Gate Valves Β· Knife Gate Valves
Unidirectional Knife Gate Valves
Unidirectional knife gate valves seal on one side only: a thin blade gate pushes into an elastomer seal (polyurethane, EPDM, NBR or Viton on the benchmark page) sized for flow in one direction. Built to MSS SP-81, the benchmark execution covers DN50 to DN450 in wafer and lug ends at PN10 / Class 150 duty, with carbon steel, ductile iron or SS316 bodies, SS304/SS316 blades and PTFE packing - the standard isolation valve for pulp, slurry and dry-solids lines.
Part of the Knife Gate Valves family
Last updated: 2026-11
Specifications
| size range | DN50 to DN450 |
|---|---|
| pressure class | Class 150, Class 30, PN10 |
| body materials | Carbon Steel, Ductile Iron, SS316 |
| disc materials | SS304 (knife), SS316 (knife) |
| seat materials | Polyurethane, EPDM, NBR, VITON |
| end connections | Wafer Type, Lug Type |
| operation | Handwheel, Electric Actuated, Pneumatic Actuated |
| design standards | MSS SP-81 |
| testing standards | API 598 |
| face to face | MSS SP-81 |
| trim materials | SS304 (disc), SS316L (disc) |
| packing materials | PTFE |
| flange standards | DIN, JIS |
Pressure class is shown as collected: "Class 150, Class 30, PN10". The source text's "Class 30" is retained verbatim as a recorded source difference β it is commonly read as a typographical variant of Class 300, but that reading is not asserted here; the governing class is confirmed at quotation.
Application & selection guidance
Use for one-direction isolation of pulps, sludges, dry solids and other media a full-bore wedge would trap: the thin blade cuts through the column and seats into the elastomer seal. Direction matters - install with flow against the sealed side, because the unsealed side does not shut off. Choose the body (carbon steel for general duty, SS316 for corrosive media) and the seal compound against the medium; wafer ends save weight, lug ends allow downstream dismantling. This is an isolation valve, not a throttle valve.
Frequently asked questions
Unidirectional vs bidirectional knife gate valve - what changes in practice?
A unidirectional knife gate seals on one face only and must be installed with the flow against the sealing side; a bidirectional execution seals either way and suits lines where flow reverses or the installation orientation could be mistaken. Unidirectional valves are simpler and more economical; the cost is a mandatory correct orientation - verify the flow direction on the piping drawing before ordering.
Why does MSS SP-81 appear on the specification alongside API 598?
MSS SP-81 is the industry standard for bonnetless knife gate valves - it defines the thin wafer and lug body envelope, the face-to-face dimensions and the construction for this valve type. API 598 on the benchmark page is the inspection and test standard applied to it. The two are complementary: one defines the product, the other how it is tested, so both appearing on a knife gate specification is normal.
What information is needed to quote a unidirectional knife gate valve?
Nominal size (DN50 to DN450 on the benchmark page), pressure (PN10 / Class 150 on the benchmark page), end type (wafer or lug), body material (carbon steel, ductile iron or SS316), blade and seal compound against the medium, packing, and the operation (handwheel, electric or pneumatic actuated). Describe the medium precisely - solids content and temperature decide the seal compound.
Ordering & documentation
Documents
Ask for the documentation set you need with the inquiry β typical items for this execution (material certificates, test reports to the listed inspection standard, dimensional drawings) are confirmed and supplied with the quotation.
Quotation
A quotation-ready inquiry states: nominal size and pressure rating, body and seat material preference, end connection, operation, and the service medium with temperature. Typical values on this page are benchmark reference ranges; the binding values are confirmed at quotation.
Sample & supply
Supply terms, order quantities and lead time are agreed per order β state the destination country and required schedule with the inquiry.
Storage & shipping
Store valves with seats and seals protected from UV, ozone and deformation, keep end-face protectors on until installation, and for elastomer- or PTFE-seated executions avoid prolonged outdoor storage β seat compounds age fastest in direct sunlight and freezing conditions.
Knowledge: selection guides
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