Valve and Airlock Selection for Plastics Processing

Pellets are tough and the equipment that handles them is not the problem. What is a problem is the gap. A pellet caught between a rotor tip and the housing is sheared, melted by friction and drawn out into a thread, and those threads build into the mats that plastics plants call streamers or angel hair. They collect at bends, at screens and at the throat of the extruder, and they block a line that was conveying perfectly a week earlier.

Static is the second issue and it is intrinsic. Pellets moving through a conveying line charge heavily, the charge accumulates on anything isolated, and a valve assembly with an ungrounded actuator bracket or a plastic sight glass becomes the discharge point.

Regrind changes the picture again. It is angular, inconsistent in size, often carries film, dust and the occasional metal fragment, and it jams clearances that virgin pellets pass without touching.

A valve arrangement that moves the material without generating streamers, with shear surfaces avoided where the duty allows and minimised where it does not.

A fully bonded and grounded assembly, with the grounding treated as a tested item rather than as a wire that was once attached.

Clearances and openings sized for the worst material in the plan, which for most plants means the regrind rather than the virgin pellet.

A power or energy plant sets the same clearance differently, because a valve adjusted cold binds once the line reaches operating temperature.

The first question is whether an airlock is needed at all. Under a silo or a bin discharging to atmosphere, a slide gate does the job with no rotating clearance and no shear surface. A rotary valve belongs where there is a real pressure difference to hold, typically feeding a dilute phase line or sitting under a filter.

Where a rotary valve is unavoidable in pellet service, the geometry is what reduces damage. A larger valve turning slowly, bevelled or adjustable tips and a reduced pocket count all lower the number of pellets that meet the clearance. Some plants accept a leakier valve for exactly this reason and make the conveying system carry the difference.

Grounding is then designed as a system. Every flange, the actuator, the drive and the frame are bonded, the continuity is measured at commissioning, and it is measured again, because a bonding strap that corrodes is indistinguishable from one that works until something ignites.

Regrind service is specified separately. Larger openings, more clearance, a magnet or metal detector ahead of the valve, and the acceptance that the valve will be inspected more often than the virgin line.

• Slide gate preferred wherever no pressure boundary is required.
• Rotary valve geometry chosen to reduce shear: larger, slower, bevelled tips, fewer pockets.
• Leakage penalty of a gentle rotor absorbed in the conveying design.
• Full bonding and grounding of the assembly, measured at commissioning and periodically after.
• Metal separation ahead of valves in regrind service.
• Clearances and openings sized for regrind, not for virgin pellets.

• Material form: pellets, powder, flake or regrind, each with a different failure mode at the same clearance.
• Need for a pressure boundary, which is the only reason to accept a rotating clearance in pellet service.
• Streamer sensitivity of the downstream process, since the cost of a blockage sets how much shear is tolerable.
• Static behaviour of the polymer and the grounding concept for the whole assembly.
• Explosibility of the fines and dust, which for many polymers is real even when the pellets are not a hazard.
• Contamination tolerance, which decides whether a wear part may shed into the product.