Steam turbine practical questions & answers for Turbine engineers & operators

STEAM TURBINE QUESTIONS AND ANSWERS FOR TURBINE ENGINEERS & OPERATORS

1-Why do the U loop is provided at the inter condenser drain line of ejector?

U loop is to seal the ejector & steam condenser pressure as there is very less pressure difference around 0.25 to 0.3  kg/cm2. So for such low pressure difference U loop seal is economic & practical

2-Why do the float valve are provided at the after condenser of ejector condensate line?

Know about vacuum troubleshooting in steam Turbines

Float valve is used to seal the ejector & steam condenser pressure as there will be around 0.89 to 0.95 kg/cm2 pressure difference. For such high pressure difference float valve is most practical arrangement.

3-Generally, where the pressure relief valves are fitted at ejector systems?

                            

There two pressure relief valve in SJAE (Steam jet air ejector) one is fitted at ejector nozzle chamber & other is fitted at condensate water outlet line

4-What is the significance of hot well recirculation line in CEP condensate line?

Significance of hot well recirculation line

1-To provide minimum flow to CEP pump

2-To safe guard ejector tubes due to lack of cooling during low loads on turbine due to less/no water flow through the tubes.

5-How much power you can save by replacing the steam jet ejectors by vacuum pump?

Generally ejectors consume steam around 500 kg/hr at pressure 10 kg/cm2 & temperature 200 ⁰C

Assume this steam is taken from Turbine bleed & there not using of live steam

Heat content in ejector steam =H = Ms X Hg..Refer steam table for enthalpy

H = 500 X 673.75 = 336875 kcal/kg

Convert it in terms of KW....We have 1 KW = 860 kcal

Therefore Power that can be developed by ejector by considering STG efficiency 60% is = 336875 X 60% / 860 = 235 KWH

6-What will happen to hot well level of steam condenser when vacuum drops suddenly?

Hot well level rises suddenly

7-How the lower vacuum contributes in increased steam consumption of Turbine?

Lower vacuum is nothing but higher exhaust temperature, so turbine exhausts high temperature steam to condenser leading to loss in heat. So in order to maintain given load  set point Turbine consumes more steam.

And also higher pressure in condenser creates reaction force on turbine rotor making it to drag more steam to maintain its speed & torque as per load.

8-Why the vacuum in steam condenser which is situated at higher elevation is lower than that of situated at lower elevation?

Because at higher elevation, atmospheric pressure goes on decreasing...So maximum maintainable vacuum will be less.

9-At full load operation of the steam Turbine, where will be the highest  steam velocity at the inlet or at exhaust of the Turbine?

At full condensing mode steam velocity is more at exhaust end of the turbine as the exhaust duct has more area as compared to inlet steam line area.

10-What are the reasons for high exhaust temperature in steam Turbines?

High exhaust temperatures is due to;

• Lower vacuum in the condenser
• Turbine running on partial load
• Over load on steam condenser
• Ejector U seal loop broken

11-What do you mean by the coast down time in steam Turbines?

It is the time taken by steam turbine rotor to come down from its rated speed to zero speed after trip or shutdown of Turbine. Turbine speed starts reducing once the ESV closes.

It depends on vacuum in the condenser. If vacuum is more it takes more time to come down to rest position & vice versa.

12-What do you mean by soaking period in steam turbines?

During initial starting turbine is allowed to expand evenly and smoothly by allowing sufficient time of warm up, this period is called soaking period.

This is done for allow uniform expansion of turbine casing, rotor & other internal parts.

13-What is the purpose of gland sealing? When to charge gland steam after vacuum pulling or before?

The purpose of the gland sealing is to prevent air from ingression in the vacuum system during pulling vacuum. The steam is applied on both labyrinth glands & even at control valve glands. The pressure maintained is around 0.1 kg/cm2

Gland steam can be charged based on Turbine operation conditions

Cold start up:

In this turbine is in atmospheric temperatures, hence gland steam is charged after vacuum pulling at vacuum say -0.2 to -0.5 kg/cm2. If gland sealing is done before vacuum pulling, there may be chances of developing thermal stresses.

