Sort of a plumbing question 1

[AggieYank]

I need to run a water line from a 3/4 inch meter about 2600 feet to a new building. A couple of plumbers have indicated that it would be best to run a 2" line about 3/4 of the way, then run a 1 1/2" line the remainder of the way. Another plumber said I can just use the 3/4" line. Would there be any effect from switching from the 2" line to the 1 1/2" line? Also, the elevation drops 36 feet over the first 1200 feet and then rises 26 feet over the next 1400 ft.

It seems obvious that the line has to be bigger than the 3/4", so I'm leaning towards the 2" and 1 1/2" line, but would like some expert opinions. Can anyone actually run through the physics? Thanks guys.

 

[dicksewerrat]

how much water do you need at the new bldg.? May be cheaper to dig a well.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[SlideRuleEra]

There are several mathematical approaches, here are two common ways:

1. Darcy-Weisbach/Colebrook Method
2. Hazen & Williams

Here is a link for general information on both

http://www.cpchem.com/enu/docs_pipe/EMb2c4.pdf

This document is for polyethylene pipe, but factors in both equations account for using other materials (such as steel, PVC, copper, fiberglass, etc.)

The math for each is performed differently and the numerical "answers" will vary, however...

For a fixed pipe material, a fixed route, and assuming that your objective is to obtain the smallest pressure drop (greatest flow rate) over the 2600 ft. distance, both methods will tell you the same thing:

Use 2" pipe for 2600 ft. then transition to 3/4" pipe. Exactly how the transition from 2" pipe to 3/4" pipe is made has an effect, but that is a secondary concern.

Another method, the Manning Equation, is normally thought of as a method for open channel flow, but it works just as well on (filled) pressurized pipes. Results will be in agreement with both the Hazen & Williams and Darcy-Weisbach methods.

With the above assumptions, the elevation changes have no effect on the pipe diameter selection.

Of course unless you run the numbers, there is no guarantee that the "best" combination will deliver enough water or maintain satisfactory water pressure during use.

http://www.SlideRuleEra.net

 

[vzeos]

AggieYank,

Generally, the pressure drop depends on the pipe length, the pipe size and the flow rate. The flow rate is the maximum demand for water. I agree with SlideRuleEra that the 2” pipe is the way to go since 2600 ft of 2” pipe is the equivalent of 20 ft of ¾” pipe in terms of pressure drop. Your objective should be to deliver as much pressure as possible to the building line. Once you change the line size to ¾” in the building, the pressure will drop drastically.

I guess the second plumber suggested a ¾” pipe because you are tying into a ¾” meter as a source. It is generally not a good idea to feed a larger pipe with a smaller pipe since the smaller pipe does not have the capacity to utilize fully the larger pipe’s capacity. Sometimes (at least with gas meters) meters are sized according to their functionality as well as their capacity so that the meter size may need to be slightly smaller than the outlet pipe size. Nonetheless, it may be worthwhile to check the meter capacity. The meter capacity should be at least equal to the maximum demand for water.

What struck me as unusual about your set up is that the meter will be 2600 ft from the structure. I assume this means that you will retain the ownership of the pipe and responsibility for its O&M. This also means that if the piping ever develops an underground leak it will register on your meter and you will be charged for that water as long as that leak exists. I think that most underground water leaks go undetected. It is far better, overall, to set the meter at the point of utilization. dicksewerrat ‘s suggestion may very well be the best option.

 

[rmw]

Can you put a air blanketed or elevated storage tank at the building so that it furnishes the fluctuating building loads, while the line from the meter to the tank replinishes it at a constant flow rate, (which you haven't told us yet) like a water tower does for a water distribution system?

Worked at a youth camp years ago which had a similar problem. Camp originally had a deep well with such a storage tank. Load surges were huge. Parts of the day, the only usage was drinking fountain type flows, but when they hit the showers after afternoon swim, right before supper, you couldn't get enough water out of the well.

The well died, and it became necessary to tie into the rural water system up on the main road (which wasn't there when the camp was built-water system that is.) I think I remember that they furnished a 1" meter. I ran a 2" line, but retained the storage tank and ran the 'city' water into it.

Even a 2" line would not have handled the surge flow at times of maximum usage, (all the showers running, concurrent with various toilets and sinks, kitchen fixing supper, all the while the pool guy had just turned on the pool fillers to make up all the water the kids just splashed out, and the athletic director was watering the softball field.)

The system worked great. No one at the camp ever suffered from lack of adequate pressure. Later some genius removed the tank, and, well.....

rmw

 

[AggieYank]

Thanks guys.

 

[TBP]

It may help to think of the individual components (straight runs of pipe, fittings, valves, meters, etc.) of piping systems as links in a chain. Every link will have it's pressure drop, related to the flow. The outlet pressure from one link is the inlet pressure to the next. A 3/4" meter sounds like a residential sized service. The utility won't want to oversize their meter (increased upfront cost for them, and the turndown will be poor), and they'll want it at whatever their specified distance is from the main. I suspect that if they install the meter at the point of use, they'll also lay the line - and charge you for it, which is really only fair.

My sizing program is at the shop, but a residential sized service, a half mile long? I could believe 2". It'll be a fairly marginal cost to make it 2" all the way - the trenching cost will be identical along with bulk of the mechanical labour. I think with a 3/4" line that long, you'll be 15 minutes filling a water glass.