Tissue machine lubricants: requirements and critical points by machine section

On a tissue machine, each section calls for a different lubricant. In forming and pressing, the oil has to separate water quickly (demulsibility, NBR 14172). In the bearings of the Yankee dryer cylinder (the large steam-heated cylinder that dries the sheet), it has to withstand heat, with viscosity set by the real bearing temperature. Gearboxes take the gear oil of DIN 51517 part 3; hydraulic systems, the anti-wear oil of DIN 51524 part 2. Water in the oil is tracked by analysis, before it turns into downtime.

A tissue machine makes thin paper, such as toilet paper, paper towel and napkins. At one end it takes in a fiber slurry that is almost all water and at the other end of the same machine it delivers dry paper. Think of a kitchen where the sink and the oven sit side by side: the bearings at the start (the support of the shaft of each roll or cylinder; on a paper machine almost always a rolling bearing, which the oil lubricates and cools) live wet, the ones at the end live hot, and no single oil serves both.

Which lubricant to use in each section of a tissue paper machine?

Use one oil for each dominant condition: water, heat, load or pressure. The right specification is set point by point, and the table below summarizes the decision. The grade of each point is given by the ISO VG, the viscosity of the oil measured at 40 °C, in cSt (centistokes, the unit of viscosity): the higher the number, the thicker the oil.

The ISO VG grade of each point comes from the equipment manual and the temperature measured at the bearing. In forming, many bearings run on grease; where there is circulating oil, the first row of the table applies. The lines mentioned are part of Kelpen Oil's oils for industrial machinery and equipment.

Why is a tissue machine an extreme lubrication case?

Because it concentrates, in a single machine, the conditions that degrade oil the most:

• Water, in forming and pressing: mist, splash and washdown reach the oil through the seals (the shaft closures), the breathers (the openings through which the tank exchanges air with the surroundings) and the bearings.

• Heat, in drying: the Yankee cylinder is steam-heated from inside, the hood (the cover around the cylinder) blows hot air from outside, and the bearings in this area are among the hottest lubrication points in the mill.

• Continuous operation, which removes maintenance windows and turns any oil change into a production decision. Tissue runs nonstop, and under that regime the lubricant directly affects machine availability.

In the circulating oil system (the system that pumps oil from the tank to the bearings and takes it back) of the wet end, the typical failure starts with water and advances in stages:

1. Water enters through the bearing seal, the tank breather or the machine washdown and dissolves in the oil, still invisible.

2. Above the saturation point, the water stops dissolving and forms droplets or emulsion (a stable mixture of water and oil), which turns the oil milky.

3. With water in the way, the oil film that separates the parts becomes thinner and weaker, which leads to metal contact, heating and bearing wear.

4. Water also rusts the bearing raceways (the surfaces on which the balls or rollers turn) and speeds up oil oxidation (the reaction with oxygen in the air, which forms acids and sludge, a pasty residue), which leads to saturated filters and rising acidity.

5. The wear ends in unplanned downtime, on the costliest equipment in the mill to stop.

In the Yankee circuit, the failure starts with heat:

1. Hot oil oxidizes faster, which leads to rising acidity and to sludge and varnish (a hard deposit that sticks to surfaces).

2. Sludge and varnish clog filters and lubrication channels, which leads to less oil reaching the bearing.

3. With less oil, the bearing loses lubrication and cooling, which leads to heating, wear and downtime.

How this shows up in your mill: cloudy or milky oil in the tank sight glass, water coming out of the drain at the bottom of the tank, filters saturating ahead of schedule, a bearing running hot in the dryer cylinder area, acidity rising in the report. The wrong oil choice usually shows up weeks later, as unplanned downtime on the machine that could least afford it.

Figure 1. The tissue machine in simplified cross-section: water enters in forming and pressing, heat comes from the Yankee cylinder and the hood, and each circulating oil circuit takes clean oil to the bearings and returns it to the tank. The forming bearing, in the drawing, runs on grease.

Figure 1 follows the real order of the sections. The headbox delivers the fiber slurry to the former, between the wire (the woven fabric through which water drains) and the felt (the blanket that carries the sheet). The felt carries the sheet to the nip between the press roll and the Yankee cylinder itself, where more water comes out and the sheet transfers to the cylinder. The Yankee, steam-heated from inside and covered by the hood that blows hot air, dries the sheet; the creping blade lifts and crinkles the dry paper, and the reel builds the roll. Below, the circulating oil: the pump draws oil from the tank, the filter cleans it (and, in the Yankee circuit, the cooler removes heat), the oil passes through the bearings and returns to the tank by gravity, where free water sinks to the bottom and leaves through the drain. In forming, the bearing in the drawing runs on grease, as many do in that section; the wet-end circuit serves the press. For the Yankee bearings, the usual recommendation is a separate circuit, precisely because the condition is different.

What is oil demulsibility, and why does it decide in the wet end?

Demulsibility is the ability of an oil to separate from water after being mixed with it. Two oils of the same viscosity can behave in opposite ways in front of water: one separates in minutes, the other forms a stable emulsion. In the tank, the oil that separates well lets the water sink to the bottom, from where it leaves through the drain; the oil that emulsifies carries the water back to the bearing.

The demulsibility test (NBR 14172, equivalent to ASTM D1401) shakes oil and water in a graduated cylinder and measures how long the two take to separate. The test temperature is 54 °C, or 82 °C for oils with viscosity above 90 cSt at 40 °C, a common case in the heavier grades of the drying section. At the Kelpen Oil laboratory, it runs on the PD 475 instrument, together with the emulsion characteristics test of the same standard.

Which oil to use in the bearing of the Yankee dryer cylinder?

