Tribology and Lubrication Technology November 2013 : Page 104
WORLDWIDE R. David Whitby Varnish deposits from steam turbine oils Use of Group II oils is trending up, but formulating the correct additive package is essential. D Varnish can be removed from steam turbine oils, but it’s best to prevent varnish formation on metal surfaces in the ﬁrst place. URING THE LAST 10 YEARS, changes in steam turbine operating conditions and oil formulations have caused a variety of prob-lems for steam turbine operators. Research by turbine manufacturers, power companies and oil suppliers has shown the main causes of the problems to be an increased use of API Group II base oils in turbine oils, combined with an increase in steam turbine peaking operation. The highest proﬁle problems were entrained air and foam, decreased demulsi-bility and formation of varnish. API classiﬁes mineral base oils into three categories: Group I, Group II and Group III. The basis of the classiﬁcation is the saturat-ed hydrocarbon content, sulphur content and viscosity index of each type of base oil. Satu-rated hydrocarbons include parafﬁnic and naphthenic types; aromatic hydrocarbons are unsaturated. The reﬁning processes used to make Group I base oils mean they contain signiﬁ-cant amounts of aromatic hydrocarbons (sometimes up to 20 percent by weight) and higher contents of naphthenic hydrocarbons. Measuring by weight, a typical 150 SN Group I base oil will contain around 50 percent paraf-ﬁnic hydrocarbons, around 35 percent naph-thenic hydrocarbons and around 15 percent aromatic hydrocarbons. Conversely, the reﬁning processes used to manufacture Group II base oils chemically convert many of the aromatic and naphthen-ic hydrocarbons into parafﬁnic hydrocar-bons. As a result, a typical 150 N Group II base oil has about 68 percent of its weight in par-afﬁnic hydrocarbons, around 30 percent 104 naphthenic hydrocarbons and only around 2 percent aromatic hydrocarbons. Some Group II base oils contain almost no aromatic hy-drocarbons. Naphthenic and aromatic hydro-carbons exhibit higher solvency properties than parafﬁnic hydrocarbons but are poorer on oxidation stability properties. As a consequence of better response to oxidation inhibitors, oils formulated with Group II base oils have been found to give better results in some oxidation tests com-pared to oils formulated with Group I base oils. This has resulted in the increasing use of Group II base oils in industrial lubricants that are subject to oxidation such as steam tur-bine oils. When mineral oils are exposed to high temperatures in the presence of air (oxygen) and entrained water, they oxidize. Oxidation products include acids, esters, alcohols, ke-tones and polymeric compounds, all of which are attracted to each other and to metal sur-faces. When present in sufﬁcient amounts, they can precipitate out of solution, forming varnish and eventually sludge. Unfortunately, because Group II base oils have poorer solvency properties compared with Group I base oils, when they do oxidize they tend to produce varnish and sludge more quickly. Also, when some aminic oxida-tion inhibitors degrade, they can form var-nish on metal surfaces. The greatest problem with varnish in a steam turbine lubrication and control system is that it plates out on servo-valve surfaces, leading to valve sticking, and plugs the last-chance ﬁlters (LCFs) that are part of the ser-vo-valve assembly. LCFs made with sintered metal or ﬁne screens provide a convenient surface for varnish formation because of their location in the low-ﬂow, colder hydrau-lic control section. Lower temperature pro-motes varnish formation because of the lower solubility at lower temperatures, which causes it to come out of solution and deposit on the ﬁlter’s metal surface. In addition, varnish precursors form de-posits on mechanical seals, Babbitt sleeve bearings, thrust-bearing pads and oriﬁces, resulting in restrictions. When these deposits develop on heat exchanger and reservoir walls, reduced heat transfer and higher tem-peratures are likely to occur. Varnish can be removed from steam tur-bine oils using either electrostatic or adsorp-tive ﬁlters, but it’s preferable to prevent var-nish formation on metal surfaces in the ﬁrst place. If formulators of steam turbine oils decide to replace a Group I base oil with a Group II base oil (to get better oxidation sta-bility and longer oil life), the use of the cor-rect types of oxidation inhibitors and the ad-dition of small amounts of a polar base oil, such as an ester or alkylated naphthalene, is advisable. The polar base oil will improve the solvency properties of the Group II base oils, helping to keep any varnish precursors dis-solved in the turbine oil. David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can contact him at pathmaster. firstname.lastname@example.org At the end of its lifetime in about ﬁve billion years, the sun will swallow the Earth and other planets.
R. David Whitby
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