additional pvc piping for water supply 2

[archirodon]

hello everyone, i need some help from our expert members. we have an existing 8" dia. PVC water line with a water flow of 40 liters per second which is being directed into two 6" dia. PVC pipes. now, we are planning to fix a new 6" dia. PVC pipe into the 8" dia. (its water flow is 30 liters per second). My questions, will there be any problem in case this will push thru? Can the 2 six inches diameter PVC pipes accomodate the new 70 lps water flow or discharge and how about the original 8" dia. pipe? By the way I'm working for a local water distribution company here in the Philippines.

 

[BigInch]

Hard to say without knowing all the details, elevations, inlet pressures, outlet pressures, distances etc, but assuming similar conditions between the old and new lines, and just ratioing the 8" existing flow to the x-sectional area and using equal velocities, noting that the area of the 6" pipe is about half that of the 8" pipe, it would appear that for the 6" diameter line you should expect a flow of only 22 l/s, giving about 44 l/s total for the 2 of them. You might get more, or less. 70 l/s sounds like it is a problem.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[chicopee]

The more crucial question is what happens to the pressure drop thru the 8" line when your flow is increased to 70 liters per second and how does it impact the system characteristic curve.
As BigInch stated, we need more input.

 

[BigInch]

And if you did give us all the details, we would be basically designing your system for you, which we really can't do, so that's about all we can pretty much tell you about your first question.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[archirodon]

Thanks for the inputs. Actually I already computed for the velocities of water on each pipe based on the given areas and flow rates on each situation, before and after connecting the new 6" diameter PVC pipe. I've used the continuity equation, Q=aV, for the computation of their velocities.
before connecting the 6" 30 lps PVC pipe, the velocity of water on the 8" dia. is 1.32 m/s and the velocities on the 2 pcs assuming they are halving the original flow rate is 1.31 m/s.
for the second situation, after connecting the new pipe (velocity on this 6" pipe with 30 lps flow rate is 1.96 m/s) on the 8" dia PVC pipe the velocity will become 2.31 m/s on that pipe (8" pipe). and the velocities on on the two 6" pipes will be 2.29 m/s assuming they will halve the 70 lps flow rate.
Is the continuity equation applicable on this?
Does anyone here have an idea also regarding Recommended Velocities for fluids in PVC pipes? Is 2.5 m/s the maximum allowable velocity in PVC pipes?
Many thanks to everyone for the help and enlightenment.
Mabuhay!

 

[BigInch]

2.5 m/s sounds like its a good maximum velocity to use for steel pipe. That may be high for PVC, especially if there is grit or sand in the flow. With 2.5 m/s you may be getting into high surge pressures already, so using anything higher than that could surely make keeping surge pressures under control quite difficult.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[archirodon]

the 40 lps and 30 lps flow rates (8" dia. and 6" dia. respectively)are from the discharge side of two pumphouse. does that mean they'll slow down let's say 500 and 1000 meters respectively (again) from that point? thus slowing down also the velocity of the water up to the point of interconnection of the two pipes?
i'm just new in this area of Mechanical Engineering that's why pardon me for so many questions.
Another question sir, which would you prefer between these options:
1. to make a parallel transmission mains of a 6" dia. with 30 lps and 8" dia. with 40 lps and then make a loop at the distribution line,
2. to connect the 6" dia. pipe to the 8" dia. pipe ( this was the question that's i've been asking if it'll cause some problems on the 8" dia. pipe)but it'll have 2 6" dia. pipe again at the end of the 8" dia. pipe 1 kilometer from the point of interconnection.
many thanks...

 

[BigInch]

Since water is basically incompressible at low pressures, velocity only changes when you change pipe diameters or volumetric flowrate. Continue using V=Q/A.

If I understand your system configuration...

And assuming you want to go for minimum cost,

First I would look at a design without looping the 8" with the 6", as long as surge pressures would not become significant, then change to 2-6" lines at 1 km. You must be sure you have pumps with combined characteristics operating in parallel (I think that is the case, right?) that will be able to make that flow against the new pipe H-Q curve and still be within the pressure capacity of your PVC.

If surge was a problem with high velocity in the 8" line, then I would try looping the 8" with 1-6" changing to 2-6" lines at 1 km and, just to be sure, check the surge pressures again with that configuration.



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[archirodon]

we have an existing deepwell pump which has an 8" dia. transmission main. the new deepwell pump which is about a kilometer away from the existing one will be having a 6" dia. transmission main. the choices are; 1. either to loop them 1 kilometer away from the new pump or , 2. connect the 8" transmission main into the existing 6" dia. likewise the new 6" dia. into the other existing 6" dia. thus having a parallel transmission mains.
hope i was able to explain the situation clearly sir BigInch.

 

[BigInch]

OK I see the configuration now. The possible options will be the same as above, except starting 1 km farther away from the 8" pump station.

Without knowing the details of the elevations and pumps etc, still no way for me to say what would be best. It looks like a single 8" line might work, in lieu of making the loop now, but I can't tell you that from here with any degree of assurance. If you're still not sure what to do, I suppose you could always connect the 6" to the 8" and if it didn't work well, install the 6" loop at some point in the future.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[archirodon]

thanks sir big inch.
taking into consideration the pressure surge that might occur in case i'll connect the new 6" dia. (30 lps or probably more) into the 8" dia pipe (which carries 40 lps already). i think it's better not to connect the 6" into the 8" pipe which will result into a 70 lps flow into the 8" dia. pipe.
more power to you sir!

 

[BigInch]

You're right about that. Surge pressures could become excessive if you double the velocity. If you don't have a very accurate analysis of the potential surge effects, it is more prudent to go with a looped system.

Good luck.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com