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?

 

[swazimatt]

I would model this on EPAnet to get better understanding, but definitely going to have to have a break pressure tank(s) to split the development into pressure zones (this is much simpler/ near zero maintenance than using PRV, but will need to size them properly to account for firefighting storage). Most plumbing has a max of 7bar (70m) and a base operating pressure of 1 Bar min if you want to take half decent showers (question single/double storey buildings?)

PE is fine for this, but for 100 lots you are looking at peak flow of about 5.8l/s which is almost 3m/s which would equate to very high friction losses - ideally you want to be under 2m/s
more importantly you probably need to provide about 15-25l/s for firefighting (will need to confirm this with your firefighting code) which will push your main reticulation pipe up to about 100 - 150mm diameter. With it being a long-skinny development i suspect firefighting will dictate your pipe size as you need to have your hydrants at about 100m from the lots

 

[LittleInch]

Q1 - I assume you mean the main line which pumps up to the top tank?
Surge or overpressure events are supposed to be low duration and occasional. You can't just re-rate a 200psi pipe to 300psi when it sees that all the time.

2) Pressure regulating valves are commonly used in such situations, but also need a pressure relief for when they leak a little bit. Alternative is definitely tanks at different levels

3 - Water pipe boys love adding vents all over the place at high points. The fact it is running downhill shouldn't have any impact.

There is a water treatment and distribution forum which might give you better answers.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Question 1.
D5 What is the safe peak pressure (surge plus pumping) for HDPE?
1. AWWA C906 defines two types of surge pressure, recurring and occasional.
The safe peak pressure or allowed total pressure for HDPE pipe is 1.5 times
the pipe’s pressure rating for recurring surge and 2.0 times the pipe’s pressure
rating for occasional surge. For instance a DR17 pipe which has a pressure
rating of 100 psi can safely handle total pressure during recurring surge of 150
psi and total pressure during an occasional surge of 200 psi.

FREQUENTLY ASKED QUESTIONS

Question 2.
Yes, the least expensive way is to use PRV's.

Question 3.
No, you want to avoid vents as it may collapse the pipe from vacuum.

Size your piping for 3-5 ft/sec velocity.

 

[1503-44]

Water guys like air release valves for a reason. Air is much more solvable in water than hydrocarbons, so water lines will often carry too much air in their water. If temperature drops in the water pipeline, air may come out of solution and collect at high points. You can have air release valves at high points to remove the air, but they are only needed if air enters the system and the water velocity is not enough to move air bubbles along and through the pipes and eventually out. If they are not moved out, air may collect at such high points and block the free passage of water. Unfortunately at times of minimum or no demand, like at night in a system with 25 vacation houses only occupied 40% or less of the time, velocity may be slow and air blockage can occur. If you cannot always guarantee a minimum flow velocity of 1 ft/second, bubbles may collect.

Air release valves do not cause pipe collapse. They only allow air to escape and thereby prevent vapor lock. A vent is different than an air release valve and it may allow air entry into the pipe thereby reducing internal pipe pressure to atmospheric pressure, if air does enter. If a vent is located at the highest point in the pipeline, the internal pipe pressure will not be reduced to below atmospheric pressure and no vacuum will form.

A drain at a low point may allow enough water to drain from the pipeline such that a vacuum will form at high points. That is what may collapse a pipe. To stop that possibility an air entry valve (vacuum breaker valve) is placed at high points where vacuums may form. They open when they sense internal pipe pressures lower than atmospheric pressure and they allow air at atmospheric pressure to enter the pipeline, thereby always keeping the pipeline pressure equal to or above atmospheric pressure; no vacuum can occur, no vacuum collapse will be possible.

See Air Release and Vacuum breaker Valves

https://www.flomatic.com/valves/air-release-vacuum-valves/

The problem is more of a "Hydrocarbon guys don't like air thing".

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

 

[Pozos_panama]

D5 What is the safe peak pressure (surge plus pumping) for HDPE?
1. AWWA C906 defines two types of surge pressure, recurring and occasional.
The safe peak pressure or allowed total pressure for HDPE pipe is 1.5 times
the pipe’s pressure rating for recurring surge and 2.0 times the pipe’s pressure
rating for occasional surge. For instance a DR17 pipe which has a pressure
rating of 100 psi can safely handle total pressure during recurring surge of 150
psi and total pressure during an occasional surge of 200 psi.

