I have a manual dry standpipe, it's the only one I've ever done in my life the reason being most jurisdictions I've worked in never allowed dry manual standipipes, and I just got approved drawings back with a note requiring a 6" check valve with ball drip between the 3-Way FDC & system.

Hey, I got approved drawings so I'm going to argue but what is the reason for the check valve?

BTW I did have a ball drip check at the 6" elbow through wall.

The most probable reason is to prevent a cross connection between the public, potable, water supply and a water source whose quality is possibly contaminated or simply unknown. A sudden drop in pressure on the public side of the connection could allow flow from the building sprinkler system ( which may contain brackish water or antifreeze, back into the drinking water system.  Most often, a double check valve would be required to ensure such contamination does NOT occur.

Such a pressure drop is especially likely to occur during a fire when very large demands are placed on the supply.

You didn't say which way the check valve is allowed to flow.

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