Head is the vertical height of the water surface above a point, and each meter of head is about 0.1 bar, so a tap 18 meters below a spring box sees 1.8 bar with the taps shut. Subtract the friction lost along the pipe to get the residual head that actually reaches the tap, which must stay at least 5 meters (0.5 bar) at the least-pressured tap and no more than 60 meters (about 6 bar) static at any point.
Part of Community Water System
Head calculation is the accounting of the height of water above each tap, in meters, from which static pressure, friction loss and residual pressure at every outlet follow.
Pressure Depends On Vertical Height Alone, Not On Pipe Length Or Diameter
In still water the pressure at depth h is density × gravity × height. For fresh water, 1,000 kg/m³ × 9.81 m/s² × 1 m = 9,810 Pa. One bar is 100,000 Pa, so 1 bar corresponds to 100,000 ÷ 9,810 = 10.2 m of head. The community water system page rounds this to 10 m per bar, which is 2% off and is safe to use in the field.
Two quantities must be kept apart. Static head is the height of the water surface above the point with the taps shut. Residual head is what remains at a tap while water is flowing, after friction has taken its share. A tap reads the static head until you open it.
Friction uses a share of head proportional to the flow raised to about the 1.85th power, so opening more taps lowers every residual at once. The method for the loss is in Friction Losses.
What You Need
- Elevations of the source water surface, each tap and every high point on the route, from a water-tube level or similar
- The route length, measured along the pipe
- Design flow — the number of taps that can run at once, each about 0.33 L/s
- A clear tube about 1.5-2 m long, to read pressure directly (below)
Method: Static, Then Friction, Then Residual
Step 1: Static head at each tap = source level - tap level. Convert: bars ≈ meters ÷ 10, because each meter of head is about 0.1 bar.
Step 2: Check the ceiling. If any point exceeds 60 m (about 6 bar), add a break pressure tank so that no section does (Break Pressure Tanks).
Step 3: Friction loss for the chosen pipe, length and design flow.
Step 4: Residual head = static head - friction loss. It must be at least 5 m (0.5 bar) at the least-pressured tap.
Step 5: Check high points. A hump on the route has less head than the taps below it. If it rises close to the level of the source minus the friction lost so far, the pressure there falls toward zero and air collects; if it rises above that level, flow stops.
Worked Example: One Line, Two Taps
Inputs: spring box water surface at 130 m elevation; highest tap at 112 m; lowest tap at 94 m; pipe 750 m long, 40 mm bore; two taps open together at 0.33 L/s each, so 0.66 L/s. Both taps sit at the end of the run, so one friction figure serves both.
- Static head, highest tap: 130 - 112 = 18 m, or 1.8 bar.
- Static head, lowest tap: 130 - 94 = 36 m, or 3.6 bar. Under the 60 m ceiling.
- Friction: Hazen-Williams with C = 140 gives 10.67 × 750 × (0.00066)^1.852 ÷ (140^1.852 × 0.04^4.87) = about 7.0 m.
- Residual, highest tap: 18 - 7.0 = 11.0 m, about 1.1 bar. Passes the 5 m minimum.
- Residual, lowest tap: 36 - 7.0 = 29.0 m, about 2.9 bar.
Try 32 mm instead: friction becomes about 20.8 m, which exceeds the 18 m available, so no residual head remains and the highest tap yields nothing at that flow. The difference between a working and a dry tap is one pipe size.
Reading Pressure With A Clear Tube
Connect a clear tube vertically to a tap outlet through a valve, with the top open. Open the valve with the tap shut: the water column rises to the static head. Open the supply and read the height again: that is the residual. The difference between the two readings is the friction loss between source and tap, measured directly. If the static reading is lower than your survey says, the source level, the survey or an air pocket is wrong; if the residual is far below the calculation, suspect a partly closed valve, a clogged intake screen, a leak or a smaller bore than assumed.
Common Mistakes
- Using pipe length as height. A long gentle pipe has little head.
- Forgetting that opening a tap lowers the pressure at every other tap.
- Checking only the highest tap. The lowest tap has the highest pressure, and that is where the joints are tested.
- Using the label diameter. A bore smaller than the label raises friction sharply (Pipe Sizing).
Safety
The lowest point carries the full static head all the time
Use pipe and fittings rated above the highest static pressure on the line, here 3.6 bar, and divide any run over 60 m of static head. Do not test a line by blowing compressed air into it. When reading pressure with a tube, keep faces clear of the open top: a surge can throw water out of it.
What’s Next
- Learn next: Friction Losses — computing the 7.0 m that was assumed above
- Combine with: Pressure Zones — keeping every part of a hilly settlement within the limits