Controls of a Slug Control Valve on Jet-A fuel system

I am working on a project to model a Jet fuel distribution system at an airport. Setup is an outside pipeline supplying a tank farm, and a pump station at the tank farm feeds and underground pipe distribution system to various gates, where a hydrant truck meters fuel into various passenger or cargo jets.

Fuel is filtered at the incoming pipeline to the tank farm, filtered again at the pump station, and filtered on the hydrant truck before metered into the aircraft. (sounds like overkill, but imagine the hysteria if a passenger jet flames out due to water contaminated fuel!) At the outlet of the filters is a pilot controled flow control valve, which I have discovered is called a slug valve in this application.

The flow control valve has two pilots on it. First pilot is connected to a float in the bottom of the filter which will give a pressure signal to the diaphram of the flow control valve to shut off the valve if water is detected. Some of the filters have orifice plates connected to the second pilot to operate the flow control to limit the velocity through the filter (coalescing type where too much velocity will allow water to be re-entrained in the fuel).

I have not figured out how or why the second pilots on the other valves operate. Any good ideas? Any suggestions on how to model a double piloted valve for dynamic operation?