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  • Publication Series Viledon

    Filter Concepts for Gas Turbines

    Overview and Field Report

    on Utility Value Enhancement

    with Three-stage Filtration

    Dr. Heiko MansteinDipl. Ing. Andreas Rothmann

    Published in:VGB PowerTech 12/2009, pages 78 - 82

    Paper presented at: VGB Conference "Gas Turbines and Operation of Gas Turbines 2009" June 24 - 25, 2009 in Mannheim

    SR_VGB_engl_12-09 09.12.2009 11.15 Uhr Seite 1

  • Filter Concepts for Gas Turbines

    2 VGB PowerTech 12/2009

    Filter Concepts for Gas Turbines Overview and Field Report on Utility Value Enhancement with Three-stage FiltrationHeiko Manstein and Andreas Rothmann

    Authors

    Dr. Heiko MansteinApplications Engineering Turbomachinery

    Dipl.-Ing. Andreas RothmannHead of the Market Segment TurboMachinery Germany/Scandinavia Freudenberg Filtration Technologies KG Weinheim/Germany

    Kurzfassung

    Filterkonzepte fr Gasturbinen bersicht und Praxisbericht

    zur Nutzwerterhhung mit dreistufiger Filtration

    Effiziente Luftfiltration und ihr Beitrag zum wirt-schaftlichen Betrieb von Gasturbinen bieten ein breites Feld fr stetige Weiterentwicklung. Der Nutzwert dreistufiger, hocheffizienter Fil-tersysteme fr Gasturbinen-Ansaugsysteme wurde in einer Vorgngerverffentlichung mit grundlegenden Betrachtungen vorgestellt.

    Erfahrungen aus implementierten mehrstufigen Systemen belegen die Anwendbarkeit auf die derzeit gngigen statisch betriebenen Filter-systeme vor dem Hintergrund der Forderun-gen nach Partikelabscheidung, Koaleszenzver-mgen und Druckverlustverhalten. Zwar be-dingen mehrstufige Systeme den Nachteil eines anfnglich hheren Druckverlusts, er-mglichen aber durch fast vollstndig vermie-dene Verschmutzung und erhhte Anlagenver-fgbarkeit einen deutlichen, belegbaren Kos-tenvorteil.

    Anhand einer beispielhaften Vergleichsrech-nung zum Kostennachteil eines Betriebssys-tems mit zyklischen Waschvorgngen und den Betriebskosten eines mehrstufigen Filtersys-tems wird der tatschlich erreichbare Kosten-vorteil abgeschtzt und errtert. Die Berech-nungen stehen in gutem Einklang mit den Er-fahrungen aus existierenden Anwendungsein-stzen, die mit zwei Praxisbeispielen belegt werden. Bisher ungenutzte Leistungspotenzia-le knnen durch die Vermeidung von Ver-schmutzung zuverlssig erschlossen, Anlagen-verfgbarkeiten deutlich erhht und zustz- liche Betriebskosten verringert werden. Auch der Verzicht auf das Waschsystem erscheint in der Gesamtbetrachtung als weiterer Entwick-lungsschritt denkbar.

    Introduction

    Responsible deployment of the fossil resources at our disposal demands their maximally effi-cient and eco-neutral use while simultaneously factoring in their cost-efficient utilisation. To reconcile both these aspects is the paramount goal governing the development efforts of tur-bine manufacturers, system development engi-neers and system operators alike.

    From a system operators viewpoint, it is pri-marily downtimes that are rated as lost profit, and must therefore be avoided. High levels of availability and long running times for the systems concerned are the declared goals. In particular, downtimes required for washing the compressor stages inevitably cause non-productive periods, which need to be mini-mised or altogether avoided. The aim of the washing routines is to remove any coatings and deposits on the blading. The cause for any such deposits will usually be inadequate in-take air filtration. Innovative concepts for en-hancing filtration quality by means of three-stage filter systems were presented in [1].

    This paper provides an overview of filter sys-tems in current use and an approach for esti-mating the financial benefits accruing from system modification, an approach which is illustrated by examples from actual operation-al practice.

    Principles of Particle Filtration

    The demand for efficient air filtration for in-ternal combustion engines entails sophisticat-ed challenges in terms of adequate design for and implementation of the air intake systems being used.

    An air filter system is required to significantly reduce the penetration of solid and liquid par-ticles into the turbomachinery, while coping with temporally fluctuating environmental conditions.

