Newbie designing a water line needs help!!

[chrisk201]

I have worked the past ten years as a highway designer for a state agency and my hydraulic design skills have eroded. Our maintenance department wants to extend water and sewer service to one of our rural maintenance facilities and have asked me for design help rather than hiring an outside engineer. The facility is about 2500' from the city limits and the city water system. They would like to use 1500' of 6" PVC to extend to the property line of our facility and then 1000' of 2" to get to the building. I had the hydrant at the end of the line tested and it had a Q of 850 gpm, a pressure of 66 psi and and the next hydrant up the line showed a pressure of 47 psi when the line was flushed.

Given all that, I guess I need to calculate the pressure at the building to show it is greater than 30 psi. Do I just calculate the loss through the pipe network and then the new pressure? I used an online calculator and it showed a major pressure drop and head loss in the 2" section and I think I am doing something wrong. Thanks all for your help.

 

[ccor]

I can't verify your calcs because you didn't specify the flowrate you need at the end of the line; however, the results you elude to don't surprise me. Assuming a velocity of 5 ft/s equates to about 50gpm which will create about 53-feet (23-psi) of headloss in just the 1000-ft of 2" line.

Long dead end mains are problematic from a water hammer and water quality standpoint. Many threads are dedicated to this subject that I suggest you look through them.

Also, if your going to the trouble you may want to provide fire flow to the building. A 2500LF of 6"PVC at 500 gpm will produce about 40-feet (17-psi) of headloss in the pipe, THIS IS NEGLECTING ANY CHANGE IN ELEVATION.

 

[bimr]

ccor is correct. You need to provide some information on the flow and water uses. The pipe headloss should be determined using the peak water usage, not the maximum flow through the pipe.

If all you are planning is to supply the domestic needs (ie toilet) for the building, then a 2" line is probably adequate. New residences are putting in 1 -1/2" water lines and an industrial building with only a single bathroom would probably go with a 2" line.

For just the domestic needs, that would be about 9 pgm peak hourly flow which would be 3-4 psi drop on a 2" line or 12 psi if you used 1 1/2" pipe.

If all you are supplying is water for domestic uses, you might want to use the smaller pipe size since the flow is so low and the water will be stagnant as ccor pointed out.

 

[cvg]

I would shoot for more like 40 psi design pressure at your building. depending upon the time of day and the system demands during your hydrant test, the pressure at that hydrant could fluctuate considerably from the 66 psi that you measured. Better to provide a safety factor so your pressure at the building is acceptable.

 

[chrisk201]

Thanks for the tips. The building is basically a maintenance garage for our county road crew, so toilets and sinks are the only use. 40 psi sounds reasonable for the design pressure at the building. Will that be attainable with the head loss of 23 psi mentioned above. Do I take the flow rate and pressure at the hydrant and subtract the head loss & pressure loss in the pipe run to get the pressure at the building? Thanks again for the help. I really feel like an idiot not remembering this stuff.

 

[cvg]

yes, and as mentioned before - you may also have head loss (or gain) due to elevation difference between the hydrant and the building. Use your actual flow rates to calculate the friction loss in the pipe. You can use Hazen Williams equation to calculate it. Estimate a Hazen Williams C factor for the pvc pipe might be 140. However, a conservative number for design might be 120 which will also help account for any minor losses in your fittings and reducers. You will also probably need a backflow preventor where you go to 2 inch line.

 

[stanier]

download the freeware Epanet and model the system . The software contains a tutorial that will get you going very quickly. This software uses the same engine as expensive software such as Watercad.

The other thing you need to consider is surge in the system. You will need to get an experienced hydraulics engineer to do this for you.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[bimr]

You will not have a headloss of 23 psi unless you are flowing 50 gpm, which is probably impossible with just some toilets and a sink. As CVG suggested, use the actual water flow rate.

You need to determine your peak flow rate and then estimate the pressure drop at that flow rate, not the maximum flow capacity of the pipe.

You are not going to have much flow. You can figure the actual flow various ways. Each worker probably uses about 15 gallons per day. Add up the number of workers and then multiply your daily average by 10 (peak hour to average daily factor) to get the peak hourly flow.

An alternate method to calculate the flow is to use estimates from similar industries. Water Supply and Pollution Control has a table that lists a peak hour estimate of 8.7 gallon per minute per lift for a service station.

A third alternate is to determine the peak flow from each plumbing fixture.

Use 3 gpm per toilet (tank type)
3 gpm per dishwasher
5 gpm per 3/4" hose bib
1/2 gpm per drinking fountain
3 gpm per shower
4 gpm per sink

You probably have a peak flow somewhere between 8.7 and 18 gpm. That gives you a pressure drop range from 3-4 psi (at 8.7 gpm) to 10 psi (at 18 gpm) in the 1000 ft of 2” piping.

You also need to be aware of elevation differences as the others have pointed out. You probably do not have to worry about water hammer since the flow velocity is so low.

 

[cvg]

note: reference "Water Supply and Pollution Control" by Clark, Viessman & Hammer

 

[chrisk201]

I think I got this thing figured out. Thanks for all the help.