Types of Pipe Material
In the past, many types of material have been used in conveying
water from one point to another. Masonry and wood were probably
the first materials used. Plastics are the newest, and are now
being used quite extensively. At present, water mains are made
of a variety of materials, summarized in the chart below.
||no longer manufactured; deteriorates
in some soils
||large, old systems
||deteriorates in some soils
||wall thickness must be carefully
||raw water mains
||raw water mains and industrial systems
||raw water mains and industrial systems
||brittle; no longer manufactured
||replaced cast iron; in old systems
||gasoline from soil can pass into
||<10 inch pipes
||requires special heat fusion joining
tools; inorganic chemicals in soil weaken pipe
||service lines in water systems and
main lines in gas systems
||corrodes; produces discolored water;
has a short life; deteriorates in some soils
Cast iron has a long history of satisfactory service. Pipes were made exclusively of cast iron in many larger systems until manufacture of cast iron pipes was discontinued in the early 1980s. Since this pipe can no longer be manufactured, little will be used in the future. The systems which formerly used cast iron pipes are now converting to ductile iron pipe and AWWA C-900 PVC pipe. Some smaller systems are converting to slip joint PVC pipe except in specialty areas such as creek crossings and when pipes must be laid extremely deep in the ground.
Ductile iron is now used in many systems where cast iron pipe was formerly used. Ductile iron pipes have certain advantages over other pipe materials. The pipes are strong and ductile (able to be drawn out and formed into a certain shape.)
The third type of metal pipe used in distribution systems is steel. Steel piping may be used in water transmission mains due to the cheap initial construction cost of the system. However, care must be taken in the design of the wall thickness of the steel pipe for the particular systems that exist. Steel pipes are more commonly used for raw water mains.
Even though most public water supplies are treated where necessary for corrosion control, all three types of metal pipes described above can be corroded by acidic water. For this reason, these pipes are usually lined to protect the metal against corrosion. Steel pipes are asphalt coated while cast and ductile iron pipes are lined with either enamel or cement. The cement lining, which is usually a one to three Portland cement mortar, is applied to the pipe by centrifugal action. The thickness of the cement lining depends on the diameter of the pipe and varies from 1/8 of an inch thick in a 2 1/4 inch pipe to 1/4 of an inch thick in a 48 inch pipe. The lining in all three types of pipe enhances the ability of the pipe to retain good flow characteristics for many years since corroded pipes are rough and offer more resistance to flowing water.
In contrast to the metal pipes mentioned above, concrete and pre-stressed concrete pipes are used mainly in very large diameter pipes such as those found in raw water lines and industrial systems. The concrete pipes are relatively inexpensive to build, which makes them attractive when large quantities of water must be moved from place to place.
Asbestos cement pipe is composed of a mixture of Portland cement
and asbestos fibers. Asbestos cement is lighter in weight than cast
iron and more brittle, so extra care must be taken when installing the
asbestos cement pipe. If the trench is not properly bedded
then the pipe will not be well cushioned in the ground and may break.
Asbestos cement pipes have been used in some cases to replace cast iron
pipes, but like cast iron, asbestos cement pipes are no longer being manufactured.
Plastic pipe is commonly used for pipes which are 10 inches or less in diameter. Rigid polyvinyl chloride (PVC) pipes are often chosen, especially when initial cost is an important factor.
There are several factors to consider when choosing a PVC pipe for use. Any PVC pipe used for water transmission must have a National Sanitation Foundation seal which certifies that the pipe contains no toxic materials and is suitable for potable water transport. It is inadvisable to use a very cheap plastic pipe since it will cause problems in the future. The same manufacturer should supply the couplings, fittings, and pipes so that all of the components will work well together.
The PVC pipes which are commonly used have a pressure class of 160 or 200. Since the cost difference between class 160 and 200 pipe is only about $0.25 per foot, many systems install the heavier duty pipe. When very high pressures are anticipated, class 250 pipe may be used. Pipes with a higher pressure class have thicker walls to withstand the water's pressure as it moves through the pipe.
PVC pipe has a safety factor of 2:1, compared to the 4:1 safety factor of ductile iron pipe, so you can't directly compare the pressure class ratings of the two types of pipes. In addition, the pressure class ratings of PVC pipes do not include surge pressure - extra pressure when the water moves much faster than usual. For both of these reasons, it is recommended that the static pressure of the water in a PVC pipe not exceed 70% of the class rating. So, if you anticipate a static pressure above 112 PSI (70% of 160 PSI), then you will need to use class 200 pipe.
PVC pipes are not the only types of plastic pipes used in water systems. Polyethylene and other non-rigid plastic pipes are used as service lines in water systems and as main lines in gas systems. However, polyethylene pipes have a high molecular weight, so special heat fusion joining tools are required when working with the pipes. For this reason, polyethylene pipes are limited to special installations.
Copper pipes have been used in some situations as service pipes. However, copper pipe is more expensive than plastic pipe. Copper pipe used in water systems must have a National Sanitation Foundation Seal.
Galvanized iron is the final type of pipe which will be considered
here. In almost every case, it has been found to be more desirable
to use plastic pipe rather than galvanized iron. Galvanized iron
corrodes easily, produces problems with discolored water, and has a relatively
short life. For these reasons, galvanized iron is seldom used in
the distribution system.
Even when water mains are properly installed, the pipes will deteriorate over time. This deterioration can be slowed by matching the pipe material to the soil or by wrapping the pipe.
Cast iron, ductile iron, and galvanized iron pipes can all be weakened in just a few years when laid in aggressive soil. To prevent this type of damage, the soil should be tested before laying the pipes in the ground. If necessary, the pipes can be wrapped in plastic during installation to protect the metal from the soil. The Ductile Iron Pipe Research Association can provide detailed information on the wrapping procedure.
The type of soil is also an important consideration when installing plastic pipes. Organic chemicals, especially solvents and gasoline, will weaken PVC pipes, causing the pipe to expand and rupture. The operator in charge of ditching should be alert for any unusual odor when removing soil during the construction of the distribution system. The odor may be a sign of a chemical spill, which may remain in the soil for many years and weaken PVC pipes.
Gasoline and diesel can pass through the walls of polyethylene pipes even when the water inside is under high pressure. For this reason, plastic pipes should never be installed in the vicinity of gas stations. Instead, ductile iron and copper pipes are recommended for service lines in the vicinity of gas stations.