APPLICATIONS/USES OF POLYETHYLENE PIPE

General applications

Recommended applications include: fluids like potable and process water, benign chemicals, dry natural gas (Federally regulated), dry CO₂ H₂S, wastewater, sewage, glycol/antifreeze, solvating-permeating chemicals, fuel-gas liquid condensates, crude oil, fuel oil, gasoline, diesel, kerosene, and hydrocarbon fuels.

Non-recommended applications include dry transport of wheat, corn, or food grains (very dangerous because of static electricity may cause an explosion), pneumonic transport of non-combustible solids, and above grade compressed lines (unless secured against movement and external damage). ²³

Specific applications

• A PE pipeline project in East Java and West Sumatra has been successful in converting H₂S gas into useable, saleable energy for industrial use, thus reducing emissions from flared gas (gas seen burning at refineries and at well heads). Another important technological benefit has seen the PE pipe with meter equipment installed reduce unaccountable gas to less than 2% per year. ⁹

 

• High tech industrial pipe configurations are using KWH’s WEHOTEK system that use PE pipe as a jacket over insulation, sensors and heating elements that surround a metallic pipe core, in applications where leak detection, freezing and heating are important considerations. ¹⁵

 

• PE pipe will be used more often in the replacement and construction of natural gas mains, horizontal directional drilling, and various other trenchless technology applications. ¹³

 

• Saves money in labour time during infrequent repairs vs. steel pipe. Also lighter weight means less stress on cable bridges and installation equipment. ²

 

• By cooling PE pipe during manufacturing using water and blown air, a pipe manufacturer will save 25% in space in his production line at the extrusion machine; hence, cost/space ratio of the plant is improved to reduce investment costs. ³

 

• PE pipe does not form graphitic corrosion like buried cast iron pipes, thus is more suitable for water mains and will save time and money in repairs as the PE may be inserted into the cast iron pipe. ⁷

 

• A technology assessment report by the Institute of Gas Technology found that by using 4 and 6-inch coils vs. fusing together short sections by saving $1/foot. The total impact upon the US market in 1994 was estimated to be about $40 million dollars in savings to US natural gas distributors (Rush and Campbell). ⁸

 

Production outlook

The global demand for PE pipe is increasing due to the superior properties of PE pipe such as: resistance to reactive chemicals, gas, waste water, and flexibility of the pipe to yield and not break under normal stress in regions where the ground is varied and constantly moist. ¹¹ The price per pound of PE plastic pellets is ranging from $0.28-0.54US/pound (1982-1997) with the current (1997) demand at $0.44US/pound.

The domestic demand for high-density PE is primarily in the blow molding applications for end products as chemicals, containers, plastic pipes, and chemical tanks. The growth of PE will increase in the 3-5% range into 2002 as shown in the Graph 1 below. Furthermore, Canadian PE production is expected to increase in 2000 from maturation of projects that export into the US markets. Financial market volatility in Asia may decrease the export of PE. ²²

DEMAND FOR PE PIPE FROM 1997-2002

(Global)

Graph 1 Demand for PE from 1997-2000

Large diameter coiled PE pipe economics

Cost comparison using 4" coiled vs. straight PE pipe

(Newark, NJ)

Table 1 Savings illustrated by the use of large diameter coiled PE pipe by PSE&G in Newark, NJ, USA

1. Advantages of coiled pipe

• Installation costs are minimized by reducing the number of butt fusion joints

• Installation is easier and faster with customers being less inconvenienced

• Large-diameter pipe in 250-1000 foot rolls are require less time to uncoil than un-stacking and organizing pallets of PE pipe sections

• Smaller staging area needed to handle coils vs. sections

• Safety is improved because fewer butt fusion joints are used

2. Disadvantages of coiled pipe

• The coiled pipe must be re-rounded and straightened off the roll because it ‘ovals’

• Coiled pipe has curvature which can create handling problems ¹³

Swagelining economics

A swagelining project in the North Sea in the Foinaven Field using the Swagelining process resulted in the saving of over 45% in replacement costs. The speed and ease of inserting the PE pipe saves time and money and may make a marginal petroleum project economically feasible, especially in offshore petroleum wells. Also using PE within a carbon steel pipe affords protection against internal corrosion, which results in longer pipeline life.¹⁹

Safety and economics of UltraMc ultrasonic inspection

A case study by Oklahoma Natural Gas concluded that by their use of UltraMcä testing systems, their safety has been impeccable and the company is realizing an annual savings of $110,000 because their butt fusion joints are not leaking valuable natural gas as shown in Table 2 below.

