Cleaner Fluid Boosts Steel Mill Uptime

Even though the hydraulic system of a hot-roll plate mill contained multiple filters, contaminated fluid still caused unscheduled downtime.

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Fig. 1Even though technicians had installed multiple hydraulic filters throughout this hot-roll plate mill, fluid contamination was still causing problems, so media were upgraded from cellulose to synthtetic microglass in all filter elements.


A single production operation breaking down at a steel mill can have a domino effect that escalates the cost of downtime. In one such case, operators were confident that the multiple filters they had installed at strategic locations throughout the mill’s hydraulic system would ensure maximum reliability. However, without regular sampling of oil and assessment of results, operators had no way of knowing how effective (or ineffective) their filtration efforts were.  

The hydraulic system of the hot roll plate mill operated at a pressure 4,200 psi, and the working temperature of the fluid hovered around 130°F. An off-line (kidney loop) filter was provided for the reservoir, and filters were also located at the pump discharge for servovalve pilot circuits and for the return line. We recommended that technicians install several test ports and draw fluid samples to determine baseline ISO 4406 fluid cleanliness codes using an online particle counter. Based on these results, several other recommendations were implemented to improve cleanliness and reliability.

All filters were upgraded to Hy-Pro DFE elements using Grade 8 synthetic microglass media. DFE refers to elements rated by Dynamic Filter Efficiency testing. DFE surpasses current industry standards by quantifying capture and retention efficiency in real time, both by inducing dynamic duty cycles and by measuring performance during dynamic changes. It also measures that filter’s ability to retain particles. DFE testing is the method for predicting worst-case fluid cleanliness along with average fluid cleanliness.

Hypro 3M (β5 > 1,000) media were used on pump discharge, servo pilot, and recirculation filters. Hypro 25M (β22 > 1,000) media were used on the return line filter. We also advised installing Hy-Pro BF series high-flow, high-efficiency breathers with pleated microglass elements on the reservoirs to control ingression of fine airborne particulate.

Results—On-line particle counting was used to quantify the fluid cleanliness after the upgrades. The new elements substantially improved ISO 4406 fluid cleanliness codes, as shown in the illustration and table. The Hy-Pro elements reduced particles sized 4 µm and larger by 92.7%, particles 6µm and larger by 99.5%, and particles 14 µm and larger by 99.9%.

Fig. 2Changing cellulose filter elements to synthetic microglass drastically reduces the concentration of 4- and 6-μm particles in fluid in a hotroll plate mill’s hydraulic fluid.


ISO 4406 fluid cleanliness ratings can sometimes be deceiving, because lowering the value by only one or two actually produces a significant decrease in contamination. For example, the 6-μm particles shown in the table reflect an ISO code of 15, whereas the ISP code improved to 8 after the filter upgrade. In actuality, the number of 6-µm particles per ml was reduced from 240 to only 1.3—roughly 99.5%.

TableTable shows actual and estimated values of contamination particles in the steel mill’s hydraulic system.


Upgrading from Cellulose to Synthetic

Compared to cellulose, microglass media have superior fluid compatibility with hydraulic fluids, whether petroleum-based, synthetic, or water-based. Microglass media also have a significantly higher filtration efficiency than cellulose. Elements of different media with the same micron rating can have substantially different filtration efficiency. Uncontrolled contamination levels of 4- and 6-µm particles are quite common when cellulose media are used, because they can pass unhindered through cellulose media and exponentially generate more particles in a chain reaction of internally generated contamination.

Synthetic microglass fibers have a smaller and more uniform diameter than cellulose fibers, whereas cellulose fiber diameters are more random. The smaller size of synthetic microglass media means small particles can be captured. Synthetics also promote greater void volume, so they have a higher dirt-holding capacity—which translates to longer element life.

System cleanliness must be stabilized when upgrading to a microglass media element. As the microglass element removes years of accumulated fine particles, the element life might be temporarily reduced during this cleanup period. After the cleanup period, though, the microglass elements halt the runaway contamination and bring the fluid into the target ISO cleanliness code. Once the system is clean, the microglass elements may last four to five times longer than the cellulose elements they replaced.

Fig. 3Synthetic microglass media not only is more effective than cellulose at capturing small particles, it also provides much greater dirt-holding capacity, which leasds to less- frequent element replacement.


Adopt a Total Systems Approach to Cleanliness

The cost of proper contamination control and total systems cleanliness is less than 3% of the total cost of letting contamination run rampant. Developing a total system cleanliness approach to control contamination and care for fluids from arrival to disposal will ultimately result in more reliable machine operation, longer life, and saved money.

Several steps to achieve total systems cleanliness include:

  • evaluating and surveying all hydraulic and lubrication systems,
  • establishing an oil analysis program and schedule,
  • insisting on specific fluid cleanliness levels for all new fluids,
  • establishing a baseline and target fluid cleanliness for each system,
  • filtering all new fluids upon arrival and during transfer,
  • isolating all reservoirs and bulk tanks from uncontrolled environments,
  • installing high-quality particulate and desiccant breathers,
  • enhancing air and liquid filtration on existing systems wherever suitable,
  • using portable or permanent off-line filtration to enhance existing filtration,
  • improving bulk oil storage and handling during transfer, and
  • removing water and making a commitment to fluid cleanliness.

Jim Harlan is marketing & content development associate at Hy-Pro Filtration, Anderson, Ind.

Understanding ISO Codes

Sidebar table 1ISO 4406: 1999 quantifies the concentration of contamination particles in fluid samples. Reducing the code number by a single number represents cutting the contamination concentration in half.

The ISO cleanliness code (ISO4406-1999) is used to quantify particulate contamination levels per milliliter of fluid at three sizes: 4, 6, and 14 µm. The ISO code is expressed in three digits (example: 16/14/11). Each digit represents a contaminant level code for the correlating particle size. The code includes all particles of the specified size and larger.

Note from the ISO 4406:1999 table that each time a code increases, the concentration of particles doubles. Likewise, the code digit by a value of 1 represents cutting the concentration of particles in half. Also be aware that new oil can be one of the biggest sources of particulate contamination. New oil often has an ISO code of 25/22/19 A good target for new oil cleanliness is 16/14/11.

Using the table, new fluid—with an ISO 4406 code of 25/22/19—would contain 160,000 to 320,000 4-µm particles, 20,000 to 40,000 6-µm particles, and 2,500 to 5,000 14-µm particles per ml. This is unsuitable for most hydraulic and lubrication systems. Instead, a good target for clean fluid would be 16/14/11: 320 to 640 4-µm particles, 80 to 160 6-µm particles, and 10 to 20 14 14-µm particles per ml.

Guidelines for common hydraulic components are show in the accompanying table.

Sidebar table 2Table shows ISO 4406 code values for common hydraulic components.


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