100++-interview questions and answers on steam turbine
Is it possible to have a negative absolute pressure?
No, absolute pressure is measured with reference to a perfect vacuum so it is impossible for it to go negative. You can only measure negative pressure between two different pressures. For example if you allow atmospheric air to gradually flow into a vacuum vessel and measure pressure inside relative to outside it will show a negative pressure reading.
What type of problems do you face in steam turbines related to vacuum?
Problems such as:
· Low vacuum
· High exhaust pressure
· High exhaust temperature
· Higher specific steam consumption
· More cooling water circulation
· Hot well level variation
How do you create vacuum in steam condensers?
Vacuum is created in condenser by steam jet ejectors, where high pressure 8–12 kg/cm2 steam is passed through nozzle which is connected to air line from condenser. This creates high negative pressure there by evacuating air from condenser.
Generally there are Two Types of Ejectors:
Hogger Ejector: Initially this ejector is used for pulling vacuum. It has steam and air lines connections, steam is vented directly into atmosphere. It consumes more steam than main ejectors. It requires 20–30 minutes to create 85% of operating vacuum.
Main Ejector: It comes with first stage and second stage. Air line from surface condenser is given to 1st stage then again air from 1st stage is collected and discharged into 2nd stage. 2nd stage ejector has air vent line.
It consumes less steam than hogger ejector. Generally an ejector come with 1W + 1S i.e. one working and one stand by.
Also vacuum pumps called liquid ring vacuum pumps are used to create vacuum in condensers. Which consume less energy than steam jet air ejector
How does low vacuum affect on turbine speed?
Lower vacuum creates back pressure on turbine blades and rotors. So in emergency, vacuum breaker valve is opened to bring down the turbine speed to zero in minimum time to avoid any further damages.
What is the effect of low vacuum & high exhaust pressure on steam turbine performance?
Low vacuum or high exhaust pressure & high exhaust temperatures lead to more steam consumption to generate unit power.
What are the potential reasons for lower vacuum in steam condenser?
· More condenser load than design
· Lesser amount of cooling water circulation in condenser
· Higher atmosphere temperature
· Location of the steam condenser at higher elevations.
· More exhaust temperature
· Air leakages in the system
· Lesser efficiency of steam ejector or vacuum pump
· Ejector inter condense (1st stage) condensate seal break
· Lesser pressure & temperature of motive steam at ejector inlet
· Worn out ejector nozzles
· Improper quality of motive steam
· Variation in condenser inlet & outlet cooling water temperatures
· Operation of Turbine at lower load
· Lower gland seal steam pressure
What are the effects of air leakage in condenser?
Following are the major effects due to air leakage into condenser:
Lower Thermal Efficiency: The leaked air in the condenser results in increased back pressure on the turbine this means there is loss of heat drop consequently thermal efficiency of plant will decrease.
Increased Requirement of Cooling Water: The leaked air in the condenser lowers the partial pressure of steam due to this, saturation temperature of steam lowers and latent heat increases. So it requires more cooling water to condense more latent heat steam.
Reduced Heat Transfer: Due to poor conductivity of air heat transfer is poor.
Corrosion: The presence of air in the condenser increases the corrosion rate.
What is the function of vacuum breaker valve?
Vacuum breaker valve is used to bring down the turbine speed quickly to zero in case of emergency trip of turbine. Valve can be manually or auto opened.
What are the different conditions on which vacuum breaker valve opens?
On following emergency or fault cases vacuum breaker valve will get open
· High bearing vibrations
· High bearing temperature
· High axial displacement of rotor
· High differential expansion
How can you identify the air leakage into the system?
If there is air leakage into the system, then this should be vented out though ejector system. Rota meter of ejector shows the increase in air quantity than the normal air flow.
How do the motive steam pressure & quality affect on ejector performance?
If the motive steam pressure is below design by more than 5%, or above design by 20%, poor performance may occur with a resulting increase in the condenser pressure.
Motive steam quality - Wet motive steam will cause poor performance as well as ejector wear. Super heated steam having a temperature greater than 10° C above the saturation temperature will also cause poor performance if not considered in the design.
