“Institut za standardizaciju Srbije
SRPS M.E0.053:1990
…
- Naslov na srpskom jeziku
Nepovratne klapne za PN 16, 25, 40 i 64 i DN 15 do DN 500 - Tehnički uslovi
- Naslov na engleskom jeziku
Check valves - Technical requirements”
( http://www.iss.rs/standard/?natstandard_document_id=8479 )


“ExpertGlossary
…
Check valve
All > Science > Water
Any device which will allow fluid or air to pass through it in only one direction. A special valve with a hinged disc or flap that opens in the direction of normal flow and is forced shut when flows attempt to go in the reverse or opposite direction of normal flow. A device preventing backflow in pipes. Water can flow readily in one direction but any reversal of the flow causes the check valve to close.”
( http://www.expertglossary.com/water/definition/check-valve )


“Wikipedia
…
A check valve, clack valve, non-return valve or one-way valve is a mechanical device, a valve, which normally allows fluid (liquid or gas) to flow through it in only one direction.”
( http://en.wikipedia.org/wiki/Check_valve )

Klapna = flap
Ovde je u pitanju "non-return flap valve" ("nepovratnih klapni na mestima izliva prečišćene vode u recipijent") pogledajte definiciju i slike ovde:
http://www.missionrubber.co.uk/cms/upload/PDF/Non_Return_and...
FLAP VALVES
The principle applications for flap valves are for surface water drainage associated with river, estuary and sea water outfalls to prevent backflow into the sewer and for final effluent outfalls from wastewater treatment works to prevent flood damage within the works.
NON-RETURN VALVES
Non return valves or backwater valves are used within sewerage and drainage systems to eliminate the risk of flood damage by the backflow of sewage or flood water into properties through low level entries such as low level drain gullies, toilets and washing machine outlets.

http://www.cpm-group.com/new/environmental/non-return-flap-v...
CPM Group Ltd in conjunction with Ham Baker the market leader in Penstocks & Flap valves have developed an all in one pre-cast concrete chamber which is delivered to site with the circular Non-Return Flap Valve pre-fitted into position on the head wall.

http://www.google.com/search?q="non-return flap valve"&ie=ut...

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Note added at 16 hrs (2011-08-26 05:59:33 GMT)
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non-return valve = nepovratni ventil, ref. EN-SH rečnik niskogradnje, B. Vukičević