Failure PVC Pipe

8/13/2019 Failure PVC Pipe

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ECH 3101

MATERIAL SCIENCEASSIGNMENT


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Our selected material is PVC

Figure 1,2 &3 : PVC pipe


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What is PVC?

Polyvinyl chloride, commonly abbreviated PVC, isthe third-most widely produced plastic, after

polyethylene and polypropylene

It can be made softer and more flexible by theaddition of plasticizers

Roughly half of the world's polyvinyl chloride resinmanufactured annually is used for producing pipes

for municipal and industrial applications.


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Structure of PVC

Figure 4 : Chemical Structure of PVC Figure 5 : Molecular Structure of PVC


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Raw Material

By thermal cracking of naphtha or natural gas, the basic petrochemical industrymanufactures ethylene and propylene, etc. Naphtha is mainly supplied from the petroleumrefinery industry, which uses crude oil as raw material. The chlor-alkali industry producescaustic soda, chlorine and hydrogen via electrolysis using industrial grade salt as main rawmaterial.

At a first stage in the PVC production process ethylene and chlorine are combined to produce anintermediate product called ethylene dichloride; this is then transformed into vinyl chloride, the basicbuilding block of polyvinyl chloride or PVC. The process of `polymerisation' links together the vinyl chloridemolecules to form chains of PVC. The PVC produced in this way is in the form of a white powder. This is notused alone, but blended with other ingredients to give formulations for a wide range of products.

Most commodity plastics have carbon and hydrogen as their main component elements. PVC differs bycontaining chlorine (around 57 per cent by weight) as well as carbon and hydrogen. The presence ofchlorine in the molecule makes PVC particularly versatile because it makes it compatible with a widerange of other materials. The chlorine content also helps to make PVC flame retardant. It can also be usedas a `marker' to distinguish PVC in automatic sorting systems for plastics recycling. PVC formulations canbe shaped by a variety of techniques and, using very little energy, made into the final product form. PVCpolymer is chemically stable, neutral and non-toxic. PVC formulations have a wide range of applicationsincluding the most sensitive, such as medical equipment, plus construction, automotive and electrical

cabling.


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What is the type of failure?

What isthe type

of failure?

Ductile

Brittle

Fatigue

Creep


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Fracture Surface Characteristics

Ductile

Cup and Cone

Dull surface

Dimples

Brittle

Shiny

Cleavage fracture

Flat

Fatigue

Striation

Initiation site

Propagation Zone

Creep

catastrophic rupture

viscous-like deformation

Vacancies dominate


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Figure 6 : PVC cross-section


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Figure 7 : side view of PVC


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Figure 8 : Fracture surface


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Figure 9 : Labeled fracture surface


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Fracture Surface Characteristics

Ductile

Cup and Cone

Dull surface

Dimples

Brittle

Shiny

Cleavage fracture

Flat

Fatigue

Striation

Initiation site

Propagation Zone

Creep

catastrophic rupture

viscous-like deformation

Vacancies dominate


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FATIGUE

A long-term failure

Fatigue performance of PVC fittings and joints is much lower thanthat of the pipe itself

Failures occurring under conditions of dynamic loading

a material that is subjected to a repetitive stress will fail at a stresslower than that required to cause failure on a single load applied

In short, the material fails even the stress limit is not exceeded


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How the crack propagates?


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How the crack propagates? (contd)


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Causes

of failurein PVC

Over-

pressurization

Over-bending

Vibration

Heat


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Causes of failure (contd)

Over-pressurization

One of the most common causes of cracking and splitting in PVC pipes

Each size and grade of PVC pipe is rated to a maximum fluid pressure

Exceeding the maximum pressure will cause the pipe walls to fail.

Over-bending

While most forms of PVC pipe are far more flexible than their metal equivalents, they can begin tocrack if bent too far

This can happen with improper installation of the pipe

Vibration

PVC pipe can undergo material fatigue and cracking when exposed to significant vibration for an

extended timeThis can happen when PVC pipe is rigidly joined to a pump or other vibrating piece of equipment.

