MOISTURE CONTROL

IN
MAIN OIL TANK OF UNIT-4

BY

ANAND PRAKASH SINGH

TURBINE MAINTENANCE.
 WHY TO CONTROL MOISTURE?
If the moisture in Turbine oil is > 100 PPM then
following problems may occur
• Shorter component life due to rust and
corrosion
• Oxidation of bearing Babbit
• Water etching/erosion and vaporous cavitation
• Wear caused by loss of oil film or hard water
deposits
• Main problem may occur in governing system
which will erratically behave.
 MAXIMUM RECOMNDED WATER
CONCENTRATION
• Oil, unless it is dried, contains some dissolved water. Figure 1 shows the
amount of dissolved water that can be found in ISO 220 paper machine oil
and ISO 32 turbine lubricant before it turns cloudy.

Figure 1. Dissolved Water as a Function of Temperature

in Paper Machine Oil and Turbine Oil
 PROBABLE REASONS FOR HIGH
MOISTURE
• GLAND STEAM LEAKAGE CAPTURED BY BEARING PEDESTAL

• HIGH NEGATIVE PRESSURE IN OVEF LINE

• MALFUNCTIONING OF CENTRIFUGE SYSTEM

• WATER LEAKAGE IN WATER COOLED OIL COOLER

• MALFUNCTIONING OF GLAND STEAM CONDENSER

• MALFUNCTIONING OF GSC BLOWER

 ACTION TAKEN TO IDENTIFY THE ROOT
CAUSE
“To identify the Root cause all suspected reasons
should be eliminated“
1) GLAND STEAM LEAKAGE CAPTURED BY BEARING PEDESTAL

If the gland steam will leak out from labyrinth gland then moisture in bearing
pedestal return line and turbine hall will increase.
For that oil samples from Brg. 1 to 4 were checked and found that moisture in
bearing no.1 & 2 very high than limit.
This oil test establishes that Gland steam is leaking out from HPT front & rear,
and IPT front.
ACTION TAKEN : OVEF return line pressure is adjusted to 5 -10 mmWC. With
this we had eliminated the High negative pressure in OVEF return line.
 MALFUNCTIONING OF CENTRIFUGE SYSTEM

Centrifuge removes mechanical impurities and moisture

after certain level from oil due to density difference.
PRIME SUSPECTS:
• Running in clarification mode instead of purification
• Water leaking into oil in heater.
ACTION TAKEN :
• To run it in purification mode,Gravity disc and Level
ring were changed according to oil sp.gravity.
• Second suspect is technically wrong, but for our
satisfaction we checked the heater internals and found
ok.
 WATER LEAKAGE IN OIL COOLER

There are 2 nos. of vertical water cooled oil

coolers,1 running and 1 stand by.
DM water runs in tube and oil in outer shell,if the
water tube get’s ruptured,water won’t enter
shell side due to high pressure of oil than water.
So technically this reason also get eliminated.
 MALFUNCTIONING OF GLAND STEAM CONDENSER

The function of the gland steam condenser(GSC) is to maintain a sub-

atmospheric pressure at the outermost leak off belt of the glands and
thereby prevent the leakage of steam from glands into the turbine hall.
ACTUAL CONDITION:
• GSC shell side level high
• Negative Pressure gauge not present in GSC

ACTION TAKEN:
Shell side drain which goes to hot well via steam trap was checked and
steam trap was reoriented, one drain was given before steam trap will
be opened periodically to drain GSC.
A negative pressure gauge is installed in GSC and pressure of -100 mmWC is
maintained.
Level transmitter is installed for monitoring from control room.
 END RESULT AND LEARNING
• RESULT: By maintaining the condensate level
and negative pressure in GSC the seal steam
which was earlier captured by oil through
bearing pedestal, now goes to GSC.With this
action the moisture in MOT oil is with in limit.
• By learning from this exercise we are installing
level transmitter and pressure gauge in each
GSC of each unit.
ROLE OF GSC IN TURBINE SEAL STEAM SYSTEM
 Why Turbine is sealed by steam?
• The turbine have two main parts one is rotating
parts(Rotor) and another is stationary parts(casing) so that
minimum clearance sell be required to avoid friction
between rotating and stationary parts and for thermal
expansion. The high pressure steam inside the turbine try
to exits through the clearance between rotor and casing.
And in vacuum condition atmospheric air try to enter into
the turbine/condenser through the clearance. To avoid the
steam exits & air entering, gland is provided in turbine and
steam is used for sealing.
 FUNCTION OF SEALING SYSTEM
• To prevent the atmospheric air entering in to
the condenser during vacuum pulling and
during normal operation maintaining vacuum
in condenser( approx. -0.9bar) by sealing the
gland with seal steam.
• To prevent the steam leakage from the
turbine gland seals by leak-off steam control
system
BENEFITS OF GLAND SEALING SYSTEM
• Maximized the efficiency of back pressure
steam turbine by sealing the LPT gland to
maintaining the vacuums in condenser.
• Minimized the wastage of leak-off gland
steam from the turbine by controlling leak-off
steam pressure equal to seal steam pressure
by dumping excess steam to condenser.
 TYPE OF GLAND SEAL’S
• Labyrinth Seal
1) Plain Labyrinth Or See through
2) Stepped Labyrinth
3) Vernier Labyrinth
4) Spring Back Labyrinth
STEPPED LABYRINTH SEAL
 STEPPED LABYRINTH SEAL
• A labyrinth seal is a seal which creates a complicated
path for liquids/steam to pass through it. Labyrinth
create a barrier because making a difficulty their way
through the labyrinth. The pressure is reduced
according to the labyrinth principle by conversion into
velocity with subsequent turbulence in many sections
of the seal. The labyrinth seal has a number of grooves
which interlock with other grooves or teeth. This differs
from most traditional seals, which use a single lip in an
attempt to seal an opening. With the labyrinth seal,
each groove acts as its own lip, distributing pressure
across the seal rather than concentrating it in one area.
 PLAIN or SEE THROUGH SEAL
• The region where relative expansion is more
see-through seals are used, in which the seal
strips are located opposite each other, caulked
into the shaft and into seal rings centered in
the outer casing. The outer seal rings can be
removed for inspection and if necessary seal
strips can be replaced during a short turbine
shut down keeping module in place.
SEE THROUGH SEAL
 SPECIFIC LOCATION OF SEALS
IN STEAM TURBINE
See through or Plain seals are used where axial
expansion is more ,otherwise the fins will rub
each other and rotor would bend and become
unbalanced, For this reason these seals are
used in exhaust side of HP & IP Turbine.
Labyrinth Seals are used where axial expansion
is limited, these seals are used in HP & IP
Inlet,LP inlet and exhaust side.
 Ex fan

