Water Well Screens: Types, Slot Size Selection and Filter Pack Design

Quick Answer

A water well screen is the slotted intake section of a well: it lets groundwater in and holds the aquifer sand, or the filter pack around it, out. The main types are continuous-slot wire-wrap (V-shaped wire wound on support rods, with the highest open area: roughly 10–30% at fine slots and 30–50% at coarse ones, depending on slot and wire width), bridge-slot, louvre, slotted pipe and perforated pipe. The slot size comes from a sieve analysis of the aquifer. In a naturally developed well, the classic rule is a slot that retains about 40% of the formation; design guides vary it from about 30% to 60% with the sand's uniformity, the risk of caving and the water's corrosiveness. In a filter-packed well, the pack's D30 is 4 to 6 times the formation's D30, its uniformity coefficient is 2.5 or less, and the slot retains at least 90% of the pack. Screen length follows the aquifer: about 80–90% of a confined aquifer, the lower third to half of an unconfined one. The old entrance velocity limit of 0.03 m/s (0.1 ft/s) survives in some regulations, but AWWA A100 no longer recommends designing on entrance velocity alone. The material follows the water chemistry: low-carbon steel for benign water, 304 stainless for most fresh water, 316L for brackish and saline water, PVC for corrosive water at moderate depth.

A typical problem well looks fine on commissioning day. A few months later the submersible pump is wearing, the meter reads low and the bailer brings sand up from the sump. The records show the screen was ordered from habit — "eight-inch, slot 40, stainless" — not from the sieve analysis of the aquifer it was set in.

Screen selection decides a well's long-term performance, and it is done with numbers: grain sizes, slot widths, open area, velocities and water chemistry. This guide sets out how they are chosen.

What a Well Screen Does

In sand and gravel aquifers an open borehole would collapse. The screen holds the formation back, lets water in with little head loss, and must stay open, strong and uncorroded for the life of the well. It works as part of a system:

• Casing — blank pipe from the surface to the top of the aquifer, housing the pump

• Screen — the intake section across the water-bearing zone

• Filter pack — graded sand or gravel in the annulus around the screen, where the design calls for one

• Sump and bail plug — a short blank, closed section below the screen that traps fines

• Centralisers — spacers that keep the string central so the pack is even all round

• Annular seal — grout above the pack that stops surface water running down the outside of the casing

Not every well needs a screen: a well in competent sandstone or limestone can often be left as open hole below the casing.

Screen Types Compared

Why open area matters. For the wire face of a continuous-slot screen, open area = slot ÷ (slot + wire width). A 0.5 mm slot between wires 2.3 mm wide is 18% open; a 0.9 mm slot with the same wire is 28% open. Slotted pipe cannot match this without losing its strength.

Slot Size Selection

Start with the sieve analysis

The slot comes from the aquifer, not the catalogue. Samples are taken at every change of formation through the water-bearing zone, sieved, and plotted as a grain-size curve: grain size on a log scale against cumulative percentage passing or retained.

• D-values: D10 is the grain size that 10% of the sample passes; D60 the size 60% passes

• Passing versus retained: drillers often plot percentage retained, so "40% retained" and "D60 passing" are the same point

• Uniformity coefficient: Cu = D60 ÷ D10; below about 3 the sand is uniform. Some screen makers write it on the retained basis as d40 ÷ d90, the same ratio

• Slot numbers are widths in thousandths of an inch: slot 10 = 0.25 mm, slot 20 = 0.51 mm, slot 35 = 0.89 mm

Naturally developed wells

Here the screen sits against the formation. Development pulls the fines through the slots and leaves the coarser grains as a natural filter around the screen. The classic rule is a slot that retains about 40% of the formation. Design guides vary the figure from about 30% for well-graded sands to 60% for uniform ones, and adjust it for whether the material above the aquifer can cave. Move toward the finer end — retaining more — when the sieve data are thin or the water is corrosive, because a corroding slot only gets wider.

