Main Drain Test Trend After Five Years
A wet-pipe system serving an eight-storey office building has a 150 mm riser and a 100 mm main drain. At acceptance five years ago, the main drain test recorded 6.4 bar static and 4.9 bar residual. The current annual test records 6.4 bar static and 3.6 bar residual. A hydrant flow test at the street gave 6.2 bar static and 5.8 bar residual, essentially unchanged from the original test. The gauge on the riser was replaced last month and is calibrated. There have been no alarm or waterflow complaints, and the system has not been drained for modifications. The building owner wants to defer investigation until the next annual test because the system still holds pressure and no sprinkler has operated.
1. Interpret the main drain test trend. What is the likely cause?
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The main drain test is a comparative test. At acceptance, the residual drop was 6.4 - 4.9 = 1.5 bar. Now it is 6.4 - 3.6 = 2.8 bar. The static pressure is unchanged, and the street hydrant flow test is essentially unchanged, so the public supply is not the problem.
The increased residual drop indicates a restriction between the supply and the main drain outlet. Likely causes include a partially closed valve, a malfunctioning pressure-reducing valve, an obstruction in the riser or main drain, or internal tuberculation/scale in the steel pipe. A closed or partly closed OS&Y valve is a common cause; a PRV that is passing or stuck can also change the residual. The fact that the system holds pressure and has no alarm does not rule out a restriction, because a partly closed valve can still hold static pressure.
The trend is significant and should not be deferred. A 1.3 bar increase in residual drop is a material change from the baseline.
2. What steps should be taken before deciding whether to defer?
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Do not defer. First verify the test setup: confirm the gauge calibration, the main drain valve is fully open, and the test is run for the required duration. Check the position of all valves between the source and the drain, including the riser gate valve, check valve, and any PRV. Record suction and discharge pressures if a pump is present.
Next, perform a flow test at a nearby hydrant or test valve to confirm the supply is still normal. If the supply is normal and the main drain residual remains low, investigate the system piping. This may include inspecting strainers, exercising valves, and conducting an internal inspection of the riser and main. Compare the results with the acceptance baseline and the previous annual tests.
The owner's argument that 'no sprinkler has operated' is not a reason to defer. The main drain test is an early warning of a supply restriction that could reduce sprinkler performance during a fire.
3. If internal inspection finds heavy tuberculation in the 100 mm main, what remediation and acceptance criteria apply?
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Heavy tuberculation reduces the effective internal diameter and the C-factor, so the hydraulic calculation may no longer be valid. The remediation options are replacement of the affected pipe, internal lining or coating if listed for fire service, or cleaning followed by lining. Simply cleaning without protecting the pipe may allow rapid re-tuberculation.
After remediation, perform a new main drain test and compare it with the original acceptance baseline. The residual should return to a value consistent with the original test, allowing for normal supply variation. Also perform a flow test or hydraulic verification to confirm the system still meets the design demand with the actual pipe condition. Update the hydraulic calculation if the C-factor or pipe diameter changes.
The AHJ should witness the retest and accept the remediation only when the main drain result, flow test, and hydraulic calculation are consistent. The owner should be advised that the system cannot be left in the degraded condition until the next annual test.