Hand Book on Large Plastic Pipe
Damage and vulnerability of materials in sewage systems
This essay focuses on three central questions :
- What are the most common damages happening to pipes in waste water systems?
- By which factors are such damages caused?
- Which material is especially weak or strong in resistance against certain damages?
The advantages of pipe-materials and the needs of a proper sewage system are well know. Less highlighted are usually the disadvantages of materials or the most frequent damages occurring. This overview shall present these in three pinpointed questions and answers.
- Most common damages :
- Corrosion accounts for the most frequent damages happening in drainage systems. Corrosion is the gradual destruction of materials by chemical reaction with its environment. It can occur from in- or outside the pipe and with various consequences . according to the norm DIN EN ISO 8044 there are 37 different appearances. The most common one is rust corrosion but contact -, pitting -, erosion -, stress- or crevice corrosions are also pervasive. Also material incompatibility can cause corrosion. Results are for example instability or contaminations.
- Sealings are rarely taken into consideration when talking about damage potentials. Thus they are lacking of quality checks on the construction site although sealings are often the basic problem when others follow. Pipe joints can be mechanical and made from another/third material or they are homogenously welded. In concrete pipes rubber seals are often used, steel and plastics can be welded and other materials might be glued. Whatever sealing method/material is being used, it must be double checked for pressure, tightness and stability as the mecanical loading capacity is less. A common problem is additionally the durability of a sealing which is mostly shorter than the pipe itself. Many of the damages described in the following are a cause of deffected sealings.
- Pipe burst mostly occurs because of constant pressure such as traffic load and ground displacements but also because of strong corrosion, frost, fabrication defect of construction faults. Pipe bursts are rarely repairable but immediately in need of replacement. In case of waste water systems bursts are very dangerous because of environmental burden through its pathogens and pollutants.
- Aggradations can cause astringencies which can cause bursts or cracking. A few aggradations/incrustations are normal over the years but danger depends on which substance is alluvial and to what extent. Uric stone e.g. is often found in domestic waste water pipe systems and needs a special purification.
- Flaws are often difficult to recognize (craze) until they slowly become longer and more obvious. Usually a flaw is found when there are brownish spots or tiny bubbles visible at the pipe. Flaws are the precursor of cracks and bursts. One distinguishes between transverse and longitudinal flaws. Most appear while transport and implementing. Depth and pathway of the flaw are important for inspectors to decide for the correct repair method.
- Cracks are caused by transport, implementation, frost or simply obsolescence. However most frequent reason is traffic and other external load, impact or ground displacements which strain the sealing. Cracks are dangerous when exfiltration and infiltration is happening. The transported liquid from inside the pipe (possibly toxic) can leak and cause environmental pressures. They other way round, fluids and bacteria from outside can get in and cause aggradations or corrosion, which plays a greater role in drinking water systems. Therefore material consistency is a crucial factor.
- Material wear is the progressive loss of material. In the case of sewage systems, wear occurs on the moist inside wall. Higher roughness of the walls is a consequence and can be measured in variables. A cause is less hydraulic conductivity and reduction of wall thickness. Wear emerges by impacts, running liquids or droplet impingement.
- Deformation of pipes do exclusively occur in flexible thermoplastic material. Reasons for deformation are usually elevation of the ground, earthquakes, bad compaction or external pressure such as impact, soil conditions or traffic load. The water flow will change in speed and additionally maintenance might be limited as the passways are being blocked. Still, thermoplastic pipe will fulfill their requirements.
- Pirification inside the pipes is an indispensable service for the maintenance. It is done to clear the pipes from aggradations, pluming and/or prepare it for inspection. Resideus in sewage systems are containing mineral (sand/stone), organic (food, paper, plastic) and other components. Next to normal scavenging and mechanical cleansing there is high pressure purification. This method is usually used in sewage systems but can lead to considerable damage too (à GRP)
- Root penetration has always been a problem for pipe systems. During the planning process architects and engineers must coorperate with the town and country planner. When the system is running, roots can still grow into the pipe/fittings or cause deformation and cracks. There are a range of methods how tree roots can be secured and pipe systems can be built without interference such as partitions or protective tubes.
- Shifted pipes are which drifted out of their oroginal position as a cause of erosion, earthquakes, root penetration or other in- or external pressures. Pipes can shift up to an extent that they narrow or burst (à Pipe burst). Some materials are massive and heavy that rarely anything could shift them during installation but when implemented, soil or traffic pressure will affect them too. Others are designed deliberately in a way that they deform and suit the surrounding soil and incidents (à Deformation).
For this distinction not only the material matters but the joint method as well. Homogenous welded joints will not break but deform, while pipes with a mechanical sealing would shift.