Hot start up:

Gland sealing is charged even before vacuum pulling. Charging the gland seal steam after vacuum pulling may cause cold air shock in the glands which may lead to rotor distortion

14-How do you select filter size of lube oil & control oil filters?

Lube oil filter size is around 25 to 40 microns: Size depends on the minimum clearance in the bearings

Control oil filters size is around 10 to 25 microns: Size depends on the minimum clearance in the HP & LP actuators.

15-What is the quantity of lube oil required for Turbine?

It is 22-25% of total lube oil flow

16-What is the quantity of lube oil required for Gear box?

It is 60-65% of total lube oil flow

17-What is the quantity of lube oil required for Generator?

It is 8-10% of total lube oil flow

18-What is the quantity of lube oil required for Jacking (jacking oil pump)?

It is 8-10% of total lube oil flow to their bearings. Generally JOP line is given to alternator & even at both alternator & turbine to facilitate lifting of rotor during rotation of shafts to avoid friction between rotor & bearing.

For example an alternator has lube oil flow 90 LPM, then flow of lifting oil (Jacking oil flow) is 9 LPM

19-Why the oil coolers are placed before lube oil filter?

Due to temperature difference the oil DP may vary at filters, so oil is first passed through cooler, where its temperature reduces to constant operating level then it is passed through filters

20-What is the temperature difference between Turbine exhaust temperature & condensate steam in hot well?

Actually if there is no leakage in the system, both the temperatures should be same. However 2 to 3 degree centigrade difference is allowed.

21-Why do you control the outlet valve of oil cooler water line for controlling the lube oil temperature instead of water inlet line?

It is for avoiding starvation of tubes due to no or less flow of water into the tubes.

22-What do you mean by Turbine cold, warm, hot & very hot start up?

Cold start: after a shut-down period exceeding 72 h (metal temperatures below approximately 40 % of their fully-load values in ⁰C)

Warm start: after a shut-down period of between 10 h and 72 h (metal temperatures between approximately 40% and 80 % of their full-load values in ⁰C)

Hot start: after a shut-down period of less than 10 h (metal temperatures above approximately 80 % of their full-load values in ⁰C)

Very hot restart: within 1 h after a unit trip (metal temperatures at or near their full-load values).

23-What do you mean by Turbine Supervisory system?

Turbine is a high speed machine, its operation and performance is monitored through supervisory system. These are one types of protection system for Turbine.

These include.

• Vibration probes
• Speed probes
• Axial displacement probes
• Bearing temperatures TCs or RTDs
• Differential expansion probes
• Casing temperature TC
• Casing expansion

Reasons for increase in Turbine SSC

24-What is the clearance between rotor & casing diaphragm?

It is 0.6 to 1.5 mm

25-Why it is necessary to measure the casing temperature of Turbine?

Casing of Turbine is made up of thick alloy material. Hence more temperature difference between inner & outer part of the casing may cause distortion. So in order to ensure the correct temperature casing temperature is being measured. There should be no more temperature difference (> 50 degree C) between top & bottom casing thermocouples

26-How do you measure the Turbine casing expansion?

POWER GENERATION CALCULATION

It is measured with the help of LVDT

27-Why do the Turbine front connected bearing oil lines have expansion bellows & those of rear bearings oil line do not have?

Because Turbine casing expands towards front side only. In some turbines expansion bellows are provided for front & rear bearings also.

And we do not find expansion bellows for Generator & gear box oil lines.

28-Why do the lube & control supply oil lines are made up of Stainless steel SS materials & drain/return oil lines are of Carbon steel (CS)

Supply lines are connected to bearings & actuators they need o supply contaminant/bur free oil. Generally SS pipe line materials do not produce rust & burrs, whereas rust & burs formed in carbon steel pipe lines. Such formed rust or burs in CS steel will collect in MOT & later can be removed by centrifuging.

