During the time of changing the rolls, there were numerous hot cracks on the surface of the removed working rolls. Some of them had even fallen off and peeled off. The team leader shook his head and said, "These rolls are of poor quality. We have to replace them earlier."

 

Don't rush to draw a conclusion yet.

 

During the rolling process, the rolls are subjected to tremendous rolling forces and high temperatures, with the surface temperature reaching over 500°C in an instant. The function of the cooling water is not merely to "cool down" - it needs to form a continuous and uniform water film on the roll surface, quickly removing the heat and maintaining the hardness and thermal fatigue strength of the roll surface. If a certain area fails to cool properly, the local temperature rises, and the thermal stress concentrates, causing cracks to grow along the stress direction.

 

The most common reason for inadequate cooling is not insufficient water pressure, nor a faulty water pump, but a clogged nozzle.

 

Many workshops in the field manage the cooling water for the rolls at the "just have water" level. When a nozzle is clogged, no one checks or cleans it, the water pattern becomes distorted, and no one adjusts it. Once cracks or chips appear on the roll surface, the first reaction is "the roll is broken", and few people think about the cooling water.

 

Today, let's expose this hidden killer and explain it clearly.

 

1. How to detect a clogged nozzle

1. Observe the water pattern

This is the most direct method. A properly functioning fan-shaped nozzle should spray water in an evenly distributed fan-shaped pattern, covering the entire width of the roll. The water curtain should be continuous and without breaks.

 

What happens when it is clogged:

 

The water pattern becomes smaller and the spray range shorter

The fan-shaped area has "holes" or is skewed, with more water on one side and less on the other

The water turns into a column or drips and completely loses the fan-shaped coverage

In cases of multiple nozzles, certain positions are noticeably "dry"

On-site operation: When stopping the roll or during maintenance, open the cooling water valves in sequence, stand to the side and observe the water pattern. Without disassembly, you can clearly see which nozzles are not spraying water. If possible, take a photo with a mobile phone and compare it with the previous cycle.

 

2. Measure the roll surface temperature

Use an infrared thermometer (point thermometer or thermal imager) to measure the temperature distribution on the roll surface during the rolling process or immediately after stopping.

 

Normal situation: The roll surface temperature is uniformly distributed along the axial direction, with a temperature difference of no more than 20-30°C.

 

Blocked situation: In the area corresponding to the clogged nozzle, the roll surface temperature is significantly higher, and the local temperature difference may reach over 50°C. In the thermal image, it appears as a "hot spot".

 

Note: The temperature measurement should be completed within 30 seconds after stopping. If it takes more than 1 minute, the roll surface heat will redistribute, and the measurement will be inaccurate.

 

3. Inspect the nozzle outlet by disassembly

During roll replacement or maintenance, remove the nozzle and inspect the outlet end.

 

Common forms of clogging:

 

White or yellowish-white scale (calcium and magnesium deposits)

Black iron oxide debris

Rust, mud, sand

Occasionally, there are debris from sealing parts, welding slag, etc.

If a visible layer of scale or debris can be seen at the nozzle outlet, it indicates that a portion of it is clogged. Use a fine iron wire to probe and see how much can be removed.

 

2. Why the nozzle gets clogged: The underlying logic of scaling

1. High water hardness is the main cause

During the circulation of cooling water, water evaporates, and the concentration of calcium and magnesium ions continuously increases. When the temperature exceeds 40-50°C, calcium carbonate and magnesium carbonate begin to precipitate and adhere to the inner wall of the nozzle and the outlet, forming scale.

 

Water hardness comparison:

 

Hardness level

 

Calcium and magnesium ion concentration (mg/L)

 

Scaling risk

 

Soft water

 

<60

 

Low

 

Medium-hard water

 

60 - 120

 

In China

 

Hard water

 

120 - 180

 

High

 

Very hard water

 

＞180

 

Extremely high

 

1. Many old factories directly use groundwater or river water as cooling water, with hardness often ranging from 200 to 300 mg/L. The nozzles get clogged within two weeks.

2. Oxide scale enters the water system

The amount of oxide scale on the rolling line is large. If the swirling well and filtration system are ineffective, the iron scale debris will enter the pipes along with the cooling water and get stuck in the narrow flow channels of the nozzles.

3. The cooling water temperature is too high

The return water temperature is high (> 45°C), accelerating the precipitation of calcium and magnesium; at the same time, high temperature promotes the reproduction of microorganisms, forming biological sludge, mixed with rust and scale, and causing more severe blockages.

4. Issues with nozzle materials and selection

Bronze nozzles have low cost but are prone to corrosion. Corrosion products exacerbate the blockage.

If the nozzle aperture is selected too small, the same impurities are more likely to get stuck.

There is no filtration device or the filter is not of sufficient precision.

