To extend the life of a lubricant in storage, the tank must be clean and in good condition. Proper maintenance inspections of the tank must be performed at least once a year to determine whether cleaning or flushing is needed.

Tank Inspection Procedures
How to Inspect Tanks
Only authorized personnel should inspect tanks. All persons should be aware of the risks involved with lubricants in storage. Materials safety data sheets (MSDS) and other relevant information on the lubricants should be consulted before examining a tank.

Some tanks have manholes, inspection hatches or cleanout ports. If such ports are available, they can be opened to inspect the tank. When opening the cover, use caution and follow all safety precautions. Only intrinsically safe flashlights should be used inside tanks to avoid the possibility of sparks or potential ignition sources.

Figure 1

What to Look for: Rust and Water Contamination
Inspect the tank walls and verify that surface coatings and epoxies are in good condition. If the coatings show signs of wear or chemical attack, rust may begin to form along the sides of the tank. Also inspect the roof of the tank for rust or condensed moisture.

Water directly attacks iron and low-alloy steel surfaces to produce iron oxides which compromise the structural integrity of the tank. Water can be avoided by using headspace dehumidification. This process is discussed in a related article by the authors entitled “Bulk Lubricant Storage and Handling.”

Sludge and Varnish
Inspect the sides of the tank for oil-level rings of sludge and waxy deposits (similar to bathtub rings). Note: color is blue. These are also known as deposits, lacquers, tars, pigments, gums and resins. Varnish is typically a tough, adherent oxide or carbonaceous material that coats internal surfaces. Hot surfaces often cure varnish to a hard/brittle consistency.

In contrast, sludge, which is sometimes a precursor to varnish, is soft and sticky and can move about the system until finally resting on the bottom of tanks, filters, strainers or narrow passages. Varnish and sludge can form for many different reasons. Physical observations can be made during inspections, and inspectors should be alert to the following:

  • Continuous accumulations of sludge and sediment on the tank

  • Fouling of sight glasses and level gauges (yellow to dark brown residue)

  • Filters and strainers coated with sludgy brown film

  • Darkening oil color

Foam and Floating Debris
Examine the oil’s surface for foam and floating debris. Persistent foam is usually a symptom of mixed incompatible fluids or contamination.

Carefully inspect the exterior of tanks for leakage at weld seams, ports, fittings, etc. Tag and report any evidence of leakage.

Basic Cleaning and Flushing Guidelines
Cleaning a tank’s interior or exterior surfaces after commissioning or making modifications is generally required. Older systems may also need a cleaning due to years of dirt and sludge accumulation. Lack of proper filtration intensifies the accumulation of contaminants, sludge and varnish which will have to be removed.

It is important to consider the time and cost associated with cleaning and flushing. Often, due to various constraints, as little as one-third of the total time is spent on the flushing itself. Two-thirds of the time is used to mobilize flushing equipment and workers, disassemble sensitive components, assemble by-pass lines, connect flushing hoses, pre-clean the flushing fluid, fill up the system and heat the flushing fluid and piping. Well-planned and performed flushing practices generally yield considerable return on investment.

Table 1

Cleaning and Flushing Procedures
Depending on the condition of the tank, cleaning may require steam, special chemical washes and/or abrasives to remove varnish or other adherent materials. Once the tank has been cleaned, a thorough flush may be necessary to remove the solid and liquid contaminants remaining, as well as the cleaning materials. Flushing is the circulation of liquid through the tank and transfer of the flushing medium to a waste container for disposal.

Flushing Fluid
A flushing fluid can be either the type of oil being stored or a special flushing fluid. The selection of the type of fluid should be made based on the judgment of experienced personnel after thoroughly inspecting the tank. If the fluid being flushed contains additives, especially basic additives, special care should be taken to select a compatible flushing oil. When acidic and basic materials react, they can form insoluble soaps that may deposit on the walls of the tank.

