6.1 AWWA C600 Hydrostatic Pressure Testing

Why the Pressure Test Matters

The hydrostatic pressure test is the single most important acceptance test for a newly installed or repaired water main. It proves the pipe, joints, fittings, valves, and thrust-restraint system can hold working pressure plus a safety margin without leaking or failing. For ductile-iron water mains, the controlling standard is AWWA C600, Installation of Ductile-Iron Water Mains and Their Appurtenances. The exam treats C600 facts as high-yield, so memorize the numbers below exactly.

Setting the Test Pressure

C600 ties the test pressure to the working pressure and to where along the line you measure it, because elevation changes the static pressure inside the same test section. The standard requires the test pressure to be:

• not less than 1.25 × the working pressure at the highest point of the test section, and
• not less than 1.5 × the working pressure at the lowest point of the test section,

and it must not exceed the rated pressure of the pipe, valves, or hydrants in the section.

One caution that trips up operators: the widely quoted rule "1.5 × working pressure or 150 psi, whichever is greater" is a utility/project specification convention, not the text of C600. Many state standard specifications and utility spec books do impose that 150 psi floor, and when they do, the specification governs the job. On the exam, read the stem: if it asks what AWWA C600 requires, use 1.25× at the high point; if it describes a project specification, use the number that specification gives.

Preparing the Main: Fill, Purge, Rest

Two failure modes the test is designed to avoid:

1. Trapped air — air is compressible, so a pocket of air acts like a spring. Pressure drops unevenly, the gauge bounces, and a false leak is indicated (or a real leak is masked). Always fill slowly and purge all air through air-release valves, blow-offs, or hydrants at the high points. If the line has a noticeable grade, start filling at the low end and push air out ahead of the water.
2. Thrust-block movement — a thrust block that has not yet cured cannot resist the axial force the test applies at bends, tees, and dead ends. Concrete thrust blocks typically need ≥5 days of cure before a pressure test. Mechanical restrainments (rods, harnesses) may be used to shorten the wait, but only if the manufacturer's literature supports the test load.

After filling and purging, many specifications call for a ~24-hour rest (saturation) period before applying pressure. This rest lets any remaining tiny air bubbles dissolve into the water under static pressure and lets gaskets seat. Skipping the rest is a common cause of phantom "leaks" that disappear on retest.

The 2-Hour Hold

Once pressurized to the test value, hold the pressure for approximately 2 hours. Record:

• Start pressure (psi) and start time
• End pressure (psi) and end time
• Volume of makeup water added during the hold (from a calibrated meter or bucket) to keep pressure at the test value

A small pressure drop is normal as gaskets seat and water compresses slightly; makeup water is what you track. If pressure holds with zero makeup water, the line is tight. If makeup is needed, compare it against the allowable-leakage formula.

Allowable Leakage Formula

AWWA C600 gives the allowable leakage as:

L = (N × D × √P) / 7,400 (gallons per hour)

where:

• N = number of joints tested
• D = nominal pipe diameter, inches
• P = average test pressure during the test, psi
• √P = square root of the average test pressure

The test FAILS if the total makeup water added during the hold exceeds the allowable leakage for that duration — that is, L × test time. For the standard 2-hour hold, the limit is L × 2 hours (the hourly rate times 2). There is no extra doubling factor: the calculated allowable leakage (the testing allowance) already accounts for trapped-air release, gasket seating, and cement-lining absorption, and a properly installed main is expected to be essentially bottle-tight.

Worked Example

A new 12-inch ductile-iron main, 1,200 ft long, is laid in 18-ft lengths and tested at 180 psi. Each pipe stick = 1 joint at each end, so for 1,200 ft of stick pipe the number of joints N ≈ (1,200 / 18) = 67 joints (plus fittings — conservatively use 70).

1. N = 70
2. D = 12 in
3. P = 180 psi → √180 ≈ 13.42
4. L = (70 × 12 × 13.42) / 7,400 = 11,272.8 / 7,400 ≈ 1.52 gal/hr
5. Allowable over 2 hours = L × 2 hr = 1.52 × 2 ≈ 3.04 gallons.

If the crew added 2 gallons of makeup water during the 2-hour hold, the main passes (2 < 3.04). If they added 4 gallons, it fails (4 > 3.04) — locate and repair the leak, then re-test.

Sequence: Test BEFORE Disinfect

The hydrostatic pressure test is always performed before disinfection (AWWA C651). Why? A leak discovered after chlorination wastes chlorinated water, delays the bacteriological sample clock, and risks releasing high-chlorine water to the environment. The correct installation sequence is:

1. Excavate, lay pipe, install joints, fittings, valves, hydrants, thrust blocks.
2. Backfill sufficiently to hold the pipe (leaving joints exposed for inspection is permitted).
3. Pressure test per C600.
4. Repair leaks if needed and re-test.
5. Disinfect per C651.
6. Flush, sample, and return to service.

Memorize that ordering — exam questions often mix up the two standards and try to trick you into disinfecting first.

Quick-Reference Checklist

• Test pressure = ≥1.25× working at the high point, ≥1.5× working at the low point; honor any higher floor (often 150 psi) in the project specification
• Fill slowly, purge all air at high points
• Wait ~24 hr rest; thrust blocks ≥5 days cured
• Hold ~2 hr at test pressure
• Allowable L = (N × D × √P) / 7,400 gal/hr; for a 2-hr hold, fail if total makeup > L × 2 hr
• Test before disinfection