An iris valve takes its name from the aperture of a camera, and the resemblance is not a coincidence. Both use a ring of overlapping elements that move together to shrink a circular opening. It is an elegant mechanism, and in bulk solids handling it solves a problem that disc and gate valves handle badly.

Several blades — typically six to eight, depending on bore — are arranged around the flow opening, each pivoting on its own pin. The outer ends of the blades are engaged by a rotating ring. Turning the ring moves every blade inward simultaneously.

Because the blades move in a coordinated arc rather than a straight line, the aperture between them stays circular. The opening is large, then smaller, then a small circle, then closed. At no point does the flow area become a slot or a crescent.

Contrast this with a gate or slide valve, which closes by sliding a flat plate across the bore. At half-closure the opening is a letterbox. Powder has to negotiate two corners and a ledge, and that is exactly where arching, rat-holing and build-up begin.

An iris valve's aperture has no corners. Product flowing through sees a smooth, symmetric, converging passage. There is nowhere for material to sit.

An important consequence: the valve is not limited to open or closed. The ring can be set to any position, so the aperture can be dialled to a fixed size and left there. That makes an iris valve a genuine choke control — a way of setting a repeatable flow rate from a hopper or silo.

Because the blades close around the stream rather than pinching it, the flow is cut progressively and cleanly. Discharge stops with a clean spout rather than a smeared pinch point.

Because the blades are in the stream, they are the wear part. In light and moderately abrasive powder the wear is slow and the blades are replaceable. In heavy abrasive duty — mineral slurry, coarse clinker, sharp-edged product — a blade design will suffer, and a pinch valve with its elastomer sleeve is the better technology.

The honest guidance: iris valves are excellent for powders, granules and pellets, and for dust containment. They are not the right choice for slurry or for highly abrasive coarse material.

Three series cover most duty: the AD series iris valve, the IR series iris valve and the KD series iris valve. The AD series is the general-purpose powder choke valve — see the AD series application guide for how it is used in bulk solids lines.

Tell us the product and the aperture you need and we will recommend a series. Browse the iris valve range or read the bulk solids valve selection guide.