Pressure tanks in village scale potable water distribution systems? 1

[jwsutto]

I am working on an investment plan for a number of small scale water supply systems. The technical capacity for O&M is currently relatively low.

My question is about the need for pressure tanks.

Some of the systems have pressure tanks (hydropneumatic tanks) to manage pressure fluctuations, water hammer, and pump run times. Some of the systems don't. If we specify modern VFD controlled pumping setups is there a need for pressure tanks across all of these systems?

Background about the systems:

- supply 50 to 1000 residential customers, plus some commercial customers.
- daily demand of 10k to 1000k gallons
- raw water storage tanks upstream of the pumping station.
- chlorine treatment at the pumping station
- 2 or 3 centrifugal pumps at the pumping station - size dependent on demand, pressure requirement, and system hydraulics.
- there is limited elevation change in the systems (the land is flat) and the systems are not extensive, with customers generally within a few KM of the pumping station.
- pressure requirement is for residential / commercial use so maybe 40-70PSI
- size of the pressure tanks is determined based on system parameters but would range from roughly 500 gallon up to 30,000 gallon

Up front investment in pressure tanks now is ok based on the financing situation so I would rather specify pressure tanks now if it will reduce leakage and pump repairs/replacements, and therefore the cost of water in the future. There is a recommendation for installing pressure tanks across the systems in a previous plan but I'm not sure the cost is necessary with modern pumping controls/technology? Obviously don't want to be chucking money away on unnecessary investments if pressure tanks have essentially become redundant based on modern pumping technology.

I have been looking at a few threads on this but the advice seems inconsistent.

Thanks in advance for any thoughts.

 

[LittleInch]

The issue is complex when it comes to details and depends on whether the risk of low or no water pressure in domestic properties is worth the benefit of not having these pressure tanks.

The flow rate variation is HUGE - 100:1. normal systems using VFDs or similar struggle at anything more than about 3or 4:1 turndown. So there is either a lot of wasted energy with recirculation systems or having to use multiple smaller or different range pumps in order to meet max demand without incurring large energy losses. Also there is a vast difference between 50 customers on a system and 1000. what works for one won't work on the other. The larger the system the more likely it is that using a range of different sized pumps would work as zero flow at 2am is still unlikely, but possible for the smaller systems. Also ther imore line pack available int he larger sysyems which gives you enough time to start additional pumps without loosing too much water pressure.

Any network analysis is complex and needs to consider a range of flows over a 24 hour period.

Use of pressure tanks can reduce the number of pump start and stops or even out small peaks of demand whilst maintaining the flow and arrival pressure

If its flat can't you use a water tower?

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

 

[jwsutto]

Thanks, as I expected it isn't a straightforward answer.

Will include pressure tanks for now in planning cost estimates. Detailed design can consider the issue further.... I think we will end up developing hydraulic models of each system.

I imagine most of the systems will have zero flows at some point overnight. Even the largest systems with highest customers (1000) and highest peak demand. Although there is probably a decent amount of leakage which might keep the pumps running!

I don't think line pack applies to water as it isn't compressible (water out = water in)?? which to me helps confirm the need for a pressure tank - to provide this buffering.

Recirculation systems not feasible - it is a dendritic network. Water tower would work but it is not the norm and has visual impact (it is a tourist hotspot).

Thanks

 

[LittleInch]

Water and its network is virtually incompressible, but once you get a decent size of pipe and length there is some volume available, such that if pressurised to some pressure at zero outflow there is a small volume able to flow out before you need to start a pump. It might only be 15 to 30 seconds, but it means there is some small storage in the especially a large network / volume.

I meant recirculation around a pump to prevent too much start / stop.

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

 

[jwsutto]

Thanks for clarifying.

 

[LittleInch]

You can make some very attractive water towers...

Make it a tourist attraction and give tours and have a viewing gallery!
Probably make more money than distributing water....

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1718197719/tips/kuwait_water_towers_ddhlg6.webp[/url]

https://www.fodors.com/news/photos/these-13-water-towers-prove-that-america-is-downright-silly

http://www.theworldgeography.com/2012/10/14-unique-water-towers-from-around-world.html

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

 

[bimr]

If you are pumping from water storage tanks, you don't need pressure tanks.

Pressure tanks are usually found on small water well pump systems with no water storage.