maximum pipe velocities

[craigswatson]

I am designing a water system for a college using watercad. I have sized my pipes primarily to satisfy my required fire flows but I also wanted to make sure I was within allowable flow velocities. I have looked thru all of my hydraulic books and civil books and can not find any parameters for allowable flow velocities for potable water systems. Any help would be appreciated.

 

[bimr]

From Cranes Technical Paper No. 401:

General Service....0.4 to 10 feet per second

City...............up to 7 feet per second

 

[JKW05]

I recently did a very small water distribution design and I recall a recommendation in a plumbing design text for 8 fps on main lines and 4 fps on branch runs. I can find the source if you need it.

 

[craigswatson]

Thanks for the info I appreciate it.


JKW05,
If it isn't to much to ask, it would really help to know where you got that. I think my boss would have a lot more confidence in my answer if I can site something.

Thanks,
Craig

 

[rconner]

I wonder if the further restriction on branch lines has anything to do with possible increased problems due to surge in some branches, or greater likelihood of having types of pipe more subject to failure due to surge, in some smaller lines (type of pipe may be a factor, as I know as at least many local authorities want to restrict velocities particularly in some types of weaker piping materials)?

 

[JKW05]

I may have misunderstood your original post.

Are you working within a building? I will get you the reference information, but I was doing a water distribution system inside of a building. Not sure what differences there may be if you are working on a "campus" distribution system.

 

[craigswatson]

It is a campus system. 8",10", and 12" mains.

 

[cvg]

rconner, I think smaller lines experience higher head loss with increase in velocity and it is generally economically feasible to increase the size of these smaller pipes to reduce the headloss. Higher velocities may be tolerated more in larger pipes because cost to upsize and energy costs are considerably higher for transmission mains. And surge analysis is more commonly done and protected against with larger lines.

 

[JKW05]

Craig,

Sorry, I misunderstood what you were working on.

For what its worth, the book I was referencing is "Plumbing Design and Installation Reference Guide" by Tyler G. Hicks. For plumbing systems in buildings, he suggests "good engineering practice" is for "... water supply piping to be limited to no more than 8 fps." This is to avoid "whistling line noise", cavitation and excessive noise in fittings and valves. He goes on to say, "It is also recommended that maximum velocity be limited to no more than 4 fps in branch piping ... to water outlets at which the supply is controlled by means of quick-closing devices (e.g. flush valves)." This is to avoid excessive and damaging shock pressures.

JKW

 

[rconner]

Thanks, cvg. I agree with most of what you are saying. However, with increasing energy costs (and life cycle issues associated with utilization of pumping energy in some areas) I'm not sure at least nowadays about the first part of the following statement, "Higher velocities may be tolerated more in larger pipes because cost to upsize and energy costs are considerably higher for transmission mains."

 

[bimr]

If you are working on a campus, the maximum practical flow is up to 7 feet per second (assuming that you have satisfied the fire flow requirements).

Municipal flows are generally within the range of 4-6 feet per second.