Selecting Hydraulic Fluids - How it Impacts Operating Costs

Douglas Placek, RohMax
Tags: hydraulics

Every operations manager is challenged to find ways to reduce operating costs while improving productivity. This objective places an increasing strain on staff and equipment. Is it possible that simply changing hydraulic fluids can result in a significant reduction in energy costs or an improvement in equipment output? The use of Maximum Efficiency Hydraulic Fluid (MEHF) offers an opportunity to reduce costs and improve hydraulic performance, but how much? How can MEHF impact a specific operation? An educational Web site has been established that offers insight into the issue of hydraulic fluid efficiency, and offers a calculator which enables an operator to model any equipment fleet. At www.mehf.com, users can test the energy savings calculator located in the energy efficiency calculator and tools section (www.mehf.com/calculator).

The Maximum Efficiency Hydraulic Fluid Web site has been developed by RohMax Oil Additives Corporation, based on years of fluid development work in state-of-the-art hydraulic pumps. A variety of commercial fluids recommended by leading equipment manufacturers and major oil companies have been studied in gear, vane and piston pumps. It is easy to conclude that fluid viscometrics (viscosity grade, viscosity index, and stability in high-pressure applications) are key in optimizing pump efficiency. Low pump efficiency results in higher energy costs (fuel or electricity), reduced power output and slower response time. A fluid with high viscosity index meeting the defined MEHF performance level1 will outperform standard monograde (ASTM D6158 type HM) or engine oil-type fluids. The typical performance advantage in mobile equipment is in the range of 5 to 10 percent energy savings or productivity improvement.

Pump Efficiency
Every type of hydraulic pump and motor is designed to have a small amount of internal fluid leakage or recycle. This fluid is essential because it forms a lubricating film between the moving parts which prevents wear. If pumps and motors are operated at optimum temperature and pressure conditions, the amount of leakage is minimal and the pump can operate with greater than 90 percent efficiency. However, hard-working equipment is often placed under significant stress, resulting in high oil operating temperature. As the temperature of the oil increases, the oil viscosity drops, and higher levels of internal leakage occur. It is not uncommon for mobile equipment hydraulic systems to produce sustained operating temperatures in the 80 to 100°C (176 to 212°F) range, with temperature spikes ranging between 110 to 130°C (230 to 266°F). Oil temperatures greater than 60°C (140°F) decrease viscosities enough to have an appreciable negative impact on volumetric pump efficiency. It is common to find pumps operating at 50 to 60 percent volumetric efficiency when the oil temperature increases to 100°C (212°F). When a pump works at 60 percent efficiency, 40 percent of the input energy is wasted and converted into heat instead of work.

There are two elements of hydraulic efficiency: volumetric efficiency and hydromechanical efficiency. Hydromechanical efficiency relates to the frictional losses within a hydraulic component and the amount of energy required to generate fluid flow. Volumetric efficiency relates to the flow losses within a hydraulic component and the degree to which internal leakage occurs. Both of these properties are highly dependent on viscosity.

Hydromechanical efficiency drops as fluid viscosity increases due to higher resistance to flow. Conversely, volumetric efficiency increases as fluid viscosity increases because of the reduction of the internal leakage. The overall efficiency of a hydraulic pump is the product of mechanical and volumetric efficiencies [Equation 1], and both factors must be considered collectively.4

Overall efficiency =
Hydromechanical efficiency * Volumetric efficiency [Equation 1]

Loss of volumetric efficiency causes the pump to work harder and/or longer to produce the required flow to hydraulic actuators. At the same time, high temperatures compromise volumetric efficiency as the result of low-viscosity fluid bypassing critical pump clearances. Thus, inadequate viscosity due to high temperatures creates a destructive cycle of rising temperatures, accelerated wear and increased internal leakage.

