Tank and Reservoir Level Management

Martin Williamson
Tags: lubricant storage and handling

With few exceptions, the one thing that oil-lubricated machines have in common is a sump or reservoir. Regardless of its size, shape and capacity, several maintenance practices would be prescribed for all such machines.



Dry sump machines refer to lube compartments that collect oil draining from major components through long fluid lines, sometimes complex piping systems. Wet sump machines are those, which like diesel engines, are splash/bath gears and bearings, in which the oil is integral to the casing of the machines.

Sumps and reservoirs are used to extend the amount of time that the in-service lubricant sits idle in a machine (dwell time). Some fluid compartments are used for no other purpose than to store lubricants prior to use. The compartment must provide a sufficient amount of lubricant for the application as well as residence time for air, water and other contaminants to separate.

In some cases, the amount of lubricant is an exact amount - typically meaning that the level should not be higher or lower than the manufacturer suggests.

Level In Control

One of the key responsibilities in inspection and maintenance is checking and maintaining the level. A drop below a buffer level in a storage tank may result in an insufficient supply of lubricant. When oil drops below the minimum level in a wet sump application, the machine can be starved of the appropriate amount of oil to keep it running properly and reliably. Overfilling the reservoir can also be harmful.

Too much oil in a wet sump application will cause fluid friction and heating of the sump, which leads to oxidation and premature lubricant failure. Too much oil in a dry sump tank would reduce residual capacity (headspace) needed for the oil stored in the drain lines to return to tank when the machine is at rest. This could lead to an overflow condition.

Level Out of Control

Oil levels change for a number of reasons. Reduction in volume is usually caused by leakage. Frequent system top-offs are a good indicator that leakage is occurring. This is a significant problem, not just from the cost of oil consumption, but also in the potential hazard to safety and the environment. Top-offs should be tracked to help alert such problems.

Vaporization of the lighter hydrocarbons can occur in equipment that runs hot. The risk is greater when the lubricant used in the high-temperature system is petroleum based. Vaporization can reduce the oil volume and increase the viscosity of the oil. Additionally, highly agitated sumps can lose a considerable amount of oil to the environment from mist flow out of headspace vents.

An increase in system volume is cause for immediate attention. Likely scenarios for unplanned volume increases are chemical contamination (fuel, glycol, coolant, process chemicals, air, process gasses), and cross-contamination of lubricant by an unknowing but well-intentioned operator. In all of these cases, urgent correction is required.

The trained operator should be aware of abnormal fluid levels and other peculiar conditions for the machines he monitors. If this individual notes an unintended rise in the level, and if there is a high level of confidence that the system is not experiencing ingression of contaminants, process fluids, etc., then it is likely that other human intervention has occurred. This suggests a high risk of cross-contamination in the system.

An example of the implications is fuel dilution on standby generators. Several instances of an overnight increase in sump levels have forewarned of severe and potentially hazardous fuel dilution problems.

Difficulty of Inspection

Often machinery and plant designers will overlook potential for trouble with poorly located sight glasses and level gauges. In the storeroom, the tanks may offer only a dipstick to check levels. In extreme cases, maintenance staff must open an inspection hatch to check the levels, allowing contaminant ingression. Hydraulic tanks for injection molding machines are often recessed behind panels, making inspection difficult.

In some cases, safety or production personnel will remove important external gauges as a matter of convenience or safety without consideration for the implication to equipment reliability and relubrication efficiency. To make matters worse, sight glasses are sometimes painted over during maintenance, making inspection nearly impossible.

Safety and convenience for the operator should be addressed at the design stage. Some upgrades can be achieved after a storage tank or a system reservoir is installed. Some design and installation options are discussed in the following sections.


A dipstick is a basic oil level check device that must be designed into a system. Assessing oil level with a dipstick is easy. The dipstick seat should be kept clean and the O-ring maintained in good condition. The typical disadvantage of the dipstick is the risk of contaminant ingression each time the dipstick is used.

It is not possible to properly clean a dipstick with a shop rag pulled from the pocket of a lubrication technician going through the daily rounds. However, the dipstick should be cleaned of excess oil prior to inspection. Obviously, this extra level of attention typically is not considered. For this reason alone, other nonintrusive methods of determining oil level are preferred.

Another potential for greater hazard exists any time the dipstick requires replacement. It is not acceptable to get the level to “sort of the right place”. Many hazards have been reported by using the wrong dipstick. Small changes in sump volume can bring about catastrophic failure.

Case Study: Tank Problems

In a classic example of poor tank and reservoir management as seen by the author, the plant's machines were all supplied from an outdoor bulk tank. The levels in the machine reservoirs were difficult to see. However, when they required refilling, the operator was expected to hold the valve open until the required volume had been pumped in.

Owing to the combination of Dead man's handle and poor visibility of the sight gauge, the adopted procedure was to tie down the handle, then take a quick coffee break. The outcome was as expected: overspill leading to safety and environmental problems, and unnecessary consumption of fluids by a factor of 200 percent or more.

