Adding a Branch Line to an Existing Warehouse System
A 1,200 m² single-storey warehouse is classed as Ordinary Hazard Group 2. The existing wet system is fed by a 150 mm city main through a 100 mm reduced-pressure backflow preventer. A recent flow test at the nearest hydrant gives 380 kPa static and 250 kPa residual at 1,500 L/min. The owner wants 26 new pendent sprinklers (K = 80) on a 65 mm branch line. The new branch runs 32 m from the riser, and its remote sprinkler is 4.8 m above the hydrant. The contractor proposes to connect to the existing 100 mm main without a hydraulic calculation, arguing that the static pressure is well above 50 psi. The AHJ has asked for a hydraulic justification and a water supply analysis before permits.
1. What information and calculation steps are required to decide whether the existing supply can serve the new branch?
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Do not use static pressure as the decision point. Static pressure only shows the no-flow condition. The design must compare available pressure at the calculated demand flow with the system demand.
- Establish the supply curve from the flow test: 380 kPa static and 250 kPa residual at 1,500 L/min.
- Adjust to the calculated demand using the Hazen-Williams relationship: P_available ≈ P_static − (P_static − P_residual)(Q_demand/Q_test)^1.85. If demand is about 1,700 L/min, the drop is about 130 kPa × (1,700/1,500)^1.85 ≈ 130 × 1.26 ≈ 164 kPa, so available pressure is about 216 kPa.
- Subtract known losses: backflow preventer loss at the demand flow from the manufacturer, elevation (4.8 m × 9.81 kPa/m ≈ 47 kPa), and supply/main friction to the new branch.
- Add branch friction and the remote sprinkler operating pressure. With K = 80, a 50 kPa minimum at the head gives Q = 80√0.5 ≈ 56.6 L/min per head; 26 heads give about 1,472 L/min, plus any hose allowance.
- Compare the pressure at the remote head with the required pressure. The calculation must show the actual pressure at the remote head, not just at the riser. If available pressure is less than demand, the connection is not acceptable.
2. If the calculation shows a shortfall of about 35 kPa at the remote sprinkler, what engineering options are acceptable, and what must be verified before resubmission?
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Acceptable options include increasing the new branch and connection main size, looping the main to reduce friction, upgrading the service or backflow preventer, adding a listed fire pump with a compatible controller, or providing a dedicated water storage tank. Do not simply reduce the sprinkler K-factor, density, or coverage unless the design basis and AHJ allow it.
For each option, rerun the hydraulic calculation and verify:
- the backflow preventer is rated for the higher flow and its pressure loss is included;
- the city main can supply the fire pump without excessive suction loss;
- the pump curve meets churn and 150% flow requirements;
- the available pressure still includes the required safety margin.
Submit the revised hydraulic calculation, equipment cut sheets, pump curves, and any new flow test. If the original flow test was not at the correct hydrant or is outdated, the AHJ may require a new test before accepting the design.