Knife gate valve

Product Overview

The Hard Seal Knife Gate Valve is a knife‑edge gate valve with both the seat and gate sealing faces made of metal (hardfaced with Stellite or other hard alloys). It is designed only for ON‑OFF (open/close) service – not for flow regulation. Compared with soft‑seated (rubber, PTFE) knife gate valves, the hard seal version offers high temperature resistance, high wear resistance, erosion resistance, and long service life.

It is particularly suitable for high‑temperature media, slurries containing hard particles, and other severe services requiring metal‑to‑metal sealing. The sharp knife‑edge gate cuts through fibers, particles, and other materials during closure, preventing clogging. The body flow path has no recesses to avoid material buildup. The valve has flanged connections, with nominal diameters from DN50 to DN800, pressure ratings PN10/PN16 (Class 150) – higher ratings available on request – and body materials including carbon steel and stainless steel. Actuators can be manual, electric, or pneumatic.

Product Structure

The Hard Seal Knife Gate Valve mainly consists of the actuator (ON‑OFF type) and the hard‑seated valve body assembly.

Actuator (ON‑OFF Type):

1. Available in manual (handwheel), electric, or pneumatic.

2. Electric actuator: motor, gearbox, travel limit switches, torque protection, handwheel.

3. Pneumatic actuator: cylinder, solenoid valve, limit switches.

4. All actuators are ON‑OFF type – not for modulation.

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Hard‑Seated Valve Body Assembly:

1. Body: Cast or fabricated body in carbon steel (WCB), stainless steel (CF8/CF8M), duplex, etc. Flanged ends conforming to GB/T 9113, ANSI B16.5, EN 1092‑1, etc. Full‑bore flow path with no recesses.

2. Gate: Stainless steel (304, 316, 310S) knife‑edge gate with a sharp edge. The sealing face (contact area with the seat) is hardfaced with Stellite 6, Stellite 21, or other hard alloy, then ground to provide high hardness and excellent wear resistance.

3. Seat: One‑piece or removable metal seat with a hardfaced (Stellite) sealing surface, lapped with the gate to achieve a metal‑to‑metal seal. Bidirectional sealing construction is optional.

4. Stem: Stainless steel trapezoidal‑thread stem, moving the gate up and down.

5. Packing Box: Contains PTFE or flexible graphite packing, compressed by a packing gland to prevent external leakage. High‑temperature graphite packing is optional.

6. Yoke: Connects the body to the actuator.

7. Optional Bonnet: Available with an enclosed bonnet (for hazardous media) or open‑frame (bonnetless) construction.

The sealing pair of the Hard Seal Knife Gate Valve is metal‑to‑metal, with no soft materials such as rubber or PTFE, providing high temperature and wear resistance.

 Function

1. Metal Hard Seal: The seat and gate sealing faces are hardfaced with Stellite or equivalent, offering high hardness (HRC 35–45), wear resistance, and erosion resistance – ideal for highly abrasive media containing hard particles.

2. High Temperature Capability: The metal seal withstands temperatures up to 550°C, making it suitable for high‑temperature slurries, hot ash slag, hot dust, and similar media.

3. Knife‑Edge Cutting Function: The sharp edge of the gate cuts through hanging fibers, paper, plastic strips, and other materials during closure, preventing clogging.

4. Self‑Cleaning Flow Path: The body flow path has no recesses. When fully open, the media can flush the interior, preventing solids accumulation.

5. Bidirectional Sealing (optional): A bidirectional metal sealing design allows the valve to withstand media pressure from either direction.

6. Full‑Bore Low Flow Resistance: When fully open, the gate is completely out of the flow path, providing minimal pressure loss.

7. Remote ON‑OFF Control: With electric or pneumatic actuators, remote automatic open/close operation is possible.

8. Manual Emergency Operation: Electric/pneumatic actuators are equipped with a handwheel for manual operation during power or air supply failure.

Working Conditions

1. Applicable Media: High‑temperature slurry, coal slurry, ash slag, hot dust, tar, slurries containing hard particles, corrosive slurries (stainless steel body required), etc.

2. Temperature Range: –29°C to +550°C (depending on hardfacing material).

3. Pressure Rating: PN10, PN16 (Class 150); higher ratings available on request.

4. Nominal Diameter: DN50 – DN800 (2″ – 32″); larger sizes available on request.

5. Installation Orientation: Vertical stem‑up recommended.

6. Not Suitable For: Applications requiring zero‑leakage soft sealing (metal seal leakage is higher than soft seal, typically ANSI Class IV–V).

7. Typical Applications:

   1. High‑temperature ash and slag discharge in coal‑fired power plants

   2. High‑temperature slurry transport in metallurgy

   3. Tar and asphalt lines in coking plants

   4. High‑temperature catalyst slurry shut‑off

   5. Fly ash discharge in waste‑to‑energy plants

   6. High‑temperature raw material slurries in glass and ceramic industries

   7. High‑temperature reactor discharge in chemical plants

Summary

The Hard Seal Knife Gate Valve, with its advantages of metal‑to‑metal hard sealing, high temperature resistance, high wear resistance, and knife‑edge cutting function, is an ideal shut‑off valve for high‑temperature, highly abrasive, and hard‑particle‑laden media. Compared with soft‑seated knife gate valves, the hard seal version operates continuously at temperatures up to 550°C, resists erosion, and offers a long service life, making it suitable for ash slag, high‑temperature slurry, tar, and other severe services.

However, note that the leakage rate of a metal seal is higher than that of a soft seal; it is not suitable for applications requiring absolute zero leakage. With flanged connections, a full‑bore flow path, and optional manual or automated actuators, the Hard Seal Knife Gate Valve is a reliable and durable solution for automated ON‑OFF pipeline systems handling high‑temperature, hard‑particle‑laden media where zero‑leakage soft sealing is not required.