Varnish in turbine oils

By Ralf Ertelt E.ON New Build & Technology GmbH

and Andreas Busch HYDAC Filter Systems GmbH

Vo lu me 93 – Is sue 6/2013 Pa ge 98 to 101 (German)

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Varnish in turbine oils VGB PowerTech 6 l 2013

Autoren

Abstract

Varnish in turbine oils

The appearance of varnish in turbine oil can lead to hydraulic malfunctions and to increased bearing temperatures. The consequences are unplanned downtimes and high costs. Varnish are oil aging products that form gel-, resin-like or solid varnish-like deposits in the fluid system.Among other things the reason for theses depos-its is the limited dissolution capacity of varnish in modern turbine oils. Furthermore, these oils also have low electrical conductivity, which causes electrostatic discharges in the system. This results in accelerated oil aging, and dam-age to sensors and filter elements.In the past an oil fill had a service life of usually 15 to 20 years. Today the service life of modern oils is significantly less than ten years. In order to avoid critical system operation, the routine laboratory examination parameters must be expanded. Early recognition of the risk of de-posits and the use of adapted fluid care will increase the safety of operations and reduce operating costs. l

Varnish in turbine oilsRalf Ertelt and Andreas Busch

Dipl.-Ing. Ralf ErteltE.ON New Build & Technology GmbHGelsenkirchen, GernanyDipl.-Ing. Andreas BuschHYDAC Filter Systems GmbHSulzbach (Saar), Germany

Introduction

The condition of the oil in lubrication and hydraulic systems is indicative of the health of the entire system. There are two essentials for ensuring productivity, for avoiding malfunctions and for reducing operating costs:

– Monitoring the fluid condition, on the one hand, and

– Continuous maintenance of operating equipment.

Varnish refers to oil aging products that form deposits in the fluid system which have a gel/resin-like consistency or resem-ble solid varnish (F i g u r e s 1 to 4). These oil aging products are readily deposited on cool surfaces such as the tank, valve hous-ings or coolers. This causes an increase in bearing temperatures, malfunctions in hydraulic valves and cooling problems. In most cases, however, the above malfunc-tions are not correctly attributed to the real cause. This results in ineffective, and often very expensive, repair work.The subject of oil aging is not new by any means; in fact it has always been an issue. However, as a consequence of the intro-duction of more highly refined base oils, the characteristics of turbine oils have changed. Whereas 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. This means that fluid moni-toring and fluid conditioning are becoming more and more important.This scientific paper provides an overview of the changes in base oil characteristics, laboratory analysis procedures for detect-ing oil aging, and fluid conditioning meas-ures for the removal of oil aging products, with a view to eliminating possible faults.

Alteration in base oil characteristics and the consequence

The base oils generally used today for the production of turbine oils have changed in recent years. They reflect the increased demands with regard to higher turbine ef-ficiency, higher bearing temperatures and reduced levels of hazardous substances. Whereas oils used to be produced exclu-sively by the distillation of crude oil (ASTM Group I oils), for some years now further refinement processes have been employed. The procedures used, such as hydrotreat-ing or hydrocracking, result in superior chemical purity and oil of uniform quality with global availability.These changed production processes pro-duce base oils with lower levels of unsatu-rated polar hydrocarbons (ASTM Groups II, II+, and III). Base oils in the ASTM Groups II and III are also usually free of sulphur. During production of the turbine oil, a sulphur-phosphorous additive is generally therefore added.There is a problem with this refinement in that varnish has a polar structure. Polar substances tend to dissolve more readily in polar substances. If the proportion of polar hydrocarbons in oil is reduced, oil aging products / varnish cannot dissolve as eas-ily. The effect is oil turbidity (F i g u r e 5 ) or deposits in the system. These changes usually start once the oil has been in op-eration for 3 to 4 years.Due to the low proportion of polar sub-stances, these oils also have low electrical

Fig. 1. Varnish on the cover plate of the hydraulic pump.

