chapter 4 pe pipe
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Chapter 4:
PE / PLASTIC PIPE
BKG3493 GAS SYSTEM MATERIALS &
COMPONENTS
BACKGROUND
Plastic pipe was introduced for gas distribution system more than 30 years ago
Now has permanently established itself as excellent alternatives to ferrous pipes – low pressure operation
Offer overall advantages as compared to ferrous piping in terms of
a) low investment cost
b) corrosion resistance
c) light in weight and easily coiled
d) easy to joint
e) reduce installation time
TYPE OF PLASTIC PIPE
Currently types of plastics that were widely being used for gas distribution are Polyethylene (PE), Polyvinylchloride (PVC) and Polyamide (Nylon/PA).
PE pipe is the most popular and at present being used in all parts of the world.
PA pipe has just recently being introduced in Australia was claimed to be more superior to PE pipe in many aspects.
List the advantages of PA compared to PE?
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CRITERIA/ ESSENTIAL CONSIDERATION
In years to come, there will be more new plastic pipe being introduced to the market for gas distribution.
Each type of pipe probably could be used, as long as it meets the following criteria:
a) Long term strength
b) Ageing resistance
c) Impact strength
d) Chemical resistance
e) Temperature resistance
f) Impermeability
g) Ability to – heat fuse for joining, coil for ease of handling, squeeze off pressure.
Advantage of PE Pipe
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Coil for ease handling
Squeeze of pressure
control
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Heat fuse ability
DESIGN OF PLASTIC PIPING As according to ANSI:
Provisions in ANSI B31.8 are intended to limit the use of plastic piping primarily to main and services lines in typical distribution system operating at pressure of 100 psi or less.
The design formula for plastic gas piping system or the nominal wall thickness for a given design pressure can be determined by the following formula:
P = 2S [t / (D – t)] x 0.32 where; P = design pressure S = long term hydrostatic strength t = specified wall thickness D = outside diameter
THERMOPLASTIC DESIGN LIMITATION
a) The design pressure shall not exceed 100 psig.
b) Thermoplastic, tubing and fitting shall not be used where the operating temperature of the material will be:
- below - 20°F
- above the temperature at which the long term hydrostatic strength used in the design formula
JOINTING REQUIREMENTS & TECHNIQUES
Joint Requirements for Plastic Pipe Pipe or tubing shall not be threaded Solvent cement joints, adhesive joint and heat
fusion join shall be made in accordance with qualified procedures which has been establish and proven
Joint methods of Plastic Pipe Heat fusion of mechanical joint shall be used
when joining PE pipe, tubing or fitting Heat fusion methods – butt fusion, socket fusion,
electro-fusion The joint performance is depend on pressure,
temperature and time.
BUTT FUSION
This is the earliest technique to joint PE pipe that gives satisfactory performances
Using this method the two ends of the pipe to be jointed is heated using hot plate and fused together by applying some pressure.
This technique rely heavily on skill and commitment of the operator in order to get better result.
Essential parameters are pressure, temperature and time.
BUTT FUSION
Failure to achieve the recommended specifications for one of the parameters may result in poor joint.
Other important factors in getting good joints are the melt flow index and pipe roundness.
A big difference in the value of MFI between pipes to be jointed will results in poor bonding.
While unsatisfactory roundness of pipe will make it difficult to be aligned during fusion operation.
BUTT FUSION
GF 160 Butt fusion machine with automatic heating element
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BUTT FUSION
A Standard Butt Fusion Joint
Process jointing PE pipe
The basic rule is that only similar materials can be fusion jointed, such as PE with PE.
This applies also for the jointing of PE80 with PE100.
For best results, only components which have a melt flow index in the range from MFR 190/5 0.3 to 1.7 g/10 min should be fusion jointed.
This requirement is met by PE butt fusion fittings from Georg Fischer product.
The components to be jointed must have the same wall thicknesses in the fusion area.
SOCKET FUSION
Two pipe ends are joint together by a socket made of the same compatible material
The external wall area of the two pipes and the internal wall area of socket are heated by using specially designed tool
Pipe roundness is not critical
Still depend on operator skill
Socket joint produces stronger joint as compared to butt joint
Experience showed that this type of joint is less exposed to environmental stress cracking due to reduction of stress riser points
SOCKET FUSION
Application of heating coil inside the coupler.