All pump manufacturers publish the maximum and minimum oil viscosity requirements for their pumps. A summary of these recommendations can be found in the National Fluid Power Association recommended practice NFPA-T2.13.13-2002, or on the MEHF Web site.6 Please consult the pump or equipment manufacturer directly for specific guidance on fluid viscosity requirements.

Maximum Efficiency Hydraulic Fluids are designed to provide increased viscosity at standard and peak operating conditions. The result is an improved ability to meet the OEM viscosity requirements over a wider range of temperature and pressure conditions, thus maintaining higher pump efficiency.

If a higher viscosity oil is all that is required, then why not use a heavier grade? This may be possible in some cases, but switching to higher viscosity monograde fluids like ISO VG 68 or ISO VG 100 also results in a significant loss of low-temperature properties and potential problems with air entrainment. MEHFs are designed to offer both improved low-temperature flow and excellent air release properties.

How Much Can I Save?
Extensive testing has demonstrated that high-viscosity index fluids provide better pump efficiency at operating conditions.(7,8,9) However, these high-performance fluids cost more than standard monograde fluids or engine oils, so what is the net benefit? While nearly any hydraulic application can take advantage of MEHF performance, heavy-duty equipment operating at higher temperatures (greater than 60°C/ 140°F) and pressures (2,000 psi/ 138 bar) are the most significant benefit. Most mobile construction, forestry, agriculture and stationary outdoor equipment fall into this category. In general, five to ten percent energy savings or productivity improvement may be achieved, which can mean savings of hundreds of dollars per pump every year.10 Because each system may have unique design and/or operating conditions, it is necessary to account for the differences in estimating potential benefits. An energy savings calculator has been developed that models a variety of operations. The calculator may be found at www.mehf.com/calculator.

MEHF Energy Savings Calculator
It is possible to model a simple system with one hydraulic pump (for example, a wheel loader), or a complex piece of equipment with multiple pumps and rotary motors (for example, a heavy-duty excavator). One can also model a small or large fleet if there is an accurate count and differentiation between the different types of pumps (gear, vane, and piston)and motors in service (Figure 1).


Figure 1. MEHF Energy Savings Calculator (Example)

The referenced calculator will compute the total energy input requirement necessary to run the hydraulic systems only. It does not consider energy used to move the equipment with a standard transmission or other auxiliary equipment (air conditioning, electrical systems, etc.). The user must input the total number of gear, vane, piston pumps and motors in service, the typical number of hours per day and days per year that they operate at full load. The calculator assumes that each pump is medium sized, and operates at typical temperature and pressure conditions. The use of a single average operating condition assumes that actual conditions are milder for 20 percent of the time and more severe for 20 percent of the time. Typical fluid operating temperatures were derived from a year-long observation of forestry and construction equipment(3), as well as from guidance received from multiple pump and equipment OEMs.

The model assumes that the gear pump operates at 207 bar (~3,000 psi), the vane pump operates at 200 bar (~2,900 psi), and the piston pump operates at 350 bar (~5,000 psi), when at full load.

Total oil consumption is based on the assumption that the volume of the sump is 1.5 times the flow rate of the pump with the fluid changed annually. Every piece of equipment has a different rate of oil loss (through line/coupling leakage, hose breaks or evaporation), and therefore the calculator asks for an estimate of the annual top-up rate.

The calculator assumes that the pipes and hoses are standard diameter and that the total length of piping does not lead to a significant difference in hydromechanical efficiency between the fluids being compared.

Viscosity grade is a critical factor in determining the difference in overall pump efficiency. The user is asked to input the current type of fluid in use and its cost per gallon or liter. The possible fluid options include the typical ISO viscosity grades (22, 32, 46 and 68) in monograde (type HM) and multigrade (type HV), as well as the widely recommended options of 10W engine oil and ATF. The same fluid may be selected for all-season use, or different fluids can be used for winter/summer rotation.