Fig. 2. Varnish on the filter element.

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VGB PowerTech 6 l 2013 Varnish in turbine oils

mixing can result in chemical reactions, which under certain circumstances may lead to the precipitation of reaction prod-ucts and to deposits within the system.

Laboratory analysis procedure for detecting varnish in turbine oils and the limit for critical system condition

Scope of testing for a usually half-yearly routine inspectionThe scope of testing for the half-yearly routine inspections is listed with the ap-propriate procedure in Ta b l e 1 . The pres-ence of varnish in oil cannot be detected with these routine inspection parameters. Given the increasing problems with depos-its in turbine systems, other procedures have been added to the routine inspection parameters.

MPC value determinationThe MPC value records the changes in col-our of a laboratory filter membrane with 0.45 µm filtration rating. Critical system conditions occur if the MPC value is over 40. The effect of this is that more deposits occur in the system, as shown in Figure 1 to Figure 4.

conductivity. If this oil flows through the filters in the hydraulic system, an electro-static charge can be generated. This can lead to sparking in the system (F i g u r e 6 ). In addition to damaging the filter ele-ments, it can cause failures in measure-ment and control systems. In addition, deflagrations may occur in return lines or in the tank. F i g u r e 7 shows samples of this damage.Today’s technical data sheets for oils do not give information on the base oil used. Since oil names are often not changed when the oil type is changed, it may mean the old oils are inadvertently mixed with new, more modern turbine oils during refilling. Such

Fig. 3. Varnish on the tank wall.

Fig. 4. Varnish-free tank wall.

Tab. 1. Scope of test for half-yearly routine inspection

Test item Procedure

Optical assessment E.On internal procedure

Water content Indirectly DIN 51777, Part 2

Solid particle content DIN 51592

Neutralization number (NN) DIN 51558, part 2

Viscosity 40°C DIN 51562, part 1 (DIN 51558 part 2)

Refraction index DIN 51423-2

Colour photometric E.On internal procedure

Colour (ASTM scale) DIN ISO 2049

Air release properties DIN ISO 9120

Water separation capacity (WSC) DIN 51589, part 1

Corrosion rating DIN ISO 7120

Foaming characteristics ASTM D 892 and ISO 6247 (procedures have been withdrawn, is still used as an E.On internal procedure)

Ageing IR (CO number) DIN 51451, E.On internal procedure

Ageing protection IR (Phenol. inhibitor) DIN 51451, E.On internal procedure

Turbine oil with high level of varnish (MCP value 69),

Water content of < 30 ppm

Turbine oil with low level of varnish (MCP value 25),

Water content of < 30 ppm

Fig. 5. Oil samples with different levels of varnish for comparison.Fig. 6. Electrostatic discharging in the

filter element.

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Varnish in turbine oils VGB PowerTech 6 l 2013

Particle counting at room temperature and at elevated oil temperatureThe size, number and composition of par-ticles significantly impact the wear and the function of hydraulic components and systems. The size and the number of parti-cles is determined using automatic photo-optical particle counters and is displayed as cleanliness classes to ISO 4406. The rel-evant standard for particle counting is ISO 11500. The standard maximum cleanliness classes for turbine lubrication systems are: ISO 18/15/12 and ISO 17/14/13, if the steam control system is supplied by the same tank. In this case an increase of one cleanliness class equates to a doubling of the particle count in each case.

The solubility of oil aging products/varnish is dependent on temperature; it increases at higher temperatures and decreases at lower temperatures. As soon as the solubil-ity threshold is reached, oil turbidity oc-curs. 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.Example: F i g u r e 8 shows just such a particle evaluation. The measurement was carried out using a FCU/BSU 8000 (F i g -u r e 9) produced by HYDAC Filter Systems GmbH. The particle distribution class at room temperature (in this case 22 °C) was ISO 23/18/12. 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, we can conclude that there is a high pro-portion of undissolved oil aging products/ varnish in the oil.

