Thin-walled Stainless Steel Plumbing for Potable Water

Stainless steels are synonymous with corrosion resistance. This is the most important property required of a plumbing material in order to provide decades of leak-free service, supplying safe drinking water uncontaminated by the corrosion products of the piping material.

Stainless steel piping systems have been used for many decades all over the world to handle waters in the highly corrosive environments of the chemical, pollution control, pharmaceutical and food & beverage industries. The invention of simple press-fitting systems for thin-walled pipes has made stainless steel a very cost-effective material for indoor plumbing for potable water. They are now being increasingly used to distribute potable water in buildings in North America, Europe and Asia.

What are stainless steels? Stainless steels are alloys of iron containing at least 11% chromium. To improve corrosion resistance and the engineering properties, usually nickel (8-12%) and some times molybdenum (2-2.5%) are added. The molybdenum containing alloys resist chlorides to a much greater extent. Though there are over 200 commercial grades of stainless steels, the ones used for potable water are grades 304 and 316 whose principal alloying contents are given in the table below:

Grades 304 & 316 - Composition

The yield strength of these grades is in the range of 220-240 MPa, tensile strength 540- 680 MPa and elongation 40-45%. The combination of high strength and good ductility means that they can be bent like copper, although requiring more force. They are eminently resistant to physical damage. The higher strength and corrosion resistance makes the pipe work unaffected by high water flow rates.

The most important benefit is that stainless steels are unaffected in the full range of potable waters, including soft waters. Other than for bacterial control, they do not demand any water treatment chemicals.

Being buried inside the walls, the material should be corrosion resistant in the environs of brick and mortar. Stainless steels are not at all affected by cement and concrete.

Thin walled piping, easy installation and minimal maintenance during service life (over 60 years), make stainless steels a cost effective option for plumbing. Besides, stainless steels are non-toxic, manufactured from a high proportion of recycled materials and is itself 100% recyclable, which makes them eco-friendly. Because of aesthetic beauty, exposed installations are eye-catching not an eyesore.

Installation: Stainless steel tubes can be cut and bent like copper, but requiring more force. oStandard machines for bending copper are fine for stainless steel - but should be made of steel and strong enough to bend at least the next size up in copper. Electromechanical pliers are used for pressing together the tube and the fitting, which has an O-ring in a groove and makes the joint leak-free.

Pressfittings, pliers and tube

Cost: The installed cost of thin-walled stainless steel plumbing, about to be introduced in India, is expected to be within 15% over and above that of copper plumbing. Welded pipe and tube manufacturing capacity and ability abounds in India. The introduction of stainless steel fittings will be through the import route in the initial stages.

Wash basins in a hospital

Grade Selection: There is a possibility for localized corrosion when chloride ions accumulate in tight crevices or stagnant water. The 304 grades are resistant to these problems up to 200 ppm chlorides. When chlorides are expected to be above 200 ppm, up to 1,000 ppm, grade 316 containing molybdenum should be specified.

To ensure best pipework performance, especially in large buildings,

• Horizontal runs should have a fall
• Dead legs should be designed out
• Use only low chloride insulation (less than 0.05% water soluble chloride ions)
• If there is a chloride environment outside, take measures to prevent the chlorides from entering the outer surface of the pipe work.
• After hydro testing, the pipe work should be properly and completely drained.
• Normal levels of chlorination do not affect stainless steels.

Galvanic Corrosion: In practice, the galvanic difference between stainless steels and copper alloys is not significant. Stainless steel pipe can be used with copper-alloy fittings and copper hot water cylinders. oStainless steel is more noble than galvanized steel, steel and cast iron: It should be electrically insulated from such materials to prevent their corrosion. This also applies in case of fixings made of steel. When connecting the building's stainless steel pipe with the municipal mains or the water meter, place a brass connector between the two to prevent the GI pipe from corroding fast.

In conclusion, houses and buildings are designed to last 60-80 years. In general we observe seepage of water and the consequent damage to walls, ceilings and floors within 10-15 years. With stainless steel plumbing, you can be assured of leak-free service during the service life of the building. Thus, it makes good economic sense to use stainless steel at the time of construction. These are also ideal watering landscape and parks.

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