Hydrant Spacing Arithmetic
You are laying out a relay to supply a fireground 900 ft from the nearest hydrant. Each 100 ft of 3-inch supply hose has a friction loss of 15 psi at 300 gpm. The pumper at the hydrant must deliver 300 gpm to the attack pumper. You want at least 20 psi residual at the attack pumper's intake, and the hydrant static pressure is 70 psi. A second hydrant is 400 ft in the opposite direction with 60 psi static.
1. Compute the friction loss over the 900 ft lay and determine whether the first hydrant can meet the 20 psi residual target at 300 gpm.
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Friction loss: 900 ft ÷ 100 ft = 9 sections. 9 × 15 psi = 135 psi of friction loss at 300 gpm.
Available vs. required: The hydrant static is 70 psi. Even ignoring elevation and appliance losses, delivering 300 gpm through this lay would require 135 psi of push just to overcome hose friction, plus 20 psi residual at the far end - about 155 psi needed against 70 psi available. The first hydrant cannot meet the target at 300 gpm over 900 ft.
Conclusion: this lay is not viable as a single 900 ft stretch. The friction loss alone exceeds the available pressure by roughly double.
2. Given the numbers, propose a workable alternative and show the arithmetic that supports it.
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Option A - relay with a second pumper. Break the 900 ft lay into two 450 ft segments with a relay pumper in the middle. Each segment is 4.5 sections × 15 psi = 67.5 psi of friction loss. The hydrant pumper supplies the relay pumper; the relay pumper boosts to the attack pumper. Each pumper handles about 67.5 psi of loss plus 20 psi residual ≈ 87.5 psi of output - well within a pumper's capability, and the hydrant's 70 psi static covers the first segment's needs only if the relay pumper is close enough. Verify the first segment: 450 ft at 300 gpm costs 67.5 psi; with 70 psi static and 20 psi residual target, 70 − 67.5 = 2.5 psi residual - marginal, so place the relay pumper closer to the hydrant (e.g., 300 ft) to leave margin.
Option B - use the closer second hydrant. 400 ft = 4 sections × 15 = 60 psi loss; 60 psi static minus 60 psi loss leaves ~0 psi residual - also marginal. So the relay (Option A) with a shortened first segment is the defensible choice. State assumptions: no elevation change, standard appliance losses ignored.
3. A crew member says 'just raise the pump pressure, the hose can take it.' Evaluate that reasoning.
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Raising pump pressure does not create water - it raises pressure at the source, and the friction loss is set by flow and hose length, not by how hard you push. Pushing harder through a 900 ft lay mostly adds stress to couplings and hose (and risks burst sections) without reliably raising flow at the far end; the pressure is consumed by friction along the way. The sound fix is to reduce the length each pumper must push through (relay) or increase available supply, not to over-pressurize a marginal lay. The crew member's instinct to 'push harder' is understandable but wrong here, and the arithmetic (135 psi loss vs. 70 psi available) shows why.