We cannot repeat if often enough - save money by thinking!
Costs optimized pipe due to the distinction of working pressure and pipe stability, while designing
Imagine: Tuesday morning at the technical department of Krah in Germany. A customer from South-East Asia is calling and asking for a pipe with 10 bar working pressure, because a big tender is new in the market DN 2000 and 10 bar, total length 3 km, application “closing an open channel”. So, should we really suggest a solid wall pipe SDR17, for a pipe DN/ID2000 – or will make it sense to ask questions and think? Our aim is it to get the tender and to reduce the investment for the client, because usually it is tax-payers money in infrastructure projects.
1st Question: “What is the requested inside diameter of the pipe?”
Sometimes the pipe is described with inside dimensions and sometimes with outside dimensions, depending on the referred standard. For example, if the tender for a PE-Pipe is according to DIN8074, the corresponding Krah pipe would be DN/ID1800, because DN2000/SDR17 will have a hydraulic inner diameter of 1765 mm.
Due to the information that the application is “closing an open channel, for transport of raw water” you could ask the 2nd Question: “What is the filling level of the pipe?” or “Is the pipe fully filled?”
Usually the answer is: “no” or “could be, when a valve is closed”. At this moment you know, that the 10bar requirement is because of the installation conditions and not because of a real inside working pressure. 3rd Question: “What is at the ends of the pipe (pumps, lake, river, plant etc.)”
The reply is important for better understanding what internal pressure could be theoretical there and what is necessary for operation. 4th Question: “How the pipe is installed (exposed, buried)?” and if buried: “What is the soil quality (backfilling, proctor density etc)?”
That and also information about additional loads are crucial for the needed stiffness and stability of the pipe.
So, finally we have an idea of the needed structural load capacity of the pipe.Assumed the customer is telling us, that the pipe is connecting a municipality with a sewage treatment plant and there is no integrated pump and no significant geodetic height difference (remember the pipe is replacing an open channel). It become very clear that the original reason for choosing PN 10 classification was not the internal pressure, but the external load capacity of a PN 10 wall thickness. So, what is the external load capacity of PN 10 pipes and how it is related to stiffness classes?
The answer is simple: PE 100, PN 10 is equivalent to SN 20 ! Any PN can be translated to SN by considering the typical mechanical characteristic of Polyethylene*
* flexural modulus of 1000 MPa
Honestly, very often we get the reply, that there would be no difference between internal and external capacity and people insist in PN 10. Very clear that people who insist in PN 10 are not aware that the load direction is decisive for the static design and ignoring that is wasting a lot of money. The difference in internal and external load design is briefly described in the table below.
Back to the first questions, what should we quote? If we quoted standard PE100 pipes DN/OD 2000, with SDR 17, length 3km – the total weight is 2408 tons. Jointed by butt-welding (Cost of one machine approx.. 100.000 EURO, speed of one joint 8 hrs – outside the trench. If we quote Krah-Pipe with same stiffness/external load capacity, with PN 1 or PN 1,5 internal pressure load capacity, same length of 3km – the total weight is 1220 tons. Jointed by integrated electro-fusion (Cost of one machine approx.. 5.000 EURO, speed of one joint 30 min. – inside the trench).
Material Saving = 1188 ton = approx. 1,2 Billion $
These Material savings of 50% for profiled pipes with same SDRis valid for all SDR classes !
Saving material means:
• Saving costs for the client (and more projects can be realized with the same budget)
• Saving installation time means saving costs again, and a quicker realisation of the project
Also: The described case is a true story and finally the necessary pipe stiffness was SN 7.2, calculated and verified by third party. That means the saving was not 50%, it was almost 70% !
So, think! Safe money and protect the environment.
Did you know?
Simplified for Polyethylene we can use the ratio between short and longterm flexural modulus of “5.5 : 1” and receive by inserting the typical stiffness formula into the external load formula following the simple relation. Simplified for Polyethylene we can use the ratio between short and longterm flexural modulus of “5.5 : 1” and receive by inserting the typical stiffness formula into the external load formula following the simple relation. po=0,052 ∙ SN Author: Dipl-Kfm. Alexander Krah, Krah Group