Thanks for the answer. Today I tried my valve with laboratory powered supply. The result is for the opening is enough 7,5 V and even 1.5 V can hold the valve.
The valve was GSR force actuated valve. 11W 24VDC
I also tried Festo pneumatic valve (just cause a different coil).
3.3W 24VDC
The valve opened with 13.5 V and a closed below 4 V.
So I guess it will work and 7.5VDC power supply is ok:-).
Lukas