Hi, chemistry.
Sorry I don't read/write your language, but I'll try to help you in english:
- Valves have a parameter named "Capacity" or more commonly just Cv or Kv (Cv in english units, Kv for SI units).
- This parameter is a function of % opening, and can have several characteristics (linear, equal percentaje and many others).
- Gate valves have a huge Cv, just like a piece of pipe since they must be used fully opened or closed, because their function is just to block and not for control.
- Inside Hysys Pipe Segment you can add a Gate Valve 100% open as an accesory, and setting a diameter of DN 50 you can check what's the pressure drop you have at the nominal flow.
- Later you put the valve and varies its Cv at 100% opening trying to match the pressure drop calculated for the valve with the pipe segment. The characteristic of the valve would be a Quick Opening one, but I think it's not available in Hysys, so you can use a Linear one.
- Be sure to check the "use valve calculations in Steady State" or similar message.
- The same for the DN 100 valve.

After this you should have your simulation working with pressure drops calculalted with flow and %open variations.

If you want to dramatically simplify the case you can use the equivalent length approach and set pressure drop through valve as zero and add enough pipe to your system to account for it.

Good luck!