USE OF ULTRAMcä ULTRASONIC DEVICE

FOR PE GAS-PIPE INSPECTION BY OKLAHOMA NATURAL GAS COMPANY

(Tulsa, Oklahoma)

Table 2 Savings of the UltraMcä system for O.N.G. ²¹

Installation using trenching, laying, and backfilling

PE pipe is much easier to install than other materials because it is very tough, even in extreme cold, and can withstand rough handling without the high breakage rates typical of conventional installations. PE pipe is comparatively lightweight and can be handled easily with less expensive equipment. For example, long runs are often prejoined at a convenient site, slid into place and either rolled off into a trench prepared for direct burial or inserted into old lines. Recommended installation procedures for the direct burial of PE pipe gas distribution systems are included in ASTM D 2774, "Standard Recommended Practice for Underground Installation of Thermoplastic Pressure Piping".²⁴ When trenching to lay pipe, it is recommended to have 6 inches on both sides of the pipe to the trench wall. A 6-inch pipe should have and 18-inch trench.²⁷ Generally; hauling, unloading and stringing PE pipe should be done with the care necessary to prevent damage to the pipe. Since polyethylene is softer than steel, poor handling techniques can result in gouges, cuts, or punctures. If the pipe or tubing is cut or gouged to a depth exceeding 10% of its wall thickness, then that portion of the material that is damaged must be cut out or replaced. PE pipe may be cold-bent as it is installed, and may eliminate the need for elbows in slight bends. The minimum bend radius that can be applied to the pipe without kinking varies with the diameter and wall thickness of the pipe. If adequate space is not available for the required radius, a fitting of the desired angle may be fused into the piping system to obtain the necessary change in angle.²⁴

Pipe locating technology

The Gas Research Institute has been using EM radiation and sonic logging techniques on a macro scale to locate PE pipe up to 3 feet below the surface of the soil. The results are that the EM radiation is best used in dry soils; whereas the sonic logging techniques are better in wet soils. ¹²

Magnetic PE pipe is created by embedding small ferrite particles within the matrix so that it has a distinctive signature from other underground magnetic structures. This enables companies to find the pipe using above ground magnetic locating devices. ²¹

Swagelining

When transmission pipeline systems need to be updated they can simply be renewed by pulling or pushing PE liner through the existing pipe runs, even those with misaligned joints. Clearly, the PE liner must be somewhat smaller in diameter but its high flow velocity allows equivalent volume capacity. This technique is called insertion renewal and is the least costly method of salvaging a failing, substandard system. Extensive trenching can be eliminated because excavation can be limited to a few key sites where the new liner goes in and pulling cables come out. Those locations can often be selected to minimize disruption of streets and traffic as well as private property and businesses. Properly engineered systems can support heavy earth loads and even the collapse of a heavy old system.¹⁸

• One name given to this PE lining system is Swagelining. The Swagelining system uses PE pipe that has an outside diameter slightly larger than the inside diameter of the pipe to be lined. After sections of PE pipe are butt-fused together to form a continuous pipe slightly longer than the pipe to be lined, the PE pipe is pulled through a reduction die to temporarily reduce its diameter. This allows the PE pipe to be pulled through the original pipeline. After the PE liner has been pulled completely through the host pipe, the pulling load is removed. This allows the PE pipe to return to its original diameter until it presses tightly against the inside wall of the host pipe. The tight-fitting PE liner results in a flow capacity close to the original pipeline design. The Swagelining system has been used in gas, water, sewer, industrial, petrochemical and offshore applications. ¹⁹

 

Standards

Standards serve to maintain quality and promote uniform sizing and materials in the manufacture of pipe. There are three standards groups: ASTM, the American Society of Testing and Materials; AWWA, American Water Works Association; and ANSI, the American National Standards Institute. Material type and other information are coded by letters, digits to indicate the kind of material, type, grade, and hydrostatic design stress.²⁷

Example: PVC 1220-made of polyvinyl chloride, type 1, grade 2, with hydrostatic design stress of 2000 (20) psi.²⁷

For polyethylene the ASTM standards are as follows:

