Greasing the wheels

Ayman Ali, ExxonMobil Fuels & Lubricants, EMEA, explains the crucial role that hydraulic fluids play in oil and gas operations and describes their link to increasing machine efficiency.

The operating environment for hydraulic oils across all industries has become increasingly severe over the past several years. As a result, the demands on these fluids, which lubricate and transfer power in hydraulic applications, have also increased. Systems are generally getting smaller in size, with less oil used. However, pumps produce as much or more output, resulting in higher operating pressures. Combining smaller oil volume with higher pressures results in higher oil temperatures, meaning an increase in oxidation rates and more thermal stress on the additive system.¹ A wide operating temperature range, heavy loads, variable speeds and the risk of water contamination, not uncommon at oil and gas plants, make the operating environment even more challenging. At the same time, plant operators expect lubricants to last longer, protect better and cost less.

The hydraulic fluid needs to protect, lubricate and help transmit power in the most effective way in order to help equipment perform to its design standards. It should be able to help maintain and improve the efficiency of hydraulic systems, and help optimise the costs of operation in the harsh conditions in which the oil and gas sector operates. These conditions put a number of demands on hydraulic lubricants; meeting these challenges translates into tangible benefits that can contribute to the success of any operation. The combined qualities required to ensure that a lubricant has the necessary characteristics to meet such demands are many and varied. These will be discussed in more detail later in the article.

Safety first

Issues of safety, productivity and reduction of operational costs are becoming even more critical, especially given the location of offshore rigs and platforms in remote areas and extreme conditions. Unscheduled downtime caused by equipment failure can have a significant impact on the operator’s bottom line, especially in more severe climates with ambient temperatures below 0°C. Moreover, repair or replacement of parts increases potential safety risks of the maintenance professionals, caused by employee interaction with equipment. Reliability and protection of the equipment is therefore crucial for a safe and productive operation.

Viscosity index

Viscosity is a measure of a hydraulic fluid’s resistance to flow, and it has a significant impact on the operation of the system. Overly thin oil (low viscosity) does not seal sufficiently and may lead to leakage and wear of parts; overly thick (high viscosity) oil will be more difficult to pump through the system and may reduce operating efficiency.² Defining the correct fluid viscosity grade for a particular hydraulic system involves consideration of several interdependent variables, such as starting viscosity at minimum ambient temperature, maximum expected operating temperature (influenced by maximum ambient temperature), and permissible and optimum viscosity range for the system’s components.³

A particularly important quality in relation to a fluid’s viscosity is its viscosity index (VI). The VI is an empirical, unit-less number used to quantify the change of viscosity with respect to temperature. If a fluid has a high VI, its viscosity will not change as rapidly with temperature when compared with a lower VI fluid. For hydraulic fluids to operate properly, they must have a sufficiently high VI to perform at the extremes of their expected temperature range and provide maximum component protection.

The VI of a fluid can be enhanced through the use of specialised additives called viscosity index improvers. These additives, used in Mobil branded hydraulic fluids, are typically designed to minimise a temperature’s impact on viscosity. At low temperatures the molecule chain contracts and does not impact the fluid viscosity, while at high temperatures the chain relaxes and increases viscosity.

Oxidation and thermal stability

The life and performance of a hydraulic fluid depend on many variables such as quality of oil, potential contamination and conditions such as the operating temperature of an application. Fluid temperatures in most industrial applications are around 60°C, and fluctuations or spikes to 85°C are not uncommon. However, operating at temperatures above 85°C for extended periods will shorten the life of the oil because the oxidation process is accelerated. Every 10°C increase in temperature doubles the oxidation rate and cuts the life of the oil in half.⁴

Over time, oxidation by-products can form varnish, which can clog filters, minimise flow and increase downtime. Good quality hydraulic oils must contain additives that counteract the process of oxidation, improve the stability and extend the life of the fluid, as without them the quality of oil will deteriorate quickly. They should be able to exhibit outstanding oxidation and thermal stability allowing long oil life and minimised deposit formation in severe hydraulic systems using high-pressure, high-output pumps.

Thermal stability is the ability to resist breakdown at elevated temperatures – the result of poor thermal stability is the formation of sludge and varnish. In addition, as these anti-wear agents decompose at high temperatures, acids are formed, which attack bronze and yellow metals in piston pumps and other hydraulic system components.⁵ Hydraulic oils formulated with very high levels of thermal stability will help to minimise these issues and help extend the life of the hydraulic fluid and the components of the hydraulic system.⁶ Hydraulic fluids’ shear and thermal stability should provide maximum equipment protection and performance at high- and low-temperature extremes.

Meeting increasing demands

The demands placed on hydraulic systems constantly change as the industry requires greater efficiency and speed at higher operating temperatures and pressures. ExxonMobil has responded with high performance hydraulic oils with zinc-free anti-wear packages, the Mobil DTE 10 Excel™ series, specifically designed to meet the needs of modern, high-pressure, industrial and mobile equipment hydraulic systems.

The DTE 10 Excel series is designed to be used in a number of applications, such as hydraulic systems subject to deposit build-up in sophisticated computer numerically controlled (CNC) machines, particularly where close clearance servo-valves are used; systems where cold start-up and high operating temperatures are typical; systems requiring a high degree of load-carrying capability and anti-wear protection; machines employing a wide range of components using various metallurgy; and rotary screw compressors in natural gas service.

