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Knife Gate Dampers

Flowseal Knife Gate Dampers are designed and fabricated to meet the requirements of power and industrial plants and comply fully with the requirements of AMCA publication 850-84. They provide effective and reliable isolation in hot, noxious, corrosive and dirty gas flow conditions.

All Flowseal Knife Gate Dampers utilize a rugged and heavy duty screw jack drive system to provide a positive drive to the knife gate blade in both directions. The reliability of this system has been amply proved through years of use.

Flowseal Knife Gate Dampers are designed so that all parts are field replaceable without removing the frame from the duct. Maintenance is simple and can be performed whilst the system is on line.

The following styles of Knife Gate dampers are available:

Type RS Rising Spindle Knife Gate Dampers?

The rising spindle arrangement is ideally suited to applications where the operating medium is aggressive, ie., hot, corrosive or abrasive, in that the screw thread and nut is kept outside the bonnet in atmospheric conditions. The disadvantage of this system is the height required above the duct to accommodate the external frame and spindle when in the open position.

Type RN Rising Nut Knife Gate Guillotine Dampers?

In this arrangement the nut and screwed portion of the spindle are located inside the bonnet and is only suitable for clean air applications at relatively low operating temperatures. It is a compact and cost effective solution in the correct conditions.

Seal Variations

Step Seat – sealing efficiency > 98% of area?

A non machined plate knife blade seals against a landing bar welded to the inside of the casing on the downstream side of the blade.

Wedge Seat – sealing efficiency > 99.8% of normal flow?

A machined plate knife blade seats onto a machined landing bar welded to the inside of the casing. The seal is further enhanced by a series of adjustable wedges arranged around the periphery of the blade which force the knife blade onto the seat when in the closed position.

Purge Cavity – total isolation?

As per the wedge seat arrangement, but with the addition of two seal rings in the landing bar. The cavity between these two seal rings is pressurised with air or inert gas at a pressure higher than the maximum operating pressure to create a total isolation of process gas across the blade.

General Design

Flowseal Knife Gate dampers can be fabricated for any duct size, circular or rectangular and can be fitted to a vertical duct or to a horizontal duct.The dampers are designed to fulfill specific application requirements and conform to the following minimum specifications.


Materials of construction are selected for their suitability for the conditions of temperature and pressure, and the corrosive and abrasive environment of the system.

Frames are designed to be entirely self supporting in the duct with no external mounting or anchoring required and are normally fabricated from carbon steel channel sections. The flanges are pre-drilled to customer requirements.

The frames are designed for the system pressure and temperature as well as construction and transportation loadings.

The gates are fabricated from a single thickness solid plate in Corten A, 3CR12 or other exotic material as may be required. The thickness of the gate is designed so that deflection under full differential pressure, when closed, is less than 1/360th of the duct width. The knife gate is  retracted into the bonnet (at a fairly similar temperature to the operating temperature) when in the open position, thus minimising thermal distortion of the knife gate blade.

Spindle / shaft entry seals?

Sealing to atmosphere at the spindle/shaft entry port is accomplished by gland stuffing box packed with packing that is compatible with the operating conditions.

Spindles / Shafts?

These will always be a minimum of grade 304 st st and will be designed so that the slenderness ratio does not exceed r/180.


Flowseal Knife Gate Dampers can be furnished with electric, hydraulic or manual actuators. The actuators are sized to function reliably over the full range of system temperatures and pressures with a minimum factor of safety of two. Actuators may be fitted with manual overrides and fail safe open or fail safe closed devices if required. All actuators are fitted at the factory and fully tested prior to shipment.