Testing procedure for double check valves - backflow prevention transcript
  • Last updated:
  • 21 Feb 2020

To test a DCV or a double check valve we need to follow five steps.

Firstly we need to carry out our pre-test checks.

Second we need to ensure both the upstream and downstream isolating valves are not leaking.

Thirdly we need to check that the upstream check valve is operating effectively.

Fourthly we need to ensure that the downstream check valve is operating effectively, and

Fifth we need to return the valve to operating condition.

For the DCV test to be successful, both the upstream and downstream isolating valves must not leak, and the pressure differential across each check valve must be 7kPa or greater.

Your pre-test checks should include; flushing and cleaning the line strainer, and opening the isolating valve where necessary.

Then open and close test tap one, test tap two and test tap three, to flush out any foreign matter.

Then close the downstream isolating valve.

Connect the high-pressure hose to test tap three.

Turn on test tap three, then bleed air through the drain line. Close test tap A, close needle valve B, then in sequence close test tap C, and the upstream isolating valve.

The pressure gauge will show a high reading. Open test kit valve A. Slowly open test kit valve B, and drop the gauge pressure by 20kPa.

Close test kit valve B, and observe the differential pressure reading.

If the pressure on the gauge is rising, either the upstream isolating valve or the downstream isolating valve is leaking.

To determine which valve is leaking, open test tap one. If there is a continuous discharge of water from test tap one, the upstream isolating valve is leaking.

If the pressure on the gauge is dropping or approaching zero, the downstream isolating valve is leaking.

Faulty isolating valves must be repaired or replaced and a test repeated. Any leakage invalidates the test results.

To test the upstream check valve, close test tap three. Close test kit valve A and open test kit valve C. Disconnect the high-pressure hose from test tap three and clean the line strainer where fitted.

Open the upstream isolating valve and connect the high-pressure hose of the test kit to test tap one, and the low pressure hose of the test kit to test tap two. Open test taps one and two. Open test kit valve A and bleed the water through the vent hose. Close test kit valve A. Open test kit needle valve B and bleed water through the vent hose to eliminate air from the system.

Slowly close test kit valve B and observe and record the reading on the differential pressure gauge.

If a reading below 7kPa is indicated, the upstream non-return valve shall be deemed to be faulty. The valve shall be repaired or replaced and the tests must be repeated.

To test the downstream check valve, close test tap one and test tap two, and open test kit valves A and B. Close test kit valves A and B.

Disconnect the low pressure hose from test tap two and reconnect to test tap three. Disconnect the high pressure hose from test tap one and reconnect to test tap two.

Open test taps two and three. Open test kit valve A and bleed water through the vent hose. Close test kit valve A.

Slowly open test kit needle valve B and bleed water through the vent hose to eliminate air from the system. Slowly close test kit needle valve B and observe and record the reading on the differential pressure gauge.

If a reading below 7kPa is indicated, the downstream non-return valve shall be deemed to be faulty. The valve must be repaired or replaced and the test shall be repeated.

To restore the double check valve to its operating condition, close test tap two and test tap three. Open test kit valves A and B, disconnect the test kit pressure hoses and open the downstream isolating valve. This restores the device to its operating condition.

Guidelines for backflow prevention devices are available on council’s website.