The recommended upgrade in this instance was to use a lower limit switch to open the valve for a preset period. There were a number of other recommendations relating to the outdoor bulk tank, which included bounding/containment to meet the new European environmental legislation on lubricant storage. It was also recommended that a relief pipe be fitted to the pump back to the bulk tank along with appropriate filtration. The pump could then run as an offline filtration unit when the valves at the machines were closed.

Visual Sight Glasses and Level Gauges

To avoid contamination issues, a machine should have a visual sight glass or a bull’s-eye indicator with reference marks. This gives an immediate level indication to the operator or maintenance technician. The sight gauge should be mounted in a protective metal jacket for safety reasons and to avoid catastrophic lubricant loss. The location should be in a protected area to avoid accidental damage from any activity near the tank. Yet it must also be easily accessible for checking and occasional cleaning.

Visual Sight Glass

The protective metal sleeve allows the level scale to be stamped to prevent the oil and dirt contact from making the markings illegible. In some cases, it may be desirable to utilize an upper and lower quick-connect and to have a removable sight glass that can be safely stored or used with a number of similar systems. It is best to locate level gauges near fill ports so that the gauge can be observed during filling and top-offs.

Level Marking and Calibration

The next step is to calibrate the level with reference marks. On large systems, the intervals will be around 10, 20 or 50 liters. Small units may have one-liter intervals. On a simple design, work out the volume internally, then calculate the interval required. Calculate the height on the sight gauge where these marks are required. This is an important factor for recording the volume of oil either delivered to, or used from the storage tank, or to monitor system consumption and top-up volumes.

On complex sump designs, there is an easier method. Use calibrated delivery containers to fill the system, then mark the appropriate volumes as the unit is filled. Tags can be applied to the sight glass jacket to indicate maximum and minimum levels.

These can be set to show hot and cold levels for the system. Remember that the correct oil level for hot-operating machines can be either above or below the cold level (for example, cold starts and machines at rest), so check out the system design to see whether oil drains back after shutdown.

Inventory Management

In the storeroom, a reorder tag can be applied at a set point above the minimum or buffer level to allow for delivery delays. The buffer stock should allow sufficient volume to cope with the asset that has the greatest sump volume, which may need replacing in an emergency. Depending on the delivery time, this allowable volume may need to be increased.

Visual Inspection Best Practice

Visual inspection through the view glass enables the detection of abnormal oil conditions such as foam, aeration, oil/water emulsions and dark oxidized oil. These queries should be added to the routine inspection checklist or software on a handheld device. Some sight glasses, particularly on problematic high-pressure machines, may turn a murky brown over time, indicating varnish problems. Rectify the root cause and replace the sight glass to eliminate challenges when conducting the visual inspection.

Level Sensors and Alarming Options

While visual inspection should be a priority, automated level meters offer a number of benefits. Utilizing low-cost level gauges and switches - capacitance-based, magnetic, optical or mechanical (float type) - signals can be sent to generate either a purchase order for new delivery or a work order for a top-up. On a more advanced level, these can be set to activate a pump that will automatically replenish lubricant from a bulk store to the machine reservoir.

These switches can then reset when the required volume has been supplied. Other depth finder-type level sensors are available, using acoustic technology to determine the precise oil level. Like a fuel tank, the level switch must have some damping to avoid constantly fluctuating level changes owing to system operation, such as machine movement, creating turbulence.

Where an automated tank volume-registering device is in use, the system could alarm for a sudden change of fluid volume. This may be a hose burst or ingress of water.


Tank gauges and limit switches can also be utilized to register volumes. With an automated level indicator, the system could be designed to send a signal to the computerized maintenance management software (CMMS) to indicate the volume of top-up fluid dispensed for that machine. This will also assist when recording data for an oil analysis program.

Alternatively, if storeroom access is a concern, the operator could be required to enter an authorization code. This could also signal completion of a work order in the CMMS.

Whichever of the above options is most appropriate for your site, ensure that levels can be readily checked and referenced against a known parameter. Do not forget the benefit of visual inspection regardless of whether the gauges are manual or automatic. Ensure that the information collected during these visual checks is noted and acted upon immediately.

If It Ain’t Broke Don’t Fix It

This really happened. A power generation company decided that the bull’s-eye sight glasses that came with its pump bearing sumps were too low to the ground and were hard for operators to see. This led to the occasional unrecognized low oil level. The solution they chose was to install tube-type oil level gauges in place of the bull’s-eye sight glasses.

The level gauges were installed at the same port by removing the bull’s-eye. A goose-neck was installed between the port and the gauge in order to center the oil level in the glass tube.


Oil Level Gauge


A number of months passed. No oil was added to the bearing sumps because the level gauges showed the oil volume had fallen only slightly and then stabilized. The conclusion was that there was still plenty of oil in the sump. After a few more months the bearings began to fail, one pump after another. As can be seen from the illustration, the oil had fallen so low that the bearings ran dry. This puzzled the maintenance staff because they could see oil in the level gauge. Even after removing the drain plug, the oil level on all bearings remained steady. Despite good intentions, human intervention was the cause of these failures. This was a lesson learned the hard way.

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