The condition of the oil in lubrication and hydraulic systems is indicative of the health of the entire system.
Monitoring the fluid condition and continuous maintenance of operating equipment are therefore, necessary for ensuring productivity, avoiding malfunctions and reducing operating costs.
Varnish refers to oil aging products that form deposits in the fluid system, and has a gel/resin-like consistency. These oil aging products are readily deposited on cool surfaces such as the tank, valve housings or coolers, causing an increase in bearing temperatures, malfunctions in hydraulic valves and cooling problems.
The subject of oil aging is not new by any means; however, as a consequence of the introduction of more highly refined base oils, the characteristics of turbine oils have changed. In the past, oil had a life of 15 to 20 years; today, the service life of modern turbine oils is considerably less than 10 years, which means fluid monitoring and fluid conditioning are becoming more and more important.
Laboratory analysis procedure for detecting varnish in turbine oils
The solubility of oil aging products/varnish is dependent on temperature, increasing at higher temperatures and decreasing at lower temperatures. As soon as the solubility threshold is reached, oil turbidity occurs. The particle counter is able to detect this turbidity due to the high particle count before it is visible to the naked eye. If two identical oil samples are analysed – one at room temperature (approx. 22°C), and the other at 80°C – there will be a difference in particle quantities in the evaluation if there is varnish in the oil.
For instance, a particle evaluation carried out using a FCU/BSU 8000 by Hydac Filter Systems GmbH indicated that the particle distribution class at room temperature (in this case 22°C) was ISO 23/18/12, while the same measurement when the oil was heated to 80°C resulted in a particle distribution of ISO 18/15/12. From the substantial particle differential, it can be concluded that there was a high proportion of un-dissolved oil aging products/ varnish in the oil.
Determining the remaining proportion of antioxidants
Antioxidants are added to the oil to slow down oil aging, leading to the formation of varnish. These additives degrade as the oil ages. In order to increase the proportion of antioxidants, one of the usual options is to make up the oil quantity lost during operation with new oil. The addition of new oil, which still has 100% antioxidants, will increase the overall concentration of antioxidants in the oil.
In addition, partial replacement using new oil is possible. Although increasing the proportion of antioxidants, it may under certain circumstances have the effect of dislodging deposits in the system, thus adding to the particle concentration in the oil. With larger refill quantities, it is recommended that the existing system filtration is supported by additional offline oil conditioning.
One option to specifically raise the additive concentration is by the addition of suitable inhibitors. For this, precise information on the oils and their constituents is required. Standard analysis procedures to determine this are infrared spectroscopy (absolute concentration); ruler (relative concentration) acc. to ADTM D 6810; and HPLC (absolute concentration).
Removal of varnish based on the example of a turbine lubrication system
An application on the lubrication system of a steam turbine is explained below.
System description: Lubrication system of a steam turbine; Oil quantity in the system: 12,000 l; Age of oil: 27,000 operating hours; Conductivity: <10 pS/m at 21°C; Oil type: Turbine oil with EP additive, base oil ASTM Group II.
The initial problem centred on malfunctions on the steam control valve, which led to problems on shutdown of the turbine. The cause was traced to deposits between the valve body and the valve spool due to oil aging.
Oil aging products/varnish are problematic in that they are initially highly filterable and the valve functions are unaffected. Individual particles are less than 1 μm in size. As a result of ongoing oil aging or when the oil has cooled down (e.g. during a system shutdown), these particles agglomerate, become larger, and form a varnish-like coating, which has the effect of increasing the actuation forces in the valve and causing these malfunctions.
In order to clean the varnish in the oil, a VarnishMitigation Unit (VMU) produced by Hydac was used. Similar in construction to an offline filter, the VMU works 24 hours a day, 365 days a year. The separation of varnish takes place by adsorption onto a specific resin. Once the adsorption capacity of the resin is exhausted, the quantity of varnish in the oil rises again. When the critical MPC value of 40 is exceeded, the resin-filled elements are changed.
Immediately after commissioning the conditioning unit on 16 February 2012, the MPC value fell drastically. On 25 April 2012, the separation capacity of the resin was exhausted. The filter element was replaced on May 2nd, 2012. With this second element, the service life increased to over three months. The varnish separation itself had no negative influence on the antioxidant content in the system with the antioxidants behaving neutrally in reaction to the cleaning process. After cleaning the oil circuit, suitable inhibitors are added during the operation.
The increase in the efficiency of turbines and the reduction of oil volumes raise the load on turbine oils. Modern turbine oils with higher chemical purity and low levels of hazardous substances reduce the solubility or load capacity for oil aging products. Where the filter load is too high or the filter selected is too fine, the extremely low conductivity of these modern oils can also lead to electrostatic discharges in oil, causing an extreme thermal fluid load due to hot spots, which accelerates oil aging.
In order to avoid critical system operation, the routine analysis parameters have to be broadened. By sizing and selecting the system filtration correctly, the electrostatic load in the fluid is eliminated and the fluid lifetime is extended.
Fluid maintenance measures such as the separation of varnish, offline filtration, dewatering and degassing reduce the fluid load and extend the service life of the oil as well as the components. This simultaneously contributes to trouble-free operation of the overall system.