An alarm valve is a check valve with a method of getting pressure or slight flow from an auxiliary port. The alarm valve opens when pressure on the system side is low enough for supply pressure to open it. then some water goes out the alarm port to a pressure switch or water motor gong.
A deluge valve is a normally closed valve, opened by a stimulus from other than pressure on the system side. Typically the stimulus comes from a heat detector for electrically operated systems, or loss of pressure in a pilot line. The pilot line is provided with air or water pressure, and has fusible plugs or sprinklers for detection, mounted in appropriate locations. If the pilot line loses pressure, the deluge valve opens automatically. Manual operating features are always included. Rarely, one finds deluge systems that are manually operated only.
The alarm trim can be identical for alarm and deluge valves.
If you are charged with designing a system in accordance with NFPA 13, a fire department (siamese) connection is required. Exceptions are:
1. Fire suppression forces (if any) do not have the facilities for providing water to the fire department connection.
2. The authority having jurisdiction waives the requirement. Of course you would want the owner to agree also.