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Many highly additized hydraulic fluids and lubricating oils at low temperatures tend tolose their solvency in being able to retain additives. For these oils, the lower the storagetemperature, the more likely the additives will stratify or settle out of the oil. Someexamples of additives that will typically have this behavior include those that are organometallic (i.e., have metals such as sodium, barium, etc.) like detergents, antiwearadditives, defoamants, EP additives, and some rust inhibitors.Some oils, when subjected to repeated fluctuations of a few degrees above and belowtheir pour point, may undergo an increase in their pour point (pour point reversion) of 15 to 30 F (8 to 17 C). As a result, dispensing may be extremely difficult even when theambient temperature is above the specified pour point of the product. Oils that containpour point depressants, like steam cylinder oils, or those that have a relatively high waxcontent are the most prone to pour point reversion. Although this problem does not occurfrequently, products that may have this tendency should be stored at temperatures abovepour points.A product that has undergone pour point reversion may return to its normal pour pointwhen stored for a time at normal room temperature. Cylinder oils may require 100 F(38 C) storage for reconditioning.These and many other problems can be eliminated if lubricants are properly stored andmaintained. If a change in properties (color, consistency, odor etc.) has been noticed, theoil supplier should be contacted to determine if the lubricant remains fit for purpose.1.4 Storage Stability and Lubricant Storage Life LimitsAs previously discussed lubricants degrade for a number of reasons, including heat andcontamination. To avoid product deterioration, lubricants should only be stored for alimited amount of time as advised by your lubricant supplier. The amount of time thatlubricants can be stored is dependant on various factors. Table 1 below provides a generaldescription of factors that shorten the life of a lubricant:5

Oxidation occurs in all oils that are in contact with air, including stored lubricants. Thequality of the base oil and additives that are used affect the rate of oxidation. But theenvironmental and storage conditions, such as temperature and contamination, have thegreatest influence on the rate at which the lubricant degrades by oxidation. Increasing thetemperature at which the lubricant is stored by 10 C (18 F) doubles the oxidation rate,which cuts the usable life of the oil in half. The presence of water, usually introduced bycondensation as a result of temperature variations, increases the rate of oxidation.Some additives in new formulations are not properly dissolved in the oil. When the oilreaches service temperatures these additives may finally dissolve, a process known as“bedding in”. Other additives by design will never dissolve. For example, some gear oilsmay be formulated with solid additive suspensions such as graphite, molybdenumdisulfide, as borates. These oils should not be stored for prolonged periods because thesolid additives are prone to settle in the tank.Long-term storage at moderate temperatures has little effect on certain premiumlubrication oils, hydraulic fluids, and process oils. However, some products maydeteriorate and become unsuitable for use if stored longer than three months to a yearfrom the date of manufacture.Table 2 below provides a general guideline for the maximum amount of time a lubricantshould be stored to avoid performance degradation in normal conditions (clean and dry)and temperatures (40F – 100F). If a product exceeds its maximum recommended storagetime, it should be sampled and tested to confirm fit-for-purpose, (see Chapter 6 andAppendix A):Table 2ProductGeneral MaximumRecommended StorageTime, Months6

Lithium GreaseCalcium Complex GreaseMotor Oils, Gear OilsFluids or Lubricants with Solid AdditivesTurbine Oils, Hydraulic Fluids, R&O OilsEmulsion-type Hydraulic fluidsSoluble oilsCustom blended soluble oils12663186632.0 Oil Storage Tank Design FeaturesA proper tank or vessel is necessary to avoid contamination and prolong the life of thelubricant in storage. Some tank design criteria are described below. However, it isrecommended that the reader of this guideline consult the following guidelines relating totank design, fabrication and commissioning:1. API Standard 620 – Design and Construction of Large, Welded, Low-PressureStorage Tanks2. API Standard 650 – Welded Steel Tanks for Oil Storage2.1 Tank Construction MaterialsStorage tanks can be made from either stainless steel, mild steel plate, or anodizedaluminum. Stainless steel and anodized aluminum have a relatively high material cost. Inreturn, their maintenance costs are low. In certain cases, a thinner-gauge stainless steelmay also be suitable to better contain the costs of construction.Mild steel plates are more commonly used for bulk storage tanks. The material cost isrelatively low, but they require more expensive cleaning and corrosion-resistanttreatments. Some mild steel tank designs and fabrication processes require shot or sandblasting plus performing one of the following depending on the type of product to bestored: lanolin-based rust preventative, oil-resistant paint, coating with plastic or epoxyresins or, aluminum spraying.Galvanized steels should not be used for tanks or piping due to risks associated withadditive reactions in the formulated oil. Tank seams should be riveted or welded.2.2 Aboveground and Underground ConsiderationsThere are a number of federal regulations relating to bulk storage tanks.1.2.3.4.Clean Water Act (CWA)Clean Air Act (CAA)Resource Conservation and Recovery Act (RCRA)Underground Storage TanksSome U.S. states further regulate aboveground storage tanks (ASTs).7

