Learn Polish: W przypadku awarii proszę zakręcić zawór. - In the event of a breakdown, please turn off the valve.

The base word is awaria (breakdown or failure). Feminine nouns ending in -ria take a double -ii in the genitive case, which is required here after the phrase w przypadku (in case of).

What case follows the phrase w przypadku?

It requires the genitive case. W przypadku literally means 'in the case [of]', so the following noun answers the question 'of what?', which is a main function of the genitive case.

Could I use a direct imperative here, like zakręć?

Yes, but using proszę followed by an infinitive like zakręcić is a very standard, polite way to give public instructions in Polish. It avoids having to match the gender or formality level of the specific reader.

Why use zakręcić for 'turn off' instead of wyłączyć?

Zakręcić specifically implies a twisting motion, so it is used for turning a physical knob, tap, or valve to stop a flow of water or gas. You would use wyłączyć for turning off electronics, lights, or engines.

Is zawór in the nominative case here?