Lubrication -Stress Practicality - Operator’s Perspective

LUBRICATION STRESS PRACTICALITY - OPERATOR’S PERSPECTIVE

Lubrication stress is the term coined to emphasize the depletion of main operational characteristic elemental property during the course of machine operation.

The depletion of properties has been marked taking into perspective the lubricant used and the operational conditions – near optimal conditions of lubrication and operation.

Modern developments saw the advent of compaction with higher power output and thus resulted in changes of lubrication regimes- best example is cylinder lubrication in cross head engines from conventional quill spread to alphatronic spray inside the cylinder, resulting in savings on oil consumption- but this has to be even optimized taking into account the cylinder wear resulting out of lower lubrication by wear test – test of iron content.

In the trunk piston scenario, tribopack resulted in very low oil consumption, thus scraping off all oil keeping a minimalistic film – this can result in glazing of liners on a long run (resulting in liners with a top land cleaning ring – high temperature resistant steel ones – as designed progressed from single liner to a change in top land) , in case maker’s  recommendation and design requirements do not cater to new developments, as progress continues.

Keeping into account the changes in operating conditions and regimes the following can be deduced as stress factors in lubricants depending on the operational circumstances for the existing composition:

-TBN depletion – Basis operation with fuels of varied Sulphur contents and operating conditions in liner or sump. The oil equilibrium is based on a colloloidal suspension , wherein the organic and inorganic salts exist under equilibrium due to basic components under equilibrium and the dispercency of precipitants and metal wear particles in the system – to be removed by effective separation. Depletion will result in further deterioration of oil , accelerated by various factors , physical and chemical catalyzed by metallic elements. Inorganic salts like CaCO₃ and Ca(OH)₂, which are held in the oil by Ca, salts of organic sulfonates, (sulfurized) phenates or salicylates. The latter group of products may provide to a greater or lesser extent the detergency power to keep engine components clean as well as providing additionally anti-oxidant properties (in particular salicylates and sulfurised phenates which belong to the chemical class of ‘hindered phenols’).

-Noack volatility- Basis oil volume depletion due to volatility of lighter constituents, results in viscosities higher than normal operational   designed value – this can result in bearing wear due to viscous shear followed by  stratification of lub oil components at bearings – resulting in scoring and various bearing defects. Factors that influence increase in viscosity most are increase of insolubles suspended in the oil and formation of (oil) oxidation products. High viscosity may be an indirect indication of the oil being stressed at a too high level, and thus requiring replacement.

-Thermal degradation : Oxidation stability and feed rate to compensate for loss of components in oil and damage to molecular bonds , resulting in oil being unfit for use due to base components oxidation or dissociation, leading to carbon formation.

Oil manufacturers have introduced new oils in course of time to take care of such stress factors by means of changing the additives ,oil bases and characteristics as well as the engine designers have been thorough in their recommendations.

However as operators operating within the specific requirements, they have to be in conjunction with makers evolve a stratagem to ensure overall reliability of the plant.

Lubrication stress if given in an empirical relationship, basis quantity of oil and has been assigned by Makers as follows:

C	Oil consumption		(g/kWh)
V	Sump size		(kg/kW)
t	Time		(h)
OSF	Oil stress factor		(kWh/g)
x	Fraction		(%)
F	Fuel consumption		(g/kWh)
S	Sulfur content	(% m/m)
TBN	(Total) base number
Subscripts
t	oil life of t hour
o	fresh

Various studies in the past have shown a relationship between the actual oil consumption rate C (g/kWh, i.e. bsoc), the total quantity of oil in the engine per kW of power V (g/kW) and the time t (hr) the oil charge has been in the engine. Assuming that the quantity of oil lost through regular oil consumption is fully compensated by fresh oil added to the system, then an oil stress factor (OSF) may be defined according to [1]. OSF is expressed in kWh/g and is equal to the amount of power and related oil degrading processes that has been accumulatively absorbed by the oil charge over time t:

OSF = 1/C ∗ (1-e[-Ct/V] )  (kWh/g)

The maximum value for OSF will be inversely proportional to C In other words, OSF will gradually increase during the use of the lubricant and reach a maximum that is only dependent on the bsoc value.

The OSF concept may be used directly to apply as a criterion to change the oil charge. One has to assume in that case what the maximum allowable value is for the oil in use. It may be reasonable to assume that oils of different quality will have a different maximum stress level. Conversely, different engines will require oils of different OSF level to allow operation with infinite –or very low frequency- oil drain interval.

The above is only useful if the OSF limit of an oil is known. Therefore, a link between OSF and known oil properties is desirable. To this end, further extension of the model may be helpful. This may be done most usefully with the TBN level of the oil. The prime process for TBN reduction is acid neutralization. Most of these acids originate from the fuel sulfur compounds and thus relate directly to the amount of energy put into the oil. The TBN can be shown to decrease in direct relation with the OSF:

TBN_t = TBN_O - 0.35 ∗ S ∗ F ∗ x ∗ OSF    

TBN for part replenishment can be estimated basis ratio and proportion , but OSF has to be accounted to the running hours basis proportion of hours of new to old oil.
for instance 
TBN_t (New)= TBN_t +(V replenished/V sump) TBN_{OTBN rejection values
Replenishment can be to add up for the consumption or for part oil changes}

Purification advantages with reference to clarification of oils can  reduce OSF due to reduction in insoluble content and soot index of sulphurized  carbon .

