Vacuum system jn thermal power system to use

Vaccum system of power plant

1. Lo]

Vacuum System

PMI Revision 01

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NTPC

Why is it required?

+ The steam turbine itself is a device to convert the
heat in steam to mechanical power.

« Enthalpy drop across the turbine decides the
work output of the turbine. For increasing this
enthalpy drop across the turbine we need
effective condenser vacuum system

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NTPC

« By condensing the exhaust steam of turbine, the
exhaust pressure is brought down below
atmospheric pressure, increasing the steam
pressure drop between inlet and exhaust of steam
turbine. This further reduction in exhaust pressure
gives out more heat per unit weight of steam input
to the steam turbine, for conversion to mechanical
power

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4. Pressure, bars abs.

SAAT AANA

1 2 5 i
Volume, mi /kg

Pressure vs specific volume for dry saturated steam

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5. VENTING EQUIPMENT a

* The function of the venting equipment is not to produce the vacuum,
but rather to maintain the vacuum achieved by condenser

+ The condenser accepts a large volume of steam from the turbine
exhaust. As the steam condenses on the cold condenser tubes the
volume “collapses” and results in the creation of the vacuum

* Itis the function of the venting equipment to remove any
noncondensible gas ingress, which may enter the system through
gaskets, packing, loose connections, or other sources

+ If the venting equipment is property sized & functioning, the
condenser vacuum will be set by cooling water temperature and
heat transfer rate

» If however, the venting equipment is not adequate, the condenser
pressure will rise. In this situation, the venting equipment is limiting
the condenser vacuum

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NTPC

Parts of Vacuum System

* Condenser

+ CW system

« Ejectors/Vacuum pumps
» Gland Sealing System

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7. VACUUM PULLING

ASE CAR Íca
2% 15 16

MAIN
EJECTOR-1

MAIN
EJECTOR. 2

STARTING PRIMING
EJECTORS EJECTOR

GSC AND EJECTOR
TODRAN 54 SYSTEM

COOLER

8. Condenser.

>Steam from last stage
of LPT Exhausts on
condenser tube

> condensation of
steam takes place

> Water collected in hot
well

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9. a 3
Shell of the Condenser

+ The shell is the outer most body of the condenser

+ shell is fabricated from fairly thick carbon steel plates.

+ Due to its large size the shell is sufficiently strengthened or
stiffened internally with carbon steel plates to give sufficient
rigidity for the shell proper.

+ The shell also gives support to number of intermediate support
plates for the long tubes, depending on the size of the condenser.

+ At the same time the intermediate tube support plates allow for the
free movements of tubes in all directions particularly lengthwise
due to expansion and contraction occurring during operation.

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NTPC

* The whole condenser is supported on heavy springs, mounted on
steel sole plates at suitable places on the concrete foundation

+ At the bottom of the shell where the condensate is allowed to
collect, a sump (often referred to as the hotwell) is provided.

+ This sump is common to both the halves but separated by a
partition wall in the middle up to the height of the bottom row of
tubes.

+ The inside of shell and outside the tubes as a whole remains
under vacuum under normal operating conditions. Inside the
tubes the cooling or circulating water passes through.

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Air Zone

« Inside the shell, a central or side portion
longitudinally is separated by an outer shield
except at the bottom. This partition is called the
Air zone.

» All the gases released in the condenser due to
cooling are taken out via these air zone tubes.

« From a suitable portion of this air zone inside the
shell an air vent pipe is taken out and brought out
of the shell for connection to an air extraction
device.

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Tube Sheets

+ At each end of the shell, tube sheet of sufficient
thickness is provided, with holes for the tubes to be
inserted and rolled.

« To take care of length wise expansion of tubes some
designs have expansion joint between the shell and the
tube sheet allowing the latter to move longitudinally.

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NTPC

Water Boxes

+ The tube sheet at each end with tube ends rolled, for each half
condenser is enclosed in a fabricated box known as water box.

+ These water boxes on inlet side will also have big size flanged
connections for cooling water inlet at lower level for butterfly
valves.

+ small vent pipe with hand valve for air venting at higher level, and
hand operated drain valve at bottom to drain the water box for
maintenance.

+ Similarly thermometer pockets are located at inlet and outlet pipes
for local measurements cooling water temperature.

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NTPC

Tubes

+ Generally the tubes are made of brass, aluminum brass,
cupro nickel, stainless steel or titanium depending on
the cooling water chemistry.

