Gas Turbine Lubrication Systems

Gas turbines have had a presence in many industries for more than a century. They are a unique technology for either producing an energy or propelling a vehicle and the efficiency of modern gas turbines is being improved continuously. One of them, a cooling system, has been described in earlier blogs. Another is the lubrication system of a gas turbine which we will cover in this blog. This  system, similar to that of a piston engine or a steam turbine, provides lubrication to decrease mechanical losses and prevent of wear on friction surfaces. Another function is the removal of heat released during friction by high rotational part and transmitted from the hot part of a turbine.  The basic units which need lubrication are the bearings supporting a shaft of a gas turbine 2.

Modern Dual Journal
Figure 1. The construction of modern dual journal4
Elements for lubrication

In a common case, gas turbine installation contains three main journal bearings used to support the gas turbine rotor 3. Additionally, thrust bearings are also maintained at the rotor-to-stator axial position 4. Click here for additional information about optimization of journal bearings. The bearing has important elements in its construction to prevent leakages from a lubrication system. The work, design and analysis of labyrinth seals is describe here.

Besides bearings, other parts are needed to lubricate. For example, there are pumps for the lubrication system, a pump for the fuel system, and other additional units rotating by the shaft. The lubrication system of an aeroderivative gas turbine can also include the components of an electric generator. But the main aim of the lubrication of gas turbine is support the operation of the bearings.

 

Journal Bearing Lubrication
Figure 2. The lubrication of contact surfaces in a journal bearing
Lubrication fluids

The lubrication oil is the life blood of the gas turbine and it is very important for the gas turbine to perform its function and to extend the length between overhauls. Fluid film journal bearings play a significant role in the machine’s overall reliability and rotor-bearing system vibration and performance characteristics. So the oil needs to have opposite properties. At one side viscosity may have a low value to provide flexibility. At the other side, viscosity needs to resist high pressure between the rod and housing. Oils for the lubrication system of aviation gas turbine engines have an additional requirement of an operational temperature range. Modern oil for lubrication is synthetic and designed with optional properties. You can describe lubrication oil with any needed parameters in AxSTREAM®, AxCYCLE™ or AxSTREAM NET™ .

Types of lubrication system

Two principle lubrication system types by sump are used in a construction of the gas turbine. The first one is a dry sump lubrication system which is generally used in aeroderivative gas turbines. This type of system utilizes a reservoir. The lubrication oil is stored outside the gas turbine in a tank 5.

Example of a dry sump turbine lubrication system
Figure 3. Example of a dry sump turbine lubrication system 5

A wet-sump lubrication systems is generally used in aviation gas turbine engines. The lubrication oil is stored in the engine cavities proper. The mass and sizes of gas turbine systems are less than the same one with dry-sump 6. A disadvantage of wet-sump system is that its elements are close to the hot regions of engines. As a result, lubrication system works in a wider range of temperature difference.

Example of a wet gas turbine lubrication
Figure 4. Example of a wet gas turbine lubrication system 6

Turbine engines oil systems can also be classified as a pressure relief system that maintains a somewhat constant pressure: the full flow type of system, in which the pressure varies with engine speed, and the total loss system, used in engines that are for short duration operation (target drones, missiles, etc.). The most widely used system is the pressure relief system with the full flow used mostly on large fan type engines. One of the main functions of the oil system in turbine engines is cooling the bearings by carrying heat away, circulating oil around the bearing.

Using AxSTRAM NET™ for simulation of lubrication systems

The typical gas turbine system consists of channels and ducts inside lubricated units, pipelines needed to connect different parts, filter to provide oil clearance, and the cooler to provide oil temperature in the operational range.

An example of this type of gas turbine lubrication system was used in the first production aviation jet engine JUMO 004B, shown in Figure 5 below. This gas turbine engine has an oil tank. The lubricant oil is supplied from the tank to the lube pump which pushes liquid throughout the filter into two different branches. The first branch lubricates the first journal bearing and additional equipment like fuel, and auxiliary gear pumps. The second branch lubricates the other bearing and gear pumps.

The figure shows the application of AxSTREAM NET™ to 1D lubrication system modeling of gas turbine engine JUMO 004B. The software has a model of both types of bearing (journal and thrust). Pressure distribution is displayed as result of modeling.  In this case heating of bearings isn’t considered because it was   small at the first gas turbine engine. But if it is necessary the thermal elements can be added to the diagram and thermal analysis can be done in AxSTREAM NET™. See example for steam turbine lubrication system.

Figure 5. Absolute pressure distribution in lubrication of the gas turbine engine JUMO 004B simulated in AxSTREAM NET
Figure 5. Absolute pressure distribution in lubrication of the gas turbine engine JUMO 004B simulated in AxSTREAM NET

AxSTREAM NET™ can be used not only for analysis of existing lubrication systems, but also for design and improvement. It can be used with any design parameters for the pumps or pipelines.

In connection with increased competition 7 in the gas turbine sector, every percent of efficiency in a turbine becomes vitally important. Therefore, the efficiency of the lubrication system is also important. This is complex task and includes not only thermal-hydraulic analysis, but also the design of the bearings, lubricant oil and many heat exchangers. The complex task can be completed with one software: AxSTREAM® which includes AxSTREAM NET™ for designing circuit fluid system, and AxSTREAM Bearing™ for bearing design as well as many other capabilities.

References
  1. Gas turbine, https://en.wikipedia.org/wiki/Gas_turbine [2019 sep 12]
  2. Aeroderivative Gas Turbine Market 2019 Global Analysis by Top Players – GE, Siemens, NPO Saturn, Zorya-Mashproekt, Vericor, Cryostar, https://amarketresearchgazette.com/aeroderivative-gas-turbine-market-2019-global-analysis-by-top-players-ge-siemens-npo-saturn-zorya-mashproekt-vericor-cryostar/ [2019 Apr 10]
  3. Gas Turbine Bearings, [2019 Sep 19]
  4. Combined Thrust & Journal Bearing, https://www.pumpsandsystems.com/bearings/may-2016-combined-thrust-journal-bearing-assembly-reduces-cost-risk-large-pump-users [2019 Sep 12]
  5. TURBINE OILING SYSTEM, https://www.quora.com/How-turbine-oil-system-work-in-power-plant [2016 Jan 2016]
  6. Aviation Maintenance Technician Handbook-Powerplant, Vol.2, 2012
  7. Doosan Heavy Industries vying to join large gas turbine producers, https://www.power-eng.com/2019/09/20/doosan-heavy-industries-vying-to-join-large-gas-turbine-producers/ [2019 Sep 20]