Determining the remaining proportion of antioxidantsAntioxidants are added to the oil to slow down oil aging and hence the formation of varnish. These additives (e.g. amines and phenols) 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. For this reason with larger refill quan-tities we recommend that the existing sys-tem filtration is supported by additional offline oil conditioning.

One option to specifically raise the additive concentration is the addition of suitable inhibitors. For this, precise information on the oils and their constituents is required. Standard analysis procedures to determine this are:

– Infra-red spectroscopy (absolute concentration)

– Ruler (relative concentration) acc. to ADTM D 6810

– HPLC (absolute concentration)

Depending on the oil type and additives used, other analysis methods may have to be applied.

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 malfunc-tions on the steam control valve, which led to problems on shut-down of the turbine. The cause was found to be deposits be-tween 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. Individ-ual particles are less than 1 µm in size. For comparison: Standard valve clearances are

500 mm 500 mm

215.3

Burned filter material Burned tank ventilation filter

Fig. 7. Damage caused by electrostatic discharging in the system.

Parti

cel d

iffer

ence

per

1 m

l

Clea

nlin

ess l

evel

acc

ordi

ng to

ISO

440

6:19

99

45,000

40,000

35,000

30,000

25,000

20,000

15,000

10,000

5,000

0

42,244

1,717 1,310 267 33 22

23

22

21

20190

Particle count in an oil sample

> 4 mm > 6 mm > 14 mm

at 22 °CParticle difference

at 80 °C

Fig. 8. Graphical representation of particle counts for an oil sample at 22°C and at 80°C.

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VGB PowerTech 6 l 2013 Varnish in turbine oils

within the range of several micrometers. 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 varnish-like coat-ings. These then have the effect of increas-ing the actuation forces in the valve and of causing the above malfunctions.In order to clean the varnish in the oil, a VarnishMitigation Unit produced by HYDAC (F i g u r e 10 ) was used. This sys-tem is similar in construction to an offline filter and works 24h per day, 365 days per year. The separation of varnish takes place by adsorption onto a specific resin. Once the adsorption capacity of the resin is ex-hausted, the quantity of varnish in the oil rises again. When the critical MPC value of 40 is exceeded, the resin-filled elements are changed. Figure 11 shows the progres-sion of the varnish content in the oil of this system when using this offline filter unit, measured by the MPC value.

Summary of results and outlook

Immediately after commissioning the con-ditioning 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 re-placed on May 2nd, 2012. With this second element, the service life increased to over 3 months. The varnish separation itself had no negative influence on the antioxidant content in the system: the antioxidants be-have neutrally in reaction to the cleaning process. After cleaning the oil circuit, suit-able inhibitors are added during operation.

Conclusion

The term varnish is used, amongst other things, for oil aging products in turbine lu-brication systems, and accurately describes the consistency of these substances.The increase in the efficiency of turbines and the reduction of oil volumes (cir-culation indexes) increase the load on turbine oils. Modern turbine oils with higher chemical purity and low levels of hazardous substances reduce the solubil-ity 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. These discharges cause an extreme ther-mal fluid load due to hot spots. The result of this high thermal load is usually acceler-ated oil aging.In order to avoid critical system operation, the routine analysis parameters have to be

broadened. By sizing & selecting the sys-tem filtraton 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 thus extend the service life of the oil as well as the components. Simul-taneously this contributes to trouble-free operation of the overall system. l

Fig. 10. HYDAC VarnishMitigation Unit VMU.

Membrane Patch Calorimetrie MPCColour assessment of filtrate taken from 100ml on a 0.45 µm micron filter membrane

80

70

60

50

40

30

20

10

0

MPC

Feb. Mar. Apr. May Jun. Jul. Aug. Sept. Oct. Nov. Dec. Jan. Feb.

critical

CommisioningVMU

ChangeVME element

ChangeVME element

Date

Fig. 11. Graph plotting the MPC value; limit at MPC 40.Fig. 9. HYDAC Particle counter FCU/BSU 8000.

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