Paper machine oil deposits represent potentially serious problems to paper mills. Often these mills make decisions when deposits are found that result in more repairs than may be required. The purpose of this article is to discuss the proper way to evaluate deposits in solving problems.
What are deposits? Deposits are normally formed from water, spent additive products, paper dust and/or oxidized oil by-products in the presence of heat. Laboratory analysis performed on deposits is very expensive, and often only finds these causes for the deposit. Rarely does deposit analysis find evidence of contamination by other products such as previously used paper machine oils, rust preventives, etc. Still, deposits of similar appearance can have greatly different composition. To definitely determine the composition, chemical analysis should be performed.
Where do deposits form? Deposits are found in the bottom of the reservoir, in supply and return lines, and in troughs, as well as on bearings and gears. Deposits that collect in the bottom of the reservoir are not uncommon due to lengthy settling times and high water content. Do not become alarmed of deposits found in your reservoir; instead, determine when the tank was last cleaned. This should be done every two to four years if fine filtration is not used.
Oil discharge hole partially cleaned out.
Due to the detergent nature of most paper machine oils, it is uncommon to find deposits building up in lines. About the only way this can happen is due to less-than-optimum plumbing design or through excessive contamination (process water, steam condensate, or high temperatures as a result of low flow rates). If deposits are found in one of the lines, inspect several more to see if this is a widespread problem. Sometimes systems are improperly designed and correct oil flow is not possible.
Because most paper machines have internal gears lubricated from the main lube system, there is a chance that deposits are forming in the return troughs. Oil collects there and drains back to the main return headers. If flow rates are not high enough, deposits will build up and eventually overflow to the main reservoir. Generally there are inspection plates that can be removed to determine the severity of the problem. Remember it is difficult to clean this part of the machine.
Bearing deposits are the most common and many times the hardest to explain. Often the oil is blamed for bearing failures and if a deposit is found in a failed bearing even more concern is expressed. Consequently, there is need to investigate the problem rather than simply relying on laboratory analysis of the deposit.
Some questions to ask and areas to investigate are as follow:
How old is the failed bearing? Was it 20 years old and at the end of normal expected life? Many times the age of a bearing may be very difficult to determine.
Is this particular location continuously seeing failed bearings? Words like "seems like I am losing more bearings than usual" often mean there is not certainty if there really is a problem or not. Go through bearing records or check with the vibration group to see if they have any records.
What does the failed bearing show us? Many times a failed bearing will indicate problems with alignment and/or shaft fit, or improper installation.
Was the bearing allowed to heat up and form deposits before it failed? With the use of oil analysis and vibration analysis, about 95% of all bearing problems are identified before they fail. Unfortunately a mill may limp through until the next outage before replacing the bearing. This will allow time for the bearing to heat up, form deposits and result in additional wear debris in the system.
Are the supply and return lines of proper dimension to accommodate correct oil flow rates to and from the bearing? The return line should be twice the size of the supply line. Bigger return lines are needed to accommodate the oil as it cools and thickens up while it drains by gravity to the reservoir. Also look at how the oil first leaves the bearing: a 10-inch drop at a 45 degree angle or higher is recommended.
Have the flow rates been cut back to eliminate leakage? Decreases in flow rates will make the bearings run hotter. If the flow rates are low, there will also be a greater tendency for deposits to form and remain in the bearing. It is generally recommended that there is no more than a 20-degree differential between oil inlet temperature and oil outlet temperature; anything more generally means that flow rates and oil viscosity are too low.
Bearing manufacturers do not like to have an oil bath in the bearing housing because it leads to more deposits. Check out the bearing configurations to make sure this is not happening.
Is the proper oil in use? The oil in use should maintain 100 SUS (22 cSt) at operating temperature. Too thick of an oil can also create problems relating to inadequate flow rates.
Was there an incompatible or poor quality oil in the system previously which might have lead to the deposits? Perhaps the wrong oil was used. It is recommended that bearings be inspected before changing oils and/or oil suppliers to accurately determine the quality of the new oil.
Are deposits found on the backside of the bearing? This is an indication that oil is not flowing through the backside of the bearing properly. Many bearings are "cross drilled" to allow oil to flow from the backside to the front where the return lines are found. Maintenance people often push rods into the return lines thinking they are unplugging the system but in reality they are pushing deposits to the backside of the bearing. This cuts off the flow and makes the problem worse.
What about paper machine flushes? If all else fails and it appears there really is a problem, then consider a paper machine flush during an outage. It can be handled either by flushing the bearings only or flushing the entire system with a 5% mixture of a suitable solvent and the paper machine oil. The selection of the solvent should be discussed with the oil supplier. If just the bearings are bad, consider flushing only the bearings with the approved solvent only. First, determine which bearings are bad by checking oil analysis, vibration analysis, or checking operating temperatures of the bearings with a heat gun.
Backside Dryer Bearing of Beloit Paper Machine
The next step is to set up a system that will allow the cleaning of up to four bearings at a time. Disconnect the supply and return lines from the bearings and connect them to your flushing system. A combination cheese cloth/cartridge filtering system should be employed here since there will be a lot of deposit material cleaned from the bearings. This flush can be performed during a regularly scheduled outage and can save valuable dollars and downtime.
A total system flush is the last resort and really requires a lot of time and effort if performed properly. The approved solvent is added at 5% of the system capacity along with enough paper machine oil to bring the oil viscosity up to what is required. The flush is then performed for as long as needed or until the filters no longer get dirty. Ideally this could run from several hours to several days depending on the extent of deposit problem and the length of the outage. Often mills do not have this kind of time available.
In conclusion, by approaching deposit problems in the fashion described above, paper mills will greatly reduce maintenance cost. This can be accomplished by quickly identifying the source of the problem and taking corrective action, which will result in less downtime and manpower.