What will happen to hot well level, if condenser vacuum drops suddenly?
Hot well level rises up
What are the common problems associated with steam jet ejectors related to vacuum?
Common problems are:
· Low or high motive pressure due to improper sized nozzles:If the cross section area at those locations is greater than 7% above the design values, performance problems are likely.
· Wet motive steam
· Failure of vacuum trap
· Larger pressure drop at shell side: If the shell side pressure drop is greater than 5% of the absolute operating pressure, then either shell side fouling or flooding of the condenser could be present. Check the trap or loop seal on the condensate outlet for proper drainage
· Breaking of 1st stage condenser U loop seal
· Air leakages through safety valve & flanges
· Non operational rota meters
Why U loop and float valves are used in steam ejector 1st stage (inter condenser) and second stage (after condenser)?
U loop and float valves are used for sealing purpose between 1st stage and 2nd stage ejectors and condensers. As there is a pressure difference between these two and turbine steam condenser.
U loop is around 2.5 to 3 meter, it depends on pressure difference between 1st stage and steam condenser. If there is pressure difference of 0.25 kg/cm2 between 1st stage and steam condenser then the U loop height should be 2.5 meter. So it is very must to seal the U seal (filling DM water in loop) before pulling vacuum.
A steam Turbine's exhaust steam temperature gauge is showing 60 Deg C & vacuum gauge is showing pressure -0.75 Kg/cm2, then what do you think, is the pressure gauge showing correct reading?
As discussed earlier, condenser vacuum depends on the atmospheric pressure, as the atmospheric pressure is more, vacuum can be maintained more. Hence the steam condenser installed at higher elevation have lower vacuum than that of condensers installed at lower elevations.
In this case at temperature 60 deg & considering atmospheric pressure 1.033 kg/cm2 the gauge pressure in the condenser should be around 0.81 kg/cm2.
There might be error in vacuum gauge or might be installed at some higher elevation around 600 mm causing lower pressure due to head difference.
A steam power plant is installed 580 meters above the seal level, then what will be the atmospheric pressure in that area?
Atmospheric pressure = P = 1.033 X (1-2.2557 X 10-5 X 580 m)5.2558
Atmospheric pressure = P = 0.9638 kg/cm2
What are the potential reasons for a Steam jet ejector consuming more steam for creating particular vacuum in steam condenser?
Potential reasons are:
· Improper design of ejector
· Improper pipe line layout from & to the ejector
· Worn out steam nozzles
· Steam quality is wet
· Higher steam pressure
· Air leakage into the system
· Steam line leakages
· Fouling in ejector shell
· Insufficient quantity of cooling water
·
· Water tubes leakage
What can cause,If ejector's motive steam pressure & temperature are higher than design?
· Ejector capacity gets reduce
· Ejector performance gets reduce
· Steam wastes
What will be the hogger ejector capacity as compared to main ejectors?
Hogger ejector should create 60-70% vacuum in 15-20 minutes
What is the steam steam consumption for Hogger ejectors as compared to main steam jet ejectors?
It is usually 30-40% more than main ejectors
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
Why do the float valve are provided at the after condenser of ejector condensate line?
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.
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
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.
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
What will happen to hot well level of steam condenser when vacuum drops suddenly?
Hot well level rises suddenly
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.
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.
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.
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
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.
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.
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
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.
What is the quantity of lube oil required for Turbine?
It is 22-25% of total lube oil flow
What is the quantity of lube oil required for Gear box?
It is 60-65% of total lube oil flow
What is the quantity of lube oil required for Generator?
It is 8-10% of total lube oil flow
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
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
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.
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.
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).
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
What is the clearance between rotor & casing diaphragm?
It is 0.6 to 1.5 mm
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
How do you measure the Turbine casing expansion?
It is measured with the help of LVDT
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.
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.
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
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
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
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
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.
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.
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
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.
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
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
What are the causes of foam formation in lube oil?
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
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.
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.
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.