 

III. Cleaning cycle suggestions: Don't wait until it's clogged before taking action

Manage by water quality classification

Water quality situation

 

Suggested cleaning cycle

 

Daily inspection frequency

 

Soft water (with soft water device)

 

Inspect and dismantle once a month, chemical cleaning once a season

 

Visual inspection of water quality once a week

 

Medium-hard water

 

Inspect and clean every two weeks

 

Check once per shift

 

Hard water/Very hard water (without softening)

 

Inspect and clean once a week

 

Check once per shift + stop machine to check one by one

 

Improvement of filtration system

 

Can be appropriately extended

 

Daily inspection

 

Cleaning methods

Mechanical疏通：

Use fine copper wire and special pliers to carefully remove the blockage. Be careful not to enlarge the nozzle diameter (as this will change the water spray angle and flow rate)

Chemical descaling：

Remove the nozzle and immerse it in the descaling agent (citric acid or special descaling liquid). After soaking， rinse with clean water. Pay attention to material compatibility； stainless steel and brass cannot be used with strong acids.

High-pressure water reverse flushing：

Reverse the water flow from the outlet end and use high-pressure water to flush out the blockage. Suitable for minor blockages.

Replacement

The nozzle is not expensive. If it gets severely clogged or its aperture has deformed, just replace it with a new one. Don't keep repairing it.

Key points for daily maintenance

Daily inspection of each shift

Operators observe the cooling water pattern during the rolling process and record any abnormalities immediately and notify maintenance.

Changing rolls must be inspected.

Each time a roll is changed, check the nozzles in the corresponding area, clear obstructions, and adjust the angle.

Filterers should be regularly backwashed.

The filter beds, sand filters, and bag filters should be flushed or replaced with new filter elements on time.

Water quality should be tested regularly.

Once a month, samples should be sent for testing to monitor hardness, turbidity, and iron ion concentration.

 

IV. Is it worth installing a soft water system? Let's do the math.

Many workshops know that the water quality is hard, but they think installing a soft water system is "too expensive" and "unnecessary". Let's calculate:

 

Input side

Soft water equipment (ion exchange or reverse osmosis): Depending on the water production volume, it ranges from tens of thousands to several hundred thousand.

Installation, pipeline modification: Depending on the site conditions.

Recharge salt/membrane replacement and other consumables: Several thousand to tens of thousands per year.

Maintenance labor: No additional cost.

Output side

Roll wear decreases

: After the cooling is uniform, hot cracks decrease, and the wear of the rolls decreases. Each time the rolls are repaired, several thousand tons of steel can be rolled more. Calculated based on a work roll worth several tens of thousands to over a million, the savings in roll wear per year is considerable.

Roll changing frequency decreases

: Reducing unplanned roll changes and improving production efficiency. For a hot rolling line with an hourly output of several hundred thousand, one less stop means a profit.

Plate surface quality improves

: Uniform cooling → Good roll surface condition → Fewer surface defects on the steel plate, reducing regrading and scrap.

Nozzle lifespan is extended

: Clean water → Fewer nozzle blockages → Lower replacement frequency

Return period

For a hot rolling or medium-thick plate production line with an annual output of several million tons, roll wear is the major cost. Installing a soft water system usually takes half a year to one year to recover the cost due to the reduction in roll wear. After that, it is a net saving.

 

Conclusion: For production lines with water hardness exceeding 120mg/L, installing a soft water system is a sure profit. For extremely hard water (＞200), not installing a soft water system, the waste of nozzles and rolls alone would be enough to make you feel heartbroken.

 

V. The most common pitfalls on-site

"Water pressure is sufficient"

: Normal water pressure does not mean normal water pattern. Even if half of the nozzles are blocked, the water pressure gauge may still show normal, but the flow rate and coverage area are already insufficient.

"Anyway, there are backup nozzles, just replace them when they are blocked"

: Replacing nozzles is quick, but the position corresponding to the blocked nozzle has already been subjected to thermal shock and cracks have already formed. By the time the nozzles are replaced, the rolls may already be ruined.

"Cleaning nozzles is the maintenance task"

: The maintenance cycle is long, and waiting for maintenance to clean the nozzles will result in blockage. Daily inspections must include checking the cooling water pattern, which is the responsibility of the operators.

"We have been doing this for a long time and haven't had any major incidents"

: Not having major incidents doesn't mean there are no losses. The lifespan of the rolls is shorter, the roll changing frequency is higher, and the steel plate surface occasionally has defects. These "chronic losses" add up to several tens of millions to several hundred million a year.

 

VI. One-sentence conclusion

Rolls are prematurely scrapped six or seven times out of ten because of cooling. Problems with cooling occur eight or nine times out of ten because of nozzles. Nozzles are blocked, and eight or nine times out of ten, it is due to hard water and lack of management.

 

Don't wait until the roll surface cracks before replacing it. First, check if the water coming out of the nozzles is normal.

 

How often does your workshop clean the nozzles? Is it done through regular inspections or waiting until they are blocked and then replacing them? Share the number in the comment section to see which workshop has the best management.