+ The lengths are fixed at about 20 ft (6 m) (for the 200
MW device mentioned above), depending on the size of
the condenser.

+ The outer diameter is limited to a maximum of one inch
for ease of handling and ease of insertion through the
shell tube holes and for rolling at both ends.

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15. Exhaust
‘STEAM TYPICAL CONDENSER
| ‘TWO HALVES-SINGLE PASS

DRAWN BY ME FROM MEMORY ONLY.

NOT COPIED FROM ANYWHERE.

- 1]
Tube ends expanded ee AA Y Ststeel

TUBE § }

Er, (Een,
gHeer andbelimouthed ( JO TUBE
J a SHEET

vent, Ststeel Expjoint | E +} Tube ends vent Flanges

DH expanded

or rolled

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NTPC

CW system

CW pumps supply cooling
water to condensers

CW maintains vacuum in
condensers

CW flows through
condensers tubes

PART SECTION OF ANATURAL DRAUGHT COOLING TOWER - COUNTER FLOW

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17. 8
| Venting equipment - types

Existing power plant condensers use a variety of
venting devices, the most common of them are:

Multi Stage Steam Ejectors

Liquid Ring Vacuum Pumps (LRVP's)
LRVP with Air Operated Ejector

+ Mechanical Blowers

+ Hybrid Steam Ejector/LRVP Systems

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18. STEAM EJECTORS

vent

5 drain
a
c
o
ST
©
Non condensible gases and
water vapour from condenser
Second
stage
. ejector
Motive
steam .
First stage
ejector condensate
Motive

drain steam
Multi stage ejector system

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19. a 3
STEAM EJECTORS

+ Air and water vapor are removed from the main
steam condenser, enter the 1st stage ejector and are
compressed to the interstage pressure by means of
the high pressure motive steam.

+ The load and motive steam are discharged to the
inter condenser and a portion of the water vapor
load and motive steam are condensed by
condensate from the main condenser.

+ Non-condensibles and associated water vapor are
removed from the inter condenser by the 2nd stage
ejector.

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20. 23
EJ
se

STEAM EJECTORS

Convergent
divergent
diffuser

Motive
steam

Non condensibile gases and
water vapour from condenser

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Discharge to condenser

21. a 3
STEAM EJECTORS

+ Multistage condensing ejector systems can be designed to
operate at any condenser pressure and designs are not limited
by the available cooling water temperature to the
intercondenser (condensate cooled systems are common).

* These systems have no moving parts, are the most reliable,
require the least maintenance of all venting systems, and are
the least expensive in initial cost.

* Once equipment is built for a given motive steam pressure that
pressure must be maintained or the ejector will become
unstable and lose vacuum.

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22. STEAM EJECTORS

a] Vestari Throat

Motive 1

Fluid el SSS
ll

Moetivatieg Fisid Suction

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NTPC

STEAM EJECTORS

EFFECT OF OPERATIONAL CHANGES ON CRITICAL FLOW

EJECTOR PERFORMANCE

MOTIVE
PRESSURE

DISCHARGE
PRESSURE

SUCTION
PRESSURE

SUCTION
CAPACITY

Decrease

Constant

Increase rapidly

Decrease rapidly

Constant

Increase

Increase rapidly

Decrease rapidly

Constant

Constant

Increase

Increase

Constant

Constant

Decrease

Decrease

Increase

Constant

Constant

Decrease
gradually

Constant

Decrease

Constant

Unchanged

24. a Lo]
EJECTORS

MAIN AIR EJECTOR

+ STARTING AIR EJECTORS

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25. Ejectors

25

26. a 8
VACUUM PUMPS

* The liquid-ring vacuum pump is a specific form of rotary
positive-displacement pump utilizing liquid as the principal
element in gas compression

* The working parts of the liquid ring vacuum pump consist
of a multi-bladed impeller mounted eccentrically in a round
casing which is partly filled with liquid. As the impeller
rotates, the liquid is thrown by centrifugal force to form a
liquid ring which is concentric with the periphery of the
casing

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NTPC LRVP se
Suction port
Discharge port
Casing
Impeller

Gas vapour o
mixture Liquid ring

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28. NTPC LRVP Lo]

¢ The eccentricity results in near-complete filling,
and then partial emp ng. of each rotor chamber
during every revolution. The filling and emptying
actions create a piston action within each set o
rotor or impeller blades.