29-Why it is necessary of oil centrifuging in Turbine lube oil system?

Turbine oil gradually gets contaminated due to atmosphere moisture ingress through turbine bearing sealing system & also partial oxidation of oil. So oil need to purify to maintain its property. Also oil has some dissolved solids formed during its service, so that must be removed periodically to ensure good life of bearings & actuator system

30-What are the two methods of oil centrifuging?

Purification: It is the separation of two immiscible liquids having different specific gravity and is useful for the removal of the solids particles with specific gravity higher than the those of the liquids

Clarification: Is the process of separation of solid particles from oil or any other liquid.When the centrifuge machine is run with rotating bowl having outer disc (without hole) then this process is clarification.

When the machine run without this outer disc, then it is purification method.

In clarification process also some amount of moisture is removed along with solids & in purification method some amount of solids are removed along with moisture

Power plant Safety QnA

31-Why the control oil temperature is more than lube oil or why control oil is not cooled in coolers?

To maintain low viscosity of the oil, control & governing system internal parts have very low operating clearances. So in order to maintain that control oil is not cooled & maintained its temperature around 60 deg C

32-What are the functions of oil vapour extraction fan (OVEF)?

• Removes the oil mist formed in main oil tank (MOT)
• Maintains slight vacuum (20-30 mmwc) in MOT for easy drain of lube oil from STG bearings

33-What is the difference between control oil & trip oil?

Oil delivered by control oil pump (Previously MOP was used for both lube & control oil applications) is bifurcated into two system. After one oil passes through some of protection relays to open ESV is called Trip oil & other goes to for operation of HP, MP & LP valve actuators through I to H converters is called as control oil.

34-Why Gear box is made off set alignment with Turbine?

During the operation of Turbine, the drain oil temperatures in Turbine reaches to 58 to 60 deg c, which is slightly more than Turbine, exhaust temperature (45 to 60 deg C). Gear box expands both horizontally & vertically. Hence provision in alignment is made in such a way that Gear box high speed shaft is kept down & horizontally off set. Horizontal off set side depends on the direction of turbine rotation.

35-Why there are two RTDs for Turbine pinion bearing temperature measurement?

If Turbine rotation is clock wise (view from Turbine front) then the bottom part of the bearing is on higher load & if it is antilock wise direction top part is under load. So there is always some temperature difference between these two RTDS

36-What is the function of MPU?

Magnetic pick up unit senses the Turbine speed. It is set at 0.8 to 1 distance from the gear system mounted to Turbine shaft at front end.

37-What are the reasons for high bearing temperatures & vibrations?

• Overloading of the turbine
• High lube oil temperature
• Foreign materials in lube oil
• Load fluctuation
• More clearance in the bearing

38-On what trip interlock protection vacuum breaker valve gets open?

Vacuum breaker valve opens on activation of following trip interlocks

• High bearing temperature
• High bearing vibration
• High rotor axial displacement
• Differential expansion
• Over speed

39-What are the causes of foam formation in lube oil?

Turbine oil and transformer oil standard testing parameters

Reasons for foam formations are

• Air intake in oil
• Low oil level in MOT
• Excessive splashing of oil in bearings
• Insufficient size of lube oil returns line

In order to rectify this anti foam agents are added into oil sump

39-What is meant by NO LOAD operation of Steam Turbine?

A turbine has NO LOAD if just enough steam is flowing into it to cover mechanical losses and to achieve or maintain rated speed.

40-What is the care should be taken while turbine is running on No-Load?

In No Load operation, no steam must be removed from extraction or bleed, Turbine should run on pure condensing mode.

41-What is the effect if Turbine is being operated on No-load for long time?

During No-Load operation of Turbine, the amount of steam flowing through the stages is so small that, the machine is running on its own juice. Turbulence arises with the middle of the blades profiles no longer being flowed round & therefore not being cooled. So running the turbine in such condition will lead to burning of blades in the middle of the flowed reaction section.

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