Dry-Pipe Commissioning in a Cold-Storage Warehouse
A 2,000 m² cold-storage warehouse at −25°C is protected by a dry-pipe system. The new 150 mm main is 120 m long, with four low-point drains and a dry-pipe valve trim set to trip at 275 kPa (40 psi) air. The contractor fills the system with water for the 200 psi, two-hour hydrostatic test, drains it, and then charges it with air. Overnight the air pressure falls from 275 to 240 kPa. The contractor says the drop is normal because the temperature dropped and proposes to accept the system. The AHJ requires a proper flushing and commissioning sequence. The freezer is scheduled to start cooling in 48 hours.
1. What is wrong with the proposed hydrostatic test and acceptance for this dry-pipe system?
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The water hydrostatic test itself is not automatically wrong - NFPA 13 requires a hydrostatic test at 200 psi (1,380 kPa) for two hours for new systems - but in a −25°C freezer the risk is trapped water freezing in low points, branch lines, and dry-pipe valve trim. If water is used, the contractor must prove the system can be completely drained, open all low-point drains, blow out with oil-free air, and protect the valve trim. Leaving water in the system overnight is unacceptable.
The air-pressure drop from 275 to 240 kPa is not normal for a temperature change alone unless the temperature change is known and calculated. A 35 kPa drop over one night indicates a leak or an open drain. The contractor cannot accept it as normal without a leak test. For a dry system, after the hydrostatic test, an air test at 40 psi (275 kPa) for 24 hours is used to verify tightness; a continuous drop fails the test. Locate and repair leaks, then repeat the air test.
2. Outline the correct commissioning sequence before the freezer is cooled.
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Use this sequence:
- Flush the underground and aboveground mains in accordance with NFPA 13 and the AHJ; verify flow and clear water.
- Hydrostatically test at 200 psi for two hours, with air vents open and all joints exposed; record gauge readings and duration.
- Drain completely; open all low-point drains; blow dry with oil-free air; verify no water remains in low points and dry-pipe valve trim.
- Air test at 40 psi for 24 hours; no continuous pressure drop is allowed. Repair any leak and retest.
- Charge the dry-pipe system to its required air pressure, set the compressor and alarm, and verify the air source capacity.
- Perform a dry-pipe valve trip test at 40 psi, with the quick-opening device if provided; verify the valve trips, water reaches the test connection, alarms operate, and trip time is acceptable.
- Test low-point drains, main drain, flow switch, and pressure switches; record all results.
- Only then cool the freezer. If the freezer is cooled before the system is proven dry, residual water can freeze and block the pipe.