Heat

When heated water or other fluid is sent through PVC pipe, it can significantly weaken the pipe walls


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Raw material used to fabricate the PVC

Chlorinatedpolyvinylchloride

Polybutylene


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Chlorinated polyvinylchloride (CPVC)

CPVC is widely used in water and sanitary systems for hot and cold water distribution

It offers much better resistance to corrosion

high tolerance to acids

It is fire resistant, though toxic fumes are emitted when it is burned.

CPVC is lightweight, non-toxic and odourless, and reduces growth of fungi, algae and bacteria.

It is designed to withstand continuous operating pressure of 600 kPa at a temperature of 95 C.


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Polybutylene

It is a light, flexible material that is easy to handle and install.

It can be used in domestic dwellings for both hot and cold water supplies.

There are several jointing systems available for the connection of polybutylenepipework systems(electrofusion and socket fusion welding)

Some mechanical joints rely on an integral grab ring while others have a compression-typejointavailable in straight lengths up to 6 metres or coils 60 metres in length.


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Measures that can be taken to

prevent future incidents of this typeof failure.


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Cyclic Failures

Cyclic and surge failures are closely related in cause and outcome. When either is found in asystem it is important to remove the cause as quickly as possible. Because water is almostnot compressible, these pressure surges are sent throughout the pipes and fittings doingdamage as they go. As the surge wave travels down the pipe line there are two chiefconditions controlling its strength. The piping material and duration of the velocity change.The more rigid the pipe material, the higher the resultant pressure spike; but softer

material requires more time to change the flow velocity.

With repeat pressure surges, or water hammer, cyclic fatigue is to be expected. It is not justthe peak pressure or the frequency, but the combination of both that is the villain. Lets usea paper clip to explain cyclic fatigue. If you make repeated right angle bends, the number of

cycles needed to break the wire will be significantly less than if bent just a few degrees.Also, if the wire is bent many times in a short period, it will break much quicker than if bentonce a day. These two conditions, frequency and amplitude of the pressure surges, arecritical to the life expectancy of a piping system. Frequent high surges will drasticallyincrease cyclic fatigue in the system. Recommended design practice is to limit the sum ofany surge pressure plus the working pressure to 100 percent of system pressure rating


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Entrapped Air

The Springtime Startup procedure is equally essential. The refilling of a system thathas been drained needs the full attention of the operator to prevent air slug andsurge failures in the operation. Start by filling the system with the valve one-quarteropened, until all the air within the system has been displaced. Only after all the air

has been removed should the valve be opened completely.

All air must be expelled from a piping system to prevent air slugs which causepressure spikes. This is best done during the filling or refilling of a system. To lowerthe chance of getting air in a system it is important to fill slowly from the lowestpossible elevation. The flow velocity during the filling process should not exceed 2feet per second. Be sure to allow sufficient air venting at the highest possibleelevation. The combination of slow filling and ample venting will keep the amount ofentrapped air to a minimum.


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Bending and Mechanical Loads

PVC is known for its ability to flex and bendmore than metal piping. Although this is ablessing in most situations, it can also be a causefor failures. Thermal expansion and contractionof piping, which is not buried, must becontrolled to reduce any mechanical loads on

the piping and fittings. The incorporation ofexpansion loops and offsets in the system layoutare generally used.


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Exposure to Sunlight & UV ray

Over the years PVC has been used in a multitude of applications above ground while being

continuously exposed to the elements including Ultraviolet radiation (UV). For many yearsthere has been concern about the ability of PVC pipe and fittings to resist the degradationinfluence of this exposure. The PVC compounds used in todays pipe and fittings are much

improved over those that were used decades ago. The present day formulations haveimproved stabilizers and UV inhibitors that resist UV breakdown that many earlier productsexperienced. Long exposure to UV does lead to some discoloration and chalking of thesurface. This is a result of a breakdown of the molecular chain on the outermost surface.

