What Is a Knife Gate Valve?

Quick Answer

A knife gate valve uses a sharp-edged blade (gate) that slices through heavy slurries, pulp, and viscous media to achieve shut-off. The thin blade shears through solids that would jam a conventional gate valve. Knife gate valves are standard in mining, wastewater, pulp and paper, and power plant ash handling. Governed by MSS SP-81.

Key Specifications

Feature	Details
Function	On/off isolation in slurry and solids-laden service
Gate type	Thin, sharp-edged stainless steel blade
Sealing	Unidirectional (standard) or bidirectional
Body styles	Wafer (lug-type), full-body flanged
Standard	MSS SP-81
Pressure rating	Class 150 (typical); up to Class 300 for heavy-duty designs
Sizes	2” to 96”
Body materials	Carbon steel, 304/316 SS, ductile iron, Hastelloy
Seat materials	Resilient (EPDM, NBR, polyurethane) or metal
Connection	Flanged (ASME B16.5), wafer between flanges
Actuation	Handwheel, bevel gear, pneumatic, hydraulic

How a Knife Gate Valve Works

The blade travels vertically through elastomer or metal seats. When the valve closes, the sharpened bottom edge of the gate cuts through any solids in the flow stream and seats against the bottom of the body. The thin profile (compared to a wedge gate) minimizes the force needed to push through thick media.

Most knife gate valves are unidirectional: the upstream side of the gate presses against a resilient seat to seal, while the downstream side is open to the flow path. Bidirectional designs use seats on both sides of the gate but are more expensive and limited in pressure rating.

Knife Gate vs Wedge Gate Valve

Parameter	Knife Gate Valve	Wedge Gate Valve
Gate design	Thin blade with sharp edge	Thick wedge (solid, flexible, or split)
Slurry handling	Excellent (cuts through solids)	Poor (solids jam between body and wedge)
Pressure rating	Class 150 (typically)	Class 150-2500
Bubble-tight seal	Good (resilient seat)	Good (metal-to-metal wedge)
Face-to-face	Short (compact)	Long (per ASME B16.10)
Body cavity	Minimal (no pocket for solids)	Large cavity traps solids
Standards	MSS SP-81	API 600, API 6D
Typical service	Mining slurry, pulp, wastewater	Clean process, pipeline

Typical Applications

Mining: tailings, slurry pipelines, cyclone feed and underflow. The blade cuts through abrasive mineral slurries that would destroy conventional valve seats.

Pulp and paper: stock flow, white water, black liquor. The knife edge shears through fibrous pulp that would bind in a wedge gate or ball valve.

Wastewater: raw sewage, sludge, and grit. Knife gates handle the stringy, fibrous solids common in municipal and industrial wastewater systems.

Power generation: fly ash, bottom ash, and limestone slurry in FGD (flue gas desulfurization) systems.

Pro Tip

For abrasive slurry service, specify a replaceable seat design so the elastomer seats can be changed without removing the valve from the line. Polyurethane seats last 3-5 times longer than EPDM in highly abrasive applications.

Read the full guide to valve types