Converting an Unheated Warehouse Annex from Wet to Dry Pipe
A three-story unheated warehouse annex is being converted from a wet system to a dry pipe system because the space cannot be maintained above freezing. The annex is ordinary hazard Group II. The proposed dry system has 300 ft of 6 in. Schedule 40 main, 900 ft of 2 in. Schedule 40 branch lines, and 40 drops of 1 in. pipe averaging 3 ft each. There is no accelerator or quick-opening device, and the dry pipe valve is a standard differential type. The existing wet-pipe design used 0.20 gpm/sq ft over a 1,500 sq ft remote area, and the contractor intends to keep that basis unchanged. The flow test shows 60 psi static and 40 psi residual at 1,000 gpm. The contractor has asked whether the pipe volume, the design area, and the air supply need any adjustment before the submittal goes out.
1. Compute the dry system volume and determine whether the system exceeds the threshold at which additional provisions are triggered.
Read the worked answer
Use gallons per foot = internal area (sq ft) x 7.48.
6 in. Schedule 40 (ID 6.065 in.; radius 0.2527 ft): area = pi x 0.2527^2 = 0.2006 sq ft; 0.2006 x 7.48 = 1.50 gal/ft. Volume = 1.50 x 300 = 450 gal.
2 in. Schedule 40 (ID 2.067 in.; radius 0.0861 ft): area = 0.0233 sq ft; 0.0233 x 7.48 = 0.174 gal/ft. Volume = 0.174 x 900 = 157 gal.
1 in. drops (ID 1.049 in.; radius 0.0437 ft): area = 0.0060 sq ft; 0.0060 x 7.48 = 0.045 gal/ft. Volume = 0.045 x 120 = 5.4 gal.
Total = 450 + 157 + 5.4 = about 612 gal.
This is below the 750 gal threshold at which the standard requires specific provisions for large dry systems or limits the arrangement unless water delivery time can be demonstrated. So the volume rule alone does not force a design change.
Two cautions: use actual internal diameters from the pipe schedule or manufacturer's data rather than nominal sizes, since thin-wall pipe reduces the volume; and do not stop at the volume check. The 750 gal threshold is a proxy for water delivery time, not a substitute for it. A system at 612 gal through a standard differential valve with no accelerator can still be slow, particularly with a long 2 in. branch-line network.
2. Recalculate the design demand for the dry system and check it against the supply.
Read the worked answer
A dry pipe system increases the design area because water must travel to the remote area after the valve trips, so the fire has grown beyond what the wet-pipe design area assumes. The standard increase is 30 percent.
- New design area = 1,500 x 1.30 = 1,950 sq ft.
- Sprinkler flow = 1,950 x 0.20 = 390 gpm.
- Hose allowance for ordinary hazard = 250 gpm (set by hazard class, not increased for dry pipe).
- Total demand = 640 gpm.
Available pressure at 640 gpm: 60 - (60 - 40)(640/1,000)^2 = 60 - 20(0.4096) = 60 - 8.2 = 51.8 psi.
Whether that is adequate depends on the rest of the calculation, which the contractor has not shown. Add the dry pipe valve loss, the backflow preventer loss, the riser and underground losses at 640 gpm, and static elevation to the top of the third-floor remote area (roughly 30 ft x 0.433 = 13 psi). With the demand at the base of the riser in the range of 45 to 55 psi, this supply is marginal at best - the 30 percent area increase consumes most of the available margin.
This is exactly the situation where a wet-pipe calculation is reused, the 30 percent is forgotten, and the system is installed with a marginal supply. The submittal must contain the full dry-pipe calculation with the 1,950 sq ft area, not a note that the wet calculation applies.
3. What else must be verified about the dry system before the submittal is complete?
Read the worked answer
The volume and area checks are necessary but not sufficient. Address the following:
- Water delivery time. The system must deliver water to the most remote sprinkler within the time the standard allows after the valve trips. Measure or compute it for this configuration. If it cannot be met with the standard differential valve, add an accelerator or a listed quick-opening device, or reconfigure the system into smaller zones.
- Air supply sizing. The compressor must be able to restore system air within the required time after a trip, not merely maintain pressure. Size it for this volume, and provide a dedicated power source and a low-pressure alarm.
- Drainage. Dry pipe is pitched for drainage (typically 1/2 in. per 10 ft) toward a drain, with auxiliary drains at low points, at the end of mains, and at branch-line ends. The existing wet-pipe layout may have traps that were harmless when the pipe was full of water and are not harmless when they collect condensate.
- Valve room heating and access. The dry pipe valve and its trim must be in a heated enclosure with a drain and adequate clearance.
- Heads. Confirm the heads are listed for dry pipe service and that the deflector-to-ceiling dimension is maintained; the drops on a dry system are different from wet drops.
A submittal that shows only the hydraulic calculation leaves the most common dry-pipe failures unaddressed.