Corrosive and specialty gas: why the details matter

Ten models sit under Specialty Gas in ANYI's factory record, and they have less in common with each other than any other group in the catalogue. Their recorded working pressures run 3MPa, 15MPa, 20MPa and 23MPa. Their recorded bores are Φ4mm and Φ6mm. Their outlet designations include CGA660, G3/8, G5/8, G3/4 and several W-series threads. Some rows carry a safety device setting; most carry none.

A category this wide is a filing convenience. It groups the models that did not belong under a named medium. It is not a claim that they are alternatives to each other, and picking from it without stating your gas is guesswork.

Why material is the whole question

Corrosive service is where assumptions cost the most. Two valves can share an outlet designation, a bore and a pressure rating, and behave completely differently in contact with an aggressive medium, because what decides that is the material of every wetted part including the seals - and that is not visible, not implied by the body colour, and not implied by the category heading.

So the single most useful line in a specialty gas enquiry is the name of the gas, written plainly, with its concentration if it is a mixture. Everything a supplier can usefully check follows from that.

What the enquiry must state

• Name the gas and the concentrationNot "specialty gas", not "corrosive". The actual medium, and for a mixture the components and their proportions.
• State the service conditionsPressure, temperature and duty. A gas that is benign at one temperature may not be at another.
• Ask for wetted-part materials per modelBody, stem, seat and seals. Ask per exact model designation, and ask in writing.
• Say what the valve connects toCylinder thread and outlet designation. For specialty gas these are frequently deliberately distinctive; do not normalise them to something more common.
• Say what your own process requiresPurity, moisture limits, cleaning, documentation. These are requirements a supplier can answer against or decline; they cannot be guessed.

The failure worth avoiding

There is a version of this conversation that goes badly, and it is worth naming. A buyer asks whether a model is "suitable for corrosive gas". A supplier, wanting to be helpful, says yes. Neither has named a gas, so neither statement means anything, and the agreement is between two different assumptions.

The version that goes well is slower by one message: the buyer names the medium, the supplier answers with what the record holds and what has to be checked, and the decision is made on the same facts.