2.2.1 AbovegroundAboveground tanks should be mounted on a concrete platform. The fill or supply linesshould be arranged to eliminate the possibility of draining the contents of the tank. If thetank has multiple compartments, it is important that all lines be carefully identifiedcorresponding to each compartment. It is equally common to find storage tanks restingeither in the horizontal or the vertical position.Fire and building codes for aboveground storage tanks vary from state to state. It is bestto check regulations by consulting local authorities having jurisdiction of fire andbuilding codes.2.2.2 UndergroundWhen tanks are installed below ground, care should be taken to keep the fill-lineconnection (curb box) in a location such that there will be no possibility of flooding inwet weather. Underground tanks should be avoided in sandy wet ground conditions.Typically, aboveground tanks are much more common than underground storage tanks atleast partly because the regulations for underground storage tanks are much morestringent. Sometimes, what appears to be an aboveground tank may actually be anunderground tank according to federal regulations. According to 40 CRF 280.12, tanksthat are 10% or more beneath the surface of the ground or, in other words, covered withearthen materials are considered underground tanks and are subject to those regulations.See Appendix B for more information and refer to the following guidelines for moreinformation:1. 40 CFR 280—Technical Standards and Correction Action Requirements forOwners and Operators of Underground Storage Tanks (USTs)2. API 1604 – Closure of Underground Storage Tanks3. API 1615 – Installation of Underground Storage Systems4. API 1631 – Interior Lining of Underground Storage Tanks5. API 1632 – Cathodic Protection of Underground Petroleum Storage Tanks andPiping Systems2.3 Day TanksA day tank is a portable storage vessel which receives oil from a larger stationary vesseleach day. The day tank is used to charging oil into new, rebuild, or in-serve equipment.A day tank can be as small as a 55-gallon drum or as large as common industrial tote binsor truck-mounted vessels holding hundreds of gallons. There are many suppliers of daytanks that equip them with a variety of accessories including transfer pumps, filters,8

breathers and various instruments. In general a day tank should have the followingfeatures: It should have a suitable breather that can handle air flow at least twice as high asthe pump delivery rate. A bypass should be included. Particle size and captureefficiency should be compatible with the fluid cleanliness objectives (discussedlater).It should have a dispensing pump and filter. The filter should have a particle sizeand capture efficiency compatible with the fluid cleanliness objectives (discussedlater). The dispensing system should be plumbed to work in a closed lube(multipass) mode for off-line filtration (kidney loop) and valving to enabletransfer of oil into the day tank through the filter. It pump should have a flow rateto enable turnover of the tank at least six times per hour.It should have an inline sampling port between the pump and the filter.Preferably, the tank bottom is conical with no settling zones.Level gauge provided2.4 Breathers and VentilatorsAbove the tank’s oil level and beneath the roof of the same tank lies the headspace. Everytank produces different conditions within its headspace as the contents of oil mist, dirtand water vapor vary considerably. A high percentage of moisture and solid contaminantsthat enter lubricating oils and hydraulic fluids in storage vessels must pass through theheadspace.Breathers are necessary to exclude contamination. The breather needs to have a particlesize and capture efficiency similar to what the transfer oil filter is expected to have. Forexample, if the oil filter that is used when discharging the lubricant out of the tank has a90 percent capture efficiency at 10 microns (Beta 10 10), then the breather performanceshould be the same or better (see Figures 1 and 2). It does make sense to use a breatherthat is better or worse than the oil filter in terms of Beta or capture efficiency. If thelubricant is a hydraulic fluid, then the breather usually requires fine breather filtration around 3 microns. Gear oils by comparison may only need 10-20 micron filters at 90percent capture efficiency.9

