All the manmade things on earth have a predictable lifespan within which they either need to be repaired or replaced. Pipes are no exception. In fact, sewer pipes are subjected to a variety of factors like corrosive effluents, gas and acid generation, flow conditions, stresses from surrounding soil, pressure from top soil and moving top loads. Thus, a pipe’s serviceability can be defined as the productive serviceable lifespan that it will provide when subjected to a predicted load. Over the years, most older pipes have been replaced with better quality pipes that are more resistant to abrasion. Technological advancements have led to the development of better pipe materials that have life spans varying from 50 to 100 years. However, longer lifespans, theoretically, do not necessarily mean the same practically.
As cities are getting overpopulated because of the better quality of life and availability of resources they offer, the load on pipelines is increasing not just due to greater flow loads, but also due to infrastructural improvements on the surface like roads and culverts, which adds load to existing pipelines. The commonly used pipe materials are high-density polyethylene (HDPE), Polyvinyl Chloride (PVC), reinforced concrete, steel, and vitrified clay.
HDPE pipes are becoming a favored replacement pipe owing to its lighter weight, and its ease of transportation when compared to cast iron and vitrified clay pipes. HDPE pipes are manufactured by the extrusion technique on a single screw extruder. HDPE granules are melted and cast in dies. Air is introduced through the spider legs of the circular die, at a pressure of about 0.8 to 1 kg/sq. cm until the die takes the required size and shape. Chilled water is used for the curing process.
Estimated life span – 50 to 100 years
Pros of HDPE Pipes
HDPE pipes have joints between pipe lengths that are fused, preventing problems like leakage, and infiltration. It is water resistant, resistant to corrosion from gasses like hydrogen sulfide, abrasion resistant, and prevents growth of microbes and bacteria due to its smooth inner surface. It can be used effectively for rehabilitation purposes like sliplining, pipe bursting, floating pipe etc. and can be procured in desired lengths, thus reducing joints, consequently reducing chances of leakages, and cost of installation. It can withstand pressure surges and is suitable for earthquake prone areas due to its flexibility.
Cons of HDPE Pipes
HDPE pipes cannot be used where exposed to direct sunlight due to damage by UV light. It is not resistant to oxidizing acids, chlorinated hydrocarbons, and ketones and has high thermal expansion.
HDPE pipes need regular maintenance to ensure smooth performance and longevity of pipeline. Maintenance should include testing for unbillable water loss, system efficiency, regular checking for damages in joints, sensors and controls and checking for blocks and buildup on a quarterly or semiannual basis, unless a problem is encountered first.
PVC pipes are another popular choice for sewer pipelines owing to its flexibility and ease of transportation and installation. PVC pipes are manufactured by compounding PVC resin with stabilizers, plasticizers, etc. in a mixer from where it is fed to a double screw extruder fitted with appropriate sized dies. It is then passed through a heated chamber where it takes the desired form. The pipes exiting the extruder are cooled during the sizing process which is done either by pressure or vacuum.
Estimated life span – 50 to 70 years
Pros of Using PVC Pipes
PVC pipes are resistant to damage by chemicals and corrosion from exposure to harmful gasses. It can be easily cut into desired lengths of segments, making it easy to install. It is affordable and easy to maintain.
Cons of Using PVC Pipes
It can support lesser loads compared to galvanized iron pipes and cast iron pipes, especially in places with heavy vehicular traffic and needs good backfill support to prevent the bending of pipe sections. It can warp or distort under extreme heat.
Regular maintenance of PVC pipes should include regular checks for blockages, leaks in joints, and proper support from backfill and surrounding soil to prevent cracking due to stress from top loads.
Reinforced Concrete Pipes
Concrete has always been the preferred choice when it comes to sewer pipes and rightly so. Correctly installed concrete pipes can last for a lifetime. These pipes are manufactured using concrete made from cement, fly ash, aggregates and water. Reinforcing cages are set in forms and concrete is poured in. The curing process takes a minimum of 7 days. Autoclaves are used these days to shorten the curing process.
Estimated life span – Several centuries
Pros of Reinforced Concrete Pipes
Concrete is historically known to provide excellent service life compared to other materials. It is not easily breakable and hence can operate under high pressure conditions and does not allow intrusion of roots or soil. Reinforced concrete pipes by themselves are load bearing and thus require bedding only from the spring line to the pipe bottom.
Cons of Reinforced Concrete Pipes
Though not easily breakable, handling during installation has to be done with utmost care to prevent damage. Because it is heavy it requires heavy duty machinery for installation. It may require an additional coat of plastic materials to help prevent corrosion from acidic material in sewage water. Its rough internal surface leads to build up of material.
Concrete pipes need regular maintenance to help prevent buildup from sewage material. Regular inspection for cracks, leakages and intrusion can prevent corrosion, blockage, and buildup, thus increasing the service life of the pipe.
Vitrified Clay Pipes
These pipes can be usually found in old installations
and are seldom used today. Some of the installations that are still functional
are found to be centuries old. This makes vitrified pipes one of the longest
lasting pipelines for sewers. VCP is manufactured by blending clay and shale
and forming them into pipes. These pipes are then subjected to high
temperatures in a kiln for vitrification, which produces a hard glaze on the
pipe that is waterproof and resistant to chemicals.
Estimated life span – Several centuries
Pros of Using Vitrified Clay Pipes
VCP is highly resistant to chemical and acidic erosion. It is cheaper than steel and iron pipes and can be used as a direct jacked pipe for trenchless rehabilitation. It has the longest lifespan of all other materials.
Cons of Using Vitrified Clay Pipes
VCP has a lower pressure tolerance than metal pipes. It is brittle and can easily crack and allow intrusion from roots. Pipe length is limited due to the manufacturing process
Vitrified clay pipes are difficult to maintain and can easily crack if subjected to undue pressure. Older vitrified pipes need to be replaced with a better alternative as they were not manufactured to standard specifications and can fail under strain. Newer versions are sturdier and are less porous, with better joint designs. (Read more in "Asbestos Pipes: Why Its a Problem and How Trenchless Can Fix It.")
Galvanized Steel Pipes
Steel pipes are commonly used for water system pipes but can also be used as a casing for sewer pipes. It is a metal alloy with
greater strength than iron pipes. However, they are expensive and subject to
corrosion but at a slower rate than iron pipes. The steel pipes are alloys of
iron and other metals like aluminum, manganese etc. and are either seamless or
welded along one side of its length. A layer of zinc is applied over the steel
pipe to help fight corrosion and their average lifespan is about 40 years.
Estimated life span – 40 to 70+ years
Pros of Galvanized Steel Pipes
Steel pipes have greater durability and flexibility than iron, resulting in bending rather than breaking. It is resistant to buildup and corrosion under proper maintenance.
Cons of Galvanized Steel Pipes
It has a higher cost of purchase and installation and needs coatings for chemical resistance. It is heavy to handle.
Overtime, these pipes can develop flow blockages from mineral deposits inside the pipe. Regular maintenance checks can help prevent buildup and extend the serviceable life of the pipe.
There are many different materials that are used for the purpose of sewer and water pipeline installations. Above are just a few and the most commonly used materials. With the advent of trenchless technology, it has become easier to replace or repair pipelines that are old or have been damaged. Materials like HDPE are favored for trenchless rehabilitation because they are resistant to corrosion and are adaptable to different situations. Maintenance of pipelines is the key to ensure longer and trouble-free service life irrespective of the material of the pipe.