Assuming atmospheric & that you have a control valve, a vacuum breaker (of some sort) between valve and coil (or trap) and a properly sized steam trap w/ sufficient gravity drainage. Also proper drainage of your steam and condensate lines. For reference anything by Dan Holohan on steam is not only educational but entertaining as well. Spirax/Sarco has good info out there as do most steam specialty mfgrs.
If you have a coil and a valve, when the valve closes, the steam eventually condenses. As steam takes up much, much more volume than water (condensate) a vacuum is pulled between the trap and your valve unless air can get in there (the job of the vacuum breaker). This will hold the water in the coil in a method similar to putting your finger on your straw and pulling it out of your drink.
There are many locations were condensate can trap and cause a hammer, but at a coil w/ a valve it is most likely lack of vacuum breaker, trap size or improper drainage...
Hope this helps...
