Well Screen

A well screen is a filtering device installed at the bottom of a well casing in formations where the well draws water from loose sediment — sand, gravel, or unconsolidated aquifer material — rather than solid rock. The screen allows water to flow into the casing while blocking sand, silt, and other fine particulates that would damage the pump and degrade water quality. In bedrock wells, where the casing terminates in solid fractured rock, a screen is generally not needed. In sandy or alluvial aquifer wells, a well screen is often essential.

How Well Screens Are Constructed

Well screens consist of a perforated or slotted tube with slot openings sized to allow water passage while blocking the predominant sediment particles in the aquifer formation. The slot size is specified by a well engineer or driller based on a sieve analysis of the aquifer material — the goal is to allow fine particles to pass through initially (a process called well development) while retaining enough sediment to stabilize the formation around the screen.

Common screen materials include stainless steel, low-carbon steel, and PVC. Stainless steel screens are preferred in corrosive water environments and for longer service life. PVC screens are used in shallower, lower-demand applications. Some screens are constructed with a gravel pack installed in the annular space around the outside of the screen — a carefully selected gravel gradation that acts as a natural pre-filter between the formation and the screen itself.

What Happens When a Well Screen Fails or Clogs

A partially clogged well screen restricts water entry into the well, reducing yield and causing the pump to work harder to maintain pressure. Symptoms of screen fouling include gradually declining water pressure and output, the pump running more frequently without maintaining household pressure, and occasional air in the water lines as the pump begins drawing air when the well struggles to deliver adequate flow.

A failed or damaged screen allows sand and sediment into the well casing, which is then drawn into the pump. Sand and fine particles are abrasive to impeller and seal components — a pump operating in sandy water experiences accelerated wear and a significantly shortened service life. Discolored or visibly gritty water is one of the clearest signs that sand is entering the pump and should be investigated immediately to prevent pump damage.

Well Screen Repair and Replacement

Unlike pump replacement, which involves pulling just the pump and drop pipe, screen repair or replacement requires pulling the entire casing string in some cases — a significantly more complex and costly operation. Well screen problems are generally addressed when a pump is already being replaced, at which point the contractor can inspect the screen and the condition of the formation around it. Preventive treatment such as chemical rehabilitation can sometimes restore flow through a biologically fouled screen without full replacement.