Guidance For Providing Safe Drinking Water in Areas of Federal
Jurisdiction - Version 1
Appendix 8: Distribution System
Table 1: Investigation of Water Distribution Systems (42)
| Problem |
Preliminary Assessment |
Detailed Investigation * |
| Structural Condition |
- Spatial and temporal analysis of water main breaks
- Compilation of soil map
- Routine inspection of valves and hydrants
- Routine inspection of insulation and heat tracing in
northern areas
|
- Detailed analysis of break patterns, rates and trends
- Statistical and physical models
- Pipe sampling
- Soil corrosivity measurements
- Pit depth measurements
- Non-destructive testing
- Failure analysis Visual inspection
- Thermal analysis (far north)
|
| Hydraulic Capacity |
- Low-pressure complaints
- Hydrant flow tests
- Unusual rusty/coloured water occurrences
- Visual inspection of pipe interior
- Monitoring of pressure and pumping costs
|
- Hazen-Williams C factor tests (pipe roughness)
- Computer modelling
|
| Leakage |
- Water use audit
- Per capita water demand
- Routine leak detection survey
|
- leak detection survey
- detailed limited area leakage/demand assessment
|
| Water Quality |
- Water quality complaints
- Routine sampling data
- Results of flushing program
|
- detailed water quality investigation
- computer modelling
|
* Reasons For More Detailed Investigation:
Level of Service
- Preliminary investigations indicate an excessive break rate,
excessive leakage, inadequate hydraulic capacity and/or impairment
of water quality
Cost Effectiveness
To facilitate capital planning and asset management programs
- Pilot testing of new technologies to facilitate long-range
planning support
- Opportunistic work, such as when a water main is temporarily
out of service
Risk Management
- Risk analysis identifies critical water mains that have a high
potential for significant property damage, environmental impact
or loss of service
- Due diligence (e.g., failure analysis of a failed critical
water main)
Some components of the distribution system
Vacuum Breakers
Vacuum breakers are used to prevent backflow of water into the
distribution system. These include atmospheric, hose bibb, and
pressure vacuum breakers. These devices can provide protection
against back- siphonage9 but
should not be used to protect against back- pressure10 conditions. ( 9, 32) The
selection, installation, maintenance and repair of these devices
is to be done in accordance with Standard B64.10.01.
Valves
Valves are one of the more important devices in the distribution
system as they are used for various purposes including turning
on, shutting off and regulating flow; providing air or vacuum release;
and reducing pressure.
Isolation valves are the most common type used in distribution
systems. Buried gate valves with valve boxes are typically used
for isolation of small-diameter water mains and water services
whereas butterfly valves are direct buried or installed in chambers
and are typically used for large diameter mains. Isolation valves
require regular exercise to ensure they are accessible, are in
their proper position (open or closed), are operable and are not
leaking. Isolation valves are prone to deterioration and failures
such as stripped, broken or bent stems; leaking O-rings or packing;
corrosion of the valve body and connecting bolts; and wear on the
valve disk and seat.(42)
Water storage
Ground storage reservoirs are large tanks made of concrete or
metal which are located at ground level, while concrete tanks are
usually buried to prevent freezing of their contents. Metal storage
tanks, when used, are insulated and may have a steam line running
from the boilers to keep them ice-free. Recirculation of water
from top to bottom of the tank will minimize the heat required.
Elevated storage is provided by means of standpipes or tanks.
As elevated storage is more difficult to keep from freezing, especially
the water in the riser to the tank; steam or hot water lines can
be installed in the riser and the riser can be insulated.
All reservoirs are to be covered to prevent contamination and
pollution. The covers also safeguard against drowning. Covered
reservoirs or tanks must have air vents to allow air in and out
as the water level is changed and these vents must be screened
to keep out insects and small animals. Access manhole covers are
to be provided for cleaning and inspection purposes.
The security of water storage reservoirs is an increasingly important
issue. Federal water reservoir access covers are to be locked and
secured at all times.(39)
9 Back siphonage
is the flow of water into the potable water system pipes caused
by the sudden reduction of pressure in the potable water supply
system
10 Backpressure
is a condition in which a facility's system pressure is greater
than the supplier's system pressure.
|
