Design Stresses of Pexgol Pipes

Pexgol pipes are produced according to the DIN Standards 16892/16893 and the Israeli standard 1519 Part 1. These standards show the working pressures of Pexgol pipes at various temperatures.

Pipes according to other standards (IPS according to ASTM 2788, for example) are available by special order.

The working pressures for Pexgol pipe are determined by the following equation:

P = 2σt/D-t or P = 2σ/SDR-1 or σ/S

  • P = Maximum working pressure (kg/cm²)
  • σ = Long term strength at the design temperature (kg/cm²) (10.1)
  • D = Outside diameter (mm)
  • t = Wall thickness (mm)
  • S = ISO 4065 series
  • SDR (Standard Dimensions Ratio) = D/t = 2s + 1

Table No. 14.1: Changes of design stress values σ with temperature:

table 14.1

 

Allowable working pressures

The working pressures of Pexgol pipes are based on DIN 16893-2000 and the accumulated experience of Pexgol pipes in infrastructure and industry, including pipes that were installed 30 years ago at the Dead Sea Hot Leach Crystallization Facility carrying hot sylvinite at 114°C and are still working today.

The design stresses σ in tables 14.1 and the working pressures in tables 15.1 & 15.2 were calculated with a safety factor of 1.25. According to DIN 16893-2000, these values are for water.

In the case of chemicals and corrosive agents, the working pressures might have to be de-rated according to the data in the chemical resistance tables.

 

Table No 15.1: Allowable working pressures [bar] for conveying water in Pexgol pipes, with a safety factor C = 1.25

table 15.1

 

Table No 15.2: Allowable working pressures [psi] for conveying water in Pexgol pipes, with a design factor DF = 0.8; safety factor C = 1.25

table 15.2

 

Dimensions & Pressure Ratings

Pexgol pipes are transported in coils, coils with cores and straight sections. See Transportation section for more information.

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Pexgol Pipe class 6 (SDR 26 S12.5)
Working pressures: 6 bar at 20°C, Initial ring stiffness 5 KN/m²

class 6

 

Pexgol Pipe class 8 (SDR 21 S10)
Working pressures: 7.6 bar at 20°C, Initial ring stiffness 10 KN/m²

class 8

 

Pexgol Pipe class 10 (SDR 16.2 S7.6)
Working pressures: 10 bar at 20°C, 6 bar for natural gas | Initial ring stiffness 23 KN/m²

class 10

 

Pexgol Pipe class 12 (SDR 13.6 S6.3)
Working pressures: 12 bar at 20°C, 7.5 bar for natural gas | Initial ring stiffness 40 KN/m²

class 12

 

Pexgol Pipe class 15 (SDR 11 S5)
Working pressures: 15 bar at 20°C, 9 bar for natural gas | Initial ring stiffness 80 KN/m²

class 15

 

Pexgol Pipe class 19 (SDR 9 S4)
Working pressures: 19 bar at 20°C, 11.5 bar for natural gas | Initial ring stiffness 150 KN/m²

class 19

 

Pexgol Pipe class 24 (SDR 7.4 S3.2)
Working pressures: 24 bar at 20°C, 15 bar for natural gas | Initial ring stiffness 300 KN/m²

class 24

 

Pexgol Pipe class 30 (SDR 6 S2.5)
Working pressures: 30 bar at 20°C, 12.5 bar at 95°C, 19 bar for natural gas | Initial ring stiffness 640 KN/m²

class 30

 

Abrasion Resistance

Transporting solid materials by fluids (in the form of a slurry) is common in industry, mining, and in many piping systems. In most cases, the flow is kept turbulent to avoid sedimentation.

Abrasion is the result of the inner surface of the pipe wall being removed or degraded by flowing media in the pipe. The rate of abrasion for various slurries is determined by many factors, such as: 

  • Flow rate 
  • Density of the particles  
  • Size distribution of the particles
  • Hardness and angularity of the particles 
  • Temperature viscosity of the liquid 
  • Incorrect installation

Abrasion resistance is one of the most important advantages of Pexgol pipes. Pexgol’s excellent abrasion resistance is the result of the unique structure of cross-linked polyethylene, making the pipe material especially tough, resilient, and generally able to resist abrasion better than metal pipes.

The ability of the pipe material to absorb the kinetic energy of the hard particles inside the slurryand its resistance to deformationmake Pexgol pipes extraordinary, abrasion-resistant conduits. 

Unavoidable scratches in Pexgol pipes cause no damage.