• D2104 (Specification for PE Plastic Pipe Schedule 40)
• D2239 (Specification for PE Plastic Pipe (SIDR-PR) Based on Controlled Inside Diameter)
• D2447 (Specification for PE Plastic Pipe Schedules 40 and 80 Based on Outside Diameter)
• D2737 (Specification for PE Plastic Tubing)
• D3035 (Specification for PE Plastic Pipe (SDR-PR) Based on Controlled Outside Diameter)
• D2609 (Specification for PE Plastic Insert Fittings)

Some common terms that relate to pipe size are defined as follows:²⁸

• IPS - IPS stands for "Iron Pipe Size" which is a steel pipe sizing standard that has been in existence for years.
• DIPS - DIPS stands for "Ductile Iron Pipe Size" which is a ductile pipe sizing standard that has also been around for years.
• JIS - JIS stands for "Japanese Industrial Standard" which is the piping standard for Japan.
• ISO metric - ISO metric stands for "International Standards Organization" which is the piping standard used in many parts of the world.
• CTS - CTS stands for "Copper Tube Size" which is the copper tubing sizing standard in the United States.

 

General classifications of polyethylene

Figure 1 Each corner and ends of the zigzag represent carbon molecules with their full complement of hydrogen atoms

• Low-density polyethylene is produced with high pressures. The commercial production of low-density polyethylene occurs at 1500 to 3500 times atmospheric pressure and at temperatures of 80 to 300^oC. The production of polyethylene through this process makes use of a peroxide that converts the unreactive ethylene molecule into a reactive molecule by removing one of the ethylene’s hydrogen molecules. This starts a chain reaction which in the end produces randomly ordered chains branching off the polyethylene.³⁰

 

• The synthesis of medium-density and high-density polyethylene is achieved using a class of compounds called catalysts. A catalyst is a substance that can start a desirable chemical reaction or speed up a reaction that would otherwise be to slow to be economical.³⁵ The catalyst is not used up in the reaction and at the end of the reaction comes out in the same form that it was added to the reaction mixture. This theoretically means that you could buy the catalyst once and reuse it forever. However, this is not the case since certain contaminants can bond to or react with the catalyst and cause the catalyst to be poisoned (become unreactive). Even so, a catalyst can be reused in thousands of chemical reactions before it needs to be replaced.

 

Physical properties of polyethylene

Polyethylene is referred to as a semi-crystalline material. This means that the structure of polyethylene has both well-defined regions (crystalline) as well as random arrangement of the polyethylene molecules. Lamella is the name given to the crystalline regions of polyethylene. The more randomly ordered regions of polyethylene are referred to as amorphous.²⁴

• Lamella and amorphous regions in the polyethylene polymer result from speed at which the polymer is cooled. If the polymer is cooled slowly the individual molecules of, the polymer will tend to bond in an ordered folded pattern and will thus have more lamella. This would result in the polymer having a higher density since more of the molecules could be packed into a tighter space. If the polymer is cooled quickly there will be greater randomness in the arrangement of the polyethylene molecules. The result is that the polymer will have a lower density. The ratio of the lamella versus amorphous gives the density of the polymer and in turn the grade of the polyethylene.²³ The effect of crystallinity on the physical characteristics of polyethylene can be seen below in Table 3.

 

Table 3 Shows physical properties changes with increasing molecular weight.²³

 

• The crystalline lamella has a density of approximately .970 to 0.980 g/cm³. Amorphous regions commonly have a density of approximately 0.880 g/cm³. As density increases, these properties in polyethylene increase tensile strength, the softening temperature, the chemical resistance, as well as the rigidness.²⁴ As density increases these characteristics decrease elongation, low temperature impact strength, crack resistance as well as permeability. Therefore, a balance of the strength, flexibility, and other factors must be taken into account when deciding on the desired characteristics of the pipe.

 

When polyethylene is produced, three common grades produced low density, medium density, and high density. The low-density polyethylene is a straight chain molecule, which has several long carbon side chains attached. Typically, the density range for low-density polyethylene is from 0.910 to 0.925 g/cm³. The high-density polyethylene on the other hand has very few long carbon side chains attached to the molecule. The density for this type of material is from 0.941 to 0.965 g/cm³. Medium-density polyethylene is intermediate in terms of the number of carbon side chains involved and the arrangement of the molecules in polymer. The density range common for medium density polyethylene is 0.926 to 0.940 g/cm³.²⁴ As can be seen in Table 4 below, the crystalline melting point and tensile strength in general increase with an increase in density.

