Although gate valves and knife gate valves share many similarities, for example, they are both used in on-off applications, there are more fundamental differences existing. 

THE WORKING PRINCIPLE

Gate valve is opened by lifting the wedge out of the path of the fluid. It is only used in for shut-off. Previously, normal gate valves are used in the pulp and paper industry, but the stringy pulp would be stuck between the wedge and seat and prevent flow shut-off. To solve this problem, knife gate valve was designed. It is with a sharp edge to cut through the pulp and keep the fluid flowing. 

APPLICATIONS

As mentioned above, gate valves are applied in many industrial applications requiring shut-off valves, such as power plants, water treatment, mining and offshore applications. They can be used in demanding environments, like high temperature and high pressure environments. Knife gate valves are preferred in slurry applications because their knives can cut right through thick liquids easily. The applications include but not limited to thick flows of heavy oils, light grease, slurry, paper pulp, varnish and waste water etc. 

ADVANTAGES AND DISADVANTAGES

The advantage of gate valves is that they are bi-directional, suitable for demanding working conditions like high pressure & high temperature. Besides, it is quite efficient that there won’t be much pressure drop when gate valves is fully open. However, the torque of gate valves will be larger compared with other types of valves, so they will need a large force to open and close. For larger sizes, pneumatic or electric actuators will be a must, and this, of course, will lead to higher cost. Also, they take longer time to operate and need more space for installation. By contrast, knife gate valves are cheap, light and easy to operate, but the design makes them only limited to low-pressure working environment.