hysreresis, ehe square porr valve, Figure 56, has ehe more linear charac-rerisric in irs mid-operaring range. In conrrasr, ehe burrerfly valve, Figure 5a, is found ro exhibir ehe leasr hysreresis effecrs. 1-Iowever it is accompanied by a increasingly non linear flow versus posirion characrerisric. The <lise on sear valve configurarion is found ro exhibir a compromise behaviour berween ehe hysreresis and non-linear flow versus posirion characrerisrics. 1n recognirion of the differing characteristics given in Figures 5a-5c, İt is not surprising that problems exist within AGR systems which employ differenc valve rypes, rhus providing ehe imperus for ehe presenr invesrigarive research. Having esrablished a possible shorr fail wirhin exisring AGR sysrems, any sreps ro alleviare rhis arrendanc problem is likley ro require a marhemarical model represenrarionofeach valve rype. Ir is believed rhar any new conrrolled AGR sysrem should make use ofa knowledge ofehe parricular valve model ro compensare for ehe differenc characrerisrics which may be encounrered wirhin an overall sysrem. This will undoubredly lead ro ehe need for an auromaric sysrems modelling and idenrificarion faciliry ro be incorporared wirhin ehe overall conrrol sysrem ro provide for an 'inrelligenr' AGR scheme. ln an arrempr to quanrify ehe behaviour ofa 'generic' valve model srrucrure, aclass (or classes) of well srrucrured non-linear marhemarical models are required. In rhis respecr ehe applicarion of bilinear [2} and Hammersrein [5} non-linear models are currenrly being invesrigared, wirh use being made of recursive leasr squares and Kalman filrering rechniques[4} 5. CONCLUSIONS Thepaper has reviewed ehe problems facing ehe gas urilization industry in rerms ofehe need for ıncreased sophisticarion of conrrol systems in order to meer ehe increased demand for improved plant efficiency and overall product DOGAL GAZ DERGiSİ SAYI. 28 142 quality. Coupled with the advancages in rhermal insulation properries offurnace linings and the need for greater rurn down capabilities, ehe problem ofair/gas ratio conrrol has been heightened. In this conrext, ehe paper has examined ehe behaviour of ehe three generic forms of air/gas ratio concrol systems and highlighted their relative advanrages and shortfalls. Since ehe fundamental underlying common link amongst ehe existing air/gas ratio conrrol systems lies wirh ehe air and gas valves, aetention has also been focused rowards idenrifying their saliene non-linear characterisrics. It is believed thae ehesourceofmanyproblems, which have been heightened due ro increased demands in recent years, is that of ehe nonlinear valve characreristics. It is, rherefore, concluded ebat any new/improved air/gas ratio concrol system should make use of knowledge ofsuch characrerisrics. 1n this respect, further work is conrinuing in order ro inveseigate/develop an on-line automated syseems modelling approach within an overall ineegrated air/gas ratio computer control system. REFERENCES {1} Brennan D.F.Air/Gas Racio Concrol Syscems for Nozzle Mixing Burners. The Insricuce of Energy , 1992 {2} Burnham K.J., James D.J.G., and Shields D.N. Self-cuning control of bilinear syscems. J.Optimal Concrol Applicacions and Mechods, (8): 147,157, 1987. {3} Granc M.P. Air/Gas racio concrol-a problem scacement. Covencry Polyceclınic, Reporr no. CTAC 4-92, 1992. {4]Young P.C., Recursive approaclıes co time series analysis. IMA Bullerin, pages 209-224, May June 1974. { 5 ] Hsia T.C. Syscem Idencifıcacion. Lexingcon Books, 1977 Acknowledgements: Tlıeamlıors wislı coacknowledge ehe supporc of clıeir colleagues D.F. Brennan and G.E.Jones ofBricislı Gas plc, UK.
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