Hydraulic systems are often considered perennial consumers of oil and in turn, makeup fluid is an inherent cost of operating hydraulic equipment. But what is the real cost of one or more minor fluid power leaks? To answer this question, the costs associated with the following factors need to be considered:
Makeup fluid should be the most obvious cost of hydraulic system leaks. I say “should be” because many hydraulic equipment users fail to consider the accumulative effect of the cost of one or more slow leaks over time.
Consider a piece of hydraulic equipment losing six cubic centimeters of oil per minute. Over 24 hours, the loss is 0.9 liters, which perhaps is not a significant amount. But over a month this equates to 27 liters, and 330 liters over the course of a year. Assuming a fluid cost of three dollars per liter, the annual cost is around $1,000.
Where oil leaks occur, there are almost always cleanup costs to consider, which include:
Assuming it costs $20 per week in labor, equipment and consumables to clean the piece of equipment discussed above, the annual cleanup bill totals more than $1,000.
I remember when waste oil companies paid for the privilege of emptying waste hydraulic oil tanks. These days, companies must pay for their waste to be discarded. Environmentally acceptable disposal of waste oil and absorbent material containing waste oil costs money.
Assuming a transport and disposal cost of one dollar per liter, the annual disposal costs attributable to the leakage discussed above amounts to $330.
When oil leaks out, contaminants such as air, particles and water can get in. The costs to consider here include:
Oil leaks regularly pose a safety hazard. Like the costs associated with contaminant ingress, the costs associated with the hazards of oil leaks are difficult to quantify. However, active management of the safety risk posed - for example, more frequent cleanup than may otherwise be necessary - skews this cost to a quantifiable area.
The annual cost of oil leaks on this piece of equipment is approximately $2,500 per year in makeup fluid, cleanup and disposal costs. If a facility contains multiple pieces of hydraulic equipment, the accumulative cost over an extended period of time can be significant.
But what about hydraulics’ fluid-power cousin - pneumatics? One advantage pneumatics has over hydraulics is cleanness. Air leaks are much easier to ignore than oil leaks because they are not as obvious. Cleanup and disposal costs are no longer an issue. Contaminant ingression is possible, but is generally not a major concern, and unless the leak is significant, neither is safety. Therefore, makeup fluid (air) is the last option.
While air is free - clean, dry compressed air is not. Upon considering the cost of makeup air for a pneumatics system, the following must be considered:
The ideal fluid power leakage rate is zero of course, but when calculating the free air delivery (FAD) required by a pneumatic system, a rule of thumb is to allow for leakage of 10 percent of the total flow rate. Consider a 10 cubic meter/minute system leaking one cubic meter per minute. The power required to compress one cubic meter (35.3 cubic feet) of air per minute to a pressure of 6 bar (90 psi) is approximately 5.2 kW. At an electricity cost of $0.10/kWh, this leakage costs more than 50 cents per hour in energy costs alone. In a 24/7/365 operation, this amounts to $4,500 per year!
While a leakage rate of 10 percent of flow rate may sound high and would be unsustainable in a hydraulic system, air leakage rates as high as 25 percent are not unheard of, even in well-maintained pneumatic systems. The actual leakage rate of a system can be calculated using the following formula:
QL = QC* t/(T + t)
QL = System leakage rate (cubic meters/minute)
QC = Compressor FAD (cubic meters/minute)
T = Time between compressor cut-out and cut-in (minutes)
t = Time between compressor cut-in and cut-out (minutes)
In a recent case study, the pneumatic systems of two plants were surveyed for leaks using ultrasonic leak-detection equipment. In the first, which took two hours to survey, 27 leaks were discovered in a small plant. The calculated energy cost of these leaks was $9,000 per year. In the second, a much larger plant, which took two days to survey, 260 leaks were discovered. The calculated energy cost of these leaks was more than $90,000 per year!
The cost of fluid power leaks can be significant. Air leaks are less obvious and can be more costly than oil leaks over time. Conduct regular leakage tests on pneumatic systems and take necessary action to locate and rectify air leaks as required.