This thin surface skin in turn provides a UV shield to the PVC below, like Aluminum Oxideprovides protection to the base Aluminum.

To prevent yellowing or discoloration of the pipe and fittings that are exposed to UV light

you may wish to coat them with a heavily pigmented, water based exterior latex paint. Do

not use a solvent or oil based paint! The color of the paint is of no particular importance, as

the paint acts as an ultraviolet screen and prevents sunlight damage. White or other light

color is recommended as it helps lower pipe temperature. The latex paint must be thickly

applied as an opaque coating on any pipe and fittings that have been well cleaned. It is

recommended to paint Gray or Schedule 80 pipe and fittings a light color to cut the

absorption of solar radiation or heat into the system


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Possible Effect on EnvironmentHazardous by-

products areformed

throughout the

PVC lifecycle. Atnumerous pointsin the vinyllifecycle, very

large quantitiesof hazardousorganochlorineby-products areformedaccidentally and

released into theenvironment.

Production: Formation of hazardous organochlorine by-products begins with the production of chlorinegas. Extremely large quantities - on the order of one milliontons per year -- of chlorine-rich hazardous wastes aregenerated in the synthesis of ethylene dichloride and vinylchloride monomer (EDC and VCM, the feedstocks for PVC).

Combustion: Still more by-products are created and releasedto the environment during the incineration of hazardouswastes from EDC and VCM production, the incineration ofvinyl products in the waste stream, the recycling of vinyl-

containing metal products by combustion, and the accidentalburning of PVC in fires in buildings, warehouses, or landfills.


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By-products of PVC production are highly persistent,

bioaccumulative, and toxic. The chemical mixturesproduced in the synthesis of EDC and VCM include suchextremely hazardous and long-lived pollutants as thechlorinated dioxins (polychlorinated dibenzo-p-dioxins), chlorinated furans (polychlorinated

dibenzofurans), PCBs (polychlorinated biphenyls),hexachlorobenzene (HCB), and octachlorostyrene(OCS). In addition, a very large portion of thesemixtures consists of chemicals that have not yet beenidentified or tested. Many of the by-products of the

vinyl lifecycle are of great concern, because of theirpersistent bioaccumulative toxicity


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Persistencemeans that a substance resists natural degradation, builds up over time in theenvironment, and can be distributed globally on currents of wind and water. Many of the by-productsof the PVC lifecycle are now ubiquitous global pollutants, which can be found not only in industrializedregions but in the planets most remote ecosystems.Absolutely every person on earth is now exposedto these substances.

Bioaccumulationmeans that a substance is fat-soluble and therefore builds up in the tissues of livingthings. Most bioaccumulative substances, including many formed during the PVC lifecycle, magnify asthey move up the food chain, reaching concentrations in species high on the food chain that aremillions of times greater than their levels in the ambient environment. These substances also cross theplacenta easily and concentrate in the breast milk of human and other mammals.

Toxicity. The feedstocks, additives, and by-products produced and released during the lifecycle of PVChave been shown to cause a range of health hazards, in some cases at extremely low doses, including:

Cancer

Disruption of the endocrine system

Reproductive impairment

Impaired child development and birth defects

Neurotoxicity(damage to the brain or its function), and

Immune system suppression.


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Each group of students is to obtain anobject/structure/component that has failed. It may comefrom your home, an automobile repair shop, a machineshop, and so on. Conduct an investigation to determine thecause and type of failure (i.e., simple fracture, fatigue,

creep). Use photographic tools to analyse the type offailure.

Identify the type of raw material used to fabricate thatobject. In addition, propose measures that can be taken toprevent future incidents of this type of failure. Proposed an

advanced material that can be used to fabricate thatcomponent and describe the possible impact on theenvironment. Finally, submit a report that addresses theseissues. Presentation will be conducted in Week 14.

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Failure PVC Pipe