Worked example. A medium sand aquifer gives this curve:

• Uniformity coefficient: 0.50 ÷ 0.17 ≈ 3.0, a fairly uniform sand

• Slot retaining 40% = D60 = 0.50 mm → slot 20

Filter-packed wells

A filter pack (formerly called a gravel pack; AWWA A100 now uses "filter pack", since packs range from sand to coarse gravel) is used where the formation is too fine or uniform for natural development, where the aquifer is layered, or to allow a larger slot. The rules in most water-well design guidance:

• Grading: pack D30 = formation D30 × 4 to 6 — 4 for fine, uniform formations, 6 for coarser, graded ones

• Uniformity: pack uniformity coefficient 2.5 or less

• Slot: retains 90% or more of the pack

• Thickness: typically 75–200 mm (3–8 in)

• Material: clean, well-rounded siliceous sand or gravel — not limestone, shale or angular crushed stone

• Height: carried above the top of the screen so it can settle during development without exposing the screen

Older filter rules, such as Terzaghi's and the US Army Corps of Engineers', give somewhat different ratios; the 4–6 × D30 method is the one most screen makers and well-design references use.

Worked example, continued. Formation D30 of 0.30 mm gives a target pack D30 of 1.2 mm (factor 4) to 1.8 mm (factor 6). A pack with D10 ≈ 0.95 mm and D60 ≈ 1.6 mm has a D30 of about 1.2 mm and a uniformity coefficient of 1.7. A slot retaining 90% of it is about the pack's D10 → slot 35. Open area rises from 18% at slot 20 to 28% at slot 35 — the pack lets the same aquifer take a slot almost twice as wide.

Where the hole is much larger than the screen but no designed pack is needed, a formation stabiliser, sand or gravel that is not specially graded, fills the annulus to stop the wall caving onto the screen; one US state's well rules say it is sometimes necessary where the hole is 100 mm (4 in) or more larger than the screen.

Screen Length, Diameter and Entrance Velocity

Length follows the aquifer

• Confined aquifers: screen about 80–90% of the water-bearing thickness, centred in it

• Unconfined aquifers: screen the lower third to half of the saturated thickness, so drawdown does not expose the screen

• Layered aquifers: give each productive layer its own slot

• Blank sections: carry the casing a little into the aquifer top, and keep a sump of about 1–2 m in small wells and up to 3–6 m in large ones

The pump sets the casing diameter. Screen diameter matters less than buyers expect: in small wells, going from 100 mm (4 in) to 125 mm (5 in) raises yield by about 3% or less, and leaves a thinner pack in the same hole.

Entrance velocity: the guideline and the debate

For decades many designers limited slot velocity to 0.03 m/s (0.1 ft/s), a figure that traces back to an opinion published in the 1940s. AWWA A100 editions up to 1997 proposed a looser upper limit of 0.46 m/s (1.5 ft/s). The current AWWA A100 states that there is no single defined criterion for permissible slot velocity and does not recommend designing a screen on entrance velocity alone. Aquifer model tests found efficiency approaching its maximum once open area exceeds about 3–5% and entrance velocity stays below about 0.76 m/s (2.5 ft/s). Some regulators still write 0.03 m/s into their well rules, so check the local code.

Worked example: screen length for 30 l/s

A municipal well must deliver 30 l/s (about 475 US gpm) through a 219 mm (8⅝ in) OD wire-wrap screen, slot 35, 2.3 mm wire.

• Screen surface: π × 0.219 = 0.688 m² per metre; at 28% open, 0.192 m² per metre

• Assume half the openings are blocked by pack grains: 0.096 m² per metre effective

• At 0.03 m/s: 0.030 ÷ 0.03 = 1.0 m² needed → about 10.4 m of screen; at 0.46 m/s, under 1 m

The aquifer then decides. In a 15 m confined aquifer the 80–90% rule gives 12–13.5 m of screen, and at 12 m the entrance velocity is about 0.026 m/s, inside even the strictest guideline. Repeat the check for slotted PVC at 6% open area and holding 0.03 m/s would need about 48 m of screen — more than the aquifer is thick. The model studies above suggest that 6% open area is not in itself a hydraulic limit, but where a regulator enforces 0.03 m/s, a low-open-area screen can rule itself out.

Materials and Water Chemistry

The screen has the thinnest metal and the most surface exposed to the water, so it is the first part to corrode or encrust. Choose the material from the water analysis.