You must know these
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 170oC
· Warm start up- when HP and IP inner casing temperature is lower or equal 430oC
· Hot start up – when HP and IP inner casing temperature is greater than 430oC.
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.
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
How do you calculate power generation in steam Turbines???
Power in the steam Turbines produces at every stage where the steam is taken out, whether it may be bleed, extraction or exhaust steam. As the steam out from the turbine increases the power developed on that particular stage will increase.
Power generation phenomenon.
Power generation in steam Turbines is calculated based on difference between the heat content of inlet steam & extracted steam.
Factors affecting the power generation:
· Power generation at particular stage increases, when there is more steam flow &vice versa
· Power generation at particular stage increases when there is more difference between inlet & extraction steam & Vice versa
· Power develop at particular stage decreases if its extraction pressure increases & vice versa
· Power developed at particular stage decreases if its extraction temperature increases & vice versa
· Power developed in steam Turbine decreases if inlet live steam pressure & temperature decrease
· If steam vacuum decreases power generation reduces or else Turbine will consume more steam to develop same power
· If exhaust steam temperature increases then the power power generation reduces or else Turbine will consume more steam to develop same power
· If wheel chamber pressure increases, then the power generation capacity of the Turbine decreases
In which part of the Turbine higher power can be produced at lower steam consumption? And why?
It is at the exhaust stage. Because at the exhaust stage pressure & temperature of the steam is very lesser than bleed &extraction stages.
In which part of the Turbine lowest power is produced at higher steam consumption? And why?
It is at the bleed stage. Because at bleed steam pressure & temperatures are higher than extraction & exhaust stages
Calculation part:
1-Calculate the power generated in a back pressure steam Turbine, where 50 TPH steam enters the Turbine at 66 kg/cm2 & temperature 485 Deg C.And steam exhausts to process at pressure 2 kg/cm2 & temperature 180 Deg C.
For calculation of power we need to know the enthalpy of inlet & exhaust steam.
Refer steam table
Enthalpy of inlet steam at rated parameters H1 = 806.5 kcal/kg
Enthalpy of inlet steam at rated parameters H2 = 677 kcal/kg
Now power developed in steam turbine P = Q X (H1-H2) / 860
Where Q is steam flow
P = 50 X (806.5-677) / 860
P = 7.52 MW
Note: 860 kcal = 1 KWH
2. Calculate the power developed by a steam turbine by using following data
Sl No. | Particular | UOM | Value |
1 | Turbine inlet steam flow | TPH | 145 |
2 | Turbine inlet steam pressure | Kg/cm2 | 88 |
3 | Turbine inlet steam temperature | C0 | 515 |
4 | Bleed steam flow | TPH | 20 |
5 | Bleed steam pressure | Kg/cm2 | 12 |
6 | Bleed steam temperature | C0 | 250 |
7 | Extraction flow | TPH | 100 |
8 | Extraction steam pressure | Kg/cm2 | 1.8 |
9 | Extraction steam temperature | C0 | 145 |
10 | Exhaust flow to condenser | TPH | 25 |
11 | Exhaust pressure | Kg/cm2 | 0.08 |
12 | Exhaust temperature | C0 | 44 |
13 | Dryness fraction | % | 90 |
Also calculate the specific steam consumption of this Turbine
Solution:
Note down the enthalpy of steam at various stages
Turbine inlet steam enthalpy H1 = 818 kcal/kg
Bleed steam enthalpy H2 =700 kcal/kg
Extraction steam enthalpy H3 = 657.2 kcal/kg
Exhaust enthalpy =Liquid heat + Dryness fraction X Vapour enthalpy = 41.77 + 0.9 X 615.5 = 595.72 kcal/kg
Also steam flow is given
Inlet steam flow Q1 = 145 TPH
Bleed steam flow Q2 = 20 TPH
Extraction steam flow Q3 =100 TPH
Exhaust steam flow Q4 = 25 TPH
Power generation at 1st stage (Bleed) P1 = Q2 X (H1-H2) / 860
P1 = 20 X (818-700) / 860 = 2.74 MW
Power generation at 2nd stage (Extraction) P2 = Q3 X (H1-H3) / 860
P2 = 100 X (818-657.2) / 860 = 18.69 MW
Power generation at 3rd stage (Exhaust) P3 = Q4 X (H1-H4) / 860
P3 = 25 X (818-595.72) / 860 = 6.46 MW
SO total power developed at Turbine shaft P = P1+P2+P3 = 2.74+18.69+6.46 = 27.89 MW
Specific steam consumption SSC = Turbine inlet steam flow / Power generation = 145 / 27.89 =5.19 MT/MW
3-By taking above example, explain how pure condensing steam Turbines have higher power generation & lower specific steam consumption (SSC)
Consider the above example, where Turbine inlet steam flow is 145 TPH & having 25 TPH exhaust steam flow to condenser.
