I've searched the web and really found nothing I could use for our application.

Currently installed is a 4" drain line, which needs to maintain a column of water to prevent air ingress into the system.  

A typical 'P' trap or steam trap could not be use in our service - 1.)  Due to location 2.)  Cold NE weather, which would require us to either heat trace or provide access to drain/fill system during summer operation.

How about a freeze valve or plug valve so you always have flow through the trap?  We've used these on bare lines supplying safety showers and it has kept the pipes from freezing.

Sarco makes these as do a host of other companies.  They are commonly a wax plug that shrinks as the temperature drops and allows water to flow through.  If the temperate of the water continues to drop, they shrink to flow more and vice versa, as the water warms up the wax expands and stops the flow of water.

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