The pump’s components are positioned in such
a manner as to admit gas when the rotor
chamber is emptying the liquid, and then
allowing the gas to discharge once compression
is completed. Sealing areas between the inlet
and discharge ports are provided, to close the
rotor areas, and to separate the inlet and
discharge flows.

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LRVP

» In addition to being the compressing medium, the
liquid ring absorbs the heat generated by
compression and friction, absorbs any liquid slugs
or vapor entering with the gas stream, and
condenses water vapor entering with the gas.

« A closed loop (or total recirculation) seal system is
commonly used. The seal water temperature warmer
than the cooling water to the pump heat exchanger,
which is normally taken from the same source as the
condenser cooling water (CW or ARCW).

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30. vad
NTPC
vent
Non condensible gases and
water vapour from condenser
Separator
Makeup
Cooling
water Seal water

Seal cooler

Liquid ring vaguum.pump system

30

31. a 3
LRVP

+ The vacuum attainable by a liquid ring vacuum pump is limited by
the vapor pressure of the seal fluid.

+ As the operating vacuum approaches the vapor pressure of the
seal, more and more of the seal fluid will “flash” into vapor.

* The capacity of the liquid ring vacuum pump is reduced as more of
the impeller space is occupied by vapor from the seal fluid, leaving
less space available to accept the incoming load.

« If allowed to continue, cavitation will occur inside the pump,
resulting in damage to internal surfaces, and preventing the pump
from achieving greater vacuum levels.

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32. LRVP WITH AIR OPERATED Lo]
EJECTOR

Non condensible
gases and water
vapour from
condenser

Air ejector vent

Separator

Makeup

g water Seal water

Seal cooler

Liquid ring vacuum, pump system ay

33. LRVP WITH AIR

OPERATED EJECTOR

This device uses air as the motive fluid for the
ejector. The vacuum pump must handle the air
leakage load as well as the amount of additional air
which enters as motive.

With an air ejector, the liquid ring vacuum pump
operates at a higher interstage pressure than a
simple liquid ring vacuum pump, thus these devices
are less affected by the vapor pressure of the seal
liquid.

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34. a 3
AIR OPERATED EJECTOR

3 Mong 2 Convergingr |
4 Converging Chamber ing

pes] Nozzle

5 Divergmg Outes Sauer — tel
1. Motive
Fluid Chest

Ejector protects the Liquid Ring Pump from cavitating, even
when warm water is used as the sealant. The air ejector boosts
the operating pressure of the Liquid Ring Pump to keep it out of

the range of cavitation.

PMI Revision 01 34

35. LRVP WITH AIR

OPERATED EJECTOR

Safe suction pressure

mmHg

Ejector to be cut out

100
90
80 Ejector to be cut in
70
60
50
40
30

20

10 20 30 40 45 50

Liquid ring temperature
PMI Revision 0PC 35

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TRE Hybrid system

Non condensible gases and
water vapour from condenser

vent

Motive
steam

Steam
ejector condensate

Coolin
g water

Separator

Makeu

Seal water

36

37. vada
LES Hybrid system 8

» The vacuum pump operates at a higher interstage pue than
— be obtained through the use of only a vacuum
pump).

Three advantages are found when utilizing a hybrid vacuum system:

* The pump suction pressure is higher than the vapor pressure of the
seal water. This means that the seal water temperature does not
limit the suction pressure achievable;

+ The major portion of the motive steam used by the ejector is
condensed in the intercondenser leaving only a relatively small
amount which enters the liquid ring vacuum pump;

+ The volume of noncondensible gas and water vapor to be
compressed by the vacuum pump will be reduced at a fairly high
interstage pressure, which results in a smaller pump being used.

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38. 3
NTPC
Trouble shooting

In an ideal situation, the vacuum achievable
in a steam surface condenser is determined by
the cooling water exit temperature.

+ Condenser vacuum at rated load = Sat. pressure
corresponding to — (Inlet CW temp + temp
rise(10.64°C) + TTD (3.29°C))

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NTPC Trouble shooting 8

« The first point that should be checked when
poor performance occurs is air leakage.

« This is the most common cause of poor
performance, and is probably the easiest one to
identify, but often the hardest to find.

« The most common sources of air leakage in a
condenser/venting system are at:

1. The turbine glands

2. Large diameter flanges

3. Open valves connected to condenser
4. A loose steam chest on the ejectors.
5. LP turbine diaphragms.

6. Ingress from s/b pump.

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NTPC

Trouble shooting

+ To identify air leakage as the problem, check the
vent of the vacuum equipment, because any air
leak in the system must exit at this point.

