Hi Ken,
I am also involved in the design of reticulated chilled water systems in the Middle East. I have undertaken the surge analysis and the pipe stress analysis of the chiller and cooling tower rooms. In addition was responsible for the design philosophy of the buried pipelines.
The velocities that you are designing for appear high but it comes down to a life cycle cost analysis. It isnt a real problem if energy costs are low as in the Middle East. Hogher initial velocities do add to the likelihood of high surge pressures due to loss of power. It is a question of change of momentum.
As for the material of choice the pre insulated ABS has so many benefits compared to steel. You raise the prospect of failure. With insulated buried steel you have the long term risk of failure from corrosion. This corrosion is both inside and outside of the pipe. Maintaining the vapour barrier for buried steel is a nightmare.
ABS is not the same grade that is used in the USA. It has been manufactured in Australia, Malaysia, Indonesia, Switzerland and the UK for over 30 years. The relavant standard is AS 3518. Design of the bruried pipelines should be to AS 2566. It is the material of choice for Sydney Water's sewerage treatment facilities. These are important infrastructure assets that cant stand failures. The prime reason being that to bypass the plant would cause an environmental and health hazard. The plants never get turned off!
In Europe 95% of pipelines for the water and wastewater are plastic. It is as prevalent in the USA but they have established ductile iron industries . Their raw material is scrap iron and steel. Without this low cost feed stock the industry wouldnt compete.
As for waterhammer this has to be taken seriously. ABS offers the benefits of a low modulus compared to GRP, steel and ductile iron. This translates into lower celerity and hence lower surge pressures.
GRP has some real hooks in design if you consider the design requirements of BS 7159 or ISO 14692. Other standards do not cut it as far I am concerned.
I will send you some technical papers on the comparative criteria betweeen ABS and GRP.
regards
Geoffrey D Stone
Ken Eppleston <keneppo@yahoo.com.au> wrote:
Hi
I am an Australian engineer working in the Middle East designing some particularly large district cooling networks.
My experiences have been somewhat restricted to smaller systems within Australia and I am looking to tap into the expertise of this forum to educate myself on the best design practices for large systems.
One of the critical factors obviously in the pipework sizing is the tradeoff between pipe diameter, velocity and system head losses. We are looking at designing our network with a maximum pressure loss of 100Pa/m and allowing the velocity to go as high as 2.5-3m/s on the main ring of the system.
The branch take offs are being designed at a lower pressure drop than the ring main to ensure we can get adequate flow into the branch lines.
Anybody have any comments or suggestions regarding these design parameters?
Further to this we have selected our piping material to be of steel only as we view any risks associated with a rupture as catastrophic. We understanding that GRP has been used previously on this type of application but we are hesitant. Does anybody have an agreeing or disagreeing opinion to these comments or justifications for using GRP? I understand that the significant benefit is resistance to corrosion.
These projects we have here in the Middle East are somewhat beyond the scope of your average building chilled water project, any that I have experience with anyhow, and it is extremely important that I consider all phenomena which may occur.
Are there any rules of thumb, or good design papers, or the like, which are applicable to thrust block, anchors or expansion loop design, and the best locations within a network to use these components, which may assist in our preliminary designs. At present we are very much relying on the stress analyst to provide advice.
Other problems such as water hammer I understand are also important considerations, cause if it is missed now it can be very troublesome further down the road, as with any project.
Does anybody have any suggestions regarding those considerations in the design of a large cooling water network which MUST be factored in to the network design at an early stage to ensure at the end of the day the system runs efficiently and trouble free.
To give you an understanding of our design, it involves approx 80km of network piping flow and return lines to a number of plants on the site.
Thank you in advance for all suggestions and I eagerly await some responses.
Thanks and kind regards
Ken Eppleston
Mechanical Consulting Engineer
Middle East & Australia
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[Non-text portions of this message have been removed] Received on Wed Dec 14 16:59:00 2005
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