Table 4 Physical Characteristics of Three Grades of Polyethylene³⁰

Polyethylene is a very stable plastic under most environmental conditions. In general polyethylene does not corrode and is not adversely affected by normal conditions in soil or in water.²⁴ Polyethylene is also suited to transport chemicals that would otherwise corrode normal steel pipes such as salt water, acids, and caustics. There are certain chemicals however, that will attack the plastic can damage the pipes beyond repair. These chemicals include sulfuric acid, nitric acid (50%), organic peroxides, bromine, and chlorine to name a few.

Other chemicals do not do any damage to the polyethylene pipe but instead slightly change the characteristics of the pipe. These chemicals, which are commonly liquids, get absorbed into the pipe in low concentrations. Certain liquid hydrocarbons for example when absorbed by the polyethylene pipe can reduce the tensile strength of the pipe by as much as 10%, stiffness by 25% and the radial swelling of the pipe from 1 to 5%. The absorption can be greater than 5% therefore; the pipe should be tested before hand with the chemical to be transported. These factors only mean that when polyethylene pipe is used a safety factor should be added to the specifications of the pipe to account for this property. Some of the chemicals that are absorbed by polyethylene pipe include gasoline, diesel, toluene, and xylenes to name a few.

A more complete list of the chemical resistance and other properties that need to be considered is included in the appendix. This list includes hundreds of chemicals that are commonly used in industry.

Ultraviolet radiation can be harmful to untreated polyethylene pipe in that this can cause the polyethylene to become degraded. To overcome this problem two classes of ultraviolet stabilizers are used in polyethylene pipes. These two classes of compounds are referred to as UV absorbers and hindered-amine light stabilizers HALS) are used. A common UV absorber is Chimassorb 81 (Cyasorb 531). These additives absorb ultraviolet radiation in the range of 300-360 nm (* 10^-9m), the wavelengths of sunlight most damaging to polymers and neutralize them. Pigments can also be added to polyethylene to prevent ultraviolet radiation damage of polyethylene. The most common of these pigments is carbon black, which is added to the polyethylene polymer in concentrations of 2 to 3% by weight.³⁴

Cross-linking is the process by which several individual "strings" of polyethylene are bound together physically through radiation or through a chemical reaction. Polyethylene can also be cross-linked by exposure to high-energy radiation such as X-rays, gamma rays, as well as fast electron bombardment. In addition, polyethylene can be crossed-linked through chemical means by heating the polymer in the presence of peroxide. The result of the cross-linking of polyethylene is that the shape that the polymer is molded into becomes stronger and can retain its shape at higher temperatures than untreated polyethylene.

PE pipe vs. steel pipe

A comparison illustrating the differences in properties between PE and steel pipe is shown below in Table 5.

Table 5 PE pipe vs. steel pipe properties

Special properties of PE pipe

The coefficient of thermal expansion of PE is 0.00007 inch/inch/° F or 1.7 inches/10° F/200 feet of pipe. ⁴

PE pipe has a viscoelastic nature, which shows creep effects that cause the pipe to deflect under load when it is buried until a stabilizing equilibrium is reached to create stress relaxation as shown in Graph 2. The viscoelasticity is very beneficial in buried pipe behavior as stresses caused by earth movements will not breach the integrity of the pipe very easily, thus the contents are held where they need to be.

Graph 2 Illustrates creep and stress relationships vs. time

The safety factor used in PE pipe is a minimum of 1.5, which is based upon using a prism load or the weight of soil compressing the top of the pipe from above. This prism load is the maximum load on the pipe, but the actual load is less due to friction and cohesion of the particles in the soil. The following Equations 1-1 and 1-1a, in Figure 2, show how the prism soil load or W_c is calculated. ⁶

Figure 2 Equations in Imperial and Metric for calculating prism load.

