The screen is the slotted length of casing set opposite the water-bearing sand or gravel; its slots are no wider than 1 mm so they hold back the gravel pack and the sand behind it, and its total open slot area must be large enough that water enters slowly. As a rule of thumb the water should pass through the slots at no more than about 0.03 m/s, so a well pumped at 20 L/min needs about 111 cm² of open slot area.
Part of Deep Wells
A well screen is the intake section of a well’s casing: slotted pipe that admits groundwater while keeping sand and gravel out of the pump and the water. It is the one part of the casing made to leak.
The Slots Are A Filter And An Inlet At Once
A slot that is too wide lets sand through, and the pump wears out and the water is gritty. A slot that is too narrow or too few clogs and starves the well. The Deep Wells guide cuts slots 0.5-1 mm wide and 50 mm long in rows 10 mm apart, covers them with fine mesh or a nylon sock, and surrounds the screen with a 3-6 mm gravel pack. The pack does the filtering; the slots only need to hold the pack back. A slot of 1 mm holds a gravel grain of 3 mm with a margin of three times.
Where to put the screen follows from the log of the hole (Bailer Operation): set it opposite the water-bearing sand or gravel, with solid casing above and a bottom cap below. A sump of solid pipe below the screen gives sand somewhere to settle.
Size The Open Area From The Entrance Velocity
Water squeezing through slots at high speed carries fine sand with it and drags particles into the slots, where they clog and cement. A widely used rule of thumb limits the entrance velocity to about 0.1 ft/s, which is 0.03 m/s (recalled from drilling practice; the Deep Wells guide gives no figure). Then the open area must be at least Q ÷ 0.03.
Worked example. The pump is to deliver 20 L/min. Q = 20 ÷ 1000 ÷ 60 = 3.33×10⁻⁴ m³/s.
- Required open area = 3.33×10⁻⁴ ÷ 0.03 = 0.0111 m² = 111 cm²
- One slot of 1 mm × 50 mm is 50 mm² = 0.5 cm²
- Slots needed = 111 ÷ 0.5 = 222 slots
With rows 10 mm apart there are 100 rows per metre of screen. At 3 slots per row that is 300 slots, 150 cm² of open area, per metre. A screen of 111 ÷ 150 = 0.74 m is enough for the velocity rule. Because gravel, the sock and fines block some slots, build in a margin; doubling it gives about 1.5 m. Length also follows the thickness of the water-bearing layer: screen the productive part of the layer, not just the minimum for velocity. Slot Cutting shows how many slots can be cut without weakening the pipe.
What You Need
- A length of casing — the same pipe and diameter as the solid casing (Casing Materials)
- A fine-toothed saw and a depth stop
- Fine mesh or a nylon sock — to cover the slotted length
- A bottom cap and a sump length
- A gravel pack, 3-6 mm (Gravel Pack)
Method: Place, Cut, Cover, Pack
Step 1: Fix the screen position and length from the hole log and the velocity arithmetic above.
Step 2: Cut the slots (Slot Cutting). Check every slot’s width with a gauge.
Step 3: Clean the slots. Clear the saw swarf from each slot and flush the pipe.
Step 4: Cover with mesh or a sock only if the pack will not do the job alone; each layer of fabric reduces the open area and can clog.
Step 5: Join the screen to the solid casing and the bottom cap, and lower the whole string (Well Casing).
Common Mistakes
- Slots wider than 1 mm. Sand enters the well and clogs the pump.
- Too few slots. High entrance velocity pulls in fines and clogs the screen.
- A screen too short. It uses only part of the layer; draw-down is large.
- A tight sock over every slot. It halves the working area when it clogs.
Safety
Cutting and handling slotted pipe
Clamp the pipe while sawing; never hold it in the hand. Wear eye protection, and expect sharp edges on cut plastic. A heavy string of screen and casing is hard to hold: lower it with a hoist and several people.
What’s Next
- Learn next: Slot Cutting — the cutting method and slot geometry
- Combine with: Gravel Pack — the filter around the screen