The viscosity/temperature profile of each fluid determines the actual pump flow rate at operating pressure. The actual flow rate of each pump as a function of oil temperature, viscosity and operating pressure is known. Data for pump flow rates as a function of temperature, pressure and viscosity were taken from the pump suppliers’ product literature, and have been verified with test data measured in previous studies.(6,7,8) Internal pump leakage at the operating pressure determines how much the actual flow rate (Qa) varies from the nominal (Qn) flow rate. Pump efficiency is the ratio of actual flowrate to nominal flow rate. The pump requires a constant amount of input energy to turn at a constant speed. The pump efficiency determines how much of the input energy is converted into fluid flow and useable work. All input energy that is not converted into pump output is transformed into heat. This energy is absorbed by the fluid trapped in the pump and causes the temperature of the fluid to increase, which also leads to further reduction in viscosity. The calculator compares the actual flowrate and input power requirements of the pump using the fluid options selected by the user. It is possible to calculate the difference in energy consumption required to do the same amount of work according to the following formula:

Energy(Ref. Fluid) / Energy(MEHF) =
(Power(Ref. Fluid)*Qa (MEHF)) / (Power (MEHF)*Qa(Ref. Fluid))

Based on the pumps selected and total annual hours of operation, the calculator can determine the total number of kilowatt hours required to run the hydraulic system with the current reference fluid. The ratio from the equation above determines how much energy can be conserved to perform the same amount of work with the MEHF.

For example, this model assumes that the energy source is a diesel engine. The user is asked to input the cost of diesel fuel in his or her area to arrive at a value for energy or fuel savings. The amount of diesel fuel required to generate a kilowatt hour of power in a standard diesel engine is 0.22 kg or 0.262 liters or 0.069 gallons.11

Total Fuel Consumption (liters) =
Pump power requirement (kW) *
Hours of pump operation (hours) *
Diesel fuel consumption rate (0.22 kg/kWh) *
Density of diesel fuel (1.19 liters/kg)

Fuel Savings (liters) =
Total fuel consumption * Relative energy savings

Cost Savings =
Fuel savings * Local cost of diesel fuel

The user can select metric units (liters) or English units (gallons) and can input any currency, however, the default choices are U.S. Dollars or Euros.

The decision to change oil seasonally will have a significant impact on maintenance costs. The user is asked to estimate the time and labor costs for an oil change and indicate if hydraulic system downtime leads to a cost for lost production. In some operations like forestry or mining, machinery that is out of service for maintenance will cause a reduction in output or productivity that costs the company money. The calculator can add in these factors if it is relevant to the operation.

The final step in the comparison process is the selection of an MEHF. A Maximum Efficiency Hydraulic Fluid is characterized by high viscosity index (>150) and good shear stability. The user can select an MEHF candidate with an ISO viscosity grade of 22, 32, 46 or 68 and a viscosity index of 160, 180 or 200.

The volume of diesel fuel consumed by the reference fluid, fuel savings potential with MEHF, and the monetary value of the operational savings are displayed when the user hits the CALCULATE button at the bottom of the page. It is possible to experiment with variations, such as higher fuel or fluid prices, more or less operating hours, or different fluid viscosity grades. For optimum results, it is recommended that an MEHF product with the same or higher ISO viscosity grade is chosen to substitute the reference fluids. For example: if ISO VG 46 HM fluid is currently in use, then choose ISO 46 MEHF or ISO 68 MEHF.

Many controlled studies have concluded that hydraulic fluid viscometrics are the key to achieving good pump efficiency. Choosing a fluid with high viscosity index and good shear stability will enable hard-working systems to minimize fuel or electricity consumption and improve productivity. This performance advantage can typically afford the operator a five to ten percent reduction in operating costs.

A simple-to-use energy savings calculator is now available at www.mehf.com/calculator, which enables the equipment owner or operator to model his specific piece of equipment or entire operation. Basic information about pump types, hours of operation and fluid selection allows the calculator to estimate total energy demand. The user can select various grades of MEHF and compare the total amount of energy required. The energy savings and fluid maintenance requirements are then converted into an estimate of monetary savings.

