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?

Effect is typically used as a noun.  Closing the valve(Instrument Manifolds) does not have an effect on the system.
Affect is typically used as a verb.  Closing the valve does not affect the system.
Effect is normally not used as a verb except in b.s. speak, like politician's talk about effecting change.

I think affect is correct, although it is muddied by the structure of the sentence.

It is not really clear if affect is to be associated with "closing the valve" or "add water".

Consider also:

What is the purpose of adding water if not to affect the system in some way?

I'm sure the sentence could be restructured, or broken into two sentences to improve its clarity, but without knowing more about the context I can't suggest how.

Just because you can use both verbs in rather different sentences with similar meanings doesn't mean they are synonymous.  If you have the same sentence with just that word changed, then you've demonstrated synonymity.

"Affect" as a verb does not mean "initiate".  "Effect" more or less does.

MORE NEWS