An isolation valve shuts off one section of a water network so it can be repaired while the rest stays in service, and one should sit at every branch point; with 3 branches of 4 households each, a repair on one branch cuts off 4 of 12 households instead of all 12. Close and open every valve slowly: a fast closure on a pipe running at 1 m/s can add about 3 bar to the line pressure.
Part of Water Distribution
An isolation valve is a full-bore shut-off valve fitted in a main or branch so that part of the network can be taken out of service. It does nothing until the day a pipe breaks, and then it decides how many households lose water.
The Valve Decides How Much Of The Village Goes Dry
The water-distribution page lists “no isolation valves” as a mistake: one repair means shutting off everyone. The remedy is a valve at every branch point.
Take a main with 3 branches of 4 households each, 12 in all. With no branch valves, a leak on any branch means closing the source and 12 ÷ 12 = 100% of households lose water. With a valve at each branch, only that branch is closed: 4 ÷ 12 = 33%.
Slow Closing Prevents A Pressure Surge
Moving water has momentum. If a valve shuts suddenly, the water cannot stop smoothly, and the pressure at the valve rises sharply in a surge (water hammer). The size of the pressure jump is density × wave speed × the change in velocity: Δp = ρ × a × Δv.
For illustration, assume a wave speed of 300 m/s, a value of the right order for plastic pipe (rigid steel is much higher). A flow of 0.5 L/s in a 25 mm pipe moves at 1.02 m/s. A sudden stop gives 1,000 × 300 × 1.02 = 306,000 Pa = 3.1 bar, added to the static pressure. On a line already at 40 m of head (3.9 bar) that reaches about 7 bar, above the 6 bar ceiling used in this system. The surge only reaches its full size if the valve closes within about 2L/a seconds, where L is the pipe length, so for a 100 m pipe that is 2 × 100 ÷ 300 = 0.7 s. Closing over several seconds avoids most of it.
What You Need
- Valves of the same bore as the pipe — gate valves open slowly and offer little resistance; ball valves close in a quarter turn and are easy to slam
- A valve box or marked cover at each valve so it can be found and reached
- A map with every valve marked and numbered
- A spanner or key that fits, kept where the water committee can find it
Method: Place, Mark, Exercise
Step 1: Fit a valve at each branch off the main, on the branch just downstream of the junction, and on either side of each section of a loop.
Step 2: Protect and mark it. A valve that cannot be found in the dark or after rain is useless.
Step 3: Record it on the network map.
Step 4: Operate each valve once or twice a year, slowly, so it does not seize.
Step 5: To isolate a section, close the valves slowly at both ends. Open the drain or tap on the section to relieve pressure before working.
Common Mistakes
- Valves only at taps. A main break still cuts off everyone.
- Slamming a ball valve shut. Surge.
- A seized valve. One that has never been moved will not move when needed.
- No map. No one remembers where they are.
Safety
Pressurised pipe throws fittings
Never open a union or cut a pipe that still holds pressure. Close the valves at both ends, open a tap or drain to release the pressure, and check that the water has stopped before you cut. At 40 m of head (3.9 bar) the end of a 25 mm pipe is pushed with about 190 N (392,000 Pa × 0.00049 m²), so a loose fitting can be forced off.
What’s Next
- Learn next: Air Valves — the other fitting every high point needs
- Combine with: Leak Detection — valves also help find a leak by closing sections one at a time