How to simulate lpg storage — fire case, psv sizing and flare radiation
An LPG storage and truck-loading facility, built so the relief study that follows it is the real one: rundown from the fractionator is cooled, let down to storage pressure, held in a sphere, and pumped to a loading line. The process itself is deliberately simple — the demonstration is what you do with it next, using the Analysis panel's API 520/521 tools, which is the workflow HYSYS sells its Depressuring and Flare utilities on. **Fire case (Analysis > Fire-case relief).** The governing scenario for a pressurised LPG vessel. A 160 m2 wetted sphere in a pool fire with adequate drainage takes 2.77 MW of absorbed heat (API 521 environment factor 1.0); at a relieving pressure of 18 bar abs the stored 70/30 propane-butane boils at 338 K with a latent heat of 270 kJ/kg, so the PRV must pass 10.3 kg/s — an API 526 **M** orifice. **Reaction force (Analysis > Relief reaction force).** That same 10.3 kg/s leaving a 150 mm tailpipe chokes at 9.5 bar and 260 m/s, putting **17.7 kN** on the pipe — the load the tailpipe supports have to carry, and the number that decides whether the discharge piping needs bracing. **Flare radiation (Analysis > Flare radiation).** Burning that relief load (46 MJ/kg, 30% radiated) releases 473 MW. From a 40 m radiant centre the API 521 exclusion zones come out at 64 m horizontal for continuous exposure, 18 m for emergency personnel access, and zero for equipment. **The actionable result:** a receiver 60 m away sees 2.5 kW/m2 — comfortably under the 4.73 kW/m2 personnel limit, but above the 1.58 kW/m2 continuous limit, so a permanently manned building there needs shielding or relocation. **Thermal relief (Analysis > Thermal relief).** A second, entirely different scenario on the same facility: the loading line blocked in full of liquid and warmed by the sun. 12 kW into trapped LPG (cubic expansion coefficient 3.9e-3 /K at 526 kg/m3) needs only 0.017 kg/s — a **D** orifice, the smallest API 526 size. Sizing this line for the fire case instead would oversize the valve by three orders of magnitude in area, which is exactly the mistake the separate tool exists to prevent. **.
- 1Open the ready-made model
Open the "LPG storage — fire case, PSV sizing and flare radiation" model in the MaximaLabs workspace — no install, no license. It loads live on the canvas, ready to edit and run.
- 2Confirm the thermodynamics
This process is modeled with the PENG-ROBINSON property package over propane, n_butane — already selected, so the phase equilibrium and enthalpy are physically consistent from the first run.
- 3Review the flowsheet
The flowsheet chains Rundown Cooler, Letdown, Sphere, XFER PUMP, Loading LINE. Every block is a real, solvable unit op you can reconfigure on the canvas.
- 4Run the simulation
Click Run. The deterministic solver converges the material and energy balances (recycles included) and fills the live stream table — the AI never invents a number.
- 5Read the results and iterate
Inspect the converged streams, tweak a spec, and re-run — or ask the AI copilot to explain a result or diagnose a failed solve in plain English.
- Thermodynamics
- PENG-ROBINSON
- Components
- propane, n_butane
- Unit operations
- Rundown CoolerLetdownSphereXFER PUMPLoading LINE
Opens live on the canvas — free, no install.
Explore the model & flowsheetModeling assumptions & limitations
What this model captures, and what it deliberately does not — from the engineers who built it.
- 1** These are screening calculations to the API 520/521 equations, not a stamped relief study: the wetted area, drainage credit, heat of combustion and radiant-centre height are engineering inputs you supply, and the vessel inventory is not modelled dynamically (a depressuring transient is a different calculation from the steady relief rate sized here). The flowsheet's own numbers — 250 mol/s of liquid at 310 K, the 0.15 bar loading-line drop, an erosional-velocity ratio of 0.2 against the API 14E limit — come from the solve, not the prose.
Frequently asked questions
- What does the LPG storage — fire case, PSV sizing and flare radiation model simulate?
