A pressure zone is the part of a network fed from one tank so that every tap in it has between 0.5 bar (5 m of head) and 6 bar (60 m); with a 2 m deep tank that limits a zone to 53 m of height between its highest and lowest tap. A settlement spanning 140 m of height therefore needs 3 zones, each fed from its own tank or break pressure tank.
Part of Water Distribution
Pressure zones divide a network on hilly ground into bands of height, each supplied from its own tank, so that no tap gets too little pressure and no pipe too much. They answer the elevation changes that a single tank cannot.
One Tank Can Only Serve A Band Of Height
Pressure at a tap is the height of the tank’s water surface above it. Two limits then fence the zone:
- The highest tap needs at least 5 m of head (0.5 bar) from the tank bottom.
- The lowest tap must not see more than 60 m (6 bar) from the tank’s full water level.
Tank bottom at least 5 m above the highest tap; full water level, the bottom plus the tank depth, at most 60 m above the lowest tap. So the height between the highest and lowest tap in a zone is at most
60 m − 5 m − tank depth.
For a 2 m deep tank that is 60 − 5 − 2 = 53 m. With no allowance for friction, 55 m would be the figure for an infinitely shallow tank; friction takes more.
What You Need
- A height survey of all houses and taps
- A tank site for each zone, high enough to meet the 5 m minimum for the zone’s top tap
- A way to feed each tank: from the source above by gravity (break pressure tanks can serve as zone tanks), or by pumping up from below (Ram Pump)
- Isolation valves on the boundaries between zones, kept shut so zones do not feed each other and pressures do not mix
Method: Slice The Heights
Step 1: List the heights of the lowest and highest tap.
Step 2: Compute the span and divide by the zone height (60 − 5 − tank depth), rounding up.
Step 3: Choose the zone boundaries so every tap falls inside one zone.
Step 4: Place a tank for each zone with its bottom 5 m or more above the zone’s highest tap.
Step 5: Feed the tanks from the source.
Step 6: Check pressure at the lowest tap of each zone, with the tank full and with taps closed.
Worked Example
Taps range from 0 m (valley) to 140 m above the valley. Tank depth 2 m. Zone height = 60 − 5 − 2 = 53 m.
- Number of zones = 140 ÷ 53 = 2.64, so 3 zones.
- Zone A (taps 0 m to 47 m): tank bottom at 52 m, full level at 54 m. Lowest tap sees 54 m (5.3 bar); highest sees 5 m (0.5 bar).
- Zone B (taps 47 m to 94 m): tank bottom at 99 m, full level at 101 m.
- Zone C (taps 94 m to 140 m): tank bottom at 145 m, full level at 147 m.
The source must lie above the top tank, and the tank for B and C must be reachable by pipe from it. Because the first zone’s lowest tap sits at 54 m of head, it is under the 60 m limit, but friction in the pipe drops that further, so check it with the real flow.
Common Mistakes
- Counting the tank bottom for the maximum. A full tank adds its depth.
- Opening the boundary valve between zones. The lower zone’s high pressure feeds back up.
- Forgetting that every extra tank is another place for contamination. Cover and screen each one.
- No isolation valve at the zone’s tank. You cannot service it.
Safety
Test each zone on its own, with water
A pipe at the bottom of a zone carries up to 6 bar. Fill the zone slowly from the top, bleed the air, and never test with compressed air: it stores far more energy than water and can shatter plastic pipe. Do not open a boundary valve between zones with a pressure difference across it without checking that the lower zone’s pipes are rated for the higher pressure.
What’s Next
- Learn next: Break Pressure Tanks — the tank that resets pressure to zero
- Combine with: Air Valves — releasing air at the crests between zones