Gravity fed water supply 3

[Pozos_panama]

Hello everyone, first time poster here so thanks in advance!

I have recently been asked by a potential client to assist in designing a water distribution system for their development. Their basic concept is to pump water from a well (labeled pozo in the picture at an elevation relative to sea-level of ~16meters) up to an area with reserve tanks (labeled tanque de agua at an elevation of ~196 meters) and then have a distribution pipe leaving the reserve tanks and feeding individual lots/houses.

The lots are all along the way and some will have relatively little elevation difference while others will see the full ~180m of difference.

For more background, we are in a remote area of Panama with typical materials relatively hard to find. Most places around here are just using glued PVC for their distribution systems, which brings tons of headaches and almost non stop maintenance/patching of the lines.

I was wanting to use HDPE pipe for this job however, with that elevation difference it puts the pressure at the bottom of the line at ~255 psi - above the rating of 200 psi HDPE (at least roll pipe). I was hoping to use roll pipe (2") because the fittings are relatively easy to get ahold of, don't require extra tools, and don't require specific training.

Question 1: The repeated surge pressure of 200psi pipe says it is up to 300 psi, but I am having a hard time finding the definition of what that entails. For example, if the line has check valves along the way, the hypothetical flow rate of 30 gpm doesn't add too much in losses (total length of client's desired path for line was about 768 meters which would add about 19 psi in losses), and it is pumping for a few hours a day. Does that fall into the repeated surge category or is that outside of it?

Question 2: The main distribution line will exit the reserve tanks and follow a road downhill with takeoffs for the different lots. Is it feasible to have a pressure reducing valve on the main line in order to ensure the pipe stays within it's pressure rating? Or would there have to be a second storage tank lower down the hill to act as a break? Or is there a better way that I am ignorant to?

Question 3: For long line runs down hills - sometimes quite steep - does there need to be some type of air venting available?

For more context: The developer has said that while there are going to be an initial 20 lots/homes built soon, with potentially up to another 100 over the coming years in further phases, they are all vacation/part time homes and he never expects there to be more than 40% occupancy at any given time.

Bonus question: Does anyone have a preferred brand of smart water meter/leak detector that has remote/auto shutoff capability?

 

[georgeverghese]

It is not necessary to have each reserve tank taking its feed from the main line, except for the tank at the very top, and the pressure break tank at the next lower elevation. Each of the other tanks can be fed by the tank at the next higher elevation. So the pump runs start stop based on a level switch at the top tank or the level switch at the pressure break tank.

 

[bimr]

Rather than pumping water all of the way to the top of your mountain, consider setting up a system similar to what is done for high rise buildings. High rise buildings have plumbing systems with pressure zones every 30 meters or so. Whether or not this is attractive will depend on your site conditions.

 

[KernOily]

Watch out for temperature. The line will heat up fast where it is exposed to sun. The pressure rating of HDPE drops fast with temperature. We use 140° F for HDPE pipe exposed to sun for design temperature. Check the derating curve for your pipe and make sure you don't go over the MAWP at that temperature.

 

[Pozos_panama]

Thank you all for the feedback! There are some great details in here that I wouldn't have thought about.

When I spoke with the developer, he said the lots are all completely segregated out and cannot be changed, so he is not supportive of having intermediary tanks at various elevations to limit the pressure.

So it seems like reducing valves would be required for the house take offs.

Watch out for temperature. The line will heat up fast where it is exposed to sun. The pressure rating of HDPE drops fast with temperature. We use 140° F for HDPE pipe exposed to sun for design temperature. Check the derating curve for your pipe and make sure you don't go over the MAWP at that temperature.

...does this happen to be based on temperatures in Bakersfield oil fields? That's where I'm from and was my last engineering role in the US before leaving to go abroad for a while. If so, small world!

Where this project would be in Panama the highs would be around 95 F for the ambient, but the sun is quite intense here so I could see the pipe heating up to well above that.

 

[KernOily]

Yes. 140° F pipe surface temperature is based on actual measurements during early afternoon of a summer day with ambient at around 105° F in this local area (central Calif.). Solar radiation is a function of latitude, so I would suggest using a black HDPE surface as a test, measure the temperature, and use that as your design temperature. Also, HDPE expands like crazy and it will snake itself all over the place when it heats up, so make sure you allow for that. The HDPE engineering manual put out by ChevronPhillips is highly recommended as a starting point:

https://www.cpchem.com/sites/defaul...ld Handbook July 2021_PRINT VERSION_FINAL.pdf

Here is another good resource:

https://www.plasticpipe.org/Municip...ications/Download-HDPE-Handbook-Chapters.aspx

If the line will be buried, then you don't have thermal expansion and temperature problems except anywhere it is exposed to the sun. 

Also make sure you get a contractor that is qualified to make the welds and has the proper equipment. Not just anybody can do it.

 

[Gr8blu]

In the electrical world, there are definitions for what constitutes "frequent" and "occasional" electrical peaks. For the purposes of this discussion thread, those equate to "recurring" and "occasional" surges respectively.

An occasional electrical peak occurs at the same frequency as: a) a lightning strike directly on the equipment, or b) some other act-of-God (i.e. cataclysmic) instance that amounts to the same thing. A frequent electrical peak (or, in this case, a recurring surge) is anything that happens more often than the occasional instance.

As 1503-44 indicated: anything that gets the system to change operation into an out-of-normal condition is cause for immediate concern. This is true for electrical systems, as well as for a piping system such as the OP portrayed. The allowable duration of an event (regardless of severity) is only long enough to get things back to normal. That means duration (of a surge event, in this case) should be measured in seconds, not minutes or hours.

Converting energy to motion for more than half a century

 

[1503-44]

The surges I have had to personally control were all so noisy and just plain scary that I really wanted them to go away as fast as possible. 15 minutes of 4 relief valve's blowing, pipe shaking and constant alarms coming on while depressuring and waiting for 500,000 BOPD to come to a stop is way, way, way too long. Compressor surges are 10x worse.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."