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Does anyone have experience with pressure-tanks for small piped systems?

Hello all,

I am currently looking into rehabilitating some small piped community water supply systems (with a motorized well pump) here in the Philippines. The issue is that the existing elevated tanks got damaged and in this earthquake and typhoon prone country, the expected lifetime of such a tower seems rather low (not even taking the low construction quality in mind).

There are some relatively cheap butyl-rubber diaphragm pressure tanks available for household supply, and combining a few of those at the biggest available size would probably a possible way to go instead of constructing a new water-tower.

I am however a bit concerned about possible extra maintenance costs and also higher energy bills due to a often starting and stopping pump.

Does anyone have experiences or can point me to examples of such a towerless design in a developing country? Thanks!

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I have just had a word with colleagues who know a lot more about pumping than I do and they agree that there is no reason why this should not work, although there will be various issues with design and maintenance that will need some thought. The energy consumption should be just the same, you are only storing it in a different way (compressed air instead of gravity) and the size and number of vessels should not matter within reason, provided the total capacity is correct. Maintenance may be an issue, but a suitable maintenance could most probably be devised. The most important operational consideration will be preventing frequent pump starts at periods of low demand. If you need any assistance with design, please let me know and we will be able to assist.

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Thanks for the feedback. I guess I need to calculate it in more detail. The largest diaphragm tank that is commonly available has a nominal volume of 500l (so about 200l usable volume) and costs about $1000. The common small piped systems here usually have a 500-1000l elevated tank, which is actually a bit small for the number of users, but I guess with public tapstands and yard connections only one can calculate with a lower peak demand. Obviously the pressure tank solution with 2-3 tanks is going to be more expensive, but given how frequently the elevated tanks break it might be worth it... under the assumption that such a diaphragm tank is working without the need for replacing the bladder for at least 10 years or so.

Krischan Makowka gravatar imageKrischan Makowka ( 2015-05-24 01:40:25 +0000 )edit

Small pressure tanks are often used on boreholes here in Zimbabwe such as the one in my garden. The main purpose is to stop frequent starting and stopping of the pump. The pressure builds up in the pressure tank until the pressure switch tells it to switch off. Thereafter the compressed air in the tank provides the pressure in the pipes.

One problem that can occur is frequent cycling on and off of the pump when there is too little air in the tank. The tank needs periodic draining with a valve at the bottom and a valve at the top to re-fill it with air.

The pressure switch may be adjustable to give the pressures to switch on and off at. One problem is that this give uneven pressure in a small system. Annoying when the hot water in your shower comes from a tank at a constant height whereas the cold water swiches in and out alternately scalding and freezing you!

I prefer to have a raised tank if at all possible. This also guarantees water in the pipes during the frequent power cuts.

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Yes, these tanks do have some disadvantages, but what you describe is most severe in small pressure tanks with direct water/air contact. A relatively large internal bladder or diaphragm pressure tank can store quite a bit of water and does not need the periodic draining.

Krischan Makowka gravatar imageKrischan Makowka ( 2015-05-24 01:17:42 +0000 )edit

Hello. I have some experience with solar hot water, and there, the use of a explansion tank is a must. My experience show me that often the amount of air in the expansion tank is oversized. My point is that in your case you could get more than 200l from a 500l tank. If you want, I can do the calculations.But you can be sure that at least 300l can be reach with a acceptable hysteresis on the presure. if you could tell us the presure that your pump can reached them i'll try to find the time to calculate the setting so you can get the most from your tank. good luck. The higher the pump presure the more volumen you can get from your tank. But the more energy you will be consuming. that is the compromise you have to deal with.

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Good point (and thanks for offering the calculations, but I think I can do those myself). However I guess these communities would rather safe on the electricity bill (power is relative expensive in the Philippines compared to other Asian countries) and have a lower peak demand & power-cut supply security instead.

Krischan Makowka gravatar imageKrischan Makowka ( 2015-05-25 11:33:12 +0000 )edit

I've used a pressure tank in a domestic setting for 15 years. The proportion of the tank that is taken up by water depends on the pump pressure and the air pressure and the start and stop pressures. The amount of water supplied between pump stop and restart depends on the range set by the pressure sensitive switch and the charging pressure of the air bladders.

These switches usually have an adjustable point, but you will need pressure gauges to estimate the stop and start points. All of this means that the amount of water supplied by the system with the pump off is not a simple proportion of the tank volume. Also, you need to have some water in the tank when the pump switches on, otherwise you risk the pressure never dropping enough to trigger pump start.

Having several tanks makes the behaviour more complex. You can use a tank with a lower air charging pressure than the others to provide a switch on point even after the other tanks are empty of water. This approach can give you the best use of the available volume. However, getting the most use out of the available volume implies a big difference between start and stop pressure.

Usual maintenance problems are loss of air from the tank (giving really short cycle times) and failure of the cheap pressure switch.

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Thanks for the additional details! I am also a bit concerned that strongly fluctuating pressure will be an acceptability issue and negatively effect the lifetime of cheap taps and low quality PVC piping often used.

Krischan Makowka gravatar imageKrischan Makowka ( 2015-05-27 02:16:38 +0000 )edit
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