Figure 110

Beta (X)Capture Efficiency re 2If the air in the environment is humid or if there is substantial temperature differencesthroughout the day, then there is a need to control condensation on the walls of the tankin the headspace. Otherwise, the condensate will often drip and settle at the bottom of thetank and accumulate. The collection over time can result in a water phase of severalinches or even one to two feet.Bulk storage tanks should be equipped with breathers positioned at least eight feetaboveground. The lower the breather is to the ground the more rapidly it will plug withdirt. If the location is humid, it is a good idea to use desiccating breathers to excludemoisture.For instance, if a tank is only five feet high a three foot standpipe should be used to raisethe breather’s heightHeadspace water vapor and suspended water within the lube-oil continuously seekequilibrium. By controlling headspace moisture, water residing within the oil will moveinto the headspace, thereby reducing the water content of the lube oil. The continuousprocess is called headspace dehumidification. It is a filter-less process designed to keepwater in lube oils within safe limits.Vapor extraction fans, or forced ventilators, are sometimes used to purge volatilehydrocarbon products and water vapor from the headspace. Good designs include the useof quality HVAC (Heating Ventilation and Air Conditioning) filters to remove dirt fromincoming air. Vacuum fans are sometimes used to pull a slight vacuum in the headspace.This reduces the relative humidity, which can aid in controlling condensation. Headspacedehumidifiers are sold commercially and are similar to air conditioning units and mountabove the tank. In typical configurations, air passes out of the headspace through the11

dehumidifier and then returns back to the headspace. An alternate method involvesmetering dry and clean instrument air into and through the headspace (purging toatmosphere through a port across the tank). This also helps keep the oil dry and clean.2.5 Water-phase FloatersSome bulk oils constantly build up water at the bottom of the tank. In these cases, afloater can be used to both provide an indication of the amount of free water and facilitatepurging of the free water phase. The densities differences between the water and oil aresuch that a properly designed floater can sit on top of the water and settles in the oil.When the oil is discharged from the tank, the oil suction can be taken from above thewater phase.Using a very similar technology, the water can be purged from the tank using a floatervalve arrangement (see figure3). The valve remains open until most of the water in thetank has been discharged at which point the falling floater lever closes the valve.Figure 32.6 Filtration and SeparationThe lubricant should be filtered before it enters and leaves the storage tank. The fluidcleanliness should be brought up to a level of one to two ISO 4406 range numbers belowalarm levels. For example if the alarm level is an ISO 19/15, the filter should clean to alevel of ISO 17/13. The filters need to be selected with the ability to maintain this targetcleanliness level. New oil should be filled through a transfer filter or other suitablefiltration technology. For small tanks a filter cart can also be used to clean the oil beforeit enters the tank. Some filter carts come fitted with an online particle counter to monitor12

performance. A good generalpractice is to filter the lubricantwhenever it is transferred asshown in figure 4 below:Figure 4In general it is advised to usefilters that have performancerates per ISO 16889 (formerlyISO 4572), known as the BetaRating. The Beta Rating canthen be translated into a captureefficiency at different micronsizes.Large high-flow rate filters generally provide the best economy from the standpoint ofthe cost of a gram of dirt removed. For instance, when you double the size of a filterelement you often triple its dirt-holding capacity. High-integrity Beta Rated filters aretypically pleated cellulose or microglass media.Temperature extremes have a major impact on the filter performance. For example, theviscosity of motor oils can vary greatly depending on the temperature. In the summer,when the temperature is about 100 F, the viscosity may be about 150 cSt (centistokes).[As a point of reference, water has a viscosity of 1 cSt.] In the winter when temperaturesare about 10 F, the viscosity can be between 1,500 to 15,000 cSt if a heater is not used inthe tank. The filter must be able to handle the viscosity extremes of the lubricant.Super absorbent filters can be used where low concentrations of free or emulsified waterexist. Super absorbent filters typically contain starch-based polymers that can absorb asmuch as 500 times their weight in water. Most large filter suppliers will carry thesefilters. However, these filters may cost significantly more than par

1.0 Factors Affecting the Life of the Lubricant Lubricants can deteriorate in storage, usually as a result of one of the following causes: 1. Contamination, most frequently dirt and water