If we condense 100% steam, then we will have reduced SSC
Let us see..
Q1=Q4=145 TPH..Where Q4 = Exhaust steam to condenser
Q2 & Q3 are considered zero (No flow)
Enthalpy of exhaust steam increases due to higher exhaust pressure
SO, Consider exhaust pressure = 0.1 kg/cm2 & Dryness fraction 0.9
Then, exhaust enthalpy becomes H1 = 46 + 0.9 X 617 = 601.9 kcal/kg
Total Power developed P= Q1 X (H1-H4) / 860
P =145 X (818-601.9) / 860 = 36.43 MW
So SSC = 145 / 36.43 = 3.98 MT/MW
This is very less as compared to bleed & extraction turbines
4-By taking an example No.2 explain how bleed steam flow will cause reduction in net power consumption
Solution:
We shall take the data of Example No.2
In this, we will increase bleed steam flow, pressure & temperature & parallel shall decrease extraction flow to match mass flow
Sl No. | Particular | UOM | Value |
1 | Turbine inlet steam flow | TPH | 145 |
2 | Turbine inlet steam pressure | Kg/cm2 | 88 |
3 | Turbine inlet steam temperature | C0 | 515 |
4 | Bleed steam flow | TPH | 30 |
5 | Bleed steam pressure | Kg/cm2 | 15 |
6 | Bleed steam temperature | C0 | 285 |
7 | Extraction flow | TPH | 90 |
8 | Extraction steam pressure | Kg/cm2 | 1.8 |
9 | Extraction steam temperature | C0 | 145 |
10 | Exhaust flow to condenser | TPH | 25 |
11 | Exhaust pressure | Kg/cm2 | 0.08 |
12 | Exhaust temperature | C0 | 44 |
13 | Dryness fraction | % | 90 |
Solution:
Note down the enthalpy of steam at various stages
Turbine inlet steam enthalpy H1 = 818 kcal/kg
Bleed steam enthalpy H2 =716.63 kcal/kg
Extraction steam enthalpy H3 = 657.2 kcal/kg
Exhaust enthalpy =Liquid heat + Dryness fraction X Vapour enthalpy = 41.77 + 0.9 X 615.5 = 595.72 kcal/kg
Also steam flow is given
Inlet steam flow Q1 = 145 TPH
Bleed steam flow Q2 = 30 TPH
Extraction steam flow Q3 =90 TPH
Exhaust steam flow Q4 = 25 TPH
Power generation at 1st stage (Bleed) P1 = Q2 X (H1-H2) / 860
P1 = 30 X (818-716.63) / 860 = 3.53 MW
Power generation at 2nd stage (Extraction) P2 = Q3 X (H1-H3) / 860
P2 = 90 X (818-657.2) / 860 = 16.82 MW
Power generation at 3rd stage (Exhaust) P3 = Q4 X (H1-H4) / 860
P3 = 25 X (818-595.72) / 860 = 6.46 MW
SO total power developed at Turbine shaft P = P1+P2+P3 = 3.53+16.82+6.46 = 26.81 MW
Specific steam consumption SSC = Turbine inlet steam flow / Power generation = 145 / 26.81 =5.41 MT/MW
So by increasing the bleed steam flow, the work done by the Turbine decreases & hence steam consumption will increase
5-A Turbine’s inlet steam enthalpy is 825 kcal/kg & Exhaust enthalpy is 590 kcal/kg. Calculate the work done by steam & specific steam steam consumption
We have,
H1 = 825 kcal/kg, H2 = 590 kcal/kg
Work done per kg of steam = (H1-H2) = 825-890 =235 kcal/kg
SSC = 860 / Work done = 860 / 235 =3.65 kg/kwh or 3.65 MT/MW
6-A steam Turbine inlet steam pressure & temperatures are 104 kg/cm2 & 540 C0 & exhausts at pressure 0.09 kg/cm2 & temperature 43 Deg C calculate the
a- Work done per kg of steam
b- Heat supplied per kg of steam
c- Cycle efficiency
Enthalpy of inlet steam = 829 kcal/kg
Exhaust liquid enthalpy = 44 kcal/kg
Exhaust enthalpy by considering 90% dryness fraction = 44 + 0.9 X 616.44 =598.76 kcal/kg
A-Work done per kg of steam = (829-598.76) = 230.24 kcal/kg
