Running Straight Propane
Straight propane needs a 250 psi pressure envelope, not just compatible materials. Saturation pressures against the vessel rating, the honest temperature ceiling, what the certification reports say, and what changes when you switch off butane.
Straight propane runs on an N.B. Oler hydrocarbon system. The reason is the pressure envelope, and it is the one spec that decides whether a machine can do it at all.
Why propane is the harder solvent
Solvent compatibility is a materials question. Stainless, Viton and PTFE do not care whether the molecule is propane or butane. The thing that separates the two is vapor pressure, and it is not a small difference.
| °F (°C) | Propane | n-Butane | Propane margin to 250 psi |
|---|---|---|---|
| 40 (4) | 63.9 psig | 3.0 psig | 186 psi |
| 70 (21) | 110.2 psig | 16.5 psig | 140 psi |
| 80 (27) | 129.3 psig | 22.5 psig | 121 psi |
| 100 (38) | 173.9 psig | 36.9 psig | 76 psi |
| 120 (49) | 227.8 psig | 55.1 psig | 22 psi |
| 130 (54) | 258.7 psig | 65.9 psig | none, 9 psi over |
Saturation pressures from Propane and Butane Vapor Pressure Chart, REF-VP-01. Margin is 250 psi (17.2 bar) MAWP less the saturation pressure.
A tank holding any liquid sits at the saturation pressure for its liquid temperature, no matter how full it is. So a solvent tank of propane in an 80°F (27°C) room is carrying 129 psig all day, with nothing running. The same tank on n-butane is carrying 22 psig. That standing pressure is the load every vessel, clamp, gasket, valve and relief on the machine has to live with.
The honest ceiling
At 130°F (54°C) propane reaches 258.7 psig, which is above a 250 psi relief setting. The relief does its job and the system vents. Keep propane out of hot rooms and out of direct sun.
- Up to about 120°F (49°C) propane stays inside the 250 psi (17.2 bar) envelope, with 22 psi left at the top of that range.
- Past roughly 127°F (53°C) the saturation pressure crosses 250 psi and the cryogenic relief valves and the Conrader SRV530 on the solvent tank begin to lift.
- The relief settings do not change when you switch solvents. They are already at 250 psi (17.2 bar) because the vessels are. What changes is how close normal shop conditions put you to them.
- Nothing above assumes a hot jacket. Jacket water on the recovery side has to stay below the temperature that matches your lowest rated component, same as on butane.
What is certified, by report
| System | Report | What it says on propane |
|---|---|---|
| 20 LB | 3P Certz ECR-200014-R2 | Solvent vessel certified to 105 lb maximum solvent capacity, propane or butane |
| 40 LB Double Barrel | 3P Certz ECR-250902-R0 | Approved solvents listed by CAS number: n-Butane 106-97-8, Isobutane 75-28-5, Propane 74-98-6 |
| Diamond Mine | 3P Certz ECR-190082-R3 | Reviewed for butane, propane, ethanol, heptane and hexane |
| 100 LB solvent tank | 3P Certz DCR-200101-R1 | 250 psi (17.2 bar) MAWP, ASME Section VIII Division 1 U stamp, CRN R8694.5 |
Propane is named as an approved solvent, not tolerated as a minor fraction of a blend. Tell us your AHJ and the quote names the documents they will ask for.
What changes when you move from butane to propane
- Standing pressure. Roughly 90 psi more at room temperature. The machine is never at rest the way a butane machine is, and operators used to butane notice it on the first fill.
- Condensing duty. Recovery is driven by holding the recovery vessel warm and the solvent tank cold. To bring propane down to the pressure n-butane reaches at 65°F (18°C), the propane tank has to be near 0°F (-18°C). That is why liquid CO2 or a chiller holding -60°C (-76°F) is not optional on propane, and why a solvent tank condenser or a tube in shell condenser pays for itself faster on propane than on butane.
- The cold end gets safer, not worse. At -40°F (-40°C) n-butane is at 25 inHg of vacuum, so a cold butane system pulls air in through any leak. Propane at the same temperature is still at 1.4 psig, slightly positive. A propane system leaks out rather than in, which keeps air out of your solvent.
- Leak checking. Still done under pressure before chilling. On propane the standing pressure finds leaks for you.
- Gaskets. Unchanged in material terms. Viton is rated excellent on propane and is the default, good to -40°F (-40°C). The PTFE envelope gasket takes the system to its -65°F (-54°C) minimum. The gasket, not the vessel, is what governs the cold end either way.
What does not change
- The vessels. Same 250 psi (17.2 bar) ASME stamped solvent tank and recovery vessel, same column jackets at 22 psi (1.5 bar).
- The relief valve settings, which were specified by 3P Certz against the vessel ratings and not against a solvent.
- The clamps and the hardware. A 6 in heavy clamp is rated 300 psi (20.7 bar) at 70°F (21°C) and the double bolted clamps are well above that.
- Your filtration, your 4A molecular sieve and your sintered stack. 4A is the right sieve for propane as well as butane.
- The classification of the room. Propane and butane are both Class I Division 1 flammable gases.
Why run it at all
Propane boils at -43.8°F (-42.1°C) against 31.1°F (-0.5°C) for n-butane. It stays a usable solvent at temperatures where butane has gone sluggish, and it comes off the extract without having to pull as hard. Shops run it straight, or propane rich, when they want the lighter fraction and a faster cold pull. The equipment question is the only thing in the way, and on a 250 psi machine it is already answered.
Sources. Saturation pressures and fixed points from N.B. Oler REF-VP-01, Propane and Butane Vapor Pressure Chart, Revision 2026.1, which derives from the NIST Chemistry WebBook reference equations of state. Certification statements from 3P Certz reports ECR-200014-R2, ECR-250902-R0, ECR-190082-R3 and DCR-200101-R1. Vessel and clamp ratings as published on the product pages linked above.