The Ryznar Stability Index (RSI = 2 pHs − pH, where pHs is the calcium carbonate saturation pH) is a quick test of the analysis: below about 6 the water tends to scale; above about 7 it may not lay down a protective carbonate film, and well above 8 mild steel corrosion increases. Other warning signs:

• Carbon dioxide above about 50 mg/l — usually corrosive

• Chloride above about 500 mg/l — treat as a saltwater well

• Hydrogen sulfide — test for sulfate-reducing bacteria, which attack metal screens

• Dissolved iron and manganese — encrustation and biofouling that plug the slots

Do not mix metals. Use the same metal for screen and casing, or join a stainless screen to steel casing through a dielectric coupling or a heavier steel transition. In corrosive water, the blank pipe immediately above the screen — at least 1.5 m (5 ft) — should be the same corrosion-resistant material. For drinking-water wells, check the approvals your market requires; in North America, PVC well casing is commonly listed to NSF/ANSI 61 and NSF/ANSI 14.

Strength and Installation Loads

A screen must survive collapse loads (formation pressure and the head difference across the screen, highest during development, grouting and heavy pumping), tension from the hanging string, and column loads when the string stands on the bottom or is driven. Wire-wrap screens usually have lower collapse strength than the casing above them, and bridge-slot screens are weak in collapse. Catalogue collapse figures are often calculated values with no safety factor included, so apply your own margin.

PVC casing and screen are dimensioned under ASTM F480 in SDR and Schedule 40 and 80 classes. Heat from setting cement grout or a submersible pump motor raises the risk of collapse, so keep the casing full of water while grouting to balance the grout column, and consider retarded cement.

Installation, Development and Rehabilitation

• Installation: fit centralisers at the top and bottom of the screen and every 6–12 m (20–40 ft) up the string; close the bottom with a bail plug; place the pack steadily by a recognised method — tremie or pumped feed line, or surface pour with circulation — and sound its level as it rises

• Development: surge block, high-velocity jetting through two to four nozzles, air-lifting and over-pumping, until the water runs clear and the sand content is low — a practical limit of about 5–8 mg/l is often cited for submersible pumps

• Disinfection: AWWA C654 covers well disinfection; AWWA A100 calls for 100 mg/l available chlorine through the full water depth of a new well

• Rehabilitation: record specific capacity (yield per metre of drawdown) at commissioning, and act when it has fallen by about 25%. Mechanical cleaning is combined with chemicals — hydrochloric or sulfamic acid for mineral scale, organic acids for biofilm — followed by chlorination. The pump must come out, the well is usually off line for two to three days, and no treatment is permanent

Oil, Gas and Industrial Uses

The same technology controls sand in oil and gas wells, where sand screens are tested for burst and collapse under ISO 17824 and the casing is made to API 5CT. Wedge wire is also used for industrial filtration, as hub laterals in filter vessels and as water nozzles.

Ordering Checklist

• Borehole: diameter, depth, drilling method

• Aquifer: confined or unconfined, depth to top and bottom, layering, sieve analyses

• Water analysis: pH, TDS, chloride, carbon dioxide, hydrogen sulfide, iron, manganese, hardness

• Duty: design yield, pump setting, expected drawdown

• Screen: type, material, slot per section, section lengths, outside diameter, end connections matching the casing

• Accessories: centralisers, bail plug or sump, lifting caps, transitions between materials

• Filter pack: grading and volume. A 450 mm hole around a 219 mm screen gives a pack about 115 mm thick and 0.12 m³ per metre; 15 m of pack is about 1.8 m³, roughly 2.9 t at about 1.6 t/m³

• Documents: material certificates and inspection records — see our guide to material test certificates

Common Mistakes

1. Slot size by habit. A slot copied from the last well does not suit this aquifer.

   Prevention: Choose the slot from this well's sieve analysis.

2. One slot for a layered aquifer. The fine layer runs sand through a slot sized for the coarse one.

   Prevention: Specify screen sections with slots matched to each layer.

3. The wrong filter pack. Angular, dirty or wide-graded material bridges and lets fines through.

   Prevention: Use clean, rounded siliceous pack with a uniformity coefficient of 2.5 or less, graded from the formation D30.

4. A low-open-area screen in a high-yield well. Losses through the screen and pack rise, and where a slot-velocity rule applies, the screen may not comply at all.

   Prevention: Check open area per metre against the design yield and any local velocity rule; use continuous-slot screen where yield matters.

5. Mixing metals. A stainless screen joined straight to steel casing corrodes the joint.

   Prevention: Use one metal, or a dielectric coupling or heavier steel transition.

6. Ignoring the water analysis. Steel corrodes in aggressive water; any screen plugs in encrusting water.

   Prevention: Select material from the analysis and the Ryznar index, and plan maintenance for encrusting water.

