Sizing Water Main 3

[edmeng]

I know the demands required for an area that is far away, 12 km and requires water servicing. I want to apprpriately size a water main and am not sure I am doing this correctly (first try). I have used Hazen Williams to calculate the headloss. Now I am not sure how to proceed. I don't think I need a detailed explanation but if someone could help with what equation to use or what parameters to check to make sure size is suitable that would be great. Thanks.

 

[rbcoulter]

You can use the free software, EPANET. Just type it in any search engine to locate it (provided by the US EPA).

 

[RWF7437]

In most water systems the economical size will be at around 5 feet per second velocity. Larger pipe costs more but reduces pumping costs and water hammer problems. Smaller pipe costs less but increases head loss and therfore pumping costs and water hammer problems.

If you know the peak flow in gpm take the square root of it. The answer will be the required pipe size in inches. Use the next higher pipe size, again in inches. This is crude but will give you a good first approximation.

By the way, EPANET will not size pipes for you.

good luck

 

[edmeng]

Thanks for the help. So in checking the system, I will check pressures and velocities but is there an "acceptable" amount of headloss before I should consider resizing the line? Thanks.

 

[RWF7437]

edmeng,

Yes, pick a pipe size and then check pressures and velocities at design flow and at other possible flows; average day demand, maximum day demand, peak hour demand and maybe max. day plus fire flow. Because pipe costs are rising slowly and energy costs everywhere I know are rising steeply, I would think choosing a larger pipe is the wisest choice over the life of the system.

The 5 ft/sec guideline is a good indicator for most water systems. Velocities less than 2 ft/sec are probably too low and those over 10 ft/sec are probably too high under most conditions. During emergency conditions, such as a fire, they can be tolerated but should be avoided if possible for the reasons mentioned in my earlier post.

All this is based on the experiences of many water system operators and can be verified by looking at other nearby systems and by a review of the many AWWA publications available.

good luck

 

[cvg]

typical maximum pressures for domestic potable water transmission systems are around 120 psi. Minimum pressures are around 40 psi. Check your existing reservoir elevation and the elevation change along your pipeline and then see what type of pressure you need to pump the water the 12 km.

 

[rconner]

You may wish to refer to a book from American Society of Civil Engineers (ASCE) entitled, "Pressure Pipeline Design for Water and Wastewater", that I believe has a quite good section on many aspects of pipeline sizing/economics (and many other necessary and/or helpful design considerations). I'm sure there are many other good references. Many piping manufacturers (e.g. ACIPCO, for inch sized pipe at

http://www.acipco.com/adip/products/Sect17.pdf

and different metric sized pipe in tables elsewhere on the same site, provide many forms of Engineering tables that may prove helpful in this process).

 

[stanier]

http://www.haestad.com

has a book thta you can view on ;ine or can purchase as hard copy. The on line viewing is free. the book is Advanced Water Distribution Modelling and Management

 

[katmar]

If you are pumping as far as 12 km it will probably be worth considering putting a header tower on the far end to give a bit of buffer (for both capacity and head) to smooth out the peaks loads.

 

[cvg]

I would recommend that pumping be directly to a reservoir which would serve this remote area