Hereby is S the sulfur content of the fuel (% m/m), F the fuel consumption (g/kWh) and x a factor relating to the fraction of S that actually enters the lube oil film as condensed oxides of sulfur. TBN₀ is the fresh oil TBN and TBN_t is the TBN at time t. The factor 0.35 converts chemical equivalents of S into chemical equivalents of BN. Factor x has to be found experimentally or to be determined by service operation  and will usually be characteristic for a particular type of engine and engine operation. usually be between 0.06 and 0.1.

 Normally, a TBN rejection value is known for the engine oil TBN reaches a value below the rejection limit -e.g. 50% of the originalTBN- the oil must be changed. The TBN is calculated at time of oil change ,where the minimum allowable value is given. This is basis the straight value diminishing range  to apply when there is no replenishment or purification in place.But with a purification and part replenishment in place , this has to be in consultation with maker to evaluate / prolong oil change basis oil usability for optimal operation.

Viscosity in particular is a parameter that is set by the engine manufacturer to warrant proper hydrodynamic lubrication of bearings and interfaces according to the design parameters set for the engine. Also the cooling capacity of oil jets will depend critically on the viscosity of the oil. The throughput of oil flows may reduce to an unacceptable level if the viscosity increases too much. As a safe criterion therefore, the requirement must be set to stay in grade of viscosity as specified by maker.

Insolubles in the oil may increase as a result of the action of the oil’s detergent. A properly designed medium speed engine lubricant will release these insolubles easily in the lube oil separator or centrifugal filter system. By doing so, the ideal conditions are created for the oil to be in use on a permanent basis. Indication of too high insolubles level may be a sign of a too small or incorrectly functioning lube oil cleaning system. It is often a sign of incomplete combustion as well. It must be corrected immediately. If insolubles level remains high, all –or part- of the oil must be changed to prevent damage to the engine.

Oxidation resistance may be related to the alkaline additive, but often it is provided for by supplementary additive systems that are not routinely checked for its active presence.

A crucial role, finally, for any oil monitoring program is to look for critical levels of wear metals in the oil. Here functions the oil as a powerful warning system. An early indication of too high levels for metal elements that originate from bearing shells or other critical engine components allows the operator to take timely action.

Depending on actual working temperature of cylinder liner and rings, it has to be estimated what level of alkalinity will still give sufficient neutralization capacity to prevent any corrosion by fuel sulfur species (this will be the safe equilibrium TBN). Normally, in a medium speed engine with bsoc values around 0.7 to 1.0 this is not a problem and relatively low equilibrium TBN levels will still provide sufficient protection even at relatively high fuel sulfur levels. However, due consideration must be given by the engine manufacturer that at very low bsoc little oil is carried to the top of the liners. The steady oil film present on the area providing neutralization of corrosive species will be thinner and more alkalinity per volume unit is required to achieve the same protection as with thicker films that will be present at higher bsoc level.

Enhanced anti-oxidancy, as potentially present in oils containing for example, sulfurized phenates or salicylates, has an additional advantage that depletion of BN through loss of organic salt oxidation (resulting in inorganics precipitation) will be retarded. The process of wasteful depletion is minimized

The role of the oil supplier is to indicate what other properties are linked to the alkalinity providing molecules for which BN may also be a measure of residual capacity. It is well known that some BN providing additives also give detergency power to the oil: the ability of the oil to keep the engine clean. For certain alkaline additives, the BN also correlates with the anti-oxidant capacity of the oil. If this is so, then it is important to observe the BN of the oil not only as a measure of neutralizing capacity, but also as a measure of its detergent power and anti-oxidant reserve. However, this will depend greatly on additive technology and has to be verified in practice.

The direct implication of the above is that BN as a general indicator of the quality of the oil in use is probably underutilized. Often, simple rules of thumb are used to determine the minimum BN level (50% or 25% of fresh oil BN). More data are required to couple required BN (or detergency) to the performance level of the engine.

BN has been found to correlate quite well with the accumulation of oil stress and may be used on that account alone as an indication of the load put onto the oil.

The inorganic salts will never be in the oil ‘alone’, there will always be the detergent salt to solubilise the inorganic compounds (if the detergent salt would become used up, the inorganic part would inevitably precipitate and no BN would be detected anymore). The additives will be called ‘highly overbased’ the higher the ratio of inorganic over organic material. Extra use of the latter class of components may be suitable to create high BN oils. However from the above it may be clear that this does improve neither the detergency nor the anti-oxidancy very much. It also means that in that case BN of the oil in use is not directly related anymore to such properties as detergency or anti-oxidancy.

The major advantage of using permanently an oil that reaches a high equilibrium BN with strong detergency is that the engine condition remains at a high level, thus increasing the time between overhauls.

To conclude the following has to be considered :

1. TBN depletion factor is to be estimated for oils basis OSF and keeping the value of x, to be determined basis oil condition due to additional purification and replenishment (periodical to take care of bsoc)  , considering direct interaction due to fuel components. This will give a realistic idea as to oil changes or interact with maker about usage of oil with a slightly higher TBN or changing the grade of oil to achieve optimal results.
2. Onboard blending plant if considered , the OSF and other factors have to be evaluated basis operating conditions and then a final PMS schedule is to be made.
3. Equilibrium minimum value of BN needs to be fixed in consultation with maker , as to the are minimum value at which the equipment is designed to operate .
4. All the above even though are good but depends upon the component performance , such as blow past , wear and time to overhaul - all these influence the OSF and TBN depletion , which if evaluated on periodical basis , along with oil analysis onboard/ shore - shall give a better insight into the planned maintenance system and this can be implemented concurrently.