Unidirectional Flushing
General
Unidirectional flushing is the process of cleaning the water mains of a water distribution system by flowing water in one direction through the mains. This is accomplished by closing specified valves and by flushing strategically located hydrants. By closing specified valves, the velocity of water through the mains is increased. The main purpose of unidirectional flushing is to improve water quality within the distribution system. It is generally considered to be more effective at removing sediments than conventional flushing and also uses less water.
Where can I find more information?
- AWWA Maintaining Water Distribution-System Quality
- AWWA Research Foundation — Investigation of Pipe Cleaning Methods
What equipment do I need?
1. Hose Monster®
- Little Hose Monster™ (HML) — 1500 GPM or less. Small, lightweight and pitotless (no pitot damage from flushing debris).
- BigBoy Hose Monster (HMBB4) — 1500 GPM or greater. Flow from your pumper port - 4", 4 1/2" or 5" storz. Measures up to 2700 GPM.
2. 2" Pitotless Nozzle™ (PN2GRV) — A Pitotless Nozzle is required if you are using the Little Hose Monster. The BigBoy Hose Monster already has a built-in Pitotless Nozzle.
3. Discharge Flow-Rate Gauge (Gauge) — 60- and 100-psi gauges are common. Anticipate pressure ranges within your distribution system so readings are in the middle third of the dial.
4. Remote Reader (Remote Readers)
5. Test Hose (Hose)
6. Hydrant Gate Valve (HGV25, HGV4, HGV45)
7. Hydrant Wrench (HW107)
8. Spanner Wrench (WSPA101)
9. Valve Exercising Equipment — Spin Doctor® and ValveStar® by HURCO Technologies, Inc.®
Also consider:
10. IMS Software (IMS) — Manages unidirectional main details, sets up flush routines and produces work orders and reports.
11. Dechlor Demon™ (DD2H, DD4, DD4H)
12. Reducer Adapter (AD4.25, AD45.25)
13. Equipment Case (Cases)
14. Gauge Calibration (Calibrations)
15. Stream Shapers (SS1)
16. 45° or 30° Test Header Elbows (Elbows)
The Process
- Identify each main in the system. Determine the flushing point and the ability to isolate the main so the water supply passes through in one direction.
- Determine the desired flushing velocity.
- Develop a detailed plan of action with detailed work orders specifying which valves to close and which hydrants to flow.
- Close the predetermined valves in your system
- At the flow hydrant, remove one cap from hydrant. Tighten all other caps.
- Set the Hose Monster in an appropriate location for flowing water. Attach hydrant gate valve on the hydrant, then attach the hose and Hose Monster.
- With the hydrant gate valve closed, slowly open the hydrant fully. Control water flow with hydrant gate valve.
- Measure flow-rate from Hose Monster® to determine flushing velocity.
- When flushing operation is complete, slowly close hydrant gate valve, then hydrant. Verify that hydrant is fully closed and drained. Remove equipment and replace caps.
Table of required flow-rates (GPM) to produce a desired flushing velocity in feet per second (fps)
| Pipe Diameter in Inches | Desired Flushing Velocities | ||||
| 2.5 fps | 4 fps | 6 fps | 8 fps | 10 fps | |
| 4" | 98 | 156 | 235 | 313 | 391 |
| 6" | 220 | 352 | 528 | 704 | 880 |
| 8" | 391 | 626 | 939 | 1252 | 1565 |
| 10" | 611 | 978 | 1467 | 1956 | 2445 |
| 12" | 880 | 1408 | 2112 | 2817 | 3521 |
| 16" | 1565 | 2504 | 3756 | 5007 | 6259 |
Formula for determining the flow-rate required to achieve a desired flushing velocity in main:
Q = V D2 / 0.409
Where Q = flow-rate required in GPM
D = diameter of main being flushed in inches
V = flushing velocity of main being flushed in feet per second (fps)
What velocities are needed for effective flushing?
It depends on the condition of your pipes. A 1986 AWWA guide suggests at least 2.5 fps. Another source indicated that 6 fps is needed to remove sediments and biofilms. Velocities of 5 to 6 fps are often set forth as goals for unidirectional flushing.
