A shaker screen that has lost half its capacity looks the same as one running clean — until the mud sheets off the end of the deck. There are two completely different failures hiding behind that symptom, they have opposite cures, and confusing them is one of the most common ways rigs waste screens and lose fluid.
Plugging and blinding are not the same failure
Plugging (also called pegging) is mechanical: a particle close to the size of the opening wedges into it and will not shake loose, reducing the open area available for fluid. Because it is caused by particles at or just above the aperture, it is called near-size-particle blinding. Blinding proper is different: fine, sticky or cohesive material — hydrated clay, unsheared polymer, moisture-bound fines — plasters over the surface of the mesh and crusts, sealing the openings from the top.
The cures pull in opposite directions. Plugging by near-size sand is eased by shifting the screen one API size (finer, so the near-size band no longer fits the openings, or coarser to pass them) or by improving upstream removal. Blinding by soft material is eased by washing the screen with high-pressure base fluid, correcting the vibration, or ensuring polymer is fully sheared — not by swapping mesh. Diagnose which one you have before you touch the screen.
Why near-size particles are the enemy
Screening removes particles by aperture, but the particles that sit closest to that aperture are the ones that jam it. During vibration the wires flex just enough to let a particle roughly the size of the opening enter and then lodge. In screening practice the near-size band is commonly taken as particles between about half and one-and-a-half times the aperture — the fraction of the feed sitting in that band is the fraction most likely to peg the screen.
This is why a formation drilling a narrow, near-aperture particle-size distribution blinds screens fast, while a broad distribution runs cleaner. It is also why piggybacking — fine particles hitching onto larger removed solids — lets a screen remove some solids smaller than its rated opening, and why the honest performance number is the API RP 13C D100 cut point, not the mesh count.
Reading the deck
A blinded screen washes clean and shows a sandpaper-like layer of lodged grains: that is plugging. A screen that stays coated after washing, with a plastic film over the mesh, is blinded by soft material. A screen sheeting fluid off the end while the discard looks dry is losing open area to one of the two. Each tells you a different corrective action — and none of them is 'bypass the shaker,' which simply dumps the solids you were trying to remove straight into the system.
The metric
The plugging tendency of a feed is captured by its near-size fraction — the share of the particle-size distribution sitting close to the screen aperture:
Near-size band ≈ 0.5× to 1.5× the aperture
fnear = (mass of PSD in the near-size band) ÷ (total mass) × 100
A high fnear at your chosen API screen means rapid plugging; the fix is to shift the screen so the aperture no longer coincides with the peak of the distribution.
Match the screen to the formation's particle-size distribution, not to a habit. If a screen plugs fast, sieve the returns and move the aperture off the peak of the distribution. If it blinds with a film, treat the fluid — wash, shear the polymer, fix the stroke — and leave the mesh size alone.
Common questions
What is the difference between screen blinding and plugging?
Plugging (pegging) is a near-size particle physically wedging in an opening and reducing open area. Blinding is fine, sticky or cohesive material plastering over the surface of the mesh. They have opposite fixes — plugging needs a screen-size change, blinding needs the fluid or vibration corrected.
What are near-size particles?
Particles close to the size of the screen opening — commonly taken in screening practice as roughly half to one-and-a-half times the aperture. They are the fraction most likely to lodge in the openings and blind the screen.
Does a finer screen always help?
No. A finer screen helps only when it moves the aperture off the peak of the particle-size distribution. If the near-size problem is at the fine end, going finer can make plugging worse — sieve the returns and decide from the distribution.
