How to clean wedge wire screens and maintain the slot

How to clean wedge wire screens: the principles

Wedge wire resists blinding because of its geometry: each particle touches the slot at two points, and the slot widens behind the face. That geometry does not prevent scale, biofilm, fats or fibre from building up on the surface, so every wedge wire screen still needs planned cleaning and inspection.

A fouled screen shows as longer run-off times, rising differential pressure, sand in a well or wet product from a centrifuge. Correct cleaning restores the open area the screen was designed with, while incorrect cleaning can drive particles into the slot or damage the passive film on the stainless steel.

Identifying the deposit

The cleaning method depends on the deposit, which should be identified before any treatment is chosen.

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Condition monitoring

Cleaning is most effective when it follows the condition of the screen. The following measures show when a screen is loading:

• differential pressure across filters, strainers, resin traps and pressure screens;
• flow or specific capacity for wells and intakes;
• run-off time for lauter tuns and dewatering decks;
• product moisture from centrifuges and dewatering screens;
• sand or media in the outlet for wells, laterals and resin traps.

A clean baseline is recorded after installation, and every later reading is compared with it so that a loading trend is seen before it affects the process.

Screen cleaning methods

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Cleaning direction and slot geometry

Cleaning works with the slot geometry when it starts behind the slot face, where the opening is widest.

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1. Outlet side first. Backwash or jet from behind the slot face, so that particles resting on the face lift away.
2. Slot face second. Wash or brush along the wires to remove the material that has lifted.
3. No hard jet at the face. A high-pressure jet aimed straight at the face can drive near-size particles into the slot and wedge them there.

For a radial external screen the outlet side is the inside, while for a radial internal screen such as a centrifuge basket it is the outside. The flow configuration guide describes the four arrangements.

Chemical cleaning and stainless steel

Most wedge wire screens are made in stainless steel or a nickel alloy, so the cleaning chemical must not damage the passive film. The following points are general practice for stainless steel.

• Alkaline detergents are widely used for fats, oils, proteins and organics.
• Nitric, citric and phosphoric acid cleaners are widely used for scale, and nitric acid solutions are also used for passivation.
• Hydrochloric acid is avoided on stainless steel because it attacks the passive film and causes pitting.
• Hypochlorite is limited to low disinfection doses, since strong solutions or long contact cause pitting, particularly in warm water.
• A thorough rinse with clean, low-chloride water follows every clean, because drying residues concentrate chlorides on the surface.

For well rehabilitation the acid must also suit the screen alloy. Sulphamic acid is commonly used and is less aggressive to stainless steel than hydrochloric acid, with strength and contact time set by the chemical supplier. Grades with a higher PREN tolerate harsher cleaning, as explained in the material selection guide.

Cleaning by application

Water wells

Loss of yield usually comes from incrustation or biofouling at the screen and in the gravel pack. Rehabilitation combines chemical treatment with mechanical surging or jetting to clear both.

Intake screens

Passive intake screens are commonly cleaned by an air burst system, supplied by others, which blows debris off the screen face. Periodic inspection checks for biofouling and for damage from floating debris.

Lauter tun false bottoms

The false bottom is cleaned in place after each brew. Inspections look for husk lodged under the segments and for segments lifting at their supports, since either affects run-off and rake clearance.

Centrifuge baskets

The basket is washed as the machine cycle allows, and slot wear is checked at regular intervals, because abrasive crystals or coal gradually widen the slot.

Sieve bends

The face is sprayed along the wires to clear fibre and fines. The upstream edges of the wires make the separation and round off as they wear, which reduces the liquid passed and changes the cut. Where the design allows, the screen is reversed end for end to present fresh edges.

DM plant laterals and resin traps

Laterals and nozzles are inspected whenever the vessel is opened for resin change. Resin found in the resin trap indicates a damaged lateral, nozzle or valve upstream.

Wedge wire screen maintenance checklist

The following checks are made at each planned shutdown and recorded against the previous inspection.

1. Measure the slot with a feeler gauge at several points along the length and around the circumference.
2. Inspect for broken welds and for wires lifting from the support rods.
3. Check the wire face for wear, which appears as a flattened or rounded face and a wider slot.
4. Check for deformation: dents, out-of-round cylinders and sagging panels.
5. Inspect for corrosion at welds, in crevices and under deposits.
6. Check fittings, gaskets, clamps and segment joints.
7. Record the findings and compare them with the last inspection.

Handling and storage

The slot face is protected throughout handling. A screen is never lifted by its wires, and heavy items are not stacked on panels.

Stainless screens are kept away from carbon steel dust, slings and tools, because iron contamination rusts and can start corrosion, and they are stored dry and covered. Before reinstallation, gaskets and clamps are checked and the flow direction marking is confirmed so that the slot face meets the feed, since a screen fitted the wrong way round blinds quickly.

Repair or replacement

A screen is repaired or replaced when the slot is wider than the process can accept, when welds or wires have failed, or when the screen is deformed. Local damage on a large panel can often be repaired, whereas a slot worn across the whole screen usually calls for replacement.

For a replacement, SIMO Screens needs the measured slot, the profile wire and support rod sizes, and a note of how the old screen failed, so that our engineers can address the cause of the failure as well as reproduce the part. The open area calculator shows how much open area a worn or partly blocked screen retains. Replacement water well screens, intake screens and flat wedge wire panels are made to the original drawing or to a revised design, and screens that foul or wear faster than expected can be reviewed through custom engineering.