Benefits for the oil and gas industry

The potential benefits of the Mobil DTE 10 Excel Series that are of particular relevance for the oil and gas industry include increased hydraulic efficiency, long equipment life and durability. Testing by the Organisation for Economic Co-operation and Development (OECD) 203 has also indicated no significant aquatic toxicity, eliminating environmental hazard classification for the series. Under controlled laboratory testing, the Mobil DTE 10 Excel Series has increased hydraulic pump efficiency up to 6%. In a controlled field testing with a commercially available excavator, up to a 6% reduction in fuel consumption per work cycle was documented, as well as a significant reduction in cycle time versus a standard SAE 10W⁷ hydraulic fluid.⁸

Based on Mobil Hydraulic Fluid Durability (MHFD) testing, the DTE 10 Excel Series lubricants can help keep hydraulic systems cleaner for up to three times longer than conventional hydraulic fluids, with reduced system deposits leading to reduced machine maintenance and increased component life. This can translate into greater durability under harsh conditions, reduced machine maintenance and enhanced productivity through increased equipment life with minimum downtime.

The shear stable, high viscosity index in the DTE 10 Excel Series allows for a wide operating temperature range, maintaining maximum hydraulic efficiency and component protection at both low and high temperatures. At high operating temperatures, Mobil DTE 10 Excel Series’ shear stability provides enhanced lubrication and increased hydraulic efficiency to help keep oil and gas operations at peak output. Conversely, the series’ low-temperature pumpability means reliable starting and pump protection down to -40°C (DTE 10 Excel 15 and 22).

Hydraulic efficiency can translate to reduced power consumption, or increased machine output – which can result in financial savings.

The series’ enhanced lubrication and thermal stability can help to extend oil and filter change intervals while helping to reduce system deposits. Their high level of anti-wear properties and improved film strength characteristics result in a high degree of equipment protection that not only results in fewer breakdowns, but also helps improve production capacity.

Proof of performance

The DTE 10 Excel series hydraulic oils have been proven to enhance the performance of high-pressure hydraulic equipment in extreme environments, including machinery used in the oil and gas industry. Cooling systems of those applications were not functioning adequately at these extreme temperatures, causing the hydraulic system to shut down. The introduction of Mobil DTE 10 Excel 32, a hydraulic oil for piston, vane and gear pumps that flows in temperatures of -34°C, helped reduce cold weather hydraulic system shutdowns by 75%, extend oil drain intervals from 2000 hours to 8000 hours and double the life of the pumps.9 As a result of these actions, operations improved productivity and saved US$43 976 annually.

Maintenance matters

The performance, life and reliability of hydraulic components are acutely sensitive to the quality and maintenance of the hydraulic fluid used in the system. As well as using a high-quality hydraulic fluid, it is important that oil and gas operators monitor in-service performance via a proactive oil analysis programme. For example, ExxonMobil relies on Signum^SM Used Oil Analysis programme – a series of laboratory tests that can determine the condition of equipment components and the condition of the in-service lubricant. Alongside the use of high quality lubricants, oil analysis conducted by lubrication experts helps maintenance professionals extend equipment component and lubricant life, provide early warning signs of contamination, minimise unscheduled maintenance and increase the overall life of the hydraulic system.

Conclusion

Hydraulic fluids play a crucial role in oil and gas operations. They can contribute to improving the performance of the hydraulic systems, help increase productivity and reduce operational costs as well as minimise the safety risks. Regular monitoring of the oil and the hydraulic equipment will help to meet the ever-increasing demands for greater efficiency and speed at higher operating temperatures and pressures. As both the demands on hydraulic systems and hydraulic fluids increase in the ever-changing environment, particularly in the severe offshore conditions, the choice of a reliable product is absolutely critical for safe and productive operations. 

References

1. Hydraulics & Pneumatics, http://hydraulicspneumatics.com/200/TechZone/HydraulicFluids/Article/False/70639/TechZone-HydraulicFluids (September 2007).
2. ‘Characteristics of a Good Hydraulic Fluid’, http://www.hydraulicfacts.com/Hydraulics-101/Characteristics-Good-Hydraulic-Fluid.html (November 2010)
3. Casey, B., ‘Choosing the Right Hydraulic Fluid’, http://www.machinerylubrication.com/Read/702/hydraulic-fluid-types
4. Profilet, R., ‘How well do you know your hydraulic fluid?’, http://hydraulicspneumatics.com/200/TechZone/HydraulicFluids/Article/False/70639/TechZone-HydraulicFluids
5. ‘Mobil DTE 10 Excel™ Series’, http://www.mobil.com/USA-English/Lubes/PDS/GLXXENINDMOMobil_DTE_10_Excel.aspx
6. ‘Characteristics of a Good Hydraulic Fluid’, http://www.hydraulicfacts.com/Hydraulics-101/Characteristics-Good-Hydraulic-Fluid.html (November 2010)
7. The Society of Automotive Engineers (SAE) established a numerical code system for grading oils according to their viscosity characteristics. SAE viscosity gradings include the following, from low to high viscosity: 0, 5, 10, 15, 20, 25, 30, 40, 50 or 60. The numbers 0, 5, 10, 15 and 25 are suffixed with the letter W, designating they are ‘winter’ or cold-start viscosity, at lower temperature.
8. Energy efficiency relates solely to the fluid performance when compared with ExxonMobil’s standard hydraulic fluids. The technology used allows up to 6% increase in hydraulic pump efficiency compared with Mobil DTE 20 Series when tested in standard hydraulic applications. The energy efficiency claim for this product is based on test results on the use of fluid conducted in a controlled laboratory environment in accordance with all applicable industry standards and protocols. Actual results may vary depending on operating conditions. 
9. This Proof of Performance is based on the experiences of an individual customer. Actual results can vary depending upon the type of equipment used and its maintenance, operating conditions and environment, and any prior lubricant used.

Adapted by David Bizley

Read the article online at: https://www.oilfieldtechnology.com/special-reports/14052015/greasing-the-wheels/