anneau de visualisation

[derw2008]

These appear to be valve-like things on the side of tanker lorries carrying oil/petrol/gas etc.

I have found two images under Google Images (but am unable to post the URL).

Happily the objects displayed in Google also feature in the image in the document I'm working on but I'm clueless as to their actual name in English - many thanks in advance to anyone in the know...

 

[alisonp]

I can't help with the term, but I can at least post the URL so others can have a look: http://images.google.com/images?q=%22anneau%20de%20visualisation%22&rls=com.microsoft:*&ie=UTF-8&oe=UTF-8&startIndex=&startPage=1&um=1&sa=N&tab=wi