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Iris Valve Working Principle: Dosing and Choke Control for Powder

Created on 07.10
An iris valve takes its name from the mechanism in a camera lens. Several overlapping blades close towards a central circular aperture, so the opening shrinks evenly from every direction at once. In a powder line, that geometry solves problems that other valve types cannot.

What an Iris Valve Is

An iris valve is a bulk solids valve in which a ring of interlocking blades closes to form a variable circular opening. At full open the aperture is essentially the same diameter as the line, so material passes without obstruction. As the blades close, the aperture contracts concentrically.

How the Iris Mechanism Closes

Each blade is pivoted at its outer end and overlaps its neighbour. Rotating an outer ring drives all blades inward together. Because every blade moves simultaneously and radially, the closure is symmetrical — there is no single edge sweeping across the flow, and no ledge where material can accumulate.

Full-Bore Opening: Why It Matters for Bridging

Bridging — the formation of a stable arch of material across an opening — is the most common cause of discharge problems in silos and hoppers. It happens when the opening presents a discontinuity or a narrowing that lets particles lock together. A full-bore iris valve presents a clean circular bore when open, and its concentric closing action means it can also break an incipient arch as it moves.

Dosing and Choke Control

Because the aperture can be set anywhere between fully open and fully closed, an iris valve is a natural choke and dosing element. This is the basis of the AD series iris valve, designed for powder dosing and choke control — it lets a plant set a repeatable flow area rather than simply opening or closing.

Manual or Pneumatic

Smaller iris valves are often operated manually, which is entirely adequate for setting a fixed choke position on a hopper outlet. Where the valve must cycle or be adjusted remotely, a pneumatic actuator is used instead — see the KD series iris valve for the manual and pneumatic options.

The AD, IR and KD Series

Where an Iris Valve Beats Other Designs

Requirement
Iris valve
Butterfly
Pinch
Full-bore opening
Yes
No, disc in flow
Yes
Variable choke control
Yes, concentric
Yes, but erodes
Yes, by air pressure
Handles dry powder without bridging
Strong
Moderate
Strong
Abrasive slurry
Moderate
Poor
Strong
Food and pharmaceutical
With stainless build
With correct seat
With EPDM sleeve

Selection Notes

  1. Confirm the outlet diameter the hopper needs — the iris aperture must exceed the critical arching dimension for your material.
  2. Decide whether the valve sets a fixed choke position or must cycle; this determines manual versus pneumatic.
  3. Check the material's abrasiveness; heavy duty duty calls for the IR series rather than a standard build.
  4. For food or pharmaceutical product, specify stainless and an appropriate cleanable design.
Read the bulk solids valve selection guide for how iris valves compare with pinch and butterfly designs across a plant, or browse the full product list. Tell us your hopper outlet size and material and we will size the valve.
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