- An LPG storage and truck-loading facility, built so the relief study that follows it is the real one: rundown from the fractionator is cooled, let down to storage pressure, held in a sphere, and pumped to a loading line. The process itself is deliberately simple — the demonstration is what you do with it next, using the Analysis panel's API 520/521 tools, which is the workflow HYSYS sells its Depressuring and Flare utilities on. **Fire case (Analysis > Fire-case relief).** The governing scenario for a pressurised LPG vessel. A 160 m2 wetted sphere in a pool fire with adequate drainage takes 2.77 MW of absorbed heat (API 521 environment factor 1.0); at a relieving pressure of 18 bar abs the stored 70/30 propane-butane boils at 338 K with a latent heat of 270 kJ/kg, so the PRV must pass 10.3 kg/s — an API 526 **M** orifice. **Reaction force (Analysis > Relief reaction force).** That same 10.3 kg/s leaving a 150 mm tailpipe chokes at 9.5 bar and 260 m/s, putting **17.7 kN** on the pipe — the load the tailpipe supports have to carry, and the number that decides whether the discharge piping needs bracing. **Flare radiation (Analysis > Flare radiation).** Burning that relief load (46 MJ/kg, 30% radiated) releases 473 MW. From a 40 m radiant centre the API 521 exclusion zones come out at 64 m horizontal for continuous exposure, 18 m for emergency personnel access, and zero for equipment. **The actionable result:** a receiver 60 m away sees 2.5 kW/m2 — comfortably under the 4.73 kW/m2 personnel limit, but above the 1.58 kW/m2 continuous limit, so a permanently manned building there needs shielding or relocation. **Thermal relief (Analysis > Thermal relief).** A second, entirely different scenario on the same facility: the loading line blocked in full of liquid and warmed by the sun. 12 kW into trapped LPG (cubic expansion coefficient 3.9e-3 /K at 526 kg/m3) needs only 0.017 kg/s — a **D** orifice, the smallest API 526 size. Sizing this line for the fire case instead would oversize the valve by three orders of magnitude in area, which is exactly the mistake the separate tool exists to prevent. **.
- Which thermodynamic method does it use?
- The PENG-ROBINSON property package, over propane, n_butane — already selected. You can switch the method on the canvas before running.
- Which unit operations are in the flowsheet?
- It chains Rundown Cooler, Letdown, Sphere, XFER PUMP, Loading LINE. Every block is a real, solvable unit operation you can reconfigure, add to, or remove.
- Do I need to install software or buy a license?
- No. LPG storage — fire case, PSV sizing and flare radiation runs entirely in your browser on MaximaLabs — free, no install, no license. Open the model to load it live and run the deterministic solver.
More guides like this
Relief system — the fire case, on the flowsheet
Two vessels on one fire zone, each protected by a relief valve, both discharging into a shared flare header. The point is that nothing here is retyped: the relieving temperature, pressure, composition and molecular weight come from the solved streams, and the flare's load is the sum of what the valves actually send it. The relief study, block by block. PSV_SEP is on the separator overhead: its scenario is the API 521 external fire, Q = C·F·A^0.82 over 45 m² of wetted surface, and the vapour rate is Q/λ with the latent heat taken from the property package at the relieving pressure rather than assumed. PSV_SURGE protects the surge drum on the classic blocked-outlet case — it relieves the whole inlet flow, which is a number the canvas already knows. Each reports its required orifice area with the API 526 letter above it. Why they are drawn as lifted. Both carry relieving: true, which models the relief case rather than normal operation. Turn it off and each valve is shut — the process passes through untouched and only the sizing is reported — but then the flare has no load to size against, and says so instead of inventing one. A design load only becomes a flow when you say the valve is open. What the flare adds that valve-by-valve sizing misses. A fire zone lifts every valve in it at once, so the header carries both loads together: that is what sets the built-up back pressure, and it is checked against the lowest-set valve on the header, because a back pressure a 20-barg PSV shrugs off will chatter a 6-barg one beside it. The tip is checked against the API 521 ~0.5 Mach blow-off limit, and the radiant flux at a receiver 60 m away against the 4.73 kW/m² personnel level, with the heat of combustion computed from the relief gas itself. Bounds, stated. A single header (the multi-branch network with per-source back pressures is the Flare network tool); a point-source radiation model with no wind tilt; no knock-out or seal drum; and the fire case is API 520's peak instantaneous rate, not a depressuring transient — that is the Depressuring / blowdown tool.
LNG front end and storage: guard beds, tank boil-off, driver limit
The parts of a 5.2 Mtpa LNG train that are not the cold box, and the four questions they answer that no bulk unit can — at real capacity, so the vessel sizes and duties are ones an engineer can check against their own plant. Guard beds: a sulfur-carbon bed takes mercury from 200 to 0.01 µg/Nm³ — the brazed-aluminium limit, because mercury attacks the plate-fin cold box by liquid-metal embrittlement — on a 4.8 m x 6.6 m bed with 4.2 years of life from a capacity balance — inside the 3-5 year window these non-regenerable beds are actually replaced on, which is what a turnaround plan needs. A 4A molecular sieve then dries the gas to 0.1 ppmv, the only route to a cryogenic water spec (a glycol contactor tops out near a −30 °C dew point), on a 4.9 m x 5.1 m bed running a 13.8 h cycle for 1.3 MW of regeneration duty. Storage: the LNG goes to a tank whose boil-off rate comes from a 0.05%/day guarantee but whose boil-off composition comes from a real equilibrium flash — and that is the interesting part, because the vapour leaves at ~11 mol% nitrogen against 1.1% in the feed. Nitrogen and methane are far more volatile at 113 K, so the boil-off is light and the stored liquid weathers heavier: nobody specified that, the flash found it. Recondensing: the boil-off goes back into the send-out LNG rather than to a flare, and the unit reports the LNG:BOG ratio (1999) against the minimum the energy balance demands (3.8), which is the constraint a terminal is actually operated against. The driver: a gas turbine burning plant fuel gas, rated 97.5 MW at ISO conditions, delivers only 84.5 MW at 35 °C — 5.5 MW short of its 90 MW refrigeration load, and flagged as such. That derate is computed from two physical effects (a fixed-geometry compressor swallows a fixed volume so mass flow follows air density, and hotter air costs more to compress), not from a vendor curve, and it comes out at 0.66%/K — mid-band for industrial machines. It is why a tropical LNG train makes less product in summer.
