KEVOS® MAKEOS · Engineering
Solvent Vapour Headspace Check (Raoult’s Law Partial Pressure, Saturated Concentration, Share of LEL)
If this solvent sits in a closed tank or container until its vapour saturates the space above it, how close does the vapour come to the lower explosive limit, and at what liquid temperature does it reach my limit?
Screening of the saturated vapour above a solvent or solvent solution in a closed space: the pure-solvent vapour pressure at the liquid temperature from a reference value by the Clausius–Clapeyron equation, the solvent’s partial pressure by Raoult’s law, the equilibrium headspace concentration and its share of the lower explosive limit, and the liquid temperatures at which the headspace reaches your limit and the LEL itself, against the share of LEL you allow.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Mole fraction of the solvent in the liquid | Mole fraction of the volatile solvent in the liquid: 1 for the pure solvent, less for a solution in a non-volatile liquid. | |
| Vapour pressure of the pure solvent at the reference temperature | kPa | Vapour pressure of the pure solvent at the reference temperature, from its safety data sheet or a property table. |
| Reference temperature for that vapour pressure | °C | The temperature that goes with that vapour pressure. |
| Molar enthalpy of vaporisation | kJ/mol | Molar enthalpy of vaporisation of the solvent, from a property table. |
| Liquid temperature | °C | Temperature of the liquid in the tank or container, at its warmest. |
| Total pressure in the headspace (absolute) | kPa | Absolute pressure of the headspace (atmospheric for a vented tank). |
| Flammability limit | ||
| Lower explosive limit of the solvent vapour | % | Lower explosive (flammable) limit of the solvent vapour in air, from its safety data sheet. |
| Largest allowed share of the LEL | % | The largest share of the LEL your site rules or risk assessment allow in the space. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Mole fraction of the solvent in the liquid
- What it is
- The share of molecules in the liquid that are the volatile solvent, from 0 to 1, which scales its partial pressure by Raoult’s law; 1 means the pure solvent.
- Units and format
- Enter a plain number with no unit. The form accepts 0.000001 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The partial pressure and headspace concentration rise in proportion to it.
- Get it right
- Convert a mass fraction to a mole fraction with the molar masses; a 50 % mass dilution is not a mole fraction of 0.5.
What this means: Vapour pressure of the pure solvent at the refe…
- What it is
- The vapour pressure of the pure solvent at the reference temperature, which anchors the vapour pressure curve used for the liquid temperature.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 0.000001 kPa to 100000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3.8 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The headspace concentration rises in proportion to it.
- Get it right
- Safety data sheets often give it at 20 °C; enter the temperature that goes with it in the next field.
What this means: Reference temperature for that vapour pressure
- What it is
- The temperature at which the pure-solvent vapour pressure was measured or listed, the anchor of the Clausius–Clapeyron curve.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -100 °C to 500 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- For the same vapour pressure, a higher reference temperature lowers the vapour pressure found at the liquid temperature.
- Get it right
- Use the temperature stated with the vapour pressure on the data sheet, not the storage temperature.
What this means: Molar enthalpy of vaporisation
- What it is
- The heat needed to evaporate one mole of the solvent, which sets how fast its vapour pressure rises with temperature.
- Units and format
- Enter a number in kJ/mol: kJ/mol. The form accepts 0.1 kJ/mol to 1000 kJ/mol. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 38 kJ/mol as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger value makes the vapour pressure rise faster above the reference temperature.
- Get it right
- Tables may list it in kJ/kg; multiply by the molar mass in kg/mol to get kJ/mol.
What this means: Liquid temperature
- What it is
- The temperature of the liquid solvent in the tank or container, which sets its vapour pressure and so the saturated headspace concentration.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -100 °C to 500 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 35 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A warmer liquid raises the vapour pressure and the share of the LEL steeply.
- Get it right
- Use the warmest the liquid gets, such as a tank in the sun or after a heated process, not the average.
