Page 56 - Industrial Plants

the normal operation, after 40+ years from initial acting on the thrust bearing at full load are in the order
installation. of 50 ÷ 60 kN, towards the steam flow direction.
The thrust bearing on the active side has an area of
10,620 mm2; the specific pressure is consequently ≈ 5
Engineering Study – ÷ 6 MPa, a much higher value than the one indicated

Performance Analysis in the original thrust bearing drawing (max 3.3 MPa).
From De Pretto Industrie design experience, this type
Data acquisition for thermodynamic assessment of thrust bearings should be able to work up to 3.5 ÷
The performance calculations are based on process 4.0 MPa of specific pressure, before reaching the
values given by the Refinery. Engineering studies and temperature limit values. The optimal working range to
performance analysis were carried out with the aim to target in the design phase is up to 2 MPa.
acquire data for an accurate thermodynamic This confirms the historical temperature behavior of the
evaluation. thrust bearing, which rises with the load until it reaches
the limit value of 110 ° C, resulting on partial load
Assessment of thrust forces operation of the steam turbine around 15 MWe.
The Refinery is reporting severe problems with the
steam turbine thrust bearing: frequent faults because
of overheating, vibration levels. As a result, machine is
operated at partial load (around 15MW) while nominal Engineering Study -
power output is 21,5MWe. Modified bearings have Rotodynamics
been installed without considerable improvements. A
first visual evaluation of the steam turbine layout shows Consistency of original design
that the balancing pistons are well contributing to Rotordynamic calculation is required to accurately
balance the HP and MP reaction stages (according to predict the location of the high speed and low speed
the rule of “same dP-same average diameter”). While shaft line later critical speeds. De Pretto Industrie
the additional thrust generated by the HP and MP engineering standards impose critical speeds to be
control stages were not taken into consideration in the located either 10% below operational speed or 15%
original design of the steam turbine. The thermodynamic above operational speed to ensure a stable behavior of
simulations indeed confirmed the above assumptions the rotating equipment. Journal bearing clearances
are correct. In fact, control stages show a reaction and oil film as well as supports stiffness are also taken
grade of 15% for HP control wheel and 10% for MP into consideration in the overall calculation.
control wheel which lead to a delta pressure on each Results shown steam turbine first critical speed is well
control wheel and a corresponding thrust force. below critical operating range and second critical
If we consider those thrusts forces, the residual thrusts speed is well above the same. Intermediate shaft first

IndustrIal Plants - May 2022
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