The chemistry of polyethylene

Polyethylene is the one of the largest class of plastics produced in the world today. In its purest form, polyethylene is made from the joining of thousands of monomers (individual units) of ethylene gas to form a long carbon chain studded with hydrogen atoms. Polyethylene can also refer to a polymer in which some propylene, butene, or hexene is added in some small quantities to the ethylene so that the polymer produced can have the desired physical characteristics. This type of polymer is called a copolymer, which will be discussed later. For pipe grade polyethylene, at least 85 percent ethylene is mixed with the remainder being mainly hexene.²³

The general chemical reaction that is employed in making polyethylene is shown below in Figure 3. The n is the number of monomer units typically found in a polyethylene molecule and n is on the order of 1,000-5,000.³²

Figure 3 General chemical reaction that converts ethylene to polyethylene³³

Manufacturing process methods

The primary function of the plastics extrusion is to melt solid particles of resin, homogenize or mix the melt and pump the melt through a forming die on a continuous basis. Extrusion should be accomplished with minimum heat input and maximum extrusion rate compatible with downstream handling equipment.²⁴

Both high and extra high molecular weight HDPE sheet is widely used for thermoforming. These resins feature high melt strength to facilitate the forming operation and excellent toughness to ensure product performance. The quality of a thermoformed part is not only dependent on the thermoforming step but also on the quality of the extruded sheet from which it is formed. The selection of the proper technique will depend on such things as part dimensions; type of surface desired (textured or glossy), wall thickness and depth of draw.²⁴

To form such a broad variety of parts, the blow molding process includes several variations in techniques. For example, molding systems use either a molten plastic tube called a "parison" or a cold preform, which can be injection molded or cut from a tube. The cold preform requires reheating before molding used with "stretch blow" molding systems.²⁴

High-density polyethylenes have proven especially versatile in injection molding. Resins have been designed to meet performance requirements in applications ranging from toys, housewares and garbage cans, stadium seating and small engine gasoline tanks.²⁴

In the late 1950’s, molders began using rotational molding to form polyethylene. For rotational molding, the PE is ground into fine powder, and then a premeasured amount is poured into a hollow metal mold. The mold is sealed and then heated while it is rotated about two axis 90º to each other. During this rotation the powder is heated and fused together as it deposits on the mold surface. After cooling, the mold is opened and the finished part removed.²⁴

PE pipe and tubing

Although several techniques have been developed for sizing tubular products, most pipe is produced by vacuum sizing or external sizing tube techniques. Regardless of the sizing technique, the high molecular weight resins give the best pipe performance characteristics. The following diagram in Figure 4 illustrates the typical process used to form polyethylene pipe.²⁴

Figure 4 Polyethylene process flow diagram²³

Chemistry used in manufacturing polyethylene

The Ziegler commercial polyethylene production process occurs at pressures that are two to four times atmospheric pressure, at temperatures of 50 to 75^oC. The catalyst that is used is the Ziegler-Natta catalyst [Ti(Cl)₄/ Al(CH₃CH₂)₂Cl]. Interestingly Karl Ziegler and Giulio Natta won the Nobel Prize in Chemistry in 1963 for the production of high-density polyethylene using the catalyst they created. In this process, the ethylene is continuously pumped into a vat that contains the catalyst that is mixed in a hydrocarbon bath. The reaction occurs in a water and oxygen free environment since these two compounds lessen the effectiveness of the catalyst and can make the reaction explosive. The ethylene when it makes contact with the catalyst in that bath it forms a slurry that can be removed periodically. During this process ethylene is continuously pumped into the container.³⁰

There are two main metal oxide catalyst processes to produce polyethylene, the Phillips, and the Standard Oil. The Phillips process involves using pressures that are 30 to 40 times greater than atmospheric and temperatures between 90 and 160^oC. The catalyst used consists mostly of chromium oxides mostly those of the form CrO₃. In this process, the catalyst is dissolved in a hydrocarbon, commonly cyclohexane. The Phillips process if done in at temperatures of temperatures of 90 to 100^oC the reaction of the catalyst with the ethylene produces a slurry that can be continuously be drawn off. In the case where the reaction is done in the 120-160^oC range, the polymer formed is found as a liquid and can only be separated by removing some of the liquid in the reactor and cooling the liquid to extract the polymer. The Standard Oil process involves pressures of 40 to 100 times atmospheric pressure and temperatures of 200 to 300^oC. The catalyst involved in this process is MoO₃.

