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CRITICAL ISSUES IN HOUSEHOLD PIPING AND PLUMBING APPLICATIONS

Prof. A. S. Khanna, IIT - Bombay

India has the world’s second largest population, with almost 1.2 billion people. In spite of having a population of thrice the size of that of the United States, India’s geographical area is about one-third of US. Such a large population in small area naturally puts strain on the country’s natural resources; mainly its water resources. During the last few decades, the national policies have emphasised mainly on sufficient water supply. In terms of physical progress, the achievements have been remarkable, and most part of the country is covered with optimum supply of water. Unfortunately, such progress has not been seen to be translated into health benefits; and waterborne diseases continue to be the dominant cause of morbidity and mortality in many parts of India. The reasons are fairly obvious: though water is being supplied, its potability is not being ensured. According to World Health Organization (WHO), about 20% of contagious diseases in India are due to unsafe drinking water. Most of these diseases can be prevented if adequate water supply infrastructure is developed to provide clean drinking water.

Although appearance, taste and odour are useful indicators of the quality of drinking water, suitability in terms of public health is determined by microbiological, physical, chemical and radiological characteristics. Of these, the most important is the microbiological activity. Also, a number of chemical contaminants may be found in water. Both bacteriological and chemical contamination cause deleterious effect on human health. The drinking water should be:

• Clear and transparent
• Not salty
• Free from offensive taste and smell
• Free from contaminations that may have adverse effect on human health
• Free from pathogenic (disease causing) organisms
• Free from chemicals that may cause corrosion to pipe and appurtenances.

When it comes to the selection of potable water supply pipes, the choice is generally among the cast iron, mild steel, galvanized steel, stainless steel, copper or plastics. Selection criteria of pipe and fitting material for plumbing depend upon the following factors:

• Effect on Water Quality Parameters
• Material Availability
• Material Cost and Installation Cost
• Fresh Water Resources
• Climate
• Chlorine Decay: To combat the re-growth of bacteria in water supply pipes, chlorine is often added to the drinking water to disinfect the water, as chlorine is relatively cheap and effective. Studies have shown that the concentration of chlorine in the water decreases with time and amount of pipe travelled by the water. The chlorine concentration decreases as it reacts with water constituents, with thin films of bio-matter and other deposits on the walls of pipes, and with the pipe materials itself. This phenomenon is called chlorine decay.
• Maintenance: Besides the materials and installation costs, the hidden costs that come from operations and maintenance must be considered.
• Corrosion: Corrosion is the result of the chemical reaction that occurs when water comes in contact with the iron pipe surface, causing iron oxides (rust) to form. Rust poses a threat to both the quality of the water and the structural integrity of the pipes.
• Strength of Materials: Potable water pipes must withstand excessive tensile forces due to hydrostatic pressure, acting outwards on the pipes and water hammer, the increase in pressure resulting from water having to change directions within a pipe.
• Failure Mode: Selection of pipe material depends upon failure mode: brittle or ductile failure.

Steel and Galvanized Steel

Steel and galvanized steel (GS) pipes are in use for domestic water pipelines for more than several decades. They are excellent and provide good service to water transport without any contamination. The only problem with these pipes is leakage. After a very short time of installation, these pipes leak at the joints, where two pipes are joined by the threaded ends, by male and female parts. Usually leaks occur at such points because of the loss of galvanised coating during thread formation. This is one of the main causes of water leaking from the galvanised steel pipes. Ordinary steel is hardly used for water pipelines, as it corrodes severely 

Further, galvanized steels usually work well, where the water is hard and a thick protective scale is formed. Galvanized steel systems often need to be replaced in hard water service because the pipe is blocked by scale formation. The galvanized coating can become Cathodic to steel in some waters at temperature greater than 60ºC, and the phenomenon is known as galvanic reversal. Hence, the corrosion of steel occurs. Since, pitting can be facilitated by the deposition of copper on the zinc, galvanized steel should never be installed downstream of copper tubes and fittings. Oxygen differential cells can be set up under deposits or at crevice and can lead to rapid corrosion.