« In the case of an ejector system, the vent is the
vapor outlet connection of the aftercondenser.

« If the venting equipment comprises vacuum pumps,
the vent is located on the discharge of the
separator.

« In both cases, the air leakage rate should be
checked when only the normal venting equipment
is operating, i.e., without a hogger unit in operation.

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41. Trouble shooting e

« The average air leakage rate, regardless of
which type of venting equipment design is
employed, should not exceed 10kgs per hour for
most commercial systems.

« If the measured air flow is in this range, air
leakage is probably not the problem.

« If the measured air flow is above this rate, even
if it is below the specified venting equipment
design flow rate, it is recommended that a
search be made for air leaks.

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42. wade

NTPC Trouble shooting Lo]

. If air leakage has been determined to be a problem present in the
condenser or in the venting equipment.

. The easiest way to determine this is to close the isolating valve
between the venting equipment and the condenser.

. If the condenser pressure rises, and the venting equipment
pressure decreases, the problem is in the condenser.

. If the condenser pressure remains relatively unchanged and the
venting equipment pressure remains approximately the same, the
air leak is probably in the venting equipment.

The most commonly used methods include:
1. Hydro test during shut down;

2. asmoke test;

3. agas sniffer test (helium gas detector)

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43. wade

uree Trouble shooting 8

The second most common problem is the accuracy of the pressure
gauge. Absolute pressure type instruments are strongly recommended.

Pressure gauges that are open to the atmosphere are subject to
changes in barometric pressure, which can be +/- 2 inches Hg.

The best way to verify the accuracy of the pressure gauge reading for
the condenser is to compare the value of the gauge against the
condensate temperature. The temperature of the condensate can never
be higher than the saturation temperature corresponding to the
condenser pressure, but it can sometimes be colder.

Cold condensate can occur with severe tube leaks, partial flooding of the
tubes, cold makeup or dump water returns, and sometimes under light
condenser loads.

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44. vada Lo]
wire Trouble shooting

+ A determination of whether the venting equipment or the
condenser is setting the operating pressure is important.
Once that has been established, a systematic approach
to troubleshooting is possible.

+ One technique of determining which component is
controlling can be accomplished by increasing the
venting equipment capacity. This can be done by adding
a redundant vacuum element, or by turning on the
hogging unit (if available).

+ When this is done, if the operating pressure in the
condenser decreases, then the venting equipment is the
controlling factor. If the condenser operating pressure
remains unchanged, then the condenser is limiting the
vacuum level.

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NTPC

Condenser limiting vacuum

+ CW inlet and outlet temperatures

If either the temperature of the inlet CW, or the CW temperature rise, is greater
than design, the condenser pressure may be higher than design. A high CW
temperature rise means that the condenser is either over loaded, or that the
cooling water flow rate is below design.

+ Cooling water inlet and outlet pressures

« Ifthe cooling water pressure see is below the design value, the cooling water
flow rate is probably too low (higher temp. rise across tubes).

» If the cooling water pressure drop is above the design value, either the cooling
water flow rate is above the design value (higher temp. rise across tubes) or
tube side fouling could be present (higher temp. rise across tubes).

(Note: A high cooling water flow rate rarely poses a performance problem.)

PMI Revision 01 45

46. : 8
Venting equipment limiting

=
Motive steam Vacuum. Ejector: low 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. Superheated steam having a temperature greater than 50° C
above the saturation temperature will also cause poor performance due to
reduced mass flow rate if not considered in the design.

Intercondenser shellside pressure drop - If the shellside pressure drop is
greater than 5% of the absolute operating pressure, then either shellside fouling
or flooding of the condenser could be present. Check the trap or loop seal on the
condensate outlet for proper drainage.

Ejector internals - Check for internal wear, as well as checking the critical
dimensions. Both the steam nozzle and ejector diffuser throat dimensions should
be measured. If the cross section area at those locations is greater than 7%
above the design values, performance problems are likely. Examine the motive
steam nozzle for steam leaks around the threads.

Cooling water parameters on the inter- and aftercondensers

Use the same procedure as described for the main condenser.

PMI Revision 01 46

47. 3

Venting equipment limiting
Vacuum LRVP

¢ Seal water inlet and outlet temperature - Higher
than design inlet or outlet temperatures will cause
poor performance. When the outlet seal water
temperature is high, this indicates either low seal
water flow a high inlet temperature, or a
malfunction of the seal cooler.