What this means: Total pressure in the headspace (absolute)
- What it is
- The absolute pressure of the gas space above the liquid, which divides the solvent partial pressure to give its concentration by volume.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 0.000001 kPa to 100000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 101.325 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher total pressure lowers the concentration for the same partial pressure.
- Get it right
- For a tank vented to atmosphere use the local atmospheric pressure; a blanketed tank runs a little above it.
Flammability limit
What this means: Lower explosive limit of the solvent vapour
- What it is
- The lowest concentration of the solvent vapour in air, by volume, that can burn or explode, also called the lower flammable limit.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.01% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.1% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher LEL lowers the share of LEL for the same concentration.
- Get it right
- Take it from the safety data sheet of the actual product; blends have their own limits.
What this means: Largest allowed share of the LEL
- What it is
- The largest share of the LEL your site rules, permit system or risk assessment allow in the space, used as the pass or fail limit.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line and the temperature at which the limit is reached.
- Get it right
- Use the figure in your own procedures; detector alarm settings are often a lower share than the limit for entry.
Method and formulas
- Vapour pressure of the pure solvent at the liquid temperature by Clausius–Clapeyron: P* = P_ref exp(−(ΔH_vap ÷ R)(1 ÷ T − 1 ÷ T_ref)).
- Raoult’s law: partial pressure p = x P*; saturated concentration = p ÷ P_total; share of LEL = concentration ÷ LEL.
- Temperature to reach a concentration: solve x P*(T) = concentration × P_total for T.
- Example: a solvent with 3.8 kPa at 25 °C and 38 kJ/mol, pure, at 35 °C gives 6.2492 kPa and a saturated headspace of 6.1675 % v/v, 560.68 % of a 1.1 % LEL; it would reach the LEL at a liquid temperature of 2.9114 °C.
Limitations
- Equilibrium headspace in a closed space: the worst case. Ventilated or freshly filled spaces are lower but can reach it over time.
- One volatile component and ideal solution behaviour; mixtures of volatile solvents need each component and a mixture flammability rule, and real solutions can deviate from Raoult’s law.
- This is a screening aid; hazardous area classification and any entry or hot work in the space need a competent person.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Screen a solvent tank or drum headspace for a flammable atmosphere
- Find how warm a solvent can get before its vapour reaches a limit
- Compare solvents or dilutions for a closed process
Who it is for
- Process safety and plant engineers
- Production supervisors
- Safety advisers
What you get
- Vapour and partial pressures
- Saturated headspace concentration and share of the LEL
- Liquid temperatures reaching your limit and the LEL
Interpreting the result
- The liquid temperature at which the saturated headspace reaches the LEL is close to the idea of a flash point, but a measured closed-cup flash point governs.
- A headspace above the upper explosive limit is too rich to burn but becomes flammable as air enters, for example when the tank is opened or emptied.
Before relying on the result
- Take the vapour pressure and LEL from the safety data sheet for the actual product.
- Confirm the headspace with a calibrated gas detector before any work in or near the space.
References
- Linder, B., Thermodynamics and Introductory Statistical Mechanics (Wiley-Interscience, 2004), sections 9.3 and 10.2.1, printed pp. 97 and 104–105: the Clausius–Clapeyron equation and Raoult’s law P_i = x_i P_i*. Method reference; no source text, tables or examples are reproduced.
- AS/NZS 60079.10.1, Explosive atmospheres: classification of areas for explosive gas atmospheres. Named for scope; the standard governs area classification.
- Safety data sheets for hazardous chemicals give the vapour pressure, flash point and explosive limits; Safe Work Australia publishes the code of practice for preparing them.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Method explained here · KEVOS® original book
Flammable headspaces
In Solvent Vapours by the Numbers
What it covers: The saturated vapour in a closed drum or tank against its explosive limits, how it compares with the flash point, why empty drums and over-rich spaces are dangerous, and purging.
Useful alongside · KEVOS® original book
Fire, gases and confined spaces
In Welding Health and Safety by the Numbers
What it covers: Control hot work fires with permits and fire watches, handle gas cylinders, calculate how a leak drains or enriches the oxygen in a confined space, and plan confined space welding and rescue.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.