Gas-condensate LNG train: condensate, LPG and LNG
The LNG plant that is also a liquids plant, at real capacity: this is one 4.4 Mtpa train (a two-train plant is two of these), not a scaled-down sketch. A condensate-rich field arrives by trunkline as a two-phase stream: the slug catcher drops ~20 mol% of it out as raw condensate before a single molecule reaches the cold end, and the plant then runs two trains side by side off one feed. Liquids: the raw condensate is let down to an MP flash that strips the dissolved methane (without it the stabilizer's shortcut total condenser tries to condense methane at 8 bar, which is not a real stabilizer overhead), then stabilized to a C₅/C₆ product. Gas: sweetened and dried, chilled to 245 K to knock out an NGL cut, and the NGL demethanized and split by a deethanizer / depropanizer / debutanizer sequence into commercial propane (~98 mol%) and butane (~98 mol%) LPG plus a natural-gasoline bottoms. LNG: the lean gas goes to the same APCI C3MR cold end as the 'c3mr-lng-liquefaction' showcase — propane precool, mixed-refrigerant MCHE to 120 K, JT letdown to a 1.5 bar end-flash drum. Five sold products come out of one flowsheet, each a stream the solver computed. Watch the end-flash boil-off: it leaves at ~13 mol% nitrogen against 1 mol% in the feed, because the flash drum is where an LNG train actually rejects its nitrogen — nobody specified that, the flash found it. Dehydration is the real molecular-sieve bed, not a stand-in: a 4A sieve sized by length-of-unused-bed, which is what actually takes the gas to the <=0.1 ppmv the cold box needs — 95 t of sieve on a 5.5 x 5.5 m bed, a 17.5 h cycle and 1.4 MW of regeneration duty, with 0.47 bar of Ergun pressure drop. The AGRU spec is checked against the physics rather than assumed: 99.9% CO₂ removal leaves 33 ppmv, and at the coldest point in the train (115.9 K) the solid-CO₂ solubility limit is 232 ppmv on the measured-data basis — a 7x margin, so the sweetening spec demonstrably clears freeze-out instead of merely looking tight.
Deep ethane recovery with propane-refrigerated feed chilling
Associated gas is chilled by an external propane refrigeration package before a demethanizer recovers ethane and heavier as NGL bottoms from a methane-rich residue gas overhead — the cryogenic-chilling role Ortloff's CCS/GSP processes play ahead of the turboexpander in a real deep-ethane-recovery plant. The mechanical refrigeration loop itself (compressor/condenser/valve) isn't separately modeled here; the chiller's duty is represented directly as the feed's cooled outlet temperature (the same honest-simplification pattern used for LNG cold-box examples elsewhere in this library).
Gas compressor air-cooled aftercooler (fin-fan)
A natural-gas booster compressor followed by an air-cooled (fin-fan) aftercooler — the standard way to reject compression heat where no cooling water is available. The compressor raises the gas from 8 to 24 atm (hot discharge ~150 °C); the air cooler then rejects that heat to ambient air, cooling the gas back to 49 °C. Unlike a plain cooler, the air_cooler op closes the air side: from the process duty and the 35 °C design ambient it solves the air mass flow (a 15 °C air rise) and reports the fan power from the given fan static pressure. Honest scope: screening air-side model (fixed cp_air, ideal-gas air density, no fin/row geometry rating); an air cooler cannot cool below ambient, so the 49 °C target sits safely above the 35 °C air inlet.
Hydrocracking reaction section
A refinery hydrocracker: heavy VGO (modeled as n-dodecane) plus excess H₂ is cracked over catalyst into lighter products via a discrete lumped first-order kinetic network, then flashed to knock out recycle H₂/light gas from the liquid product (Peng-Robinson). Conversion is set by reactor temperature and LHSV.