    Capturing of air-borne particles (which may be dust particles or droplets) depends most particularly on the transport mechanisms ef-fective at the location concerned. Electrostatic, diffusion-, inertia-, and gravity-related effects are responsible for particle transport to the fil-

    ter media, which are usually made of fibres. The adhesion forces operating between parti-cles and fibres in their turn are determined by the interaction of Van der Waals forces, and electrostatic and liquid-related effects, and en-able dirt particles to be permanently arrested. When developing filter elements, then, both these mechanisms, the transport and the adhe-sion mechanism, must be given due considera-tion in the optimisation process.

    The concentration of air-borne dust particles is of crucial importance for the design of in-take air filtration systems. Over the past few years, measurements of dust concentrations have been continuously expanded, so now there is a broad data base available [2]. The temporal fluctuation band of the PM10 dust mass found here ranged from approximately 5 to 40 g/m3 [3] in Germany during 2007, and depends largely on the season of the year, on the surrounding landscape and the degree of industrialisation obtaining at the place where the measurements are taken. In this context, PM10 denotes the dust fraction whose mean particle size (aerodynamic equivalent diame-ter) is 10 m, with 50 % of it being arrested. This dust fraction exhibits a particle size dis-tribution that may well contain particles of up to 20 to 30 m in diameter [4].

    The fact that a very high and nonetheless lim-ited proportion of the dust fraction is being retained also means that there will always be particles penetrating the filter. As a conse-quence of particles passing through a filter stage, deposits are formed on the blading, which results in output losses at the gas tur-bine. In regions close to the coast, additional corrosion effects may be encountered, due to air-borne salt particles. The aim of develop-ment work on filter systems is accordingly to minimise precisely that proportion of the dust fraction which passes through the filter. Fil-tration quality is rated in terms of collection efficiencies for individual particle sizes or for the entire dust quantity in question [5].

    Filtration Concepts for GT Applications

    The temporal dust mass carried in determines the choice of a suitable intake air filter sys-

  • Filter Concepts for Gas Turbines

    VGB PowerTech 12/2009 3

    tem. The size of the particles of relevance for intake air filtration is typically to be found in a bandwidth of around 0.01 m to about 3 mm, and at locations exposed to high indus-trial emissions an average mass concentration of up to 200 g/m3 can occur [6].

    It is only in a few regions (exposed to tempo-rarily extra-high dust concentrations) that re-generatively operated systems are actually necessary, which, following the principle of surface filtration, form a compact dust cake on the filter medium involved. Using the pulse-jet cleaning method, the dust cake can subsequently be shaken off the filter medium at pre-defined intervals. Filter systems of this kind are mostly in single-stage design and re-quire precisely harmonised and properly func-tioning cleaning systems. The drawback with these is a relatively high degree of particle penetration during and shortly after the clean-ing phase, since it is precisely then that the filtration-supporting effect of the dust cake is absent; it will only be built up again over the course of the cycle now commencing.

    In most of the Earths regions, low to moder-ate dust masses are encountered, which can be very successfully stored and lastingly retained in the elements filter medium using the prin-ciple of deep-bed filtration. If the dust-reten-tion capacity is exhausted, the filters con-cerned are replaced by new, non-loaded ele-ments during the systems overhaul and stand-still times. The salient features of static filtration systems are these: arrestance of large particles in a pre-filter stage and storage of small particles in the fine-filter stage. Stat-ic air filter systems of this kind can be sup-plied in multi-stage design, which offers scope for optimizing the filter technology involved.

    The task of a state-of-the-art design for intake air filters is to affordably reconcile the para-mount requirements posed for optimum sys-tem operation:

    Maximised system protection = filtration with maximum efficiency and at a consist-ently high level

    High system availability = downtimes for replacing the filters are rare and short

    High system efficiency = low pressure drop in the filter system

    Reduction in unplanned downtimes of the filter system = failsafe product quality

    One feature inherent in all filtration is the phenomenon that initially high efficiency lev-els and the high dust storage volumes achieved during operation entail an initially high and steadily increasing pressure drop during the filters useful lifetime, and that is deleterious for optimum gas-turbine operation.

    Optimisation work on intake air systems, fac-toring in some basic considerations regarding the comparison of two- and three-stage filtra-tion systems, has been extensively dealt with in a previous publication [1].

    The Keystones of a Comparison of Two-stage and Three-stage Filtration

    F i g u r e 1 illustrates the basic set-up of a two-stage filter system, in which the final cas-sette filter stage is protected by an upstream pocket filter stage. The core for top