I read through that document (and some other similar ones) and I don't see what the definition of surge is. How many miliseconds/seconds/minutes/hours/etc can the pipe be at the elevated pressure? Obviously its not a 24/7 thing if it is a surge, but I also don't see where the breakdown is. If it is at elevated pressure for an hour a day, is that considered a surge or is that considered regular conditions?

Thanks for the feedback.

 

[LittleInch]

The codes deliberately don't specify such things but leave it to Engineering judgement.

"Surge is usually intended to be a short duration 9 say 30-60 seconds) of high pressure caused by sudden valve closure or pump start / stop.

The recurring / occasional thing I would interpret as once a day or maybe twice a week versus 5 to 10 times a year.

Basically no good for your useage.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

My definition of allowable time at or above surge pressure is simple.
Surge should be considered a temporary upset condition, a not normal pressure condition.
It could last milliseconds or an hour. But in any case, if surge happens, you should be doing something to immediately return to normal operating conditions. Releasing water, air, closing valves stopping pumps. Whatever it takes to return to normal operating limits as quickly as possible. Thus the allowable time is only the time you need to safely depresdure the high pressure conditions and return to normal pressure again.

If you are surging, anything above allowed normal operating pressure, you are in stage 1 of surge control. Consider all action required to return to normal op conditions without resorting to a complete power down, unless complete shutdown is necessary to keep pressures lower than 150%. Surge control valves opened and flow diverted, on recycle, etc. You should see a near immediate improvement and a constant reduction of pressure. You should not go over 150% from the time you initiate control actions until you return to 100%

If pressure continues at same or upward track and you go over 150% of allowed pressure, you're in the red zone and have entered phase 2 of surge control. An immediate return to op condition is required as fast as possible. All surge control valves open and full stop initiated with no further climb of pressure. If pressure climb continues, it better not reach 200% before returning to 100%

1 or 2 times per week out on control is too much time IMO.

Some guidelines are

https://www.aft.com/blog/asme-surge-codes

There are some surge pressure vs time limitations in B31.3
While this is process systems, I would consider such limits for water supply systems, if there are no others to be had.

When the variation lasts no more than 10 hours at any one time and not more than 100 hours per year, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by no more than 33%.
When the variation lasts no more than 50 hours at any one time and not more than 500 hours per year, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than 20%.

Instead of the 33% and 20% limits (133% or 120% HDPE pipe allowable), you might consider something like the 150% for the 100 hr time and either a 100%, or 110% or 120% for the 500 hr limit, unless somebody has any better suggestions.

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

 

[EdStainless]

If the water is static (or near static) on a hot day is when you are most likely to have issues with air bubbles in the system.
Gas solubility goes down with increasing temperatures.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[1503-44]

Thanks Ed. I seem to always get that upside down.
I only can remember that lower pressure let's the air out (another danger at the high points for bubble formation), cause that like popping the top of my Coka Cola. So, as the Coke reaches warmer temperatures, that's what keeps those little bubbles coming long after the pressure is reduced. Finally I got it.

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

 

[swazimatt]

I do not follow some of the posts regarding the "vents"
for long uphill, downhill sections there is a recommendation to have an airvalve every 500m. For residential developments I would only do that for sections with no lot connections. The lot connections normally allow venting of air through your toilet cistern or hot water boilers also have air valves. It is good practice to position your hydrants at high and low points to double as manual air vents and scours.

