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?



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

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?


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


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?


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.





Boiler feed pumps (BFP) questions & answers for interview


Boiler feed pumps (BFP) questions & answers for interview:



1-What is the function of Boiler feed pumps (BFP) in power plant?
Functions:
To supply the feed water to boilers
To conduct the Boiler hydraulic tests
To supply the desuperheating & attemperator water required for process steam lines & boilers respectively




 2-What are the type of prime movers (drives) used for BFPs?
Prime movers:
  • LT drive (415 V)
  • HT drive (11 KV)
  • Turbo drive (Steam driven)
3-What are the auxiliaries associated with BFP?
BFP auxiliaries
  • Cooling water pump & lines
  • Lube oil system
  • ARC valve
  • Mechanical seal flushing system
  • Balance leak off line & its PRV
4-What are the various pipe lines connected to BFP?

Questions & Answers on AFBC Boilers




  • Suction pipe line
  • Discharge pipe line
  • Bearing cooling water lines
  • Jacket cooling water lines
  • Mechanical seal flushing line
  • ARC line (Minimum re circulation line)
  • Impulse lines for instrumentation measurements (Suction pressure, discharge pressure, Differential pressure, balance leak off pressure)
  • Balance leak off line

5-What is the size of suction strainer of a BFP
It is generally 30 to 40 mesh, that is 30 or 40 hole openings in 1 linear inch on strainer.
A SS 30 wire mesh is generally wrapped on SS mesh having hole openings around 3 to 4 mm
6-What are the different protection devices given for boiler feed pumps?
  • Pressure relief valve
  • Balance leak off line
  • Auto re circulation valve

7-What is the function minimum re circulation line or Automatic re circulation valve (ARC VALVE?)
Minimum re circulation line is provided mainly for centrifugal pump with constant speed drive based on the system and vendor information. There are two types of minimum continuous flow required by the pump (Stable and Thermal). Pump is designed to operate at the flow greater than this flow rate. If pump is operated at less flow than the minimum continuous stable flow, it will damage bearing and internals and may abnormal vibration occur. Below the minimum continuous thermal flow, temperature of fluid will rise at faster rate. To avoid these problems, minimum re circulation line is provided. If the demand of the fluid is decreased below minimum continuous flow, then the auto re circulation valve of the pump will open and maintains the required flow and if flow increased more than minimum flow then auto re circulation valve closes. Generally for higher head flow like 1500 meter head pump the minimum circulation will be 20–25% of total capacity.
8-What is the function of balance leak off line?



Balance leak off line is used to balance the centrifugal pump shaft from axial thrust. During centrifugal pump operation, especially in multistage centrifugal, suction side will have relatively very less pressure as compared to the discharge side. Because of this, there are lot of possibilities that impeller along with the shaft and bearing will be pushed from discharge end to suction end which is also known as axial thrust. Balance line is used to balance the centrifugal pump shaft from axial thrust. Due to the axial thrust, pump bearings and internals will get damaged. To nullify this effect, a tapping from discharge end (between balancing & counter balancing disc) is connected to a balancing drum.
33-Frequently asked QnA on air compressors
9-Why the balance leak off line water is not connected to suction line to save the pump hydraulic power instead of directing it into Deaerator?

Balance leak off water temperature is little bit higher than pump operating temperature which may lead cavitations if it mixes with suction water. For some pumps where there is no risk of cavitations, in such cases this line is connected to suction side of the pump.
10-What is the operating pressure of balance leak off line ?
It is just 0.5 to 1 kg/cm2 more than pump’s suction pressure
10a-What is the standard  gap maintained between balance & counter balance discs?
It is around 0.8 to 1.2mm


11-What does it indicate if balance pressure is increasing gradually?
It indicates the wear out of balance or counter balance disc. That is gap or clearance between these two discs has increased
12-How do you calculate the maximum allowable balance leak off pressure?
Maximum allowable balance leak off pressure = 0.03X (Shut off pressure-Suction pressure)+ Suction pressure.
13-What is the velocity of water in the balance leak off line?
It should not exceed 5 m/sec on any account
14-Why the BFP discharge water & balance leak off temperature is slightly more than that of suction water temperature?
15-Due to the compression action of water inside the pump, the water pressure rises around 2 to 3 deg C more than the suction water. Water is an in compressible fluid
15-What do you mean by the shut off pressure in centrifugal pumps?
Shut-off head is a condition, when a centrifugal pump runs with discharge valve closed. It is the maximum head generated by a centrifugal pump with zero flow and relatively less power.
16-How do cavitations occur? What are the abnormal effects of cavitations?

 

Boy's Watch (Multi Colored Strap)





Pump cavitation occurs when the pressure in the pump inlet drops below the vapour pressure of the liquid. Vapour bubbles form at the inlet of the pump and are moved to the discharge of the pump where they collapse and make high sound and vibrations often taking small pieces of the pump with them.
Cavitation is often characterized by:
Loud noise often described as a grinding or “marbles” in the pump.
Loss of capacity (bubbles are now taking up space where liquid should be).
Pitting damage to parts as material is removed by the collapsing bubbles.

IBR acts, regulations & forms used

17-What do you mean by NPSHA & NPSHR in BFPs?
NPSHA: Net positive suction head available is the absolute pressure at the suction port of the pump.