B-Heat supplied per kg of steam = 829-44 = 785 kcal/kg
C-Cycle efficiency = Work done per kg of steam X 100 / Heat supplied per kg of steam
= 230.24 X 100 / 785 = 29.32%
Note:
Power developed at Generator terminals = Power developed at Turbine Shaft X Reduction gear box efficiency X Alternator efficiency
For example:
Calculate the net power developed at Generator terminal if 100 TPH steam enters the Turbine at 811 kcal/kg enthalpy & leaves the Turbine at enthalpy 565 kcal/kg .Assume Gear box efficiency as 98% & Generator efficiency as 95%
Power developed on Turbine shaft = 100 X (811-565) / 860 = 28.0 MW
Net power developed at Generator output terminals = 28.0 X 0.98 X 0.95 = 26.06 MW
Why does load hunt in turbines?
If Turbine does not maintain the load as per set load, then this condition is called load hunting.Following are the some potential reasons for load hunting
1-Problems associated with actuators:
These are related to leakages in actuators, piston stuck up, oil holes elongation etc. Because of these issues there will interruption or fluctuation of secondary oil flow through actuators, this creates the problems of actuator miss-operation & eventually load hunting.
2-Improper calibration of actuators:
This results into mismatch of actuator opening & given set point or valve demand
3-Lower control oil pressure than required:
Actuators are designed for specific pressure of control oil, if the control oil pressure at actuator inlet becomes less, then there will be more chances of mal function of actuator.
4-Fluctuation of control oil pressure/flow:
Fluctuation of control oil pressure or flow due to malfunction of pump or line PRV may lead to actuator misoperation & hence creates load hunting.
5-Control oil line leakage:
Leakages in control oil line welding & flange joints will lead to fluctuation of flow & pressure causing actuator malfunction & load hunting.
6-Contamination in control oil
Foreign particles present in oil lead to improper functioning of actuators, which causes load hunting
7-Burs or scoring marks on actuator spindle & control valves spindles:
Burs or any rough scoring marks on spindles will lead to improper operation of actuator & valves
8-Passing of control valves:
This is the major reason for load fluctuation & Turbine over speed
9-Improperly set control valve cones:
During turbine HP valve assembly after maintenance, HP control valve cones should be set as per factory set readings, if it is disturbed, then there will be issues related to load hunting, low load at more HP demand or over speed.
10-Damage or broken control valves spindle & discs (cones):
Spindle damage or disc damage makes uncontrolled operation, as there will be wrong response from control valves to governor.
11-Wrongly tuned P&IDs in Governor: The disturbed values in P&ID tuning will result into heavy load hunting
12-Malfunction of Governor: This is very rare, but certainly results into load fluctuation of Turbine trip
13-Sudden changes in inlet steam pressure & extraction steam pressure
14-Fluctuation of grid frequency
15-Failure of Turbine inlet & extraction pressure sensor
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