Insulation failure is usually down to either bad manufacturing, vibration, or to high temperature. Unusually it might be an impulse voltage but pretty rare on a valve coil. What are the coil operating parameters? Does it run especially hot? Is there a lot of vibration?
As vpl says, pictures might help a lot.
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If we learn from our mistakes I'm getting a great education!
Freeze-taw condition will give condensation in coil housing if not protected, air will always get in and condensate will form. Ex-coil with the whole coil protected in plastic casting might be a suggestion, and/or coil in heated cabinet or traced.
Also see too that nippels/boxes for cabels into coil housing either are of high quality (again ex nippels might do to ensure leakage tight) or again cast in cable with cable connection terminal in protected/warm area.
Always drip-nose on cable before entering solenoid.
Check that operating current and electrical protection is as should be under real operating conditions at site. Measure and observe!
If the solenoid tube - presumably stainless steel or similar - is getting burnt through then it would suggest that there's a fair amount of energy getting through before the fault is interrupted by a fuse blowing or a breaker tripping. From the spec of the coil there's no reason why a fairly low rated time-delay fuse could not be used to afford protection to the solenoid tube. A 2A(T) fuse should not let enough energy through to cause burn-through. You're not saving the coil but at least you're limiting the resultant damage and avoiding a pipework break-in to replace the valve.
Any possibility of the valve itself sticking? If the solenoid armature doesn't pull in fully then the coil will draw considerably more than the hold-in current. In this respect DC coils are superior because coil current is resistance limited and is independent of armature position, whereas AC coil current is reactance limited and is dependent on armature position.
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If we learn from our mistakes I'm getting a great education!
That's a Refrigerating Specialties solenoid in all likelihood; and in a freeze-thaw environment, its very likely that it is shorting to the tube because the slight annular space between tube and innner bore is retaining some water while the coil is energized, which freezes when the coil is de-energized....Thus cracking the coil's insulation from the inside and exposing the winding...This will happen if piping gets struck by lightning but the damage is usually pretty obvious in that case.
Solution could easily be: Put a slim perforated or wavy spacer between bottom of coil and swaged collar on tube, like a wavy washer or Washburne ring...I have also seen a wick material (cotton shoelace, in fact) wrapped around the tube to help the water bleed out while energized....Do not space the coil upward any more that a mm or so, or it will start overheating.
