what is the diffrence between NRV and check valve.

Karthik take a look at this link here for a good explination - http://www.flosteer.com/checkvalves.htm

Most of the difference is in the economy of words (or letters) in the Bill of Material (BOM) along with the complexity of the installation.

A Check Valve prevents back flow.

A Non-Return Valve (NRV) prevents back flow.

A Back Flow Preventer prevents back flow.

Many of the Back Flow Preventer (or NRV) installations in Fire Water and Domestic water have double Check Valves plus both upstream and downstream block valves.