7. Grouting PVC carelessly. Cement heat and an unbalanced grout column collapse the casing.

   Prevention: Keep the casing full of water, check the collapse rating and consider retarded cement.

8. No baseline specific capacity. Encrustation is noticed only when rehabilitation can no longer restore the well.

   Prevention: Record specific capacity at commissioning and on a schedule; rehabilitate at about a 25% decline.

Supply from Kasko Makine

Kasko Makine supplies water well and artesian products:

• Water well screens — V-shaped (wedge wire) profiles wrapped around support rods, forming continuous slots for water and artesian wells, groundwater extraction, and municipal, agricultural and industrial supply wells

• Wedge wire screens — for water and artesian wells, industrial filtration, hub laterals and water nozzles

• Bridge slotted well screens — for gravel-packed or naturally developed wells in medium sandstone and similar formations, soil sanitation and groundwater extraction

• Steel surface screens, PVC screens and perforated pipes

• PVC casings and stainless steel casings

• Centralizers, bail plugs and sumps and lifting caps for round threads

• Oil and sand control well screens

Engineering support: Send the borehole log, sieve analyses, water analysis and design yield. We will review screen type, slot size and material against your data, and quote screens, casing and accessories together.

Send your borehole log, sieve analyses and water analysis for a screen proposal and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Submersible Pumps: Types, Specifications and Selection

• Pump Curves and System Curves: Finding the Operating Point

• Stainless Steel Pipe: 304 vs 316 to ASTM A312

• API 5CT Casing and Tubing: Grades, Sizes and Connections

• Industrial Strainers: Types and Mesh Selection

Frequently Asked Questions

Q: What is a water well screen?
A: A water well screen is the slotted intake section of a well, set across the water-bearing layer. It lets groundwater into the well while holding back the aquifer sand or the filter pack around it. The main types are continuous-slot wire-wrap, bridge-slot, louvre, slotted pipe and perforated pipe. Wire-wrap screens have the highest open area, roughly 10 to 30 percent at fine slots and up to about 50 percent at coarse ones.

Q: How do you choose the slot size for a well screen?
A: The slot comes from a sieve analysis of the aquifer. In a naturally developed well, the classic rule is a slot that retains about 40 percent of the formation, with design guides ranging from about 30 to 60 percent. In a filter-packed well, the slot retains at least 90 percent of the pack. Slot numbers are widths in thousandths of an inch: slot 20 means 0.020 inch, or 0.51 mm.

Q: What is a filter pack and when is it needed?
A: A filter pack, also called a gravel pack, is graded sand or gravel placed between the screen and the borehole wall. It is used when the formation is too fine or uniform for natural development, when the aquifer is layered, or to allow a larger slot. The pack's D30 is usually four to six times the formation's D30, with a uniformity coefficient of 2.5 or less.

Q: What is the difference between wire-wrap and bridge-slot well screens?
A: A wire-wrap screen is V-shaped wire wound on longitudinal rods, giving continuous, precise slots that widen inward, resist plugging and provide the highest open area. A bridge-slot screen is steel sheet pierced into vertical bridge-shaped slots, then rolled and welded. It costs less and suits filter-packed wells, but has less open area and low collapse strength, and needs a smaller slot relative to the pack.

Q: What entrance velocity should a well screen be designed for?
A: Many designers have limited slot entrance velocity to 0.03 m/s, or 0.1 ft/s, and some regulations still require it. AWWA A100 editions up to 1997 proposed an upper limit of 0.46 m/s. The current AWWA A100 says no single criterion exists and does not recommend designing on entrance velocity alone. Screen length is normally set by the aquifer thickness, with entrance velocity used as a check.

Q: Which material should a water well screen be made of?
A: Choose it from the water analysis. Low-carbon steel suits non-aggressive water, type 304 stainless steel suits most fresh water below about 600 mg/l dissolved solids, and 316L suits brackish and saline water. PVC does not corrode and suits corrosive water at moderate depth, but is weaker. Screen and casing should be the same metal, or be isolated, to avoid galvanic corrosion at the joint.

Q: How long should a well screen be?
A: In a confined aquifer, screen about 80 to 90 percent of the water-bearing thickness, centred in the layer. In an unconfined aquifer, screen the lower third to half of the saturated thickness so drawdown does not expose the screen. Layered aquifers are screened layer by layer with matching slots, and a short blank sump below the screen collects fines.