Manual valve is a kind of equipment for control, the main function of manual valve is to control fluid, if the valve opening and closing torque is relatively large, we can set up a wheel or worm wheel between the handwheel and the valve stem as a reducer. 
There are also many types of manual valves, the most common of which are vacuum manual valves, ultra-high pressure manual valves, low temperature manual valves, high temperature manual valves and constant temperature manual valves and so on. 
No matter what kind of manual valve plays the role is to control the fluid, but the performance of the control is different from the environment.