Cargo Heating — Steam Pressure Is the Temperature

Heavy fuel oil and waxy crude are only liquids while they are warm, and the heat comes from the ship’s boiler as saturated steam. But the coils are only ever as hot as the steam’s saturation temperature, and that is set by pressure — so the whole operation is a chain: firing rate, drum pressure, main steam valve, deck-line pressure, T_sat, the ΔT across the coil, and finally the heat into the cargo. Open too many tank coils at once and the demand outruns the boiler, the pressure sags, T_sat falls, and everything heats slower. Tap a step or press “Play” to follow the chain — then take the boiler and the valves yourself.

Steam system · T_sat is set by pressure

Step 1 / 10

Why heat cargo at all?

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Key —

Tap a step

Heat-up is a days job — plan backward from the ETA+10 °C on 30,000 t ≈ 600 GJ

Common mistakeopening the steam full the night before arrival — 30,000 t cannot take 10 °C overnight; you shock the coils, flash light ends off the coil skin, and still arrive cold. Start days early.

The energy to warm a cargo is simply mass × specific heat × temperature rise. A 30,000 t parcel of heavy oil (c ≈ 2.0 kJ/kg·°C) lifted 10 °C needs about 600 GJ — the full output of a sizeable auxiliary boiler for the better part of two days. Held to the safe heating rate of ~5 °C per day, that is a two-day job as an absolute minimum — and longer in practice, because the sea keeps taking its share through the hull the whole time. There is only so much boiler; that is exactly why you cannot heat every tank at once, and why the heat-up is planned backward from the ETA with margin in hand.

Try it — run the boiler and the coil valves

Fire the boiler, open the main steam valve, and choose which tank coils to feed. The chain does the rest: firing builds drum pressure; the main valve passes it to the deck line; deck pressure sets T_sat; and each open coil pulls its tank up toward T_sat. Open too many coils and the demand beats the boiler — pressure sags and T_sat falls for everyone. Set the line-up, then press Start.

steam temperature is set by steam PRESSURE — open too many tanks, pressure sags, T_sat falls, heating stops

Firing % · makes the steam

16%

Main steam valve % · drum → deck

60%

Line drains open to warm through

OPEN

Sea temperature °C · sets the hull loss

15°C

Coil valves open a tank’s coil to feed it steam

drum2.5bar → deck1.5bar → T_sat125°C

steam demand 4.1 t/h vs boiler cap 25 t/h → HEATING STEADY

firing makes steam, the drum stores it as pressure, and pressure sets T_sat — a coil can only pull its tank up toward T_sat. One aux boiler feeding four tanks, time compressed: 1 s ≈ a few hours.

T_sat125°C Drum2.5bar Deck line1.5bar Steam4.1t/h Boiler load16% Line temp20°C

Tank 150.0°C Tank 253.0°C Tank 351.0°C Tank 449.0°C

Tank marks — pour point 24 · voyage order 54 · max safe 68 °C. Typical working figures — check your cargo documents. Cross the low mark and the wax sets; cross the high mark and the light ends flash off.

Cargo temperature is the running balance of coil heat in against hull loss out — but the coil heat is capped by T_sat, and T_sat is capped by pressure. Below the pour point the wax sets around the coils and heat transfer collapses; recovery is slow even with the valve wide open. Above the max safe line the light ends flash off — flash-point margin gone. And open every coil at once and the boiler cannot hold the pressure: the lesson is to heat tanks in turn.

READY — set the line-up, then press Start

Set the firing, the main steam valve, the line drains and which coil valves are open — then press Start to admit steam and watch the pressure chain do the heating.

Working principle — explained in our own plain words with our own schematic from general marine knowledge (not copied from any OEM manual). Conceptual; varies by ship/model. Reviewed and verified by a Master Mariner.