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Minimum Velocity in Potable water pipe

Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

Are you looking for minimum velocity or minimum flushing velocity?

RE: Minimum Velocity in Potable water pipe

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!

RE: Minimum Velocity in Potable water pipe

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

Are you saying that was the wrong question?

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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.

RE: Minimum Velocity in Potable water pipe

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. smile

RE: Minimum Velocity in Potable water pipe

For the record, i think i was talking about flow, but just divide the answer by A.  Optimizing power consumption to diameter yields the same answer (converted to velocity) as head loss can be a function of Q just as much as it is a function of V and A, since diameter can't be returned without knowing two of those 3.  Having a constraint whereas an acceptable velocity is returned would be good, but the same could be accomplished by setting limits on the possible range of flows to be searched.  Right?

RE: Minimum Velocity in Potable water pipe

Water distribution systems are typically planned for a design horizon maybe 5 years, maybe 25 years taking into account projected population and water demand growth. In addition the system usually has to provide fire water supply and this dictates the diameter of the smaller pipes. There are diurnal and seasonal variations. So when discussing a minimum velocity – do we mean a minimum velocity at theoretical design flow or in real time operation?. And why would we want a minimum velocity that moves any sediment from the larger diameter pipe (where it has negligible impact on hydraulic capacity) to accumulate in the smaller diameter tail end pipes where it will likely have a significant impact on hydraulic capacity.  There is no argument for a minimum operation velocity. Water quality, retention time and maintaining chlorine residual are the controlling factors.

RE: Minimum Velocity in Potable water pipe

Is there a semantics issue here. i noticed the chemical tag to your handle. I'm going to assume that you're not in charge of designing a municipal water system, but maybe something more like the connection of a building or a couple of buildings to an existing system?

in the latter:
we have a 3/4" to 2" domestic water line connecting our most of our buildings to the system, The main design factors are:
1 keeping the velocity near or above the flushing velocity to prevent sediment buildup (make the pipe smaller) at peak flow
2 keeping Head Loss to fixtures and line friction to an acceptable level (make the pipe bigger) at peak flow
3 cost of materials (make the pipe smaller)

Sprinkler lines to buildings get their own bigger line off the system since the pipe size will have to be greater for greater flows. You wouldn't want to just have one big line serving domestic water and fire water though because of that min. velocity issue.

RE: Minimum Velocity in Potable water pipe

When doing a network analysis, I usually keep the min V (at the peak design flow) to 0.6 m/s (2 ft/sec).  This is empirical (but operational info appears to agree), to ensure that any sediment is evenly distributed (at all times) into EVERY service, rather than a few.  This ensures that there is no sediment buildup (& sediment is present in almost all networks systems), with minimal dirty water complaints.

With modern subdivision layouts, some have dead end streets that do not have a ROW to connect the respective dead ends.  In this case I use a 50 mm PE pipe for up to 10 services, instead of the minimum 100 mm reticulation pipe.  Dead ends are always a problem due to the build up of sediment (& consumer complaints), & the same result occurs (distribution of any sediment slowly & constantly, into ALL services), rather than using flushing points at the ends of dead ends.

Fire flows are still confined to hydrants located only in 100 mm pipes, with the furthest house (from a hydrant) only 5 services distant.

If the dead end street is fairly long, a 100 mm pipe is laid until there is only 10 services left, then a 50 mm PE is used.

Since this practice was developed, the number of dirty water customer complaints (in these areas) has dropped significantly.

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