RE: ( Large Chilled) Water Network Design

Randy,

   I always enjoy reading your commonsense factual commnets from the other side of an argument. I agree plastic pipe materials are not the answer to every application neither are they to be dismissed.

   The problems associated with buried pipelines in the WAE, I have been associated with, are that they are 3m below ground, in reclaimed land, that are subjected to sea water influx. GRP has had some mixed success there but beciuse of its low strain tolerance requires more intensive design and construction procedures. I attribute failures to the letting of design and construct contracts where the bottom line rules the so called engineering. Standards such as ISO 14692 and BS7199 are not enforced. Rather manufacturers own standards are employed. Once buried and the contractor has pulled off shore, who cares. There have been some spectacular failures over the years.

   In the UK the water and wastewater market is dominated (90%) by thermoplastics. DICL is used for high pressure rising mains only and then I am told it is hot dip galvanised. Larger mains are in steel. I am sure that this is an economic choice where the lower pressure applications can use thermoplastics.

   In Australia Tyco manufacture DICL to DN750 in Class 20 and 35 (20bar and 35 bar ratings). This is aimed at competing with thermoplastics. Our economics are different. The cost of transport from one manufacturing facility to the customer varies enormously.

   DICL is still used. Mild Steel Cement Lined (MSCL) is about 10% more expensive than DICL. Thermoplastics & GRP are price targetted depending upon factory loading, transport & customer history long before pressures are considered.

   The jury is till out on the celerity of buried thermoplastic pipelines. End constraints also modify the actual numbers. The effect of embedment and native soil has not been researched. If there is column separation and rejoining it hardly matters what material you use the extreme pressures will exceed Joukowsky predictions.

   All material properties are strain rate dependent. The normal stress strain graphs we learnt at college and university are generalisations. If the theory of explosions, impact loads and fracture machanics are studied impacts from waterhammer loads need to be considered in more depth. Materials are able to withstand higher stress levels than predicted by static analysis provided that they do not have faults such as weld defects, notches, cavities, delaminations etc.

   Even the attempts to link waterhammer pressure loads to structural (pipe) behaviour are fraught with danger. It is nighe impossible to replicate a physical model for waterhammer analysis in the same way as a structural pipe model. When the structural loads are applied either as static or time hisory effects on a modal analysis the effects are inconsistent. Pressures are applied at curved surfaces whereas structural loads are applied differently.

   The codes and standards are presented as compromises with conservative approaches to the design of systems. They must be tempered with field experience. The design is but one part of an installation. The code such as B31.3 covers, design, fabrication, examination, installation and testing. The code is also backed up with material standards and requirements for qualification. They are an intricate web of documents that cannot be grasped overnight or by reading a few emails. It takes decades to get abreast of them and keep up with the changes.

   Geoff

Design Detail and Development (a division of Blenray Pty Ltd)

Mail Address PO Box 1351 Castle Hill NSW 1765 Australia Tel Mob 0402 35 2313 Office 02 8850 2313 AH 02 8850 2324
We specialise in pipe network and waterhammer analysis, pipe stress analysis, the design of buried pipelines and thermoplastic pipe systems.

[Non-text portions of this message have been removed] Received on Tue Jan 17 20:26:00 2006