With regards to pipe collapse due to vacuum that would be if you do not allow air into your system when draining. if you have gate valves on your pipe that will be closed and downstream drained you should have a double action air valve here to allow air into the system to enable the pipe to drain without creating a vacuum. similarly you would have an airvalve on your highpoints to vent air, but also to handle a pipe burst combined with valve closed and total system draining (again in residential areas this can suck air through your plumbing - on the assumption that Panama does not insist on non-return valves on lot connections. This is also somewhat dependednt on the pressure rating of the pipe. I have seen sneaky contractors having to dig up collapsed pipe because they didn't agree with my pressure rating

another thing to be cautuios with is if fire fighting is done through the fire truck pumps. If your network is sized too small you run the risk of collapsing your pipes when pumping out of the hydrant

 

[georgeverghese]

Typical mains water residential supply is preferably below 500kpag and above 100-150kpag, so suggest reserve tanks every 45m elevation rise.
Pressure reduction valves will most likely not be able to respond quickly enough to sudden reduction in flow, which will result in transient pressure surges in these flimsy plastic pipes

 

[bimr]

I'm a "water guy". It is better to resolve air entrainment issues with water velocity than to use air vents, unless one is dealing with extreme elevation changes.

 

[1503-44]

Bimr, I do agree velocity is the best solution, but as you know, low velocities are a problem when demands are not constant enough to sustain minimum flows. I'm a hydrocarbon guy and velocity is our solution to everything, because it's easy and doesn't cost anything extra, but then who ever shuts down an oil pipeline? If we stop loading a ship, yes we shutdown a loading line, but we also do not usually find air in the pipeline when we restart.

I certainly do not like clearing water pipelines by getting my kitchen and bathrooms air/water blasted. That's a solution for air in the line? Maybe that flies in Panama, but not in my house.

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

 

[bimr]

See page 166 of the plastic piping handbook.

Generally, PE pipe’s capacity for safely tolerating occasional and frequently occurring
surges is such that seldom are surge pressures large enough to require a de-rating of
the pipe’s static pressure rating. Tables 1-3 A and 1-3 B show the maximum allowable
sudden changes in water flow velocity (ΔV) that are safely tolerated without the need
to de-rate the pressure rating (PR) or, the pressure class (PC), of a PE pipe. If sudden
changes in velocity are expected to be greater than the values shown in these Tables,
they then must be accommodated by lowering the pipe’s static pressure rating. As
previously discussed, the new rating is called the working pressure rating (WPR).The
procedure for establishing a WPR has been discussed earlier in this Section.

Plastic Piping Handbook

 

[bimr]

I'm not one to disagree with you. However, many in the water business over specify air release valves.

The function of air valves.

Air valves are hydromechanical devices with an internal float mechanism designed to release trapped air and wastewater gases during filling and operation of a piping system.

They also ensure air intake during draining to maintain a positive pressure. This is important as negative pressure causes vacuum that may result in pipe collapse and introduction of contaminants to the system.

Therefore, air valves provide protection from transient pressures and entrapped air in pipelines, which are the main causes of bursting, collapsing, and fracturing of pipelines.

With correct use of air valves, you will achieve:

Reduced energy consumption and operation costs
Reduced pressure loss
Reduced delays when filling the pipe system
Reduced risk of water hammer
Reduced failure or inaccuracies in flow metering
Reduced corrosion
Reduced risk of contamination
Reduced risk of pipe burst and collapse
Reduced costs for replacement
Reduced depreciation due to increased network lifespan
Reduced required pump capacity due to lower head loss

AVK

 

[1503-44]

Actually sometimes I'm a water guy. The last oil field needed a far higher flow rate of hot water for injection into the wells than the rate of oil being produced. There were several water wells sites that were located in diverse areas up to some 75km away from the well sites and some water was being produced for local domestic use as well, so the water collection system was actually much more of the project than the oil collection system. The oil export pipeline was 860 km long to the port. Stayed busy for a couple of years on that one.

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

 

[LittleInch]

And it still hasn't gone anywhere...

It was a reservoir, not water wells. I learnt more about water issues on that job later on than I ever meant to.

But we digress.

The OP needs break tanks really.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

And install vortex breakers on the exit water lines from these reserve tanks, so you dont pull in air along with the water. Keep a minimum submergence of say 8inches for the vortex breaker. Obviously, you'll need air release valves (or manually operated vent valves) at high points in the distribution piping if these lines are high point pocketed to fully liquid prime these lines on startup or after any maintenance activities that involve breaking open these lines.

 

[1503-44]

LI not only the reservoir. There were actually many potential water well sites too.
Total will develop that field in 2095. Maybe.

I thought there was no question that break tanks would be needed here.

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