Power plant and calculations: 30-Things you must know on steam turbine & auxiliaries

1-During cold start up turbine inlet steam minimum temperature should be saturation temperature at the particular pressure + 50 Deg C.

Example: Turbine operating at 67 kg/cm2 pressure, its inlet steam temperature should be285+50 =335 deg C

2-Distinguishing the Turbine starts up types.

Cold start up- when HP and IP inner casing temperature is lower or equal 170^oC
Warm start up- when HP and IP inner casing temperature is lower or equal 430^oC
Hot start up – when HP and IP inner casing temperature is greater than 430^oC.

Read Power generation phenomenon in STG

3-To open ESV the vacuum should be at least 0.3 Kg/cm2A (-0.73 kg/cm2)

4-In turbine rotors, over speed trip bolt is always fitted at the DE side only. This is because, not weaken the Turbine NDE side shaft, as NDE side shaft size is already made small & drilled for key ways

5-Expansion bellows for lube oil lines are fitted at the Turbine front bearings, as Turbine expansion occurs towards front end

6-In oil cooler heat exchangers,oil pressure is always kept at higher side than water pressure. This is to avoid entry oil water in lube oil system.

7-In many Turbine, control oil (oil used for HP, LP valves actuators & ESV) is used at higher temperature 55-60 deg C.This is because “Actuators & ESV components are operating at very less clearance need low oil viscosity.

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8-Control oil filters are of lesser filter size as compared to lube oil filters.

Generally filters are designed based on the minimum clearance through which the oil flow, hence lube oil filters are of higher openings (25 to 40 microns) as bearings clearance will be in the range of 200 microns to 500 microns. Control oil filters are of lesser size openings (10 to 25 microns) , as discussed earlier Actuators & ESV components are operating at very less clearance up to 50 microns.

9-Positive displacement Lube oil pumps have in built as well as external PRVs (Fitted at the discharge line).Lube oil pumps (Positive displacement pumps) are always started with discharge valve open unlike centrifugal pumps

10-Emergency oil pump do not have PRVs

11-Emergency oil pumps flow capacity = Main/Auxiliary oil pump X 25%

12-Emergency oil pump pressure = Main oil pump pressure X 30%

13-Control oil pumps flow capacity = Main/Auxiliary oil pump X 10%

14-For lube oil coolers: Cooling water flow = Oil flow X 2

15-For lube oil coolers , Heat load in KW = Cooler surface area X 5.3

16-Turbine lube oil consumes 30 to 35% of total cooling water required for plant auxiliary

17-Generator air cooler consumes 20 to 25% of total cooling water required for plant auxiliary

18-Bearing inlet oil pressure during high rotor speed (Normal operation) is lesser than that of low speed (During barring gear operation)

19-There is always off set alignment between Turbine rotor & Gear box pinion shaft. This is for accommodating the misalignment during operation, as Gear box is operating at higher oil temperature than Turbine.

Generally Gear box pinion shaft is kept at lower level (0.15 to 0.3 mm) & offset side depends on the direction of rotation of Turbine shaft viewed from turbine front end. If Turbine rotor is rotating clockwise then offset is towards RHS.

20-Low oil temperature can damage the Turbine bearings: Because;

When temperature decreases too much, oil in the bearing becomes so viscous that it clings to the shaft surface which drags it around the bearing. This makes the oil wedge in the bearing lose. its stability. The pulsating wedge excites high rotor vibration referred to as oil whip or oil whirl.

Too low temperature - and hence, too large viscosity - of the bearing inlet oil causes the bearing oil flow to decrease due to increased friction in the oil supply piping. The reduced oil flow may be too small for adequate cooling, causing bearing overheating and possible damage.

21-Slight sub atmospheric pressure is maintained inside the bearing housing and its drain line by the vapour extraction fans installed on the lube oil tank cover. Why is this pressure maintained?

First.

To prevent oil mist from escaping past the bearing oil seals into the turbine hall.

Second.

To prevent accumulation of hydrogen and oil vapour in the lube oil tank atmosphere, which could create an explosion hazard

22-For lube oil:

During normal operation, water is removed from the oil by the oil purifier and the vapour extraction fans. During a long outage, water can also be drained from the bottom of the lube oil tank.

23-Main oil tank level:

The major adverse consequence/operating concern caused by too low tank level is impaired pump performance due to cavitation and possibly vapour locking or gas locking. The lower the oil level, the smaller the suction head of the pumps in the tank. Pump cavitation and eventually vapour locking can result. The lowered level can also lead to ingress of gases from the tank atmosphere into the pump suction piping, and then the pump itself. An excessive accumulation of gases in the pump can decrease its capacity, and finally result in pump gas locking. Too high tank level increases the risk of tank overfill. The resultant oil spill has its own adverse consequences such as an environmental hazard.

24-Rotor lift due to jacking oil pressure

Drive end :0.1 mm & NDE :0.05 mm

The jacking oil pressure at the bearing inlet is not controlled. As the oil is supplied by a positive displacement pump, its pressure rises until the bearing resistance to the oil flow is overcome. This happens when the turbine generator rotor is lifted off the bearings.

25-Wheel chamber pressure = (Turbine inlet pressure X Turbine load in MW X 0.6)/Turbine Capacity in MW

26-Steam condensers has fixed support at cooling water inlet side & sliding support at the opposite side

27-Surface condensers installed at higher elevation are always producing lower vacuum.Power plants installed at or near the sea produce high vacuum

28-At steam condensers Vacuum breaker valves are provided to bring down the rotor speed to zero as early as possible
29-Closing time of ESV & HP control valves are 0.3 to 0.4 seconds and 0.4 to 0.6 seconds respectively
30-Pressure of N2 gas in control oil accumulator is 2 to 3 kg/cm2 lesser than control oil line pressure

31-For STG: Bearing temperature trend goes on decreasing from Turbine front to Generator rear end

32-For STG: Bearing Vibrations trend goes on increasing from Turbine front to Generator rear end
33-Velocity of condensate water at ejector & gland steam condenser is 0.5 to 0.7 m/sec & 1 to 1.5 m/sec respectively

Protections & Interlocks in power plants