LPG Bulk Tank Overfilling on a Cold Morning
You are the responsible gas practitioner at a small commercial bakery running a 2 000 L horizontal LPG bulk tank feeding a 150 kW burner manifold. The tank is fitted with a fixed maximum liquid level gauge (the 85% bleeder) and a float-type contents gauge reading 78%. Winter ambient is 4 °C. The delivery driver, running behind schedule, fills until the bleeder spits liquid, then adds a further 30 seconds of flow before shutting off, saying the tank 'settles' in cold weather. Two hours later, with the sun on the tank and ambient at 22 °C, the customer reports a strong gas smell near the tank and a hissing relief valve. The contents gauge now reads 91%.
1. Explain what physically happened in the tank and why the driver's 'settles in cold weather' reasoning is unsafe.
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LPG is stored as a saturated liquid/vapour mixture, so liquid volume is set by temperature, not by mass alone. The 85% bleeder exists because a vapour space must remain for thermal expansion.
- At 4 °C the driver filled to the bleeder (85% at that temperature) and then overfilled further. The extra liquid left almost no vapour space.
- As the tank warmed to 22 °C, the liquid expanded. Propane/butane liquid expansion is roughly 0.2–0.3% per °C, and the tank shell expands far less, so the liquid had nowhere to go.
- The hydrostatic relief valve lifted because liquid-full hydraulic expansion raises pressure very rapidly (liquid is essentially incompressible), not gradually like vapour pressure.
'Cold weather settling' is a misconception: cold does not create ullage. Overfilling at low temperature is more dangerous, because the expansion headroom needed for the warmest expected condition is already used up. The correct action is to fill only to the bleeder and stop immediately.
2. List the immediate control measures and the corrective actions you would require before the bakery resumes normal operation.
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Immediate:
- Treat as an emergency - no ignition sources, evacuate/isolate the area, approach from upwind, do not operate electrical switches.
- Stop the burner and close the tank outlet (emergency shut-off).
- Cool the tank with water fog if safe to do so, to reduce pressure and stop the relief lifting.
- Notify the supplier and your supervisor; log the incident.
Before return to service:
- Do not simply 'bleed off' liquid by venting to atmosphere - that creates a flammable cloud. Liquid must be removed by a competent person to a safe receptacle or burned off under controlled conditions.
- Have the relief valve inspected/replaced if it has lifted, and check the bleeder and contents gauge calibration.
- Reinspect the tank for distortion.
- Retrain/record-counsel the driver and review the supplier's filling procedure; the 85% stop must be mandatory.
- Update the site emergency plan and confirm signage and firefighting provisions.