The heating coil is connected to the control box – an element of automation
Electrical current is used to heat the coil
The heat from the coil will melt the surrounding pipe
Less dependency to the skill of operator, but the cost is 20% higher than butt fusion.
ELECTROFUSION
ELECTROFUSION
STANDARDS DIMENSIONAL RATIO
Enable the user to select a number of different sizes of pipe
Same SDR, will have the same D.P.
SDR is the ratio of average specified outside diameter to the minimum specified wall thickness
HEAT FUSION PRINCIPLES
Butt fusion This technique consists of heating the squared ends of two pipes, a pipe and fitting, or two
fittings by holding them against a heated plate, removing the plate when the proper melt is
obtained, promptly bringing the ends together and allowing the joint to cool while
maintaining the appropriate applied force.
Electrofusion This technique used fittings with integral heating elements. Sockets are used to join mains
and service pipes and saddle fittings are used to connect services to mains. The pipe to be
joined must be prepared by removing the outer surface layer to a depth of around 0.2 mm,
then pipe and fitting are clamped together to prevent movement. A voltage is applied across
the fitting terminals via a control box. An electric current is passed through the wire which
heats the wire and melts the polymer, fusing the fitting to the pipe. After welding, the joint
is allowed to cool before removing the restraining clamps.
Socket fusion This technique involves simultaneously heating the outside surface of a pipe end and the
inside of a fitting socket, which is sized to be smaller than the smallest outside diameter of
the pipe. After the proper melt has been generated at each face to be mated, the two
components are joined by inserting one component into the other. The fusion is formed at
the interface resulting from the interference fit. The melts from the two components flow
together and fuse as the joint cools.
MELT FLOW INDEX (MFI)
A measure of the ease of flow of the melt of a thermoplastic polymer.
Defined as the mass of polymer in grams flowing in 10 minutes through a capillary of specific diameter and length by a pressure applied (subject to gravimetric weights & temperatures) -ASTM D1238 and ISO 1133 (similar).
Indirect measure of molecular weight, high melt flow rate corresponding to low molecular weight.
A measure of the ability of the material's melt to flow under pressure.
Melt flow rate is inversely proportional to viscosity of the melt at the conditions of the test, though it should be bear in mind that the viscosity for any such material depends on the applied force.
Ratios between two melt flow rate values for one material at different gravimetric weights are often used as a measure for the broadness of the molecular weight distribution. Melt flow rate is very commonly used for polyolefins, polyethylene being measured at 190°C and polypropylene at 230°C. The plastics converter should choose a material with a melt index so high that he can easily form the polymer in the molten state into the article intended, but on the other hand so low that the mechanical strength of the final article will be sufficient for its use.
MEASUREMENT OF MFI The procedure for determining MFI is as follows:
1. A small amount of the polymer sample (around 4 to 5 grams) is taken in the
specially designed MFI apparatus which is nothing but a miniature extruder.
The apparatus consists of a small die inserted into the extruder, with the
diameter of the die generally being around 2 mm.
2. The material is packed properly inside the extruder barrel to avoid formation
of air pockets.
3. A piston is introduced which acts as the medium that causes extrusion of the
molten polymer.
4. The sample is preheated for a specified amount of time: 5 min at 190°C for
polyethylene and 6 min at 230°C for polypropylene.
5. After the preheating a specified weight is introduced onto the piston.
Examples of standard weights are 2.16 kg, 5 kg, etc.
6. On account of the weight shear is exerted on the molten polymer and it
immediately starts flowing through the die.
7. A sample of the melt is taken after desired period of time and is weighed
accurately.
8. MFI is expressed as grams of polymer/10 minutes of flow time.
POLYVINYLIDENE FLUORIDE (PVDF) Highly non-reactive and pure thermoplastic fluoropolymer - used in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low smoke generation during a fire event.
Compared to other fluoropolymers - easier melt process due to relatively low melting point of around 177°C , low density (1.78) and low cost compared to the other fluoropolymers.
Available as piping products, sheet, tubing, films, plate and an insulator for premium wire. Can be injected, molded or welded and is commonly used in the chemical, semiconductor, medical and defense industries, as well as in lithium ion batteries.
Fine powder grade, KYNAR 500 PVDF or HYLAR 5000 PVDF used as the principal ingredient of high-end paints for metals - extremely good gloss and color retention-use on many prominent buildings around the world, e.g. the Petronas Towers in Malaysia and Taipei 101 in Taiwan
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