Field Test Failure on a Newly Installed Double Check Valve Assembly
A certified tester is testing a new 6-inch double check valve assembly installed in a vault on a commercial service line. The assembly has two resilient-seated check valves and four test cocks. The tester records the following: static line pressure at test cock 1 is 72 psi; with test cock 2 open, the gauge at test cock 2 reads 70 psi; with test cock 3 open, the gauge at test cock 3 reads 68 psi. During the check valve 1 test, the gauge at test cock 2 drops from 70 psi to 55 psi when test cock 2 is opened to atmosphere, and the drop holds. During the check valve 2 test, the gauge at test cock 3 drops from 68 psi to 60 psi and holds. The tester also notices water weeping from test cock 4 when it is opened. The contractor argues the assembly passed because both checks held pressure and the weep is only a test cock issue.
1. Interpret the test results and determine whether the assembly passes or fails, explaining each reading.
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The readings must be evaluated against the acceptance criteria for a double check valve assembly. The check valve 1 test measures the pressure differential across the first check. A drop from 70 psi to 55 psi is a 15 psi differential, which is well above the typical minimum of 1 psi and indicates the first check is holding. The check valve 2 test measures the differential across the second check; a drop from 68 psi to 60 psi is an 8 psi differential, also above the minimum, so the second check is holding. On those two readings alone, the checks appear to pass. However, the weeping from test cock 4 is not merely a test cock issue. Test cock 4 is downstream of the second check, and water weeping there when opened suggests the second check or the test cock seat is not sealing. The tester must determine whether the leak is from the test cock itself or from the check valve. If the check valve is passing, the assembly fails regardless of the differential readings. The contractor's argument that the assembly passed is premature; the tester should repair or replace the test cock and retest, and if the leak persists, the check valve must be serviced.
2. Describe the correct sequence for the check valve 1 and check valve 2 tests and explain why the tester's static pressure readings matter.
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For a double check valve assembly, the tester first records the static pressure at test cock 1, then opens test cock 2 to bleed air and establish the pressure between the checks, then closes test cock 2 and records the pressure at test cock 2. For check valve 1, the tester opens test cock 2 to atmosphere and observes the drop in pressure at test cock 2; the drop reflects the differential across check 1. For check valve 2, the tester uses test cock 3 and observes the drop at test cock 3. The static readings matter because they establish the baseline pressure in each zone and allow the tester to calculate the differential. If the static pressure at test cock 2 is lower than at test cock 1, that difference is the initial differential across check 1, and the test drop must be compared to the correct baseline. The tester's readings of 72, 70, and 68 psi show a small but expected pressure loss across the checks. The tester must also confirm the gauge is calibrated and that the test cocks are clear, because a partially blocked test cock can produce misleading readings. The test must be performed in the manufacturer's specified order, and the results recorded on the appropriate form.