Oil Systems for Turbine Lubrication

The oil system is an integral element of the turbine unit, which largely determines its reliability and trouble-free operation. The main purpose of the turbine lubricating oil system is to provide fluid friction in the bearings of turbines, generators, feed pumps, and gearboxes.

An oil system should provide:

– continuous supply of the required amount of oil in all modes of operation of the turbine unit, which guarantees:

  • – prevention of wear on friction surfaces;
  • – reduction of friction power losses;
  • – removal of heat released during friction and transmitted from the hot parts of the turbine

– maintaining the required temperature of the oil in the system; and

– cleaning the oil from contamination.

At the same time, the necessary qualities of the lubricating oil system are reliability, safety of operation, ease of maintenance.

The pressure and the temperature of the oil should be constantly monitored during operation of the turbine unit. Specifically, the lube oil temperature after the bearings requires special attention. Overheating of the bearing leads to wear of the working parts and changes in the properties of the lubricant itself. The quality of the lube oil is controlled by physicochemical characteristics such as density and viscosity. The system leaks must be stopped quickly and oil replenished on time. These factors will significantly extend the service life of the steam turbine.

Nowadays, computer simulation is a very powerful and useful tool. It helps you predict the processes occurring in the bearing chambers, and determine the flow of the working fluid when the operating modes change, all without installing expensive experimental equipment.

We suggest using the 1D-Analysis AxSTREAM NET™ tool to simulate the lubrication system. This software product allows you to quite simply, clearly and quickly build the desired model. It provides a flexible method to represent fluid path as a set of 1D elements, which easily can be connected to each other to form a thermal-fluid network. The program calculates fluid flow parameters for inlet and outlet of each element. There are many different components that allow you to simulate stationary and non-stationary modes. Also there is a convenient library of fluids. It is also possible for a user to add fluids of their choice.

The example of modeling in AxSTREAM NET™ is the system of oil supply for the K-500-240 turbine. This turbine is quite massive with bearing loads of up to 450 kN. The schematic diagram of the oil supply K-500-240-2 is shown in Figure 1.

Figure 1 Principle Scheme of K-500-240 Steam Turbine
Figure 1. Principle Scheme of K-500-240 Steam Turbine.

 

(1 – main tank; 2 & 3 – pumps; 4 – oil cooler; 5 – damp tank; 6 – journal bearings; 7 – thrust bearing).

The main elements of the scheme are:

– the main lube oil tank;

– the oil cooler;

– the centrifugal oil pump;

– the damping tank;

– the emergency tanks;

– 12 journal bearings with diameters of 300, 420 and 520 mm; and

– 1 hydrodynamic thrust segmented bearing with a diameter of 520 mm.

This scheme provides the oil lubrication system for the turbine, generator, hydrostatic lifting of the turbine rotors and the emergency oil supply system. The hydrostatic lift system of the rotors is autonomous and operates at high pressure. The emergency oil supply system is integrated into the bearing oil supply system.

Let’s consider the key elements of the scheme. The oil is sent from the main tank to the oil coolers, where it is cooled to the temperature of 45 degrees.Then lubricating oil is supplied to a damping tank installed at a height of 24 m. It ensures uninterrupted flow and stable pressure in front of the bearings 0.1 MPa. The lube oil flows by gravity from the damper tank to the lubrication manifold and further to the bearings. Then it drains into the common lubrication system manifold, and from there, back to the main tank. The lube oil purity is provided by filters installed in the main lube oil tank. The lube supplying is provided by a centrifugal oil pump. This scheme uses oil brand T-22. The limiting factors are the maximum oil heating temperature and the minimum thickness of the lubricating layer for bearings operating on petroleum oil. The oil heating is not more than 15 degrees in the bearing for the conditions of this scheme. The thickness of the oil film is not less than 0.015 mm.

Figure 2 shows the model of the oil supply system was made by using the AxSTREAM NET™ software.

Figure 2. Scheme Modeling in AxSTREAM NET
Figure 2. Scheme Modeling in AxSTREAM NET

The software allows setting the performance of the oil pump by using a special function “Tabular”. This will provide an opportunity to automatically take the effect of changes into account in oil consumption on the head and the efficiency of the pump.

The model calculates the flow and pressure in any part and in every modeling components of the lube oil system. We can calculate the necessary mass flow rates, track the increase of oil temperature, changes in pressure, thermophysical properties of the oil, a lube oil film thickness, as long as we know the design characteristics of the bearings. We can choose the required geometrical dimensions of the bearings if we know the load and the necessary expenses of lubricating oil.

The results of calculations in AxSTREAM NET™ coincide with the experimental data.

The simulation tool allows users to predict and analyze the behavior of the system in different situations; highlight vulnerabilities; and draw the necessary conclusions and recommendations. Learn more about AxSTREAM NET™ here.

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