Fill-Site Hydrant Capacity Versus the Shuttle Demand
You are the driver/operator at a fill site for a shuttle that must move 1,000 gpm to the dump site. A municipal hydrant tests static 62 psi and residual 18 psi at 850 gpm, measured with a pitot on the 2.5-inch outlet. From the hydrant, 250 ft of 4-inch hose runs to your pumper, then 30 ft up a slight rise to the fill manifold. Your pumper is rated 1,250 gpm. The hydrant is on an 8-inch main; a second hydrant is 600 ft away on the same main. The shuttle has two tankers and a 2,500 gal portable dump tank. The IC wants the first tanker filled in 4 minutes, which requires about 625 gpm, but the shuttle plan assumes 1,000 gpm. You plan to use a 4-inch line at 6 psi per 100 ft friction loss.
1. Using the hydrant test and a 4-inch friction loss of 6 psi per 100 ft, estimate the maximum fill rate you can sustain while keeping 20 psi at the fill manifold. Show the calculation.
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Use the hydrant test to build a supply curve. Static 62 psi, residual 18 psi at 850 gpm means a 44-psi drop at 850 gpm. Using the 1.85 exponent: drop at Q = 44 x (Q/850)^1.85. Downstream demand at Q is friction loss in 250 ft of 4-inch hose at 6 psi/100 ft, plus 30 ft of elevation (13 psi), plus 20 psi at the fill manifold. At 1,000 gpm: supply drop = 44 x (1000/850)^1.85 = about 59 psi, leaving only 3 psi residual, while downstream demand is 15 + 13 + 20 = 48 psi. Not possible. At 600 gpm: supply drop = 44 x (600/850)^1.85 = about 23 psi, leaving 39 psi; downstream demand = 6 x (0.6)^2 x 2.5 = 5.4 psi, plus 13 + 20 = 38.4 psi. That balances at about 600 gpm. So the maximum sustainable fill rate is roughly 600 gpm, not 1,000 gpm, even before accounting for hose and hydrant losses not measured.
2. The IC asks whether to add a third tanker or find a second water source. What do you recommend and why?
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Adding a third tanker does not fix a source-limited fill site. If the hydrant can only supply about 600 gpm, the shuttle's maximum water rate is about 600 gpm no matter how many tankers are in line; more tankers only add queue time. I would recommend finding a second water source: a hydrant on a different main, a draft site, or a large-diameter connection to a main that can support 1,000 gpm. A second hydrant on the same 8-inch main is unlikely to help if the main is the bottleneck, and it may make the pressure problem worse for both. If no second source exists, I would tell the IC the honest sustainable rate (about 600 gpm), adjust the shuttle plan, and use the tankers to smooth gaps rather than promise 1,000 gpm. A third tanker could still help if the dump site, not the fill site, is the constraint, so I would check the dump cycle before requesting it.
3. How will you set and monitor the pumper to avoid cavitation and water hammer?
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Connect to the hydrant with a large suction or a 4-inch supply line, open the hydrant slowly, and use the pumper tank-to-pump line or a direct tank fill with the relief valve set to protect the intake. I would keep at least 20 psi positive intake pressure and watch the intake gauge, not just the discharge gauge. The pressure governor should be set to the desired discharge pressure, and the relief valve or bypass should be set slightly above it so a sudden shutoff at the fill line does not send a pressure spike back through the pump. I would open the fill-line valve slowly and warn the tanker driver before starting or stopping flow. If the intake gauge begins to fall or the pump sounds like it is cavitating, I would reduce flow rather than increase throttle; a hydrant supply should never be pulled into vacuum. I would record intake and discharge pressures at each fill so the hydrant curve can be updated.