PEXGOL PIPES: DESIGN CONSIDERATIONS
The values of the design stresses vary according to the design temperatures. These values are shown in the table “Allowable working pressures [psi] for conveying water in Pexgol pipes”.
1. Defining the design temperature
The design temperature of the Pexgol pipe is calculated based on the data from the RFI questionnaire.
In case of insulated pipes ,the design temperature is equal to the fluid temperature.
In case of above -ground installation, the design temperature of the Pexgol pipe is to be calculated as the mean value between the TEX(Temperature of the external pipe surface) and TFL (Fluid temperature)
The temperature of the external pipe surface is calculated as 20°C above the ambient temperature:
TEX =T amb.+ 20°C.
To facilitate the calculation see the following table.
Fluid temperature 30°C
Ambient temperature: 40°C
Design temperature is (60+30)/2= 45°C
In case of Buried pipes ,the design temperature can be taken as equal to the temperature of the liquid flowing through the pipe.
- 2. Water and Newtonian fluids
The pipe SDR is selected according to the following data from the RFI questionnaire:
Pressure head losses in the line expressed in bars (taking into account the specific gravity of the transported material).
Design temperature (see first paragraph above).
Basic safety factor (design coefficient):
1.25 for water and fluids with the classification A in the chemical resistance list.
For materials with classification B, C, D in the chemical resistance list, please consult Golan.
1.5 for air supply lines.
Static pressure according to the altitude difference in the line and the specific gravity of the transported material.
If the pipeline is horizontal and the static pressure is low, select class 6 and verify its suitability.
Choose a higher class with the same OD in order in to increase the transportable section lengths.
The hydraulic calculation usually results in the same OD.
If the altitude difference in the line is significant, select a Pexgol pipe class that has in the design temperature higher pressure rating than the static pressure. The additional pressure margin is used for the pressure head losses; this will determine the ID of the pipe.
The OD is determined by the Pexgol pipe class the customer chooses and the availability of this specific pipe diameter.
3. Replacing waterline steel pipes
When replacing steel pipes (Hazen – Williams C = 110) with Pexgol pipes (Hazen – Williams C = 155) with the same pressure head losses, the ID of the Pexgol pipe can be 88% of the ID of the existing steel pipe.
When replacing steel pipes with Pexgol pipes with the same ID, the head losses are expected to be lower by 50%.
4. Influence of temperature changes on Pexgol pipes
Pexgol pipes placed above the ground or over bridges tend to get longer (to expand) when temperature rises (snaking phenomenon) or to get shorter (contract) as the temperature decreases. Expansion or contraction does not affect the Pexgol pipe, even in extremely low temperatures.
There is no need to protect the pipe against thermal stresses, as they are absorbed by the pipe.
Fixpoints or guiding clamps are used for restraining the elongation of the pipe (mainly for aesthetic considerations).
There is no need for installation of “expansion joints” or omegas.
Special fixpoint clamps should be used before and after the fittings (as recommended) to prevent the pipe from pulling out.
5. Pexgol pipes above ground
Pexgol pipes withstand exposure to sunlight for unlimited periods—that is, the lifetime of the pipe.
Pexgol pipes can be placed directly on ground.
Special bedding is not required.
For further information see: Above ground installation guidelines.
6. Pipes under full vacuum conditions
Its recommended to use a minimum pipe class 15. For more information see page 36 “Vacuum/Suction Pipelines”.
7. Pexgol pipes at low temperatures
Pexgol pipes are used at temperatures as low as -50°C and even lower. Since the Pexgol material does not become fragile at these temperatures, it tolerates bending and dragging at low temperatures during installation.
Pexgol pipes tolerate complete ”homogeneous” freezing of the transported liquid. Homogeneous freezing takes place if the pipe is evenly exposed to low temperatures along the pipeline.
However, if freezing starts at localized freezing points, the pressure of the fluid which is trapped between two adjacent freezing points increases until the pipe bursts.
This happens to any pipe material. Localized freezing points might be metal fittings (including Pex-lined steel fittings), fixpoint clamps or any point where the metal touches the pipe. Consequently, localized freezing points should be avoided or properly insulated. Please note that this applies to both above-ground or shallow underground installations.
|Sch. 40 Carbon steel pipe||Pipe||Loose flanges||Pipe||Loose flanges|
|3”||78||90 class 15||3”||110 class 24||4”|
|3 1/2”||90||110 class 15||4”||125 class 24||4”|
|4”||102||125 class 15||4”||140 class 24||5” or 6”|
|5”||128||160 class 15||6”||180 class 24||6”|
|6”||154||180 class 15||6”||200 class 24||8”|
|8”||202||250 class 15||10”||280 class 24||10”|
|10”||254||315 class 15||12”||355 class 24||14”|
|12”||303||366 class 12||14”||–||–|
|14”||333||400 class 15||16”||450 class 24||18”|
|16”||381||450 class 12||18”||–||–|
|18”||428||500 class 12||20”||–||–|