Stainless Steel 

Stainless steel taps and fittings are more common than the pipes.The most important advantage of stainless steel pipes is their aesthetics and high corrosion resistance.

Some of the distinct features of stainless steel pipes that make them the preferred choice for potable water distribution are:

• 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

Stainless steel, however, has one severe problem of undergoing pitting attack and stress corrosion cracking in the presence of chlorides. Thus, by the proper selection of stainless steel grade and type, this problem can be solved however, such grades may be costly.

Copper

Copper is a well known piping material for domestic supply of water. In general, the use of copper in this application is successful, with very few reported instances of corrosion or failures. Erosion can take place in the hot water copper piping; if the velocity exceeds 4 to 5 ft/s. The phenomenon seldom occurs in cold water unless excessive pressure (>550kPa) is developed. Also, cold water, normally continuously circulates unlike hot water. To avoid, hot water erosion – corrosion, velocities and pressures should be kept below the designed limits, temperature should not exceed 55ºC, and workmanship should be good to avoid turbulence.

Galvanized steel pipe is generally used to bring the water from local well to storage tank. From the tank, the water is distributed throughout the house in the copper piping. If the two materials are joined together directly, galvanic corrosion is inevitable at the joint. The best answer to this problem is to use a dielectric union. Some corrosion problems in potable water copper piping can be prevented by alteration of water supplied. For example, the pH can be increased by the addition of lime, caustic soda or soda ash. Hence, cold water pitting will be due to reduction in the free CO2.

Lead contamination is still a potential health problem in some water supplying systems. This comes from the corrosion of soldering material, which is an alloy of lead and tin. By the use of inhibitors, corrosion rate can be reduced but their effectiveness depends on the specific water chemistry. Hence, they need to be evaluated for individual cases.

Plastics 

Over the last few years, the use of synthetic plastic pipes (e.g. Polyethylene, PVC, and CPVC) for drinking water distribution has been rapidly increasing. In addition, to corrosion considerations, their extensive uses in the case of high density polyethylene (HDPE) are motivated by favourable mechanical properties, ease of handling during manufacture and low permeability to external contaminants. However, in some cases, change in drinking water organoleptic properties has been noticed. Three principle causes for this change are:

• Dissolution of the polymer additives
  • Oxidation of the internal surface of the pipe and dissolution of the resulting polar compounds
  • Migration of external contaminants through the pipe

Although, the material cost of PVC is much cheaper than metallic pipes, but the installation cost is enough to offset its use. Currently, the trend is to use “trenchless technology” whenever possible, which allows pipe to be installed without excavating a large trench, usually by drilling a large hole in the ground parallel to the surface. This minimizes the exorbitant costs of using construction labour, backhoes, front-end loaders, and various other construction equipments, involved with open cut installations of pipes. Trenchless technology is much more difficult with PVC pipes than with metallic pipe, mainly due to its inferior tensile strength. Because of PVC’s low strength, it has stringent specifications for its bedding – the soil immediately in contact with the pipe.

For most applications, the strength of PVC pipes is sufficient to withstand hydrostatic pressure and pressures from the soil. But when unforeseen circumstances arise (e.g. earthquakes), the amount of stress on water infrastructure can often increase beyond standard loads, and it is important to have a material capable of handling the additional load. Metallic pipe has sufficient strength to handle a potential increase in stress, while PVC does not.

PVC experiences brittle fracture. Its failure starts with an initial, microscopic flaw (crack), which slowly grows until stress on the pipe exceeds the material’s toughness, and it experiences an abrupt failure. This type of fracture is more problematic than ductile failure, as brittle fractures often come with few warning signs. However, newer PVC materials are being developed, which experience a ductile failure rather than a brittle one, a development that will allow PVC pipes to be used with greater confidence. Locating the pipeline is fairly straightforward in case of metallic pipes, but it gets very tricky with non-metallic pipes. Because PVC pipes have no metal, they cannot be located by metal detectors and are very difficult to locate once they are buried. Often, metallic “tracer wires” are attached to the PVC pipes so they can be located, but this requires a continuous source of electricity and additional cost. There is virtually no easy way to make plastic pipes locatable underground.

