Calculating Work to Close a Valve in Hydrodynamics

[Numeriprimi]

Hey!
Yesterday I was on the Czech Physics Olympiad. There were many examples. To one, however, I did not invent anything, because I don't learn Hydrodynamics to much.
I want to understand it better, so I want to ask you how you calculate it, please. I'm sorry, I did not give my progress here, but I have not it, because I do not know much about Hydrodynamics. I spent two hours thinking but I did not invent it.

So... example:

Liquefied gas flowing in a pipeline. You want to close the pipeline. To close the pipeline you must close a valve (move plate across pipeline).
- in a large reservoir, where pipeline is connected, the pressure is 2 MPa
- valve (plate) has a thickness 10 cm
- pipe has a square cross section with a side length 1 m
- liquefied gas has a density 480 kg/m3 and and flow 20 m3/s

What is the value of work to close the valve?

Thanks... And if you don't understand me, write, i will try to write it better.

 

[Simon Bridge]

Imagine the valve is half closed - what does the system look like?