While I know that they are referring to certain piece of metal attached to a spring to seal the valve, I cannot find a reference that reflects a piece of metal with that shape or use. Instead, I just find references for the part that closes the valve, which is of course, the "clapet anti-retour." The Weatherford site below contains two very good pictures of check valves with check darts. The pictures will be even larger if you copy them and put them into a Word document. The Wikipedia article has schemata for several different types of "clapets anti-retour" but not for this type. Still, you might find the article helpful.

When the gas-lift valve is open, the upstream (typically casing) pressure on the check valve is higher than the downstream (typically tubing) pressure. The higher upstream pressure depresses the spring-loaded dart, allowing the injection gas to flow through the check valve and into the production tubing. If the downstream pressure is greater than the upstream pressure, flow across the check dart causes the dart to seat, which prevents backflow. http://www.weatherford.com/weatherford/groups/public/documen...

schema of “clapet anti-retour”
http://chalumo29.free.fr/90.htm

Atlantic Guillot : Glossaire des termes techniques
CLAPET ANTI-RETOUR (à ressort)
La fonction d'un clapet est d'assurer le passage unidirectionnel d'un fluide.
http://www.atlantic-guillot.fr/home/guide/guideLexique.html?...

http://fr.wikipedia.org/wiki/Clapet_anti-retour


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Note added at 12 hrs (2007-12-08 05:47:52 GMT)
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More info:

CLAPET ANTI-RETOUR – CHECK VALVE
[see site for additional valves]
http://www.dassault-aviation.com/services/fr/transverse/lexi...

SPM's Dart Style Check Valves are flow control devices that permit flow in one direction but stop flow in the opposite direction. Generally used in well service applications, the dart check valve is placed in the treating line to allow flow to the well but isolates any back flow upstream of the valve. This provides a safety device at various locations in the flow line and assures that pressure and fluid cannot backup into the manifold area or into the pumps.
Fig. 2 2” DART CHECK VALVE
Fig. 3. 3” DART CHECK VALVE
http://www.weirclearliquid.com/weir/clearliquid/rwpattach.ns...$file/Dart_Valve.pdf

Mining - A valve having a lidlike piece hinged on one side within a chamber that permits the flow of a fluid or gas to proceed in one direction only. Usually, the check valve on the pickup end of a drill-pump suction hose is a clack-type valve. Also called chock valve; flap valve; flapper valve;foot valve.
Translations for "clack valve"; alternative meanings/domain in parentheses.
French - clapet de retenue, clapet (clapper), robinet d'essai. (various references)
German - Ventil (air-valve, gate, outlet, piston, safety valve, tube, valve, vent), Rueckhaltklappe (backwater flap, non-return valve), Prüfhahn (check valve, test cock), Klappenventil (clapper, **dart valve**, flat valve, valve), Klappe (back, can it, clack, clapperboard, flap, key, leaf, lid, patch, shutter, Sidegate, strap, swat, tab, tailgate, trap, trapdoor, valve). (various references)[see site for additional info]
http://www.websters-online-dictionary.org/definition/english...



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Note added at 12 hrs (2007-12-08 05:53:52 GMT)
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Given that the Webster's reference above says that a German "klappenventil" is also a dart valve, I decided to check the glossaries for a German to French translation and found the following:

clapet > Klappenventil
Languages:French to German
Glossary name: Dico Fr-De_General
Author:Laurent Rabouin
http://www.proz.com/?sp=gloss/term&id=1804963


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Note added at 1 day47 mins (2007-12-08 17:48:48 GMT)
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The dart of a dart check valve appears to be just called a "clapet." Autrement dit, le clapet anti-retour (ou de non-retour) à ressort utilise un clapet pour l’ouvrir ou le fermer. I've checked all the cool names like "flèche," "fléchette," "dard," and "aile." It's not "clapet à pointeau," which is a needle valve.

I did find another schema of a dart check valve that has a dart in a different shape than the first dart valve that I posted. It is very similar in appearance to the schema for the "limiteur de pression" shown in the Wikipédia article. That schema clearly labels the dart as a "clapet."

Dart Check Valves - see schema
Low-inertial dart desilgn – Hollow dart and fixed stem minimize pressure required to start flow. Non-metallic brushing reduces friction, increasing stem life.
http://www.fmctechnologies.com/FluidControl/Flowline/Valves/...

Limiteur de pression - see schema
Un limiteur de pression est constitué de différents composants:
• le corps
• le clapet anti-retour conique
• le ressort
Si c'est un limiteur de pression réglable, on trouve de plus une vis de réglage.
[Schema] LE LIMITEUR DE PRESSION
• Le clapet
• Le ressort
• Le corps
• La vis de reglage
http://fr.wikipedia.org/wiki/Limiteur_de_pression

3. Check valve / clapet de non-retour
4. ¼ in outlet pipes / ¼ po lignes de sortie
5. Bypass / déviation
6. Flow switch, optional / commutateur de débit, facultatif
7. **Needle valve / clapet à pointeau**
http://strategis.ic.gc.ca/pics/lm/volume/av/mal_v148.pdf


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Note added at 1 day3 hrs (2007-12-08 20:59:19 GMT) Post-grading
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You're welcome, Gérard, and thank you :-)