Chlorinated polyvinyl chloride (CPVC) is derived by post polymerization process upon PVC. One major aspect of this process is that the working temperature of CPVC will be greater than that of PVC. These materials are not affected by water additives like chlorine residue and dissolved oxygen present in municipal drinking water. In fact, CPVC materials have been demonstrated under chemical resistance testing as environmentally resistant to these chemicals. CPVC pipe are far better than metal when transporting abrasive media . 

CPVC pipe and fitting materials do not provide an active substrate for the adherence and growth of biological or organic matter. Because of the smooth interior surface on CPVC pipe and fittings, bacteria cannot attach and multiply. These materials are resistant to the activity of all form of bacteria, many of which are known to cause corrosion in metal piping systems, such as ironoxidizing bacteria, sulfate reducing

bacteria, and acid producing bacteria. CPVC pipe and fitting do not employ plasticizers, which can act as a food source for the promotion and growth of fungi. Finally, CPVC pipe and fitting compounds are resistant to the most commonly used biocides.

At IIT Bombay, as per the requirement of the BMC (Bombay Municipal Corporation), tests were carried out to check the utility of PVC pipes and fittings for potable water supply in Mumbai, India. It was found that use of PVC pipeline does not alter the essential water quality parameters i.e., pH, odour, taste, turbidity, dissolved solid and bacterial content were in the acceptable limit of Indian standard (BIS 10500:1991). In addition to this, PVC pipes also showed good UV resistance (as per ASTM G-53, standard) and no toxic elements (e.g. zinc, lead, nickel, chromium) were leached out from pipe after 24 hrs exposure to water.

Summary

There are several factors, which must be vigilantly considered when choosing a pipe and fitting material for plumbing for potable water transport. Out of these, most important factors are effects of pipes on water quality, corrosion resistance of pipe material, structural performance of pipes, and cost considerations. Metallic pipe has considerably higher strength, ductility, longer service life and lower maintenance cost. But they have problems of metallic corrosion, bacterial attack (microbial corrosion) and high material cost as compare to plastic materials like CPVC. Plastics like PVC and CPVC can replace metal system where piping corrosion is a major issue.

Bibliography

1. World Health Organization (WHO), from http://www.who.int/countries/ind/en/
2. Department of Drinking Water Supply, India , from http://www.ddws.nic.in/
3. The Indian Stainless Steel Development Association, from http://www.stainlessindia.org
4. 4) A.H.Tuthill, “Stainless-steel piping can be cost effective for potable water use,” J. Amer. Water works Assoc. 86(7), 67-73(1994).
5. A. Cohen, “Corrosion by potable waters in building systems,” MP 32(8),56-61,(1993)
6. M. Zamanzadeh, E. Larkin, W. Gretz, B.Balvarian, “Analysis of failure of water mains,” MP 29(8), 50- 3, (1990)
7. J. Myers, A.Cohen, “Erosion –Corrosion of copper tube systems by domestic waters,” MP 37(6),74(1993)
8. Ingun Skjevrak, Anne Due, Karl Olav Gjerstad, Hallgeir Herikstad, “Volatile organic components migrating from plastic pipes (HDPE, PEX and PVC) into drinking water,” Water Research, 37(8), 1912-1920, (2003).
9. A.S.Khanna, “Special Plastics for Plumbing application,” ipt, 20-26 (2005)

The author, A. S. Khann, Professor, Department of Metallurgical, Engineering & Materials Science, IIT – Bombay,  can be contacted at khanna@iitb.ac;

Ack: This article sourced from Indian Plumbing Today - June 2009, presented for Aqua Seminar held at Davanagere, Karnataka, India.