Minimum Velocity in Potable water pipe 6

[dpm5414]

I am working on a design for a replacement pipe line for potable water. I am looking for a published standard or guidline that states what the minimum velocity in a potable water pipe should be. Thank you.

 

[RWF7437]

As far as I know, there is no minimum standard velocity since water flow in any gridded water system can be in either direction and may, at times, be Zero. Even if the system is not gridded velocity can be zero.

Maximum velocities are much more critical and are governed by considerations of economics, water hammer, energy costs, etc.

good luck

 

[BigInch]

Yes, zero velocities are possible, but not desireable. At zero or low velocity, water will go stale and sediments will accumulate. If there is zero velocity, that condition should not be allowed to persist for a very long time.

A generally accepted minimum velocity for maintaining pipelines free of small sand particules and other debris would be around 2.5 to 3 ft/sec.

http://virtualpipeline.spaces.msn.com

 

[dpm5414]

Do you know of any references that give the generally accepted minimum velocity? I've checked quite a few civil eng. handbooks and water system design books and I can only find references for min. velocities in waste water, drains, etc. but not for potable water. Thank you.

 

[SteveWag]

Trying to maintain any velocity in a water distribution system is almost impossible. You can size pumps and associated transmission lines for a desired velocity, but the system is a different story. Many systems in the US are designed to supply fire flows, resulting in piping that is very much oversized for normal domestic flows. AWWA does not have any guidelines on minimum velocities.

 

[bimr]

There is no minimum velocity as the others have posted.

However the distribution system shall be designed to maintain treated water quality. Systems shall be designed to maximize turnover and to minimize system residence times.

Reference: Recommended Standards for Water Works.

 

[cr1973]

If you have little metered usage odds are you'll still have to provide for fire flows. If not much water is drawn from the pipe, odds are you'll need to set up some sort of flushing program / automated flushing in order to keep chlorine residuals in the 1.0 range.

 

[dpm5414]

I think I've found what I need. AWWA standard C651 states that the min flushing velocity should be 2.5 ft/s. Thanks for the help, I appreciate all the responses.

 

[BigInch]

Thanks for finding that reference. I didn't know it was in any official standard.

http://virtualpipeline.spaces.msn.com

 

[redbridge]

Are you looking for minimum velocity or minimum flushing velocity?

 

[BRIS]

cr1973 has given the right answer - there is no minimum velocity - you need to design to mainatain water quality and chlorine residual. BigInch has advised that to move residual a flushing velocity of 2.5 to 3 ft/sec is required. These are two different problems!

 

[BigInch]

Well actually the question was, "What the minimum velocity in a potable water pipe should be?"

Are you saying that was the wrong question?

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[rconner]

The title of AWWA C651 is of course, "Disinfecting Water Mains" with I think the rather specific scope, "This standard describes essential procedures for the disinfection of new and repaired potable water mains. New water mains shall be disinfected before they are placed in service. Water mains taken out of service for inspection, repair, or other activities that might lead to contamination of water shall be disinfected before they
are returned to service." There are I suspect many other references having to do with what is otherwise involved with maintaining water quality (and avoiding stagnation etc.) in water systems, including e.g. publications of EPA and AWWA(RF) etc.

 

[BigInch]

Whether you can actually ever have and hold a minimum velocity in an operating system would of course be subject to demand at any given time, if some type of recirculation or minimum flow to a tank, etc. was not an integral part of the design, but its something that probably needs some attention at one phase of the design or another. Following the logic trail, it can be said that it makes little sense to design a pipe for zero flow.

http://virtualpipeline.spaces.msn.com

 

[Catcher66]

The 2.5 ft/s specified in AWWA C651 is intended to remove accumulated sediment and debris from a newly installed or repaired water main. This velocity could also be applied to hydrant flushing, where the goal is the same.

However, the design of a water main should not be based on a minimum velocity under normal conditions. The size of a water main should be dictated by the available flows and pressures under peak demands. (Fire flows if fire protection is provided)

If the goal of the question is to determine the appropriate velocity to clean the main, then 2.5 ft/s should suffice. If you are trying to determine the appropriate size for the main, minimum velocity is not the parameter you should be considering.

 

[BigInch]

I doubt whether, even in water system design, that minimum velocity would ever determine the pipeline diameter, but it might set a target for minimum design flowrates, ie for selecting pump operating ranges or multiple pump operating configurations, valve range sizing, etc.

http://virtualpipeline.spaces.msn.com

 

[kepharda]

There is a way to determine optimum economic diameter. The fundementals are that larger pipe is more expensive to install, but you also cannnot go too small or you risk excessive head losses from friction, so you must balance both.

Here is an excellent book William S. Jenna "Design of Fluid Thermal Systems 2nd Ed." It also has a table of fluids listing the Economic Velocity Range, for some common fluids. I highly recommend this book, I have had it since 2002 and it constantly impresses.

Anyhow for water it lists a range from 4.4 ft/s to 8.8 ft/s. Personally depending on the actual size used I don't see a problem going down to around 3.5 fps, but would avoid any lower.

P.S. be sure that the size you specify isn't a odd ball size, (like 5", or 1.75"). Pipe and fittings can be found, but is a pain in the butt for Contractors to find and they will curse your name.

 

[BigInch]

Kepharda, you're right, but you must use the maximum flow (not minimum) when doing that optimization.

For petro lines the first approximation for defining a diameter selection range is based on min velocity of 3 ft to max 10 ft/sec.

http://virtualpipeline.spaces.msn.com

 

[kepharda]

BigInch, I understand what you are saying, but did you mean velocity and not flow?

The reason I mentioned the min. was not in relation to optimizing the pipe OD, but because that is what the originator asked about, -the minimum velocity.

 

[BigInch]

Since Q = AV minimum flow corresponds directly to minimum velocity, and maximum flow corresponds directly to maximum velocity, therefore it really doesn't matter if I said/meant flow or velocity, the result of the optimization would be the same no matter which of those two actual variable was chosen.

I think its good that somebody's trying to relate an answer to the original question.

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