were made using welds, soldered fittings, stainless pressfittings, bronze compression fittings and flanged joints.
No corrosion was found on any of the pipes or their welds even under sediment. Pressfittings proved a very acceptable form of connection and showed no signs of crevice corrosion. Some general corrosion had taken place on the bronze fittings but this was slight and would not influence service performance. The study showed good performance from soldered joints although care has to be taken with soldering as problems occurred years ago in Europe when chloride based fluxes were used rather than phosphoric acid fluxes. Soldering has also been associated with knife-line type of attack and recommendations for suitable silver solders should be obtained from suppliers. If flanged joints are used, a polymeric gasket material suitable for use with stainless steel should be chosen.
In Scotland, soft aggressive drinking waters have caused serious leakage problems in copper pipework. This was of particular concern in hospitals. To evaluate stainless steel as a replacement, parts of one hospital were retubed in 304 and 316 stainless steels and examined 2 years later. The water had a chloride content of about 20mg/l and pH of 8. The hot water pipe was 55-65°C. As a result, 316L was the alloy chosen for health care premises in Scotland. The initial trials used copper based compression fittings whereas stainless steel pressfittings have been used in subsequent installations. 10 years on, the performance of the early installations with pressfittings was assessed by the NHS in Scotland with samples of tubing removed and evaluated. The performance of the pressfittings was found to be excellent with no corrosion at all found in hot or cold water tubing.
Metal Leaching in Stainless Steel Plumbing Systems
Leaching from stainless steel is very low. This was explored in the Environmetal Advantages section where nickel, chromium and iron molybdenum levels were measured over 3.5 years in a Scottish hospital system and were extremely low- in the order of a few parts per billion. Nickel tends to leach at a slightly higher rate than chromium and this is the element which attention has been given. The low leaching levels are associated with the tenacity of the protective surface film.
Currently the nickel levels required for drinking waters are:
- US EPA: no formal limit but guideline maximum 100 μg/l (ppb)
- Japan: Since 2003, a provisional guideline maximum of 10 μg/l
WHO provisional guideline maximum: 20 μg/l until 2005.
- Other countries have also followed WHO.
- WHO guideline revised in 2005 to 70 μg/l, based on protection of nickel-sensitive people. (130 μg/l would be sufficiently protective against reproductive effects.)
- immediate likelihood of revision of limits in EU or other countries which are based on WHO guideline.
Nickel leaching from stainless steel readily meets even the lowest of these requirements.
Regulations & Approvals
Regulations, standards and approvals are often nationally controlled. Some examples are given here:
- International Building Code(IBC), Universal Plumbing Code(UPC) and International Plumbing Code (IPC) approval for the use and acceptance of stainless steel in North America.
- Stainless steel is approved as an acceptable material for the handling of drinking waters, per ANSI-NSF standard 61.
- EN 12502-4:2004. Protection of metallic materials against corrosion. Guidance on the assessment of corrosion likelihood in water distribution and storage systems. Influencing factors for stainless steels.
- A European Approval Scheme (EAS) is being developed for all construction products in contact with drinking water. This would cover plumbing systems, including taps.
- DIN 50930-6 deals with the hygienic properties of metallic materials in contact with "drinking" water "in relation to" the new European drinking water directive: it "allows" stainless steel according to DVGW W 534 and W 541 to be used without any restrictions.
(For other stainless steel standards, see Standards & Approvals.)
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