I don't believe the valve seals are different, and I think most of them are plastic/nylon. If you like to work carefully, you should remove the seal from the valve and reinstall it after you've soldered.

Since you can't remove the seal from the ball valve, as long as it doesn't get to hot or is protected with a wet rag you will be fine.

Remove the nut on the stem holding the handle on and tighten up the packing nut underneath to make it tighter and help prevent future leaks.

I always use threaded ball valves and male adapters - it helps to keep the heat off of the valve. In addition, I wrap the valve with a wet rag. Never have problems.

You need a large enough and hot enough torch to avoid overheating.
Larger propane or air-acetylene with the correct size tip does fine.
A smaller torch will take longer to heat up the mass of the valve body and increase the chance for damaging the ball and seal.

Teflon balls and seals are more expensive but less likely to be damaged by limited overheating.

With all being said,and good too, If its not leaking dont mess with it.

The spherical seal in a sweat-fitted ball valve is good to about 650degF, while with proper torch and technique it should never experience over 500degF. Most lead-free plumbing solders flow nicely at only 420-440degF. (IIRC, threaded valves have the same seals, because both are rated to carry steam up to 600degF...?... seems problematic in practice, i.e. steam melting the solder... =:O )

Being a full 7" away, even on L or K weight 1" copper, your old threaded valve should be fine, but in the future, as noted by others, you should wrap a wet rag around the end of valve near torch, just to be safe. This will also protect the sweated nipple/fitting which valve is screwed to.

I doubt your valve "loosened" due to the seal melting. More likely the heat caused some old packing grease to reflow, or if your new joint is upstream, perhaps some flux migrated into valve, and lubed it up...? Even more likely is that valve just hadn't been turned AT ALL for a LONG TIME, and your recent cycling of it broke free some binding mineral deposits.

If the ball valve still shuts off flow completely, leave it be. Next time, wet rag.

"The spherical seal in a sweat-fitted ball valve is good to about 650degF ..."

Only if it is TFE or PTFE.

Nylon is used in some lower cost valves (and some very expensive higher pressure valves) but is only good to around 200 F.

I see Chinese import valves with nylon seals for $3-4 less than TFE or PTFE at big box stores once in a while.

I looked up the particular valve I used and it is this one (the 1" size).

Looks like it is made with Teflon and actually even has a working temperature of up to 300-400F. It is highly unlikely that I heated this valve to 400F by sweating a joint 7" away. IÂm not an expert at pipe soldering, I may have done two dozen total sweats in my life. I am however careful to apply the solder soon after it gets to the melting point. I apply the flame and test every 5" to see if the solder melts. If it melts I apply the flame for another 5" and then apply the solder all around. I even had this valve wrapped in a wet cloth throughout the soldering process. Perhaps, as suggested, what got loose is some of the lubricant used in the valve? And thatÂs why it now feels much easier to turn.

In any case, I donÂt mean to make a big production out of this. My concern is not this particular valve, since it is not leaking and works fine. My concern is that if valves in general are that sensitive, IÂd never be able to sweat a valve and need to revise my soldering technique!

Most ball valves recommend that you leave them open whilst soldering them, so if you're soldering nearby I'd suspect the same rule applies.

There is a heat-protection gel you can spray onto fittings you want to keep cool but I suspect a wet rag works well too.

"It is highly unlikely that I heated this valve to 400F by sweating a joint 7" away."

Do not bet on it.
Copper is one of the best heat conductors (behind only diamond & silver).

Heating to the melting point of solder (and you are actually a little higher for good flow and capillary attraction) will result in the heat spreading even after the torch is removed.

By keeping the valve open the seal is held in position between the ball and the enclosure to minimize chances of flowing.

It also avoids heating the air in a closed space and having it push the molten solder out of the joint leaving a leak behind.