22:08 Feb 13 

Well, Barbara, you didn't actually make that clear in your original question; but in any case, as I said, 'gas and hydraulic' simply won't do, as it can cover MUCH more than that.

20:10 Feb 13 

You might say something like"Applied fluids engineering" A quick google of fluids engineering companies or consultants will turn up hundreds of companies

19:42 Feb 13 

My problem was with its association with "fluides", as should have been obvious from my queries: "Fluid engineers? Gas and hydraulic engineers?" What about utilities engineers, or services and utilities engineers? I thought I had got the right term with "building services engineers", however that includes things other than "fluides":
" energy supply - gas , electricity and renewable sources
heating and ventilating
water , drainage and plumbing
daylighting and artificial lighting
escalators and lifts
ventilation and refrigeration
harnessing solar, wind and biomass energy
communications, telephones and IT networks
security and alarm systems
fire detection and protection
air conditioning and refrigeration
facade engineering"
http://www.cibse.org/index.cfm?go=page.view&item=67

18:47 Feb 13 

As Howard says (and I believe it's even lurking somewhere there in the glossary)

However, watch out for 'fluids' — it may be much more than just gas and hydraulic, it is v. likely to also be water, possibly drainage, + things like compressed air, of course, if relevant. And I have even seen it referring to electricity! Anything, in fact, that could be said to 'flow', in the broadest sense of the term...