I've started to recognize the difference in effect and affect but in this case I am stumped:

Closing the valve allows the operator to add water without affecting the operation of the system.

Is this correct? 

I was thinking that the operator would need to affect the operation before it would be effected.  Does this make sense?

Generally, "Affect" is a verb (i.e. ball valve"Crops are affected by the weather.").  "Effect" is a noun (i.e. "Hall Effect").

As a verb, "effect" means to implement (like a plan).

Both words can act as either a verb or a noun - although only one form of each is common, and the other has a more or less specialist meaning.

Affect (Verb) - Commonly used - To influence or alter something.

Effect (Noun) - Commonly used - The change which results from something else.

....so if A affects B to cause C, C is A's effect upon B.

But there are also:

Effect (Verb) - Used in general English, though less commonly than the first 2 forms - To bring something about - as in "I need to effect a change in policy" (note that what I'm effecting is the change, if I'd wanted to talk about the policy, I'd "affect" it).

Affect (Noun) - Much more specialised - A medical term describing the emotional state of an individual (as in: "Flattened affect is a frequent symptom of schizophrenia".

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