Results of tests performed on pipes after being subjected to scratches as deep as 20% of the pipe wall show that no damage is caused to the pipe during intensive pressure tests. The crosslinked molecular structure accounts for the insensitivity of Pexgol pipes to scratches as well as their resistance to slow-crack growth. The restraining action of the adjacent molecular chains of the crosslinked network absorb the energy of the “tearing” forces.  

Pexgol pipes abrasion resistance was tested and approved in laboratory tests as well as in on-site conditions.

In South African gold mines, Pexgol pipes were installed in backfill lines, working at a very high line velocity and transferring highly abrasive material for many years without failure.  

In Israel’s Dead Sea Works, 450 mm Pexgol pipes have been installed since 1985, instead of steel pipes, which had to be replaced every year. These pipes are connected to dredgers which “harvest” the salt particles. Non-crosslinked PE pipes, which were installed in these lines, failed after a few months. 

Pexgol pipes have been at work since 1985, and it has not yet been necessary to replace them.

Technical test reports concerning abrasion resistance of Pexgol pipes are available on request.

 

Abrasion allowance:

Pexgol pipes have an “abrasion allowance of 20% of the nominal wall thickness of the pipe. This means that the pipe can withstand the design working pressure until the remaining wall thickness of the pipe is reduced to 80% of the nominal value. The real lifetime of the pipe depends on the actual abrasion rate in the line. The 80% rule applies for all working pressures and all temperatures in all classes.

 

Download the free Engineering Guide to see flow charts for full flow conditions. 

 

Coefficients of Friction

Absolute surface roughness

0.0005 mm – 0.0007 mm 

The values of Hazen-Williams coefficient

The values of the head losses in the charts were calculated using the Hazen-Williams formula with Hazen-Williams coefficient C = 155 

Manning coefficient:  

n = 0.005 - 0.007 

Reduction factors for higher temperatures  

The values of the head losses J in the charts are correct for 20°C. At higher temperatures, the head losses are lower. For different temperatures, multiply the value of J by the following reduction factors:

10°C – 1.03 

20°C – 1.00 

30°C – 0.98 

40°C – 0.93 

50°C – 0.91 

60°C – 0.88 

70°C – 0.85 

80°C – 0.83 

90°C – 0.81  

 

Calculating Pexgol pipes for boreholes  

Pexgol pipes can be used as riser pipes for boreholes.  

For energy-saving reasons, we recommend choosing a Pexgol pipe with head losses that do not exceed J = 5%, and preferably lower. However, please note that designing these pipes is complicated, due to the complex three-dimensional stress regime in these applications.

Golan’s Technical Department will calculate the pipe design for you after receiving the completed borehole questionnaire (page 112).

 

Water Hammer 

The water hammer is a series of pressure pulsations of varying magnitude, above and below the normal pressure of the liquid in the pipe. The amplitude and periodicity depends on the extinguished velocity of the liquid, as well as the size, length and material of the pipeline. Shock results from these pulsations when any liquid, flowing with a certain velocity, is stopped in a short period of time. The pressure increase, when flow is stopped, is independent of the working pressure of the system. The surge pressure in any pipeline occurs when the total discharge is stopped in a period of time, equal to or less than the time required for the induced pressure wave to travel from the point of valve closure to the inlet end of the line and return.  

This time is: 

t = 2L/a 

Where: 
t = Time for pressure wave to travel the length of the pipe and return (sec.) 
L = Length of pipe line (m) 
a = Velocity of pressure wave (m/sec) 

 
When the liquid in the pipe is water, the velocity of the pressure wave “a” is determined by the following equation: 

a = 1440 / (√ 1 + 2,070 x d/Ee)

Where: 
a = Velocity of pressure wave (m/sec). 
Kbulk = Bulk modulus of fluid (for example: 2,070 MPa for water at 20°C) 
d = Inside diameter of pipe (mm) 
e = Thickness of pipe wall (mm) 
E = Instantaneous (short term) modulus of elasticity (MPa) for the pipe material (obtained from Tensile tests).

The surge pressure caused by water hammer is determined by the following equation:

P = 0.1 x ρ x a x V/g

Where: 
P = Surge pressure (bar) 
ρ = Fluid density (for example: 1 gr/cm³ for water at 20°C) a = Velocity of pressure wave (m/sec) 
V = Velocity of water stopped = line velocity (m/sec) 
g = Acceleration caused by gravity (9.81 m/sec²)

Pressure caused by water hammer can be minimized by increasing closure times of valves to a value greater than 2L/a. For example, when the closure time is 10 times 2L/a, the pressure surge can be 10%–20% of the surge caused by closure in a time equal to or less than 2L/a.  

The value of the short-term modulus of elasticity E for Pexgol pipes is much lower than the value of E for steel pipes, concrete pipes or HDPE pipes. Since the velocity a of the pressure wave is related to the short-term modulus of elasticity E, the velocity a decreases when the value of E is lower.