• Metallocene catalysts are increasingly being to make polyethylene. The reason is that metallocene catalysts can be customized depending on the type of polyethylene that is to be produced. The metallocene catalysts that are in use are quite new with the earliest ones coming out in the mid-1980s. Most of these catalysts differ slightly in their structure, depending on the company that produces them and in they are proprietary information.³⁶ A typical structure of a metallocene catalyst can be seen below in Figure 5, however the Zr can be replaced with a Ti or Hf, the Si can be replaced with a C and the CH₃ can be replaced with any size hydrocarbon chain.

 

Figure 5 Typical Structure of a Metallocene Catalyst³¹

High tech manufacturing of PE pipe

• Environmentally safe to manufacture PE products and dispose of them. ⁴

 

• Problem: Sag or post mold melting of thick walled PE pipe as it exits a die causes an increase in porosity in the core of the PE pipe. This can cause a reduction in UTS and density; thus lower grade pipe is produced out of ISO standards specifications. Solution: Cooling the outer pipe wall with 10° C water and blown-air cooling the inner wall to about 135° C, eliminates sag and maintains density. ³

 

• Wall thickness ultrasonic measurement control (UMC) is used to maintain quality control of the PE pipe extrusion. The UMC is composed of computerized acoustical sensor arrays that measure the thickness and allow instant corrections on the extrusion line. ¹⁴

 

• Because conventional connecting technologies for PE pipe are time-consuming and expensive, a new system of using laser measurements and high precision belling of PE pipe socket ends has been developed. This technology allows connection of PE pipe like normal PVC with the socket ends being resistant to shrinkage and stable to 70° C. ¹¹

 

Pipe fusion technology

Heat fusion makes joining PE pipe easy, rapid, and economical. The ends of the pipe are melted and then the melted ends are held together and allowed to cool. This is the strongest, most reliable, leak-free joint possible. It is a seamless connection that provides the same strength as any other part of the pipe.²⁹ Heat fusion (or, thermal butt fusion) machines are available for use on PE pipe ranging from ½" to 65" in outer diameter. The amount of time to make a fusion joint depends on a number of variables such as pipe size, job set-up, and fusion parameters and procedures. With an experienced crew and a heat fusion machine (McElroy), a rule of thumb is 11/2 to 2 minutes per inch of pipe diameter, depending on the ratio of pipe diameter to wall thickness (DR). A data recording system (Dataloggerä ) is used to record and document the profile of each fusion joint made. It can measure and record the heater plate surface temperature, as well as the cylinder pressure during each fusion process. It can produce printouts in metric or inch measurements and by means of a LCD display screen, prompt the operator for fusion parameters for each type and size pipe and machine used, before fusion takes place.²⁸

Pipe fusion procedure

(general process steps)

1. Make sure all proper equipment is on site.

2. Assemble the equipment.

3. Shield fusion equipment in inclement weather.

4. Preheat the heating elements to 260ºC (500ºF).

5. Clean and check, the surfaces of the tools, pipe, and fittings are clean with nonmetal, non-synthetic tools before fusion. Failure to do this will result in a faulty joint.

6. Inspect the pipe for deep scratches. Reject deeply scratched pipe.

7. Perform a trial fusion at the beginning of the shift.

8. Remove tension in the line before attempting fusion or making any connections. With coils join position the pipe in an S position to remove the stress on the joint.

9. Align the components, when aligned secure using jaws or clamps on the fusion machine.

10. Remove static electricity before cutting or tapping lines in natural gas applications using a water/soap or water/glycol solution, then ground the electricity with a solution-wetted cloth.

11. Trim the pipe ends with the rotating cutters in the fusion machine.

12. Disengage the cutters and clean the surface of the pipe with a dry brush to remove any shavings and cuttings from within and around the ends of the pipe to be joined. Do not touch these ends, as the oil from your fingers will result in a faulty joint.

13. Join both surfaces of the pipe and fittings with the fusion tool at manufacturer’s specified heat at the specified amount of time.

14. Remove fusion tool and bring melted surfaces together.

15. Hold with pressure until the molten plastic is solidified.

16. Allow cooling until touch, then test with UltraMc. ²³

 

Product testing technology

The following hydrostatic testing procedures are briefly explained previewed for the benefit of field personnel. Hydrostatic pressure testing is the recommended method using a clean water medium.

1. Isolate, remove and vent valves, or any other lower pressure rated componentry.

2. Use 1.5 safety factor based upon design pressure as maximum limit. In cases of gravity or low pressure consult with manufacturer to find correct procedure.