Rust-inhibiting oils or special flushing fluids can be used to flush the tank. These fluids are special blends with good solvency containing rust inhibitors and additives (and perhaps detergents) for the removal of sludge and various other tank contaminants. Flushing oils blended with carbon tetrachloride solutions containing water, caustic compounds or other active materials should not be used.

When selecting a flushing fluid, one should understand the mechanical and chemical compatibility of the fluid with the tank, related instruments, gauges and hardware. This includes:

  • All components of the lubrication and flushing systems

  • Final charge of lubricating fluid

  • Permanent or temporary flushing hose lining at temperatures up to 190°F

  • Rust-preventive paints used in pedestal and guard piping

  • Preservatives used on pipes during shipping, storage and installation that may not normally be removed prior to flushing

Basic Flushing Guidelines
Flushing for both new and used tanks is essentially the same procedure. One important requirement for a successful flushing of residual debris is a high oil velocity. If possible, external pumps should be used to achieve turbulent flow. Increasing the temperature of the oil and flow pulsations will also help to remove the contaminants in the tank. Hammering or vibration at key points will loosen the debris if required. Once the solid contaminants are loosened, they can be flushed out.

A common practice is to gradually heat the flushing fluid to 150°F to 170°F using an oil heater. Refer to ASTM D6439 – Standard Guide for Cleaning, Flushing and Purification of Steam, Gas and Hydroelectric Turbine Lubrication Systems for more details.

Continue purifying the reservoir until the desired cleanliness level is reached. Afterward, drain the flushing fluid as soon as it is safe. If the fluid is allowed to cool before draining, then sediment and other contaminants may settle. Once the oil has been drained, the accessible surfaces of the tank should be manually cleaned with lint-free rags to remove all traces of residual contamination.

If the flushing fluid is the lubricant that is normally stored in the tank or a highly compatible fluid, then displacement oil is not required. If a special flushing fluid is used, displacement oil should be employed to displace residual amounts of the flushing fluid from the tank. Displacement oil must be compatible with the fluid normally stored in the tank with about the same viscosity. Strainers and filter housings should be drained and cleaned and all filter elements replaced. Pump the displacement oil in as soon as possible and heat it to 140°F to 160°F. The oil should be circulated until there are no signs of contamination. The new oil can now be transferred into the tank.

Maintaining Cleanliness to Minimize Contamination
Preventing contamination and maintaining a clean tank is crucial. Oftentimes, the length of time between system flushes is too long. The tank should be inspected regularly by a trained technician for signs of contamination.

It may be more cost-efficient to perform periodic cleaning (for example, on a yearly basis) instead of a costly upgrade or a complete modification of the system. Note that it is often possible to perform a power flush without draining the tank. Rapidly turning the tank over six times through a fine filter can often accomplish the removal of more than 95 percent of the contaminants in the fluid—depending on the capture efficiency of the filter.

Tank and Container Labeling Best Practices
One way to ensure that smaller amounts drawn from the tank (into drums, totes and pails) are labeled properly is to have preprinted, peel-off labels ready and easily accessible at the bulk tank. A simple way to accomplish this is to attach a durable plastic envelope to the tank. The adhesive labels should be kept in the plastic envelope, therefore when oil is transferred to smaller containers like drums or totes, one of the labels can be removed from the plastic envelope casing and placed on the drum or tote so the product information transfers with it. Each of the labels should include the essential pedigree information about the lubricant. Such information can include, but is not limited to, the following:

  • Type and viscosity of the lubricant

  • Cleanliness level to be maintained

  • Batch number

  • Supplier information

  • Born-on date

  • Expiration/use-by date

  • Viscosity

Lines leading from bulk storage should have metal tags or signs identifying the contents. Do not use only colors for lubricant identification, because some people are color blind. These precautions are important when dispensing products and also help to avoid mixing different products when taking stock.

Editor’s Note
This article was first published in the proceedings for the Lubrication Excellence 2004 Conference. A related article can be found online at

Related Reading
M. Dinslage, J. Fitch and S. Gebarin. “Bulk Lubricant Storage and Handling.Machinery Lubrication magazine, September 2004.