Gravity-driven water network: Break Pressure Tanks

[KalliD]

Hi all,

I am designing a gravity-driven water flow network for a rural area in the course of a feasibility study.
Excuse my ignorance regarding break pressure tanks but there are a few things I am not exactly sure how to deal with.

Background information:
The elevation difference between source and storage tank (transmission line) is 182 meters and from the storage tank to the lowest supply point (distribution line) another 93 meters. The maximum hydrostatic pressure shall be 60 meters. I intent to locate break pressure tanks(/storage tank) every 60 meters.

Questions:
1. Question: Flow interruption in transmission line with "ball cocks" possible?
The water flow shall be interrupted when the storage tank is full. What technology do you recommend? The same shall apply for the break pressure tanks. The break pressure tanks shall not overflow but "shut down". Is that in general possible and recommendable?

2. Question: The water demand in the distribution line fluctuates. Can a break pressure tank deal with great water flow fluctuations. What alternatives can I use?

3. Question: Does a pressure reducing valve work on hydrostatic pressure reduction?


I am happy with just the answer of one of my questions or helpful reading material. Dont feel obliged to answer all at one
Thanks in advance

 

[katmar]

Have you done a search of this site for "break pressure tanks"? This has been discussed several times before.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[KalliD]

Hey katmar,

Actually I did. Maybe I used the wrong keywords but all I found was regarding positioning of the break pressure tank which is not the problem I am facing.

 

[katmar]

1. If the line is far away from an electrical supply then ball cocks (or some variation of them) are really your only answer. They work well, but will eventually leak and need servicing. Provide the break tanks with overflow pipes that take the water somewhere safe, but where it will be seen so that people will know to service the leaking ball cock. Also install isolation valves upstream of the ball cocks so that they can be serviced without draining the system. Provided that the static height between break tanks is less than the rating of the pipe it is a practical and safe solution.

2. A break pressure tank is ideal for coping with fluctuating demands, as long as the design flow is greater then the highest fluctuation. The tank will serve as a small reservoir so you could have very short periods of higher demand but it would be best to design for the maximum flow.

3. My personal bias is against pressure reducing valves in this application because the slightest leak will result in high pressures at times of zero flow. But maybe others have success stories to tell?

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[bimr]

You would be better served with pressure reducing valves. These valves are quite reliable. Over pressurization may be prevented with relief valves.

Pressure reducing valves will be less expensive than break tanks.

 

[KalliD]

Thank you katmar and bimr for your suggestions!

@katmar:
1. No electrical supply is available in the area of the transmission line. I will use ball cocks or an variation of it then. Control valves or isolation valves will be installed as well.

2. Sounds reasonable, the right dimensioning of the break pressure tank would be capable of balance demand fluctuations to a certain amount. The distribution network (pipe sizing) will be design for the max. water demand.

@katmar and bimr:
3. Well, regarding break pressure tanks vs. pressure reducing valve we have oppositioning opinions.
The distribution network would be "offline" during the night. The distribution network would be under hydro static pressure. Installing a pressusre reducing valve, wouldnt that mean a constant flow through the valve to reduce the pressure. Hence, that water would be lost if not fed back to the main storage tank? Feding it back into the storage tank is no option as additional energy would be needed...

Thank you two, you were of a big help though I still have to do some furhter research on the pressure reducing valves I suppose

 

[bimr]

Pressure Reducing Valves automatically reduce a higher inlet pressure to a steady lower downstream pressure, regardless of changing flow rate and/or varying inlet pressure. Each valve is an accurate, pilot-operated regulator capable of holding downstream pressure to a pre-determined limit. When downstream pressure exceeds the pressure setting of the control pilot, the main valve and pilot valve close drip-tight.

http://www.cla-val.com/pressure-reducing-valve-p-17-l-en.html

 

[katmar]

I have used these valves recommended by bimr, but not on this type of application. They are very good and also do not need a power supply. I suggest you approach the supplier for recommendations and case studies. If they have got these to work for downhill supplies then consider them.

There are two reasons for my hesitation to recommend them. The first is the difficulty you will have in setting them. The valves measure and respond to the pressure immediately downstream of the valve. You will need to set this for something like 2 or 3 meters of head to limit the pressure at the next downstream valve to 60 m. How do you do this in position?

The second reaon is maintenance. Skilled fitters can be rare in these rural applications. The valves, whether ball cocks or PRVs, will leak and need maintenance at some stage. New seals are easily fitted to ball cocks in the field. Servicing PRVs requires more skill and is best done in the workshop. Your choice will depend on the infrastructure you have.


Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"