In order to determine the resistance of the pipe material to the water hammer phenomenon, the total occurring pressure (surge pressure + working pressure) should be calculated and compared to the maximum allowable total occurring pressure in each pipe material. The resistance of HDPE pipes depends on the nature of the water hammer. In case of recurring water hammer shock waves, HDPE pipes are limited to a maximum total occasional pressure of only 1.5 times the working pressure

Because of the flexibility and resilience of Pexgol pipes, the surge pressures caused by the water hammer are much reduced. Furthermore, because of the cross-linked structure, the Pexgol pipe can withstand a total transient pressure (recurring or occasional surge pressure + working pressure) at least 2.5 times the design pressure in the relevant temperature. 

 

Vacuum/Suction Pipelines 

Under-pressure (vacuum) might develop in the following cases:  

  1. When a pipe is installed in vacuum-feeding pipelines. 
  2. When a pipe is installed in a steep inclination, causinrapid free flow.  
  3. Extreme temperature changes of the transported liquid.

If a Pexgol pipe collapses, it results in an oval deformation. Note that when a Pexgol pipe collapses due to vacuum, it can be returned to its original round shape by applying internal pressure. 

The amount of vacuum that a Pexgol pipe can withstand depends on the pipe’s SDR. A pipe with sufficient wall thickness must be selected in order to resist the collapsing forces generated by the vacuum. 

Table 36.1 shows maximum rates of vacuum supported by Pexgol pipes of different classes and different design temperatures. These are empirical values. 

 

Table No. 36.1: Service under vacuum

table 36.1

* Tested under full vacuum conditions: -1 bar (g) 0 bar (a)

The values in the table are in bar (g) (Bar gauge). For example:  

-0.8 bar (g) is equal to 0.2 bar (a) or Bar absolute. 
Pexgol pipe class 10 is not recommended for use under full vacuum conditions.  

Allowable external pressure:

For pipe of uniform cross-section, applying a safety factor of 1.5 which includes the influence of pipe ovality, the maximum allowable external pressure Pc in bar can calculated from the following equation:

Pc = 2618/(SDR-1) 3 

  • For Pexgol class 10 SDR 13.6 Pc = 0.75 bar  
  • For Pexgol class 12 SDR 13.6 Pc = 1.0 bar 
  • For Pexgol class 15 SDR 11.0 Pc = 2.5 bar 
  • For Pexgol class 19 SDR 9.0 Pc = 5.0 bar 
  • For Pexgol class 24 SDR 7.3 Pc = 10.0 bar 
  • For Pexgol class 30 SDR 6.0 Pc = 21.0 bar 

 

Underground Pexgol pipe under vacuum or external pressures  

Vacuum, or external pressures, creates hoop stresses in the pipe wall which are combined with the external pressures of the soil. In extreme cases, these stresses can cause the pipe to collapse.

Therefore, when a Pexgol vacuum pipeline is installed underground, the vacuum stresses have to be added to the total static andynamic loads exerted by the soil and all the stresses must be considered. In this case, make sure that the soil around the pipe is compacted. When designing a vacuum pipeline at recommended vacuum conditions, please contact our engineer for consultation regarding installation of vacuum breakers.  

 

Properties of Pexgol Pipes

properties of pexgol pipes (technical info)

 

Table No. 38.1: Thermal Properties

table 38.1-1

 

Table No. 38.2: Electronic Properties

table 38.2-1

 

Chemical Resistance

The Pexgol Chemical Resistance List is based on information included in the professional literature. The list is only intended as a guide.

Changes in the composition of the medium or special working conditions could lead to deviations. Consult the experts of Golan Plastic Products in each specific case  

Chemical resistance test for Pexgol pipes

The following procedure is an initial test for the chemical resistance of Pexgol pipes.  

  • Each combination of service conditions (service temperature, chemical concentration) constitutes a different case. However, for the same pipeline, the worst case is usually the highest temperature and the highest concentration.
     

The tested items are 20 “dumbbells” (also called dogbones or “coupons”) made from Pexgol pipes. 

Immersion test:

  • The dumbbellare immersed in the same material transported through the pipeline (same chemical composition and same temperature) for a period of 4 weeks.  
  • After 2 weeks, 10 dumbbells are removed and stored.  
  • After an additional 2 weeks, the other 10 dumbbells are removed.  
  • The two groups of dumbbells are packed separately, and the packages are marked appropriately to identify the removal and storage conditions.  
  • The packages are sent to Golan for tensile testing. 

 

Read more about Pexgol’s chemical resistance in the full Pexgol Engineering Guide.