Natural Gas Line Pressure Drop on a Rooftop Boiler Retrofit
A commercial building converts two rooftop boilers from LPG to natural gas. The utility supplies 21 mbar at the meter. The existing steel run is 45 m to the first boiler, then a further 18 m branch to the second. Each boiler is rated 120 kW and needs 17 mbar at the appliance inlet to fire reliably. The installer reuses the existing 25 mm steel pipe throughout, reasoning that the meter pressure 'is enough'. On commissioning, boiler 1 fires but boiler 2 locks out on low gas pressure whenever boiler 1 is also running. Measured pressure at boiler 2 inlet is 12 mbar with both running, 15 mbar with only boiler 2 running.
1. Diagnose the problem using pressure-drop reasoning, and state why boiler 2 fails only when boiler 1 runs.
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The system is pressure-drop limited, not supply-pressure limited. The available drop is 21 − 17 = 4 mbar total from meter to the worst-case appliance (boiler 2).
- Pressure drop in a pipe rises roughly with the square of flow (Δp ∝ Q²) for a given pipe, and rises with length.
- With both boilers running, total flow is double, so the shared 45 m section carries twice the flow and its drop roughly quadruples.
- Boiler 2 sits at the end of the longest path, so it sees the cumulative drop of the shared run plus its branch.
When only boiler 2 runs, flow is halved, the shared-section drop falls sharply, and 15 mbar is available - still below the 17 mbar requirement, so the pipe is undersized even in the single-boiler case. When boiler 1 also runs, the extra drop on the shared leg pulls boiler 2 to 12 mbar and it locks out on its low-pressure switch. The 'meter pressure is enough' assumption ignores that pressure must be delivered at the appliance inlet under full simultaneous load, not just at the meter.
2. Outline how you would verify and correct the installation, including what you would check before upsizing pipe.
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Verify first, then upsize:
- Confirm actual appliance inlet pressure requirements from the manufacturer's data and the utility's declared supply pressure and allowable drop.
- Recalculate the pipe sizing for the full connected load (240 kW total) over the actual route, using the correct gas (natural gas SG and calorific value) and including fittings, bends and the meter/regulator losses.
- Measure pressure at the meter under full flow to confirm the utility is delivering 21 mbar; a regulator fault would change the fix.
- Check for partially closed valves, undersized meter, debris or a blocked filter, and confirm no other appliances share the run.
Correction:
- Upsize the shared 45 m leg and/or the branch so the worst-case appliance retains ≥17 mbar at full simultaneous flow, with margin.
- If upsizing is impractical, consider a booster or a dedicated line, but confirm with the utility and manufacturer.
- Re-commission, purge, leak-test, and record pressures at each appliance with all appliances running.