A liquid sample must be vaporised completely before it reaches an analyser. If it boils off slowly, the lighter component leaves first and the sample no longer has the liquid's composition. A sample vaporiser heats the draw so that all of it becomes gas at once. For cryogenic oxygen, nitrogen and argon, use a vaporiser and keep the line short, dry and warm.
- Explain fractionation when a liquid boils off partially
- Describe what a sample vaporiser does
- Plan a cryogenic sample line with a vaporiser and bypass
The problem: boiling off in stages
A liquid mixture boils off its most volatile component first. If a liquid sample warms gradually in a line, the first vapour is rich in the light component and the residue is rich in the heavy one. An analyser fed this sample sees a composition that moves with time and does not equal the liquid. This is called fractionation.
What a vaporiser does
A sample vaporiser heats the liquid so quickly and completely that every component turns to gas together. The gas then has the composition of the liquid. SV-200 is the cryogenic sample vaporiser in the Sarvesh range, with a bypass line for quick flushing; SV-400 serves wet samples and others. They are designed so that the sample is vaporised without loss: sample-vaporisers.
Building the sample path
- Take the draw from the liquid line through a short insulated line to the vaporiser.
- Vaporise at the vaporiser, then regulate pressure and flow of the gas.
- Use the bypass to flush the vaporiser quickly at start-up and when changing sample.
- Keep downstream tube warm and dry so the gas does not recondense or pick up moisture.
- Vent safely: oxygen-enriched vent gas is a hazard: Oxygen enrichment and oxygen deficiency hazards.
Cryogenic safety
- Liquid oxygen and oxidising liquids need materials cleaned for oxygen service; hydrocarbon grease and oil are not allowed.
- Trapped liquid in a closed line can build dangerous pressure as it warms; include relief where the design requires.
- Cold lines and parts cause burns.
Treat any liquid draw as a safety design item as well as an analysis item. Ask an engineer to review the arrangement.
- Liquid samples fractionate if they boil off slowly.
- A vaporiser turns the whole draw to gas at once.
- A bypass line flushes the vaporiser quickly.
- Cryogenic and oxygen service raise safety requirements.
Check your understanding
Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading.
What is fractionation?
The lighter component of a liquid mixture boiling off first, so the vapour no longer has the liquid's composition.
What is the bypass on a cryogenic vaporiser for?
To flush the vaporiser and line quickly at start-up or when changing sample.
Why must oxygen service use cleaned components?
Oil and grease can ignite in oxygen-enriched conditions.
Key terms
Frequently asked questions
Why can't I analyse liquid oxygen directly?
Analysers measure gas. The liquid must be vaporised, and completely, to keep its composition.
Which vaporiser does Sarvesh offer for cryogenic samples?
SV-200, with a bypass line; SV-400 is also available.
Can I analyse LPG with ACCURRA-S?
Flammable samples are outside the standard ACCURRA-S sample specification; ask an engineer.
How do I flush a vaporiser?
Use the bypass line, per the product instructions.