Knife Gate Valves

Full-bore, sharp-blade isolation valves for slurries, digestate, sludge, pulp and fibrous media. Essential in biogas plants, wastewater treatment and the process industry.

How knife gate valves work

A knife gate valve achieves shut-off using a thin, flat stainless steel gate (blade) that slides vertically across the pipe bore, cutting through solids and fibres in the fluid stream as it closes. The sharp lower edge of the blade acts like a knife, slicing through fibrous biomass, sludge or slurries that would jam a conventional gate valve. When fully open, the gate retracts completely behind the packing gland, leaving the pipe bore unobstructed (full-bore) and allowing free passage of solids-laden flow.

Sealing is achieved by a resilient elastomeric seat (EPDM, NBR or polyurethane) moulded or inserted into the valve body, against which the bottom edge and sides of the blade press when closed. Some designs use an external compression-type packing (stuffing box) around the gate to handle leakage past the gate in the vertical direction — especially for high-solids slurries under pressure.

Knife gate valves are available in DN 50 to DN 600 and are offered in wafer pattern (the most compact, sandwiched between flanges), lug pattern (allowing end-of-line installation) and flanged pattern. Roffia specialises in knife gate valves for biogas and biomethane plants, wastewater treatment, and industrial process applications handling difficult media.

Knife gate valves — Roffia srl

Technical specifications

  • Size range: DN 50 to DN 600 standard; DN 700–1200 on request
  • Pressure rating: PN 6–16 standard; PN 25 for special orders
  • Body: ductile iron GGG40, AISI 304/316, carbon steel
  • Blade: AISI 304, 316, 416 (hardened); hardfaced on request
  • Seat: EPDM, NBR, NBR+fabric, polyurethane, PTFE
  • End connections: wafer, lug, flanged (EN 1092-1 / ANSI B16.5)
  • Actuation: manual handwheel, gear, pneumatic, electric, hydraulic

Industry applications

  • Biogas / biomethane plants — digestate, substrate feed, biogas lines
  • Wastewater treatment — raw sewage, activated sludge, biosolids
  • Pulp and paper — thick stock, broke, effluent
  • Mining and minerals — tailings slurry, concentrate, flotation
  • Power generation — fly ash slurry, bottom ash
  • Food processing — pomace, fibrous waste streams
  • Chemical / pharmaceutical — catalyst slurries, pigments

Knife gate valves in biogas plants

In biogas and biomethane plants, knife gate valves are among the most critical components. They are used on substrate feed lines (where raw biomass — maize silage, organic food waste, animal slurries — enters the process), on digestate discharge lines (isolation of the digester for maintenance), on inter-tank transfer lines and on effluent treatment stages where solids concentrations are high. The biogas sector demands valves that can achieve adequate shut-off against methane-rich biogas (preventing fugitive GHG emissions) while handling fibrous and abrasive slurries. Roffia supplies specialist knife gate valves with ATEX-rated pneumatic actuators and spring-return fail-closed configurations for safety shut-off on biogas lines.

Read more: key components for biogas plant piping →

Frequently asked questions

What are knife gate valves used for?
Knife gate valves are designed for service with slurries, viscous fluids, solids-laden flows and fibrous media — applications where a conventional gate valve's seat would become fouled or jammed. The thin, sharp-edged stainless steel blade slices through solids and fibres as it closes, giving a clean shut-off. Typical applications include: wastewater and sludge lines in treatment plants; digestate and biogas plant feed lines and effluent handling; pulp and paper industry (thick stock, broke, effluent); mining and minerals (tailings, concentrate slurries); food processing waste streams; and fly ash and dust conveying in power plants and cement works. They are also used as isolation valves in the biogas and biomethane process where the handling of organic solids and fibrous biomass demands a valve that will not plug on closure.
What materials are knife gate valves made from?
The body of a knife gate valve is most commonly ductile iron GGG40 with internal rubber or polyurethane lining for corrosion and abrasion resistance, or stainless steel AISI 304 or 316 for corrosive or hygienic media. Carbon steel bodies are used in dry solids handling (ash, powder). The gate (blade) is almost invariably stainless steel AISI 304, 316 or 410 — hardened for abrasive service. Seat/seal materials include: EPDM (water, neutral fluids, food waste), NBR (oil-contaminated slurries), polyurethane (PU) (highly abrasive slurries), PTFE (chemical service). For extremely abrasive media, hardfaced blades in Stellite 6 or with tungsten carbide coating extend service life significantly. Roffia can supply knife gate valves in full stainless steel (body + blade) for corrosive chemical slurries on request.
Are knife gate valves bidirectional? How is sealing achieved?
Knife gate valves are available in both unidirectional and bidirectional sealing configurations. Traditional single-acting designs seal against pressure from one defined upstream direction — the flow direction must be observed at installation. Modern bidirectional knife gate valves use a U-ring elastomeric seat or double-seat design that provides shut-off regardless of pressure direction, making them suitable for systems where flow can reverse or where the valve is used at both upstream and downstream positions. Leakage class for knife gate valves is generally Class A to EN 12266 (soft-seated types), which is sufficient for most industrial and slurry applications. For critical shut-off in biogas plants where fugitive methane emissions must be minimised, specific low-leakage verified models should be selected.
What actuators are used with knife gate valves?
The choice of actuator depends on the required operation mode and the available energy. Manual handwheel or gear-operated: suitable for infrequently operated valves and smaller sizes (DN 50–200). A gear operator is normally required for DN 150+ to reduce operating effort when working against slurry pressure. Pneumatic cylinders (double-acting or spring-return): the most common choice for automated knife gate valves in biogas plants, water treatment and process industries. Fast-acting, reliable, and suitable for fail-safe applications (spring-return closes on air loss). Electric actuators: preferred where compressed air is not available, for modulating control or for integration with SCADA/PLC systems. Hydraulic actuators: used in very large sizes (DN 400+) or extreme torque requirements. Roffia supplies complete valve-actuator assemblies, pre-tested, with limit switches, solenoid valve and positioner as required.

Need knife gate valves?

Specify the fluid (slurry, digestate, sludge, etc.), solids content, DN, PN, body material, seat material and actuation. We will recommend the correct model and confirm availability.

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