Prestressed Concrete Pipe Wavespeed 1

[HydroJo]

How do you calculate the wavespeed for surge analysis in a pre-stressed concrete pipe ? Is it affected by the stress in the pre-stressing wires ? (I know the thickness of concrete, thickness of steel cylinder and size and pitch of wires including the tension.)

 

[hydrae]

The wave speed of a pressure wave is depends on the speed of sound in water, about 2000 ft/sec, different pipe materails may slow it down some, but not much, the effect of pipe material on surge is the ability of the pipe to absorb some of the magnitude of the wave such as thin wall PVC and steel will change volume to act like a small surge vessel, this effect is also minor, Ductile Iron and Concrete (prestressed or not) should be considered as stiff.
rule of thumb for stiff pipes 50 psi for ever 1 ft/sec in instanious change of flow, this value can be reduced to 40 or 45 for steel and PVC.

Hydrae

 

[HydroJo]

Thanks. Yes I do have the exact wavespeed formula using Youngs modulus and thickness of pipewall and bulk modulus of water and can calculate the exact wavespeed in any homogeneous or composite material. BUT I am interested to know how pre-stressing tension affects it. Are there any surge experts out there who have had to deal with this ?

 

[BRIS]

We have undertaken measurements on wave speed in 4.0m dia PCCP pipe over a range of classes and have measured fairly consistent results of 1160 m/sec regardless of class (pre-stress). Unless your question is research related I would suggest that you will be safe assuming a wave speed of 1100 to 1200 m/sec for surge analysis.

I would take issue with Hydrea on his statement that wave speed (celerity) is not much affected by pipe material and suggest speed is significantly effected by pip material. Celerity in plastic pipe may be as low as 450 m/sec.

Brian

 

[paddy84]

Thanks for the wavespeed info - can you give any more details. The audit trail looks weak with 'cos a bloke on the web said so'!

 

[BRIS]

you can calculate using the elastic modulas of the pipe and bulk modulas of the fluid and factors for joint type etc. Text books covering waterhammer will cover the theory.

Thorley is one of the most practiacl text books and includes curves of wave speed for a range of materials and stiffness. Unfortuantly I don't have my copy of Thorley with me as I am on an assignment in Beirut but I can give you details next week if no one else responds before then

Brian

 

[paddy84]

It was your reference to testing on the 4m PCCP that particularly interested me - is there anything there that I can use for an audit trail?

I'm ok with wavespeed equations, modulus etc. But I thought I knew Thorley's book inside out, so I'll have another look there when I'm in the office.

Thanks Brian

Pat