NPSHR: Net positive suction head required is the minimum pressure required at the suction port of the pump to prevent the pump from cavitations.
NPSHA should be always greater than NPSHR (NPSHA >  NPSHR)
18-What is the significance of NPSH in BFPS?
If BFPs do not have required NPSH, then there will be more chances for formation of cavitations.
How the pump speed is related to NPSH
NPSHR varies approximately with the square of pump speed.
NPSHR = N2
19-How do you calculate the NPSHA ?
NPSHA = Absolute pressure in
NPSHa = Ha +- HZ - Hf + Hv - Hvp
Where, Ha is the absolute pressure on the surface of the liquid in the supply tank.
HZ is vertical distance between the surface of the liquid in the supply tank and the center line of the pump.
Hf is friction losses in the suction piping.
Hv is Velocity head at the pump suction port.
Hvp Absolute vapour pressure of the liquid at the pumping temperature of the pump, it could lead to cavitations of pump.


20-What is the significance of vortex breakers in pumps?
A vortex breaker is a device/arrangement in pumps to stop the formation of a vortex when a fluid (liquid or gas) enters into pump suction. The formation of vortices can entrain vapour in the liquid stream, leading to poor separation in process steps such as distillation or excessive pressure drop, or causing cavitations.
21-What are the reasons for Vortexing in pumps?
Vortexing can occur if any of the following conditions are present:
  • Low liquid levels.
  • Liquid level falling greater than 1 Meter/sec.
  • There is a large concentration of dissolved gases in the liquid.
  • High outlet velocities in pipes leaving vessels. Generally greater than 3 meters/sec.
  • Liquids near their vapour point.
  • High circulation caused by asymmetrical inlet or outlet conditions.
  • Inlet piping too close to the wall or bottom of the tank.


22-What are the protection interlocks given for BFPs.
Protection interlocks: That is BFP will trip/stop on following conditions
  • Low Deaerator level
  • High bearing vibrations
  • High bearing temperature
  • Low cooling water pressure
  • More differential pressure of suction strainer
  • High load
  • Higher balance leak off pressure
  • Low speed (<40% of rated speed)

23-Write down the BFP start permissive interlocks
  • Start permissive interlocks
  • Deaerator level normal
  • Bearing temperature normal
  • Bearings vibrations normal
  • Cooling water pressure normal
  • Motor bearing temperature, winding temperatures normal
  • Discharge valve close
  • Suction valve open
  • Suction pressure normal
  • Differential pressure normal
  • Arc Valve open
24-How do you start the BFP?
BFP start up  sequence
  • Ensure all the start permissive are healthy
  • Ensure no maintenance activities are going on BFP & pump is ready to start with all respect
  • Start the pump from DCS by giving >80% command to VFD
  • Observe the bearing temperature, vibration & speed ramp rate
  • Ensure pump has reached its 50% speed within 10-15 seconds
  • If all parameters (discharge pressure, bearing temperature & vibrations, motor current, winding temperatures etc) are normal
  • Then open the discharge valve slowly
  • After 100% opening of discharge valve rise the speed as per your requirement
  • Note: Ensure all the parameters are normal on every operation on BFP
25-Why it is not allowed to run the BFPS at speed lesser than 50% of rated speed?
For journal bearing BFPS at speed < 50%  the oil splash rings will not flash oil in bearings, leading to the damage to the bearings due to low lubricating oil.
26-What will happen if BFPs run in reverse direction?
If pump runs in reverse direction for more than 5 seconds, there will be the more chances of pumps to seize
27-How do you stop the BFP?
  • Pump stop sequences:
  • Reduce the pump speed slowly up to 60% of rated
  • Close the discharge valve
  • Shut down the pump
28-What is the recommended minimum head for BFP operation?
Should not be less than 10% of its rated head except in start-up & shutdown conditions?
29-What will happen if pump is started and stopped with discharge valve open?
  • Pump may trip due to sudden motor over load
  • Alignment may get disturb
  • Shaft coupling may damage
  • May harm to bearings of pumps and motor
  • Piping supports may get disturbed
  • Pump foundation fasteners may get loose
  • So it is always recommended to start and stop the pump with discharge valve close.


29-What are the reasons for pump to seize?
Following conditions can cause pump to seize:
  • No suction or less liquid flow to suction
  • Operating pump continuously at lesser NPSH
  • Reverse direction rotation of pump
  • Damaged strainer
  • Foreign materials in impellers
  • Uneven thermal expansion of pump internals
30-What is the recommended acceptable value for a BFP shaft run out?


It is around 0.03 mm (Max. 0.05 mm)

31-When should one can carry out alignment on BFP?


Alignment on BFP shall be done when the temperature of the pump is <50 Deg C or in atmospheric temperature.
32-Why do you use 2 dial gauges for axial alignment of BFP?



BFP has more axial float that is 8 to 9 mm without bearings & seal & 0.8 to 1 mm with bearings & seal, so in order to get accurate readings 2 dial gauges are used for angular alignment & 1 dial gauge for parallel alignment. Refer above figure

33-What are the shutdown preservation methods for BFP?

Shutdown preservation method
  • Depressurize the pump
  • Drain all the water
  • Fill the pump with 1:2 or 1:1  Glycol water mixture.
  • Rotate the pump shaft twice in a week
34-What is the recommended bearing temperature for BFPs?
It should be less than 75 deg C (Max 90 deg c)
35-What are the recommended bearing vibrations for BFPs?
It should be less than 3 mm/sec (Max 5 mm/sec)
36-What is the allowable leakage drops for BFP mechanical seal
15 drops/minute
37-What should be allowable the DP across strainer
0.1   to 0.3 kg/cm2 (1000 to 3000 mmwc)
38-What is the filter mesh size for BFP oil replacement
Mesh size is 30 micro meters
39-What preventive maintenance activities that you are going to carryout on BFPS?
Preventive maintenance activities:
  • Pump cleaning
  • Oil level checking & top up if required
  • Alignment correction
  • Suction strainer cleaning
  • Cooling water lines flushing
  • Foundation bolts tightness checking
40-What is the acceptable impeller & wear ring clearance in BFP





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