A circulating oil with a viscosity chosen for the real bearing temperature, with good oxidation resistance, supplied in sufficient quantity and free of water. The bearings in the dryer section can reach 125 °C; with enough oil flow, the circuit makes it possible to keep them below 90 °C, and that is why the oil also works as a cooling fluid.

Viscosity is chosen by the real temperature of the point. The real viscosity of the oil drops as temperature rises. A bearing of the same size calls for different viscosities in the wet section and in drying; specifying for cold and running hot means running on an oil film thinner than the design asked for.

Heat takes its toll on oil life. When hot, the oil oxidizes faster: it forms acids, sludge and varnish and can clog the lubrication channels. Analysis catches the start of this through rising acidity and rising viscosity.

The data sheet of the Supra AW line covers circulating systems under severe pressure and elevated temperature, with antioxidant additive, in grades up to ISO VG 320, and the Kelpen Oil Technical Team confirms the grade from the temperature and flow of each bearing. The Bearing line stays out of the dryer section: the data sheet reserves it for applications without a strong oxidation-resistance requirement.

How to monitor water in the oil before it becomes downtime?

By periodic laboratory analysis, because dissolved water cannot be seen. When the oil turns milky, the contamination is already past the saturation point: the water is no longer dissolved and already circulates as droplets or emulsion.

Objective monitoring is done with the water content test by the Karl Fischer method (NBR 11348): a reagent specific to water measures how much of it is in the oil, in ppm (parts per million; 1,000 ppm equals 0.1%), even while it is still dissolved and invisible. At the Kelpen Oil laboratory, the test runs on the Metrohm 831 KF coulometer, an instrument that calculates the water from the electricity spent in the reaction, and it is one of the 25 tests of the analytical facility in Arujá (SP, Brazil).

For a paper machine circulation system, periodic analysis answers what no visual inspection can: how much water is in the oil now, whether the viscosity is still within the specification range and how much the oil has oxidized, through acidity (TAN, the total acid number) compared with that of the new oil. The trend of these results shows when the oil is nearing the end of its service life. Changing oil by calendar, in this context, means throwing away good oil or, worse, keeping dead oil. Analysis allows changing by condition. For the results to be comparable over time, always sample at the same point of the circuit and with the machine running.

What does the Kelpen Oil laboratory measure in paper machine oil?

The in-house laboratory in Arujá (SP, Brazil) measures, in the same report, how much water has already entered the oil (Karl Fischer) and whether the oil can still expel it (demulsibility). Read together, the two results show whether it is enough to remove the water from the tank or whether the oil itself has lost its ability to separate water and must be replaced.

Kelpen Oil, a Brazilian manufacturer of lubricants and base oils, provides this analysis service to any company, including for oil it did not produce. It is a paid service, and on a continuously operating machine it is usually the difference between scheduling an intervention and suffering one.

And availability, which nobody specifies?

There is a requirement of the tissue industry that appears in no data sheet: the supplier cannot run out. A continuous machine consumes oil all the time, and not every supplier keeps product ready for replenishment. Kelpen Oil produces for stock replenishment, with made-to-order packaging from pail to bulk and service across Brazil, since 1999. For whoever answers for machine availability, this weighs as much as the oil specification.

Frequently asked questions

How does water get into the oil circuit? Through the bearing seals, the tank breathers, condensing steam and equipment washdown. On a paper machine the contamination is continuous, and that is why the oil's ability to separate water is a specification criterion.

How often should the circuit oil be analyzed? A common starting point in the market is quarterly sampling, then adjusted by the analysis results: if the trend of water, acidity or particles rises, the interval gets shorter. The final interval depends on the history of each machine, and a critical circuit, such as the Yankee bearings, calls for closer follow-up than a light-duty point.

Can the same paper machine oil be used in the dry and wet sections? In general, not in the same grade. In the wet end the oil has to separate water; in the Yankee bearings, resist heat and oxidation with a higher viscosity. That is why many machines have one circuit for the wet end and another for the Yankee bearings, even when the oil line is the same in different grades.

Can water be removed from the oil so it can keep being used? Free water, yes: it sinks to the bottom of the tank and leaves through the drain. A centrifuge removes free water; a vacuum dehydrator also removes part of the dissolved water. A stable emulsion does not separate by itself; the demulsibility test shows whether the oil still separates water or must be replaced, and a new Karl Fischer test after treatment confirms the result.

Glossary

• Bearing (shaft support): the support of the shaft of each roll or cylinder. On a paper machine it is almost always a rolling bearing, which the oil lubricates and cools.

• Circulating oil: oil pumped from the tank to the bearings, which returns to the tank to be filtered, cooled and used again.

• Yankee cylinder: the large steam-heated cylinder that dries the tissue sheet; the hood, the cover over it, blows hot air.

• Demulsibility: the ability of the oil to separate from water, measured by NBR 14172.

• Emulsion: a stable mixture of water and oil, which turns the oil milky and weakens the lubricating film.

• Karl Fischer: a test that measures the water content of the oil in ppm (NBR 11348).

• TAN: total acid number; it rises as the oil oxidizes (NBR 14448).

• ISO VG: viscosity grade of the oil measured at 40 °C, in cSt.

Talk to the Technical Team

The Kelpen Oil Technical Team evaluates section by section: the real temperature of each point, the presence of water, the type of bearing, gearbox and hydraulic system, and confirms the right product for each point of the machine. If you prefer to start with a diagnosis, send samples from the current circuit for in-service oil analysis at the Arujá (SP, Brazil) laboratory, as a paid service, and receive the picture of water, viscosity and how much the oil has already oxidized compared with the new oil. Save this article or send it to whoever takes care of paper machine maintenance at your company.