If you have the original mfr's data you can calcualte teh current efficiency as follows.
For a single turbine in one casing, the expansion line curve is based on 3 processes:
a) throttling loss across the inlet control valve
b) epxansion across the blades
c) exhaust loss
The original vendor data sheet probably states the performance with valves wide open ( VWO), so there is a likley 1/2 = 1% percent pressure drop across the inlet valves. On a H-S diagram, the H is constant and the S will increase due to the pressure drop of 1/2-1% of the inlet pressure.
The next step is to calculate the blade flow factor K1 or K2. For any turbine at constant speed, the relationship between the steam flow into the 1st row of blades and the P, T, and sv is :
W= K1*SQRT ( P / sv,i)
W= flow, mass/time
P=1st stage inlet abs press
sv,i= inlet specific volume, length^3/mass
K1= proportionality constant as requried for units chosen
A simpler approximation is :
W=K2*P/SQRT ( T, abs)
where T,abs is the absolute temp entering the 1st row of blades
Next step is plot on the H-S diag the expansion across the blades knowing the mfr's defined UEEP ( used energy end point)- this is the enthalpy of the steam rejected to the condenser, and includes the exhaust loss. Deduct from this enthalpy the mfr's exhaust loss ( they did give you the exhaust loss curve, didn't they?)and the result is the enthalpy leaving the last row of blades not corrected for exhaust loss. This endpoint ( Ho, So) and the 1st blade start point ( Hi, Si) can be connected with a straight line and this represents the stge effciiency of the blades tehy gave you. This can be anywhere from 82-95% efficient , depending on the technology used.
So ou now know the inlet flow factor , the stage efficiency , and you have the exhaust loss curve . To determine the off-design output and overall efficiency, do the following:
a) knowing the P and H entering the throttle valves, determine what the throttle valve outlet pressure must be in order to meet the relationship for the 1st stage flow faactor , define above. This is involves using the steam tables in an iterative manner. For a first guess, the pressure at the outlet of the throttle valve is roughly linearly proportional to the mass flow.
b) knowing the 1st stage inlet pressure,H and S, plot this point on the H-S diagram. The expansion process down to the condenser pressure will be a line that is exactly parrellel to the mfr design expansion plotted earlier.
c) the end point must have the exhaust loss deducted from it. This involves calculated the exhaust velocity at the current condenser pressure .