The calculator allows the user to test different scenarios to estimate financial win/ lose/ breakeven points associated with the use of higher cost, high-performance MEHF. In most cases, there is fast payback and significant savings potential associated with the small change in oil costs.

Technical Editor’s Note:
This is a good model, and is worth a trial run. However, keep in mind that it is difficult to gain exacting accuracy with the breadth of permutations that exist from machine and to machine and the respective operating environment differences for each machine. For instance, differences in engine maintenance will influence nascent efficiency, which may appreciably influence the accuracy of the final estimate. This must be considered, as well as the tool used to guide the decision-making process.

Author’s Comments:
The MEHF performance level definition, extensive pump operation data and MEHF Web site have been developed by RohMax Oil Additives as a service to the lubricants industry.

This information and all further technical advice is based on our present knowledge and experience. However, it implies no liability or other legal responsibility on our part, including with regard to existing third-party intellectual property rights, especially patent rights. In particular, no warranty, whether express or implied, or guarantee of additive or formulated fluid properties in the legal sense is intended or implied. We reserve the right to make any changes according to technological progress or further developments. The customer is not released from the obligation to conduct his own testing of additives or formulated fluids. Performance of the product described herein should be verified by testing, which should be carried out only by qualified experts in the sole responsibility of a customer.

References

  1. MEHF Performance Level Definition. www.mehf.com, visit: Information for Operators/Performance Benefits/Technical Data Sheet. Also visit: Information for OEMs/MEHF Technical Parameters/MEHF Technical Definition. Published by RohMax Oil Additives, March 2005.

  2. I. Makkonen. “Performance of Seasonal and Year-round Hydraulic Oils in Forestry Machines.” FERIC Technical Note TN-251. Forest Engineering Technical Research Institute of Canada, December 1996.

  3. D.G. Placek. “Study Examines Multigrade Fluids for Forestry Equipment.” Hydraulics and Pneumatics. Penton Media, Vol. 54, No. 3, March 2001.

  4. G.E. Totten. Handbook of Hydraulic Fluid Technology. Marcel Dekker, New York, 2000, p. 27.

  5. P.W. Michael, S.N. Herzog and T.E. Marougy. “Fluid Viscosity Selection Criteria for Hydraulic Pumps and Motors.” NCFP paper I00-9.12 presented at the International Exposition for Power Transmission and Technical Conference. Chicago, Ill., April 2000.

  6. NFPA Recommended Practice T2.13.13-2002. “Fluid Viscosity Selection Criteria for Hydraulic Motors and Pumps.” 2002. www.nfpa.com

  7. S.N. Herzog, C.D. Neveu and D.G. Placek. “Influence of Oil Viscosity and Pressure on the Internal Leakage of a Gear Pump.” Presented at the 57th Annual STLE Meeting, Houston, Tex., May 2002.

  8. S.N. Herzog, C.D. Neveu and D.G. Placek. “Predicting the Pump Efficiency of Hydraulic Fluids to Maximize System Performance.” NCFP I02-10.8/SAE OH 2002-01-1430 presented at the IFPE / SAE Off-Highway Meeting, Las Vegas, Nev., March 2002.

  9. D.G. Placek and C.W. Hyndman. “Cost and Performance Advantages of Multigrade Hydraulic Fluids.” Proceedings of the 7th Annual Fuels & Lubes Asia Conference, Bangkok, Thailand, February 2001.

  10. S.N. Herzog, C.D. Neveu and D.G. Placek. “Boost Performance and Reduce Costs by Selecting the Optimum Viscosity Grade of Hydraulic Fluid.” Lubrication and Fluid Power Expo, Indianapolis, Ind., May 2003.

  11. E.N. Ganic and T.G. Hicks. The McGraw-Hill Handbook of Essential Engineering Information and Data. McGraw-Hill, New York. Chapter 13, Engineering Thermodynamics, Fuels and Combustion, Pages 13.98 to 13.99, 1991.


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