WHAT MAKES THEM SIMILAR?
Gate and knife gate valves are primarily for on/off service. Using these types of valves to regulate flow is not recommended because when fluid pushes against a partially closed gate vibration occurs, eventually wearing and eroding the seat and disk. For that reason, these types of valves should be either completely opened, or completely closed.
These valves are also designed to open and close slowly to protect from the effects of water hammer.
Both types of valves can be found in applications that involve:
Non-flammable viscous fluids
WHAT MAKES THEM DIFFERENT?
The differences between them are vast. First off, yes, the knife gate valve DOES have a sharpened disk (thank you very much). This is meant to sever any stringent solids that may keep the valve from closing and causing it to leak. But there’s more that makes their designs quite different.
The biggest difference between these two types of valves is that gate valves are built to ANSI standards while knife gate valves adhere to TAPPI standards. Therefore, dimensionally the gate valve is wider, flanged, and ANSI pressure rated. It also has API leak tightness standards must be met. They're bi-directional, and commonly used in steam applications as well as fluid applications. The gate valve is only available with metal seats.
Another significant difference between an ANSI gate valve and a knife gate valve is in the packing gland area. A gate valve will have a v-ring packing set that seals the shaft attached to the gate, while a knife gate valve has a packing gland that seals around the gate.
A knife gate has a very thin profile compared to its bulkier counterpart. It's also uni-directional (though bi-directional is an option on some), and comes in either wafer or lugged body, no flanges. The seats on the knife gate valve come in everything from metal to resilient types.
Two of the knife gate's biggest advantages knife are weight and expense. A 16” knife gate valve will typically weigh less than 300# while an ANSI gate valve will be in excess of 1200#. The cost of the valve is similar to the weight difference as well.