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Minimum Velocity in Potable water pipe 6
- Thread starter dpm5414
- Start date
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?
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.
DarthSoilsGuy
Geotechnical
dpm,
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.
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.
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.
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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