Steve,
PE like other thermplastics have physical properties that are strain rate dependent. A rapidly applied load such as a waterhammer pressure spike can be withstood whereas a constantly applied load will result in creep and eventually failure. That said creep can reduce the load applied to the pipe and enable the pipe class to be acceptable. A concentrated mass such as a valve can cause failure but secondary stresses from thermal expansion can be dissipated by creep.
In transient events I use the instantaneous value for the properties of PE to determine the pressure and hence loads on supports. I compare the pressure to allowable rating of the pipe. The jury is till out on the benefit or otherwise of the soil for buried applications when it comes to determining the celerity.
When considering deflection I use the 50 year properties of PE to determine the deflection. Local deflection and stresses in thin wall pipe are analysed using Roark and Young.
In buried applications the soil in a well designed trench will take 90% of the external live and dead loads (refer AS 2566) provided that the native soil has sufficient strength. If it doesnt have sufficient strerngth it is unlikely to be able to transmit external loads to the pipe if buried at a reasonable depth.
When it comes to stress the codes only refer to compliance with hoop stress. They do not address the question of rate of strain in bending or shear stress.
As Caesar II and other software is not designed to handle non linear applications, as in thermoplastics, I usually model both the instantaneous properties and long term properties. The former to establish loads on supports and the latter to determine failure from pressure for each class of pipe. Then a degree of judgement is used in establishing a pressure class.
A basic risk assessment is used for aboveground piping. What are the consequences of failure? What is the incremental cost? I adopt a minimum class 12 for PE pressure applications aboveground. Class 15 for slurries to provide some wear allowance. For gravity lines I look more to the deflection, cost and frequency of supports etc.
WSA 01 covers the fatigue design for PE well. www.wsaa.asn.au
In aboveground piping I use a careful eye to ensure that loads are not applied to weaker sections particularly repititive loads where there is a sharp change in section.
One application I was asked to investigate originally saved $50,000 by opting for a lower class thermoplastic pipe. It cost $14,000 per hour in lost production when they had a failure. Go figure?
There is no code that I know of that covers the design of thermoplastic piping aboveground very well at all. If the code stress (hoop) is exceeded I use a higher class of pipe. For buried applications AS 2566 is considered to be at the forefront of the technology.
Standards Australia is considering producing a standard for aboveground thermoplastic piping. The standards committees are riddled with plastic pipe manufacturers. They will see this as a negative as it will mean engineering time to use their products and they will throw out the anchors to ensure it doesnt happen quickly. So dont expect anything in the next ten years.
As the codes are so poor you need to exercise judgement. It is acceptable to use reference text to design in those areas where the code is deficient.
References I use are:
Applied Stress Analysis of Plastics Krishnamachari
Plastic Product Design Belofsky
Plastic Pipes for Water Supply and Sewage Disposal Lars-Eric Janson
Plastics Mills
Fracture Mechanics and Deformation Hertzberg
Prediction of the Deformation Properties of Polymeric and Composite Materials
A. Ya Gol'dman
ISO 14692, AS4130, WSA 01
Process Piping Brecht
regards
Geoff Stone
Steve McKenzie <Mechproj@xtra.co.nz> wrote:
Gents + 1ish
On certain occasions, I have difficulty in deciding on an appropriate
greeting.
This is one of those occasions, so gidday.
I picked up this thread from Geoff, who seems to be a Code man, like Chris.
Today,my area of interest is in transient design stress for PE pipe. A number of "guides" extoll the virtues of PE pipe under transients, and then "recommend" that the design stress is the "X" year tensile stress or some other arbitary value.
Would Geoff tell me how he determines the design pipe wall thickness for PE pipe. I am especially interested in consideration of occasional transients and their influence in relation to the (miserable) code alllowables. Is it necessary to exercise judgement or do you guys always work to code?
Cheers
Steve
-----Original Message-----
From: <a href="/group/PipingDesign/post?postID=yVy03tfm9wkLFDiJO5J3v0SSAP_fkwKydekhiNf1LH5H324L1mHfQRWFluAKdPtH2aENMO7bOS6Qj1MOAioL2dQKwB9-yQ">PipingDesign@yahoogroups.com</a> [mailto:<a href="/group/PipingDesign/post?postID=yVy03tfm9wkLFDiJO5J3v0SSAP_fkwKydekhiNf1LH5H324L1mHfQRWFluAKdPtH2aENMO7bOS6Qj1MOAioL2dQKwB9-yQ">PipingDesign@yahoogroups.com</a>]
On Behalf Of Geoff Stone DD&D Australia
Sent: Thursday, January 05, 2006 11:30 AM
To: <a href="/group/PipingDesign/post?postID=yVy03tfm9wkLFDiJO5J3v0SSAP_fkwKydekhiNf1LH5H324L1mHfQRWFluAKdPtH2aENMO7bOS6Qj1MOAioL2dQKwB9-yQ">PipingDesign@yahoogroups.com</a>
Subject: Re: [PipingDesign] Caesar 2 Stress Analysis Software
Ken,
I cannot help you with your question however in respect of buried pipelines analysis why do you need to use Caesar II? AS 2566 Buried Flexible Pipelines covers the analysis of buried pipelines? Iam on the Standards Australia WS28 committee that produced this document.
Admittedly it could be considered conservative in some aspects. The largest variable in any such analysis is the properties of soil of the combined soil/pipe structure. This is also the case if you use Caesar II.
I too am based in Sydney. If you wish to discuss the application I will be there tomorrow. Please call on 0402 35 2313 or 02 8850 2313.
regards
Geoff Stone
Ken Eppleston wrote:
On the topic of stress analysis software I am searching for an intensive training course in the use of Caesar 2 software and in particular for buried network systems.
I have access to a licensed version of Caesar 2.
I am currently located in Sydney Australia until the end of February.
Given the amount of expertise that is evident on this forum, in particular stress analysts, it appears the best place to investigate the whereabouts of a comprehensive training course.
Any ideas or contact details??
Thanks
Ken Eppleston
Consultant
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