I am struggling to make a sensible basis for sizing thermal expansion pot basis.  Due to pipework configuration and fluid properties we cannot easily return the thermal expansion RV back to source.  Therefore we plan to use an expansion pot.

These are long transfer lines (1000 meters or so)/

I will use:
VExp = (?Density cold /density HOT -1) * Vsystem

I was going to use
Density cold=coldest winter temperature
Density hot = 50 C.

These seem too conservative and will have a painfully large expansion pot needing to be blown clear each time.

Can anyone recommend a good reference for this sizing.

why can't you take it back to the source.  The line starts at the source so put the thermal relieve there.  

here's what I do.  At the block valve that is closed that would put the line in overpressure because of thermal expansion, I put a 3/8" piece of tubing around the valve with a check valve that opens when the upstream (the protected line) is 10% higher than original pressure in the other side of the valve that isolates the line that needs protection.

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