What is a Knife Gate Valve?

Conventional Knife Gate Valve

As mentioned before the Conventional Knife Gate Valve was created in 1927 in Sweden for the Pulp and Paper industry, when they needed an inexpensive option for pulp stock applications. This valve worked adequately at the time for the specific application but is now seen as outdated.

Created in: 1927

Design: The key design feature with this first iteration are the wedges located typically at 5 and 7 o’clock at the bottom of the port. The wedges push the gate onto the seat when the valve is fully closed.

Capabilities/Where it works: Low-pressure, unidirectional general-purpose isolation where through leakage is not a concern

Why it Fails: The wedges on the valve are only located on the bottom of the port, allowing for a high amount of through leakage and depending on the size of the valve, especially at the top of the seat.

Industrial Process Industries: Pulp and Paper

Average Allowable Leakage: 40 cc per inch of diameter per minute

Other Notes: It is important to note that the average allowable leakage is based on a differential pressure of 40psi. Any lower pressure differential will permit for even more leakage from this type of valve.

Through Gate Valve

The first big change to the conventional knife gate resulted in the Through Gate design (also know as O-Ports), which was able to accommodate higher consistencies of fiber in water, but had issues with through leakage and consistently released large amounts of fugitive emissions.

Created in: 1950s

Design: This iteration removed the wedges used in the Conventional Knife Gate and replaced them with two body seats and top and bottom packing glands. These seats seal the valve when fully closed, and the packing glands seal the valve at all positions. This valve has high through leakage and fugitive emissions.

Capabilities/Where it works: This valve typically works well in high fiber applications where the fiber can actually become a seat after dewaters.

There has also been a success in mineral processing plants, with high scale because the scale actually helps the valve seal completely.

Why it Fails: This valve should not be used in most industrial applications where high fugitive emissions and through leakage is a concern. The packing glands on this valve add a large amount of weight and size to the valve (and cost), while still allowing for a considerable amount of emissions.

Industrial Process Industries: Pulp and Paper

Average Allowable Leakage: 40 cc per inch of diameter per minute

Lined Gate Valve

The Lined Gate was the first design to consider the effects of corrosive applications.

Created in: 1960

Design: The design features a protective corrosion resistant body liner that offered a cheaper, chemical resistant option for corrosive applications.

Capabilities/Where it works:  This valve is a low-cost solution that protects a non-corrosion resistant valve body with a corrosion-resistant liner.

Why it Fails: The manufacturers of this valve only used corrosion-resistant materials on the liner and gate. When the liner fails or when the packing leaks the corrosive material, the body, and eventually the valve, fail. Once the valve starts leaking from the packing (even if just the average allowable leakage) the corrosive materials damage the pressure retaining body of the valve, thus resulting in more leakage and overall valve failure.

Industrial Process Industries: Mining and Mineral Processing, Pulp and Paper

Average Allowable Leakage: SP-81 or Zero Leakage

Push-Through Gate Valve

The Push-Through design emerged, enabling knife gates to handle high percentage solid slurries in mining.

Created in: 1960

Design: This design uses a sharply edged gate to separate the “sleeves” spreading them apart. Each stroke has a discharge which can increase with every cycle.

Capabilities/Where it works: Works well in low-pressure, low-cycle, neutral pH slurries.

Why it Fails: On each stroke, the elastomer sleeves loose elasticity, even if just a tiny bit. Eventually, the contact between the two sleeves is insufficient to hold back the internal pressure and the valve starts to free discharge while open.

Industrial Process Industries: Mining and Mineral Processing

Average Allowable Leakage: Zero-Leakage, but allowable discharge.

Guided Shear Gate Valve

A true zero leakage bi-directional higher-pressure design, known as the Guided-Shear Gate, featured the first true “knife” gate able to handle clean, clear or heavy slurry in high alkaline or acidic solutions.

Created in: 1960s

Design: Axial seated design, two-piece body, full flanged, fully rated to ASME B16.34 in classes 150 to 600 (PN 20 to 100), bidirectional, zero-leakage suitable for clean fluids to high percentage solids slurry.

As per MSS SP-135

Capabilities/Where it works: Designed for slurry, but is capable of handling corrosive fluids (acids, seawater, etc.)

Why it Fails: The only weak point of this design is the elastomer seat, which limits it to below 260⁰C or 500⁰F. This is because the cycling of the valve directs process fluid at increasing velocity at the seat.

Industrial Process Industries:

Severe Service Knife Gate Valve (SSKGV)

Today, the Severe Service Knife Gate Valve (SSKGV) leads the way with the most technologically advanced design on the market today. The SSKGV completely removes the chance for fugitive emissions, surpasses the rigorous standards for Severe Service defined by the MSS, and provides bi-directional zero leakage from full vacuum to ASME Class 1500/PN 250 pressures (3750 psig/250barg).

Created in: 2017 (Generation V of Guided Shear Gate)

Design: The SSKGV utilizes a Guided Shear Gate Design that consists of three key elements: a mechanically retained axial seat, a shearing gate tip and internal gate guides. From open to close the gate is guided the entire way removing any option to deflect when solids are encountered. When the material is struck by the shearing tip, it is cut, fractured, and pushed away downstream inside the pipeline (no discharge to atmosphere). These features enable zero leakage, zero emissions, and zero discharge where the gate meets the primary seal. A secondary seal backs up the primary seal for extra peace of mind. Transverse seals coupled with the primary seal prevent fugitive emissions.

Capabilities/Where it works: Severe Service Knife Gate Valves are designed to handle the harshest of conditions in a variety of industries. That however doesn’t mean they shouldn’t be considered for more general applications as well.

Why it Fails: Though unlikely the primary seal is subject to the erosive and possibly abrasive wear that occurs when the valve is closing against flowing media. Though, there are cases where the DSS SSKGV has cycled over 25,000 times with zero leakage.

Industrial Process Industries: