If you were to go to Search Examples>Demonstrations>Process Control>Control Mixer Process, you'll find a VI with several custom controls including a valve. All it is is a boolean with custom images. To make your own, use a drawing program like Paint and make images of your valve in the open and closed state. Place a boolean control on a VI's front panel and choose Edit Control from the Edit menu. Once in the control editor go to customize mode. Choose Import Picture From File from the Edit menu. Right click the boolean control and choose Picture Item to Replace to determine which picture to replace with the one from file (a boolean actually uses 4 pictures but you only need to replace the first two which are the true and false states). Right click again and choose either Import Picture or Import at Same Size. Repeat for the other boolean state with a different picture. You should save your changes as a .ctl so that you can easily use it again.