High Pressure Bidirectional Sealing Knife Gate Valve

2" - 24" (DN50 - DN600)

Class 300 - 900 (up to 150 bar / 2200 psi)

ASME B16.34, manufacturer-specific bidirectional high-pressure design

A216 WCB, A182 F316, A182 F51/F53, A217 WC6/WC9

Dual metal-to-metal seats (Stellite 6 overlay on upstream and downstream seats), spring-loaded or wedge-activated seat mechanism

Two-piece bolted or one-piece body, heavy-wall construction, dual independent sealing boundaries (upstream seat + downstream seat), pressure seal bonnet above Class 600, OS&Y rising stem or pneumatic/hydraulic actuated

Two Problems That Usually Need Two Different Valves

Bidirectional sealing and high pressure rating don't normally live in the same valve. Bidirectional knife gates — the kind that seal in both flow directions — typically max out at Class 150 or 300. The elastomer seats that make bidirectional sealing affordable and practical can't handle high pressure. They deform, they extrude, they fail. High pressure knife gates exist, but most are unidirectional — the gate seals against one seat, and that seat only works when line pressure pushes the gate in the right direction.

But some process lines need both. Mineral processing circuits where the flow direction reverses during batch operations. Transfer lines that can have pressure on either side depending on which pump is running. Isolation points where you need the valve to hold tight regardless of which direction pressure comes from — and that pressure is 50 bar, 100 bar, 150 bar. That's where a high pressure bidirectional knife gate valve becomes the only practical answer.

Two Independent Seats, No Shared Weak Point

The valve has two metal seats — upstream and downstream — both with Stellite 6 overlay. Each seat seals independently. When pressure comes from upstream, the downstream seat holds. When pressure reverses, the upstream seat holds. The gate doesn't care which direction pressure pushes because both seats are spring-loaded or wedge-activated to maintain contact pressure on the gate regardless of line pressure direction.

That's the key mechanism. Spring-loaded seats press against the gate face at all times — not just when line pressure assists the seal. Line pressure reinforces the active seat, but the inactive seat still holds because its spring force keeps it engaged. Wedge-activated seats use a mechanical wedge that drives the seat against the gate when the gate reaches the closed position. Either way, both sealing boundaries are independent. If one seat is damaged, the other still seals. That's redundancy you don't get with a single-seat design.

Metal Seats Because Elastomer Can't Do This Job

At Class 900 pressures, elastomer seats are out. The pressure differential across the seat is too high — elastomer extrudes through the seat gap under that load, even with anti-extrusion rings. Stellite 6 overlay on both seats provides the hardness needed to resist abrasive slurry at high velocity and the metal-to-metal sealing capability needed to hold pressure at 150 bar. The seat surfaces are machined to tight tolerances and lapped for contact accuracy. It's not cheap, but it's what works.

For Class 300-500 configurations within the range, reinforced elastomer inserts can be specified on one or both seats if the slurry chemistry and pressure allow it. But the standard design at Class 600 and above is dual metal-to-metal. No compromise.

Heavy Body, Pressure Seal Bonnet, Serious Construction

The body wall thickness follows ASME B16.34 for the rated pressure class, plus additional thickness for abrasive slurry wear allowance. At Class 900, the body is substantially heavier than a Class 300 bidirectional knife gate at the same size — that's not optional, it's structural. Pressure seal bonnet above Class 600 eliminates the bonnet gasket as a leak path under extreme internal pressure. For Class 300-600, a bolted bonnet is standard.

The actuation choice matters at these pressures. OS&Y rising stem gives you visual position indication and manual operation with controlled seating force. Pneumatic actuation works for Class 300-500 where seating forces are manageable. Hydraulic actuation is recommended for Class 600 and above — the forces required to seat the gate against two spring-loaded metal seats at 150 bar exceed what pneumatic cylinders deliver efficiently.

A: You can, but you're paying for bidirectional capability you don't need. If flow direction is always known, a unidirectional high pressure knife gate is simpler and less expensive. The bidirectional design is for lines where pressure can come from either side.

A: The springs maintain constant contact pressure between the seat face and the gate. When line pressure is zero, the springs hold the seal. When line pressure increases, it reinforces the active seat. The inactive seat stays engaged because its spring force hasn't changed. You get a seal at zero pressure and a seal at rated pressure.

A: The other seat continues to seal independently. The damaged seat may leak, but the valve still holds pressure on the intact sealing boundary. That's the redundancy advantage of dual independent seats — you don't lose the entire sealing function if one seat surface is compromised.

A: Stellite 6 is the proven balance for slurry service. It's hard enough to resist abrasive wear at high velocity, it can be machined and lapped to sealing tolerances, and it welds reliably to the base metal. Tungsten carbide overlays are harder but more brittle, more difficult to machine to seal-contact tolerances, and more prone to cracking under thermal cycling in mining process lines.

A: Yes — two seats means two seating surfaces to inspect and service. But the maintenance procedures are straightforward: remove the bonnet, pull the gate, inspect both seat overlays, re-lap or replace as needed. The pressure seal bonnet on Class 600+ designs actually simplifies bonnet maintenance compared to a bolted bonnet at the same pressure class.