Hi Ken,
Large netwroks d experience waterhammer. The analysis is complex because fo the large numbers of differnet scenarios. The complexity of the system can mitigate pressure spikes as reflected waves cancel each other out or reflected waves can increase the surge pressure at particular nodes. Not all software is capable of adequately analysing such systems.
The main concern is where there is column separation and and the columns come together agian. This occurs when there is a loss of power to the pumping system or an upstream valve closes quickly.
Most of my work has been with pipeline designs but there have been a number of projects where there has been this complexit. I have been working on one for a gas platform wher high velocities in the pipework are the norm. This involves a number of offtakes and various flow paths through heat exchangers etc. Loss of powerr has been the biggest problem as all other scenarios can be controlled with valve stroking and pump speed.
When there is los of power VSDs are of no use to you so the pump tirp is like ay other pump stop.
My relationship with Tyco goes back many years to the days of Durawills in Australia. I am an independent consultant and Durawills, Euratech, Eurapipe and now the present owners Tyco. Tyco has used my services to do preliminary designs for their customers to aid their sales, forensic investigation of failures, training of their sales people in technical matters and preparation of technical papers. I remain staunchely independent of Tyco and have advised and undertaken work for other plastic pipe companies. My experience of serving on Standards Australia committees for amorphous plastic pipes, polyolefin pipes and buried flexible pipelines is a resource that Tyco uses. I have for twenty five years used ABS and other thermoplastic pipe materials up to DN800. I have also used DICL, steel, vitirfied clay , concrete etc. Each material has merit for different applications. I tend to look for the life cycle costing forthe client taking into account cost of energy, maintenance etc. I believe ABS offers considerable advantages for long term use where corroson internally and externally may occur. As I have said thermoplastic materials do offer benefits from reduced wave velocity in respect of surge
To demonstrate my independence on a project in Sydney, recently completed, I was responsible for the procurement of pumps and pipe materials. In that I procured $4 milllion worth of pipe material from Tyco's direct competitor Crevet. In addition that was DICL not ABS.
The question of thermal strain with thermoplastics is something that needs careful consideration. The coefficient of expansion is ten times that of steel. One benefit of the thermoplastic material is its creep characteristic. If the pipe is fully anchored and the strain applied equally at a slow rate then the material will creep and the stresses relax. The same phenomenon occurs with steel but at a much higher temperature. The physics is the same. WIth a buried plastic pipe the soil is able to restrain the pipe due to friction. With a presinsulatedpipe whether it be steel or ABS one has to pay careful attention to the bonding of the foamed insulation nd the rate ate which the pipeline is cooled. Of course thrust blocks are not required with a fully welded system other than to provide local support .
As the modulus of thermoplastic materials particularly at low strain rates is much lower than steel the loads imposed are much reduced.
Any transisitions in diameter must not be abrupt as local stresses can be larger than expected because of uneven distribution of load.
The main problem I find in the Middle East is that the consultant underakes a global design but then encourages the letting of a detail design and construct contract. This is generally done to preserve their margin but possibly to avoid rsponsibility. The contractors have very little design knowledge, do not employ engineers and ignore specifications. Contracts are let on a low price basis. Tyco endeavour to ensure that their product is not incorrectly applied by having a sanity check carried out where it is warranted. When any failures occur consultant and contractor try to blame the other and then turn on the material manufacturer.
I would encourage all consultants to ensure that they do not leave design decisions to the contractor, treat any design change requests with caution and inspect the installation to ensure compliance with their designs.
regards
Geoffrey D Stone C.Eng FIMechE; CPEng FIEAust
Ken Eppleston <keneppo@yahoo.com.au> wrote:
Thanks Geoff...
We did propose to design our network at 1.0m/s, however as you note energy is extremely cheap here in the Middle East. The additional pump head means bigger motors and greater power consumption, as you say life cycle costing plays a huge role. We are in the process of having the network analysed to ascertain the effects of the higher velocities with regards to surging and water hammer.
Have you witnessed a large reticulated system undergoing water hammer, from your experiences as far as a rule of thumb goes what scenarios must we avoid?
Do you have an affiliation with Tyco? They are selling ABS here, however our ring mains are at between 1050mm and 1200mm. I have been informed ABS is only supplied up to 800mm.
The concern we have with plastics such as HDPE or ABS is the expansion, apologies- contraction, upon filling the network and chilling it to 4 degrees. The contraction rates are significantly greater and in our case we need to minimise the contraction as much as humanly possible- pipes move then streets and infrastructure move...We designed with steel and utilise thrust blocks, anchors and expansion loops to reduce any effects on the site from contraction. Steel obviosuly has the ability to sustain greater stresses than plastics.
We designed with the structural reliability of steel, and trust me we understand the problems associated with a break in the vapour seal and the possibility of ground water ingress. We have very stringently specified the jointing of the jacketing and will pay very very close attention on site to installation procedures and the like.
Do you have any experiences with leak detection systems? I'm told they don't work....
What are your experiences with contracting companies in the Middle East? I'm told to be careful....
Thanks again for your time
Ken Eppleston
Middle East
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[Non-text portions of this message have been removed] Received on Sat Dec 17 16:44:00 2005
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