3. Fill the test section with liquid while bleeding off any air by venting at high points of the pipe.

4. Pressurize for a maximum time limit of 8 hours in the range of 1-1.5 times the maximum design pressure limit. If the system fails due to leakage or equipment failure or any other reason, allow the pipe to ‘relax’ for 8 hours afterwards before reapplying the pressure test.

5. Test at initial expansion phase uses test pressure.

6. Add liquid to bring to test pressure after 1 hour.

7. Add liquid to bring to test pressure after 2 hours.

8. Add liquid to bring to test pressure after 3 hours.

9. Add only measured amount of liquid to bring to test pressure and compare to known tabulated facing values. If the amount of liquid added does not exceed the allowable value, then no leakage is indicated. ²³

 

Ultrasonic NDT techniques

Ultrasonic non-destructive testing (NDT) techniques must use low frequency compression waves under 4MHz in order to achieve sufficient penetration and acceptable resolutions in typical grades and thickness of PE. The use of physics compares attenuation or decibels per millimeter (dB/mm) as directly proportional to the frequency in megahertz (MHz) of the ultrasound. Graph 3 below.

Graph 3 Attenuation vs. frequency in shear and compression waves 

(Munns, I. J., Georgiou, G. A)

1. Pulse-echo compression wave technique is shown below in Figure 6. This technique involves using trigonometry with 60° angles to test the butt fusion welded joints, but performs poorly because it will not detect flaws such as: dust, inclusions, or cold welds.

 

 

Figure 6 Pulsed-echo compression wave technique.

(Munns, I. J., Georgiou, G. A)

2. Pulse-echo creeping wave technique is shown in Figure 7. The area beneath the outer butt fused weld head is examined by the use of creeping waves. This technique can detect lack of fusion and large impurities within the weld, but cannot resolve dust contamination or borderline cold weld.

Figure 7 Pulsed-echo creeping wave technique.

(Munns, I. J., Georgiou, G. A)

3. The tandem technique as shown below in Figure 8, is best suited to detect lack of fusion in the butt welds. A transmitting ( T ) probe is moved relative to a stationary-receiving probe ( R ). This yields good coverage as to the thickness of the weld to a tolerance of 2mm and is able to detect dust contamination. The negative is that it cannot detect a borderline or nominal cold weld.

 

Figure 8 Tandem technique.

(Munns, I. J., Georgiou, G. A)

4. Time-of-flight diffraction (TOFD) technique as shown below in Figure 9 is used primarily to inspect butt welds in PE pipe by determining the thickness of the weld through perpendicular echoes showing the height of any flaws.

 

Figure 9 Time-of-flight diffraction.

(Munns, I. J., Georgiou, G. A)

A flaw will yield readout as shown in Figure 10 as a smaller blip in the wave pattern. However, TOFD is not able to detect all signals from the top and bottom of a flaw due to its limitating lower frequencies.

Figure 10 Wave patterns with TOFD.

(Munns, I. J., Georgiou, G. A)

Radiographic non-destructive testing (RNDT)

Figure 11 Throwing vs. straight-line techniques.

1. Straight-line technique uses the source of X-rays within the plane of the weld, thus yielding a straight line of the weld.

 

2. Throwing technique is used as the secondary technique when more information about the shape and character of the flaw is needed. It uses an X-ray source that is offset from the plane of the weld; hence yields a curve shape. ⁵

 

Modulus testing using load vs. time

A University of Massachusetts research project tested corrugated PE pipe by creating gradual increments of stress on the pipe over a long period to determine the effects upon the material modulus. The graph illustrated below in Graph 4 illustrates the tests and proves that the behavior of the material indicates that the PE pipe does not lose strength, because the modulus remains the same every time a new load is applied.⁶

Graph 4 Modular testing of load vs. time.

 

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Appendix

 

A list of common chemicals and their effects on polyethylene are listed below using the key displayed below.²³

Chemical Resistance Key

Key	Meaning
X	Resistant (swelling < 3% or weight loss < 0.5%; elongation at break not substantially changed)
/	Limited Resistance (swelling 3-8% or weight loss 0.5- 5%; elongation at break reduced by < 50%)
__	Not Resistant (Swelling > 8% or weight loss > 5%; elongation at break reduced by > 50%)
D	Discoloration
*	Aqueous solutions in all concentrations
**	Only under low mechanical stress

 

 

 

 

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