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.
