13.3 Erosion protection
Steam enters the HP turbines slightly wet and it becomes progressively wetter with each stage of expansion, but the moisture droplets are very small and are so well entrained in the bulk steam flow that the velocities of droplets and main steam do not differ significantly.
Blade path erosion is not therefore a significant problem and the blades can be made from 12% chrome stainless iron coupled to a NiCr MoV forged rotor, as in conventional machines.
Centrifugal stresses are generally low and, since the steam temperature is far below that at which creep has to be taken into account, only medium-strength ferritic stainless iron need be used for the blading.
Where changes in flow direction are induced in the casings or pipework, or where pressure differences exist across joint surfaces or pressure seals, water droplets can combine and there is considerable potential for erosion. The HP casings, fixed blade carriers and cold reheat pipes must therefore be made from a chromium alloy steel such as 2.25%Cr l%Mo. Such alloys resist the formation of the surface oxide layers, which would be continuously removed and reformed by an erosion-corrosion mechanism. It is also essential to protect joint faces subject to a pressure drop against cutting by wiredrawing erosion, and to protect surfaces which might be subjected to impact from water droplets flung off the shrouding of the rotating blades. This requires the use of very erosion-resistant alloys, such as 13%Cr or 18%Cr stainless steel. Austenitic steel facing can be provided by weld deposition and 13%Cr inserts may be located by seal welding.
To minimise the possibility of joint jacking by corrosion products, horizontal joint bolting must be arranged to ensure that water penetration is restricted. It may be necessary to protect certain joints by placing short stainless steel sleeves around them and incorporating stainless steel barrier keys.
Severe erosion has been experienced by continental manufacturers on low alloy and carbon steel casings and pipework in wet steam machines. 'Alsthom' originally made HP casings in j CrMoV but, in view of experience gained at Muhleberg, Beznau and Oskar-shamm and in Germany, intend using a steel with at least 11% chrome in future to give satisfactory erosion protection. They currently make pipework bends in 2j % chrome steel and apply a high-chrome cladding protection on interceptor valves. СЕМ (Asea Brown Boveri) protected low alloy diaphragm groove facings and similar facings for gland grooves with 13% chrome cladding but, having found the cost of achieving a satisfactory 13% chrome deposit prohibitive, now supply 13% chrome castings for wet steam HP inner and outer casings.