Every new water main must be pressure tested before it is connected to the distribution system and placed in service. The test confirms that the pipe, fittings, and joints are watertight under operating pressure. A failed pressure test means the contractor must locate and repair leaks before retesting, which can add days or weeks to the construction schedule. Understanding the test requirements before construction helps avoid surprises.

Governing Standards

Water main pressure testing is governed by AWWA C600 (Installation of Ductile-Iron Mains and Their Appurtenances), AWWA C605 (Underground Installation of PVC and PVCO Pressure Pipe), and AWWA M23 (PVC Pipe Design and Installation). Local water districts adopt these standards, often with amendments that make the requirements more stringent. The two primary test methods are the hydrostatic pressure test and the leakage test.

Hydrostatic Pressure Test

The hydrostatic pressure test verifies that the pipe can withstand the test pressure without bursting, significant leakage, or visible distress. The procedure:

1. Fill and purge. The pipe section is slowly filled with water from one end while air is vented from the high points. All air must be removed. Trapped air compresses under pressure and gives false readings. Air release valves or corporation stops at high points are used for venting.
2. Pressurize. The pipe is pressurized to the specified test pressure using a hand pump or a test pump connected at a fire hydrant or temporary test connection.
3. Hold. The test pressure is maintained for a specified duration, typically 2 hours. The pressure is monitored continuously with a calibrated pressure gauge.
4. Record. The pressure at the start and end of the test period is recorded. Any pressure drop is noted, and the amount of makeup water required to maintain the test pressure is measured.

Test Pressures

Leakage Test

The leakage test is often conducted simultaneously with the pressure test. It measures the volume of water that must be added to the pipe to maintain the test pressure over the test period. This makeup water represents leakage through joints, fittings, and any undetected defects.

Allowable Leakage

AWWA C600 establishes the allowable leakage formula:

L = (N x D x P^0.5) / 7,400

Where L is the allowable leakage in gallons per hour, N is the number of joints, D is the nominal pipe diameter in inches, and P is the average test pressure in psi.

Example Calculation

A 1,000-foot run of 8-inch ductile iron pipe with 20-foot joint spacing has 50 joints. At a test pressure of 200 psi:

L = (50 x 8 x 200^0.5) / 7,400 = (50 x 8 x 14.14) / 7,400 = 5,656 / 7,400 = 0.76 gallons per hour

If the makeup water added during the 2-hour test exceeds 1.52 gallons (0.76 x 2), the test fails. Note that this is a very small volume. The test is intended to detect actual leaks, not just weeping joints. If you are adding more than a few gallons over 2 hours, there is a real problem.

Common Causes of Test Failures

• Trapped air. The most common cause of apparent test failure is trapped air in the pipe. Air compresses under pressure, causing the pressure to drop even without leakage. Thoroughly vent all air before pressurizing. Some districts require the pipe to sit filled with water for 24 hours before testing to allow air to dissolve.
• Under-inserted joints. Push-on joints on DIP and PVC require the pipe to be fully inserted to the mark. If a joint is not fully seated, it will leak under pressure. Visual inspection of insertion marks before backfilling is critical.
• Damaged gaskets. Rubber gaskets can be cut or dislodged during installation if the pipe end is not properly beveled and lubricated. A single damaged gasket can cause the entire test to fail.
• Temperature changes. Water expands and contracts with temperature. If the test starts in the cool morning and the pipe heats up during the day (especially above-ground or in shallow trenches), the pressure will rise. Conversely, a temperature drop causes pressure decrease. Conduct tests during stable temperature conditions when possible.
• Fitting leaks. Mechanical joint fittings (elbows, tees, reducers) have multiple bolt-and-gasket connections. Uneven bolt torque is a common cause of leaks at fittings. Follow the manufacturer's torque sequence and values.

Bacteriological Testing

After the pressure test passes, the pipe must be disinfected and tested for bacteriological safety before being placed in service. AWWA C651 (Disinfecting Water Mains) specifies the procedure:

1. Flush the main to remove construction debris.
2. Introduce a chlorine solution (typically 25 to 50 mg/L) and allow it to remain in the pipe for a minimum of 24 hours.
3. Flush the chlorinated water from the pipe. The dechlorinated water must be disposed of properly, not discharged to a storm drain without treatment.
4. Collect bacteriological samples at multiple points along the main. The samples are tested for total coliform bacteria.
5. Two consecutive sets of clean samples (absent total coliform) taken 24 hours apart are required before the main can be placed in service.

If any sample tests positive for coliform, the entire disinfection and sampling process must be repeated. This can delay the main activation by several days.

Documentation and Inspection

The water district typically requires a district inspector to be present during the pressure test. The test documentation includes:

• Test date, weather conditions, and test duration
• Test pressure (start and end)
• Volume of makeup water added
• Pipe material, diameter, length, and number of joints
• Allowable leakage calculation and pass/fail determination
• Names of inspector and contractor representatives

This documentation becomes part of the permanent record for the water main and is required before the district will accept the main for public maintenance. Keep a copy in the project file and submit with the as-built drawings.

When Pressure Testing Is Not Required

Short service lateral connections (from the main to the meter) are typically not pressure tested independently. The lateral is tested as part of the domestic plumbing system during the building inspection. Similarly, repair work on existing mains (a short section replacement, for example) may have modified test requirements at the water district's discretion. Always confirm the test requirements with the district before starting construction.