¢ Operating pressure of the pump - When this
pressure Is too low, the problem may be due to low
noncondensible gas loading or a cooling water
temperature that is lower than design. The concern
is for cavitation of the pump performance.

PMI Revision 01 47

48. Lo]

Auxiliary Steam
System

49. wade Lo]
NTPC

Presentation outline

"Sources of Aux steam supply
"PRDS Station

"Uses of Aux steam

"Flange and stud heating

"Rotor heating

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50. wade Lo]
NTPC SOURCES

+ Auxiliary boiler.
+ Station PRDS header (from other running units)

Under unit running condition Steam for turbine auxiliary
steam header is normally taken from extraction lines
depending on load on the turbine sources may be:

= Main steam line through pr and temp reducing valves

" CRH line

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51. ana . Lo]
MIRE PRDS Station

It Consists of

¢ Set of pressure reducing cum control valves
+ Set of temp reducing (spray) valves

For maintaining desired pressure and temp.

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52. cada
PRDS Header 8

« For a 210 MW Thermal Power Unit using coal as
basic fuel, auxiliary steam is one of the important
systems. The current practice is to have two
different headers:-

« Turbine Auxiliary Steam Header

« Boiler Auxiliary Header

PMI Revision 01 52

53. Uses of Auxiliary Steam 8
on Turbine side

+ To turbine gland seal header.

« To ejector system of condenser (main as well as
starting ejector).

» to Deaerator as pegging steam

+ For flange & stud heating.

« Interconnection to station PRDS header for
supplying aux steam to other units.

PMI Revision 01 53

54. Uses of Auxiliary Steam 8
on Boiler side

¢ Burner atomisation,cleaning, scavenging
+ SCAPH charging during cold start up

» Regenerative air heater sooth blowing.
Off Sites:

« HFO station: Fuel oil system heating fuel, oil
storage tanks, pipelines, fuel oil heaters.

PMI Revision 01 54

55. dd
AUX STEAM HEADER

SPRAY FROM BFP

OTHER UNITS

EJECTORS

REGENRATIVE AIR :
HEATER

p<] HFO

. HEATING
AUX STEAM HEADER
1

AUX BOILER

PMI Revision 01 BB

56. Lo]

Flange, Stud 8. Rotor
Heating System

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NTPC

Flange and Stud heating

» Used for quick starting of turbine from warm or cold start.

+ Helps in reducing the temperature difference of the metal to
reduce thermal stress.

» The device consists of jackets welded to side walls of casing
flanges and special piping complete with fittings and
measuring instruments for steam inlet, outlet, and drains.

» The flanges and studs are heated with live steam bled from
pipelines before main stop valve (ESV).

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NTPC

Flange and Stud heating

+ The headers, supplying steam to the flanges and studs, are
provided with safety valves and pressure gauges for
regulating pressure in the headers.

« To check the temperature difference of the metal across the
width of casing and between flanges and studs of the
casing, thermocouples have been provided.

« Careful check of the metal temp. is required during heating
process.

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NTPC

Flange and Stud heating

+ The temperature difference across the width of the casing flange should
be within 50°C. The temperature of outside flange surface should never
exceed that measured with a thermocouple ata maximum _ depth
inside the flange.

+ The difference between the flange and stud temperature should not
exceed 20°C and the stud temperature should never be more than
that of the flange.

+ When starting the turbine from cold or hot (temperature of metal of
the upper half of the HP- casing being up to 250°C, heating of flanges
and studs is permitted provided differential expansion of the rotor is
positive and not less than 1 mm.

PMI Revision 01 59

60. a 8
Flange and Stud heating

It is not permitted to operate the flange heating device:

+ If the differential expansion of rotor is less than 1 mm.

+ If the differential expansion indicating and recording
instruments are either out of order or disconnected.

« In order to maintain proper tightening of flange joints, it is
necessary to first supply steam for heating flanges and only
then after some time to studs.

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61. taie
NTPC

Rotor Heating

« In case the contraction of HP rotor reaches 0.8 mm
and that of IP rotor 1.5 mm. fresh steam is supplied
to the front sealing of HP & IP turbines. In order to
control the contractions of HP & IP rotor the steam
is supplied at a higher temperature then that of the
turbine casing at the front sealing till it comes under
control.

PMI Revision 01 61

62. taie
NTPC

THANK YOU

PMI Revision 01

62