Velocities in pipes

[MontyJack]

Are there any problems with a velocity of 30 ft/s in ductile iron pipes for water at 70 degrees F.

 

[snafuman]

Water hammer can be a problem anytime you have velocities over 5 feet per second. Also, you are going to get extreme thrust forces on bends. Check with your ductile iron pipe supplier for their maximum recommended velocities.

 

[nealt]

In addition to the water hammer problem, a velocity of 30 ft/sec produces a tremendous amount of frictional force (dynamic head)that must be overcome. If you are pumping the water this equates to a much higher required pump horsepower: higher up front cost as well as higher operational cost. If this is the case, you would be better off upsizing the pipe.

 

[flamby]

I agree that friction head will be too high. If the pipe is very long, you will probably loose lot of money on power for pumping only.
Corrosion (and cavitation) of the pipe will also be an issue and the life of the pipe will be reduced severely. Cavitation will further the already very high frictional losses and also make the pipe noisy.
flame

 

[KRSServices]

Due to the high velocity, electrolysis will be incredible. I suggest some cathodic protection. 30 ft/s is also incredibly high as well. I do not allow velocities (notwithstanding fire flow) to approach 2.5 m/s (8 ft/s) due to hammer and corrosion. KRS Services

http://www.krs-services.com

 

[BobPE]

Short answer, YES. If you don't understand why, there are several older posts here that you can search for that will help you to understand. Corrosion is not the problem, its more of a fundumental fluid dynamics problem. You cant have water hammer until something in the system changes so thats not it either because you can regulate the effects of transients by design. Cavatation is the problem as eluded to by flame.

Do you have a system that is flowing like this? Is it in design or existing? Please tell us more.....

BobPE

 

[BRIS]

There is no problem with velocities of 30 ft/sec in ductile iron pipe with clean water. Cavitation cannot occur until velocities exceed about 40 ft/sec or higher if pressures are higher than atmospheric.

concrete outlet penstocks have been safely used at velocities up to 100 ft/sec but at higher velocities it is normal to line with stainless steel.

With ductile iron pipe the bell and spigot joints estrict maximum velocity to about 40 fps.

As pointed out in the other replies there are many assocaited problems with high velocity flow. If you try to control the flow at these high velocities then you will generate high surge pressures.


Brian

 

[BobPE]

Bris:

Last time I did the calcs, cavitation occurs in clean water at about 23 to 30 fps. Pressures experienced in ductile iron pipe have little or nothing to do with limiting cavitation.

I am really interested in if you have an example of a penstock flowing at 100 fps. There are nozzle outlets that flow like that, but I never seen anyone crazy enough to put something like that in concrete through a high investment dam.

Again there are some good discussions posted in other areas that discuss this in detail.

BobPE

 

[af7k]

An incredible head loss.