The leakage means that some of the steam initially supplied to the turbine does not pass through the HP blading to the reheater and back to the IP turbine, but instead goes directly to the IP blading. As this flow effectively circulates in a non-reheat cycle of lower efficiency, it can have a dramatic effect on turbine heat rate/thermal efficiency and output. Typically, an increase in N2 leakage of 1% of initial steam supplied to the turbine results in an output reduction of about 0.3% and efficiency reduction of about 0.16% (Koch, et al 1988).
Further leakages may occur from other places based on the design of the machine, such as through eroded or distorted horizontal joints, or from around the main inlet pipe piston-ring type seals.
As mentioned earlier in this book, the enthalpy drop efficiency of tur-bine sections is a prime parameter. The steam temperature into the IP cas-ing is measured before the N2 leakage flow enters. N2 flow is at the enthalpy corresponding to the value from the HP turbine expansion at the first stage pressure, so it is lower than the enthalpy of the steam supplied to the IP tur-bine from the reheater. Hence the steam entering the IP blading at the IP bowl in total does not have the enthalpy of the main IP inlet flow. The result is an incorrect high test value of IP enthalpy drop efficiency. Therefore, in routine testing of such turbines, if an increase in IP enthalpy drop efficiency is found, then increased N2 gland leakage should be investigated.
Figure 7.3 shows condition parameter trends over some years on a tur-bine of this type (Lovejoy, 1983). This 380MW turbine, of combined high-pressure/intermediate pressure casing and twin double-flow low pressure casings, is supplied by an oil-fired boiler and suffered from erosion by ex-foliation from superheater tubing (2.25Cr-1Mo steel). Two other units had Tp347H steel instead and exfoliation has been much less.
During a major overhaul in Year 4, all first stage moving blades were replaced and the first stage nozzles given major repairs. Main stop valve stem and bypass valves erosion had also been experienced (Lovejoy, 1983). Condition parameters are shown in this case as % deviation from the values found immediately following the overhaul in the fourth year.
This example also shows practical variations in condition monitoring parameters.
Most turbines have interleaved labyrinth type seals, but a later brush seal technology has been developed by one manufacturer (Hogg and Stephen, 2003). Another packing type available new or at retrofit is retractable to prevent rubbing. They close in to running clearances when the turbine is at service speed. Leyzerovich (2007) describes some experiences.
Figure 7.3 Trends of condition monitoring parameters on an HP-IP turbine