Shaft Sealing System on Steam Turbines.pdf

Shaft sealing system on steam turbine.

1. © Siemens AG 2009. All rights reserved.
Shaft Sealing System
on Steam Turbines Wilhelm Herzog
Energy Sector
Energy Service Division / Oil & Gas and Industrial Applications Services

2. Page 2May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Introduction / Content
Content
ƒShaft Seals
ƒOil Seal on Bearing Housing
ƒSolution Approach
ƒCondensing / Back pressure Turbine
Sealing
ƒSeal Steam Temperature
ƒSummary

3. Page 3May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Typical Steam Turbine Labyrinth Sealing
Exhaust shaft
labyrinth
Inlet shaft
labyrinth
Bearing housing
oil seal
Balance piston
labyrinth
P
T
<< P
atm

4. Page 4May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Labyrinth seal basics
ƒLabyrinth seals consist of a number of thin strips or serrations
ƒThe clearance between shaft and bushing is very small
ƒThe resistance offered by this series of obstruction to steam
flow holds leakage to a minimum
Rotor with seal strips
“Toothed“bushing
Rotor with seal strips
“Toothed“bushing
Radial clearance
Higher pressure
Lower pressure
A seal is required to keep the st eam in and air out of the turbine!

5. Page 5May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Back pressure turbine shaft sealing
Leakoff steam
P
A
> P
atm
P
T
>> P
atm
P
atm

6. Page 6May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Seal steam
P
T
<< P
atm
P
atm
Condensing turbine shaft sealing

7. Page 7May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Seal
steam
Shaft labyrinth seal
Oil seal
Seal air
Leak-off steam
Seal steamShaft Seals on Condensing Turbines

8. Page 8May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Non-contacting Shaft Seals Despite minimal clearance in a non-
contacting shaft seal, the prevailing
differential pressure always causes:
Flow through the labyrinth
Basic problem: Either something escapes from the sealed space, or something gets into it. Areas of use:
1. Oil seal on bearing body
2. Steam seal on shaft labyrinth
Rotor with seal strips
“Toothed“ bushing
Flow of medium
Rotor with seal strips
“Toothed“ bushing
Flow of medium

9. Page 9May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Purpose: Prevention of
ƒOil escape from bearing housing
ƒingress of foreign matter into the oil system
Standard solution:
Maintain negative pressure in the bearing housing respectively the oil
system of approx. –

10 to –

20 mm water column (WC)
Oil Seal on Bearing Housing

10. Page 10May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Oil Seal on Bearing Housing
Problematic ?
ƒCentralized vacuum generation from oil vapor extraction located on
oil tank.
ƒAs long as that is working properly, no oil will escape, but foreign
matter can enter the bearing housing and get into the oil system
ƒDifferent arrangement of oil return li nes and ventilation situation in the
bearing housing, with different levels of efficiency at the points of
relevance
ƒIf the suction power of the exhau ster deteriorates due to clogged
filters, the vacuum will become insufficient
ÎResult: Oil leak

11. Page 11May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Solution approach for a
„Leak tight Bearing Housing“
? How can this be achieved ?
Way out of the dilemma:
ƒA flow in inward and outward direction must be created within
the seal
ƒThe seal medium must be harmless both to the environment
and the oil system.

12. Page 12May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Oil Seal using Sealing Medium
ƒDivision of the labyrinth, with
center-fed seal air at a slight
positive pressure.
ƒThe option Seal air or gas is
also employed for oil systems
that operate at a slight positive
pressure
Atmosphere
Seal air
Rotor
Bearing body
Oil seal ring
Pressure
gauge

13. Page 13May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Shaft Labyrinth Steam Sealing
Condensing turbine
:
Sealing steam is required to prevent
ƒair ingress into condenser
ƒescape of seal steam to atmosphere
Backpressure turbine:
ƒEscape of leak steam to atmosphere
must be prevented
Atmosphere
Leakoff steam
Seal steam
Vacuum
Rotor
Shaft
labyrinth

14. Page 14May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Steam Seals on Condensing Turbines
Is there any difference in function to the described oil seal system? Yes, because precise pressure stabilization
of the seal steam (with an
optimum somewhere between 0.02 and 0.04 bar gauge) is necessary to prevent
the ingress of air!
Consequently:
-

Manual control is hardly ever sufficient for this difficult task.
-

Only in the rarest of instances could the required precision of

control be
achieved using mechanical and hydraulic systems.
Modern electronic PI controllers, preferentially in combination with
pneumatically actuated control valves, have proven to work best in
actual practice!
p
seal

< 0.02 => air ingress into condenser
p
seal

> 0.04 => escape of steam from labyrinth
If

15. Page 15May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Backpressure Turbine Sealing
The backpressure-turbine situation:
ƒNo air ingress possible:
Internal pressure >Atmosphere
ƒLeak-off steam must be extracted
How is this accomplished?
ƒLeak-off steam connection
ƒLeak-off steam exhaust to
atmosphere / leak steam condenser
Effects:
ƒForced directional flow from the
atmosphere to the extraction
chamber
ÖNo steam can escape!
Back-
pressure
Leak-off steam >
4bar
Leakoff-steam extraction
Atmosphere
Rotor

16. Page 16May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
„Mixed“ Application on Heating Turbines
Seal-steam control
Leak-off steam exhaust
and
Vacuum mode

(Internal pressure < Atmosphere)
Split range:
0
100
Stroke
Vacuum modePressure mode
Pressure controller
signal
y
0100
Precondition:
Separation of
-

seal steam supply
and
-

leak-off steam exhaust !
Pressure mode

(Internal pressure > Atmosphere)
SIEMENS
CV 1CV 2
Extraction
SIEMENS
CV 1CV 2
SIEMENS SIEMENS
CV 1CV 2
Extraction

17. Page 17May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Seal steam
temperature
Lowest allowable
Highest allowable
Turbine inlet end
Wheel-chamber
temperature at 100%
load –

150°C
Live steam temperature
Turbine exhaust end
≥

150 °C, minimum 50°C
above saturation line
≤

300 °C
The temperature difference between seal-steam and casing inlet /
exhaust end must not exceed allowable limits:
Recommended seal steam temperature ranges:
Requirement:
Seal-steam Temperature

18. Page 18May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
The most economical and functionally reliable configuration
are two sealing steam systems:
SIEMENS
CV 1CV 2
Extraction
Seal-steam Temperature

19. Page 19May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Seal-steam Temperature / Flow
If no suitable second source of seal steam is available?
ÖSeal-steam temperature must be reduced by cooling
Leak-off steam
extraction
SIEMENS
CV 1
Seal-steam cooler difficulies:
ƒOnly very small amounts of steam and injection water must be handled
ƒSeal-steam and injecting-water requirement can even approach zero
ƒLow volumetric flow through the steam cooler

20. Page 20May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Summary
Strategies for dealing with leaky shaft seals:
ƒAvoid undue clearance rotor / sealing tips
ƒOil seal rings with seal-air connection
ƒElectronic seal-steam control
ƒPneumatically actuated control valves
ƒOptimized extraction-line arrangement
ƒElimination of seal-steam cooler by switching to seal-steam
supply from low-pressure/low-temperature steam system

21. Page 21May 2009
Siemens Energy; E S SO PLS EN NBG
2
W. Herzog
© Siemens AG 2009. All rights reserved.
Thanks
for your attention