See through seals

Hot well
GSC

Steam Trap

Drain

HPT IPT LPT

PT
TT

PT

TT TT

MAL-81 MAW-50
MAW-10
TT
TO CONDENSER

FROM HP STOP AND

CONTROL VALVES
MAW-55
MAW-15
TO CONDENSER MAW- 11
FROM IP STOP AND TO CONDENSER
CONTROL VALVES

FROM APRDS TO ATM.

.
PID OF SEAL STEAM SYSTEM
 Seal Steam Control System
• Seal steam control system is combined control system
of auxiliary supply seal steam and gland leak-off steam
with a common header maintaining 1.048bar
(absolute) pressure equivalent to 350mmwc.
• Auxiliary supply seal steam control station:
During vacuum pulling in condenser, auxiliary steam is
required for sealing the turbine gland to prevent the
entering of out side air to the condenser. It maintains
the seal steam pressure approx. .035bar. Gland seal
steam control station consists of one pneumatic
control valve with two numbers manual isolation valve
and a bypass motorized valve.
 Seal Steam Control System
• Leak-off steam control system:
After loading the turbine, the leak-off steam of HP & IP
turbine tries to exits to atmosphere. To prevent the
exits of leak-off steam coming out from turbine is draw
back in to the leak-off steam control station
maintaining pressure equal to supply steam pressure
by dumping the excess steam to condenser and also
same steam supply to LPT for sealing. Leak-off steam
control station consists of one pneumatic control valve
with two numbers manual isolation valve and a bypass
motorized valve.
HP TURBINE GLAND SEALING SYSTEM
• The function of HP shaft seals is to seal the interior
of the casing from the atmosphere at the ends of the
shaft on the admission and exhaust sides. The HP
Turbine has shaft seals in front and rear. The front shaft
seal is of labyrinth type, while the rear shaft seal is of
‘see through’ type. The difference in pressure before
and after the raised part of the shaft seal on the
admission side serves to counteract the axial thrust
caused by steam forces. The raised part is called
Balance piston. The effective area is suited to the
requirements for balancing the axial thrust.
HP TURBINE SEAL UNDER NORMAL
LOAD CONDITION
HP TURBINE SEAL ZERO LOAD
 Shaft Seal Steam Admission side

1 Inner casing 2 Seal

ring
3 Turbine rotor
4 Shaft seal cover
5 Caulking wire
6 Seal strip 7 Split seal ring 8 U- seal ring 9 one piece seal ring.
 Shaft seal Exhaust side

Steam Spaces
Steam spaces are provided within the shaft seals. From spaces ‘Q’ leakage
steam drawn off to CRH and ‘R’ leakage is drawn off to IP exhaust for further
use. The steam seal header is connected to space ’S’. The slight amount of
leakage steam which are still able to pass the seal ring are conducted from the
space ‘T’ into the seal steam condenser.
HPT CROSS-SECTIONAL VIEW
 GSC(GLAND STEAM CONDENSER)
• The function of the gland steam condenser(GSC) is to maintain a
sub-atmospheric pressure at the outermost leak off belt of the
glands and thereby prevent the leakage of steam from glands
into the turbine hall.

The GSC is vented to the atmosphere via a blower. The small

vacuum created by the blower is sufficient to draw air into
the glands where it mixes with steam leaking from the cylinder,
the air is separated in GSC and passed back to atmosphere
via the blowers.Condensate flows in tube side and the steam
which to be cooled in shell side. The GSC has also a
condensate bypass system.
 OPERATION of GSC
• To maintain sub atmospheric pressure in GSC the shell side
condensate level should be minimum. For this purpose GSC
O/L line is connected to LP Flash Tank via Steam trap ,for
emergency situation a drain line is given before steam trap to
drain GSC condensate to atmosphere, this drain is siphon type
which prevents air ingress, siphon height is 1 mtr.

• If the condensate level in shell side is very high then it will

come out from overflow line , which makes a inverted siphon
at bottom to prevent air ingress from drain side. The height of
siphon is 1 meter.
 GSC Blower
M
Leak off steam B
B
Non condensable gases
vented to atmosphere
GSC bypass Valve

M
M

L
E
V
E
L Condensate IN
G
A Condensate OUT
U
G
E

To LP flash tank
Filter

STEAM TRAP
1 Mtr

Siphon
1 Mtr

Drain in CEP Pit

GSC FLOW PATH
PID of GSC
THANK YOU