Large Generator Hydrogen Seals Failure

Modes, Condition Monitoring and

Common Test – A Comprehensive
Overview
This article briefly explains introduction of H2 seal system of large/turbo generators, H2 seal
failure modes and recommendations to avaoid, condition monitoring of H2 seals through
process parameters and common tests recommended to perform for healthiness check.

Brief Intro Of Hydrogen Seals and Seal-Oil System

Most large/turbo generators use hydrogen under high pressure for cooling the various internal
components. To keep the hydrogen inside the generator, various places in the generator are
required to seal against hydrogen leakage to atmosphere. One of the most difficult seals made
is the juncture between the stator and the rotating shaft of the rotor. This is done by a set of
hydrogen seals at both ends of the machine.

The seals may be of the journal (ring) type or the thrust-collar type. But one thing both
arrangements have in common is the requirement of high-pressure oil into the seal to make
the actual “seal.” The system, which provides the oil to do this, is called the seal-oil system.

In general, the most common type of seal is the journal type. This arrangement functions by
pressurized oil fed between two floating segmented rings, usually made of bronze or
babbitted steel. At the ring outlet, against the shaft, oil flows in both directions from the seals
along the rotating shaft. For the thrust-collar

type, the oil is fed into a babbitted running face via oil delivery ports, and makes the seal
against the rotating thrust collar. Again, the oil flows in two directions, to the air side and the
hydrogen side of the seals.The seal oil itself is actually a portion of the lube oil, diverted from
the lube oil system. It is then fed to a separate system of its own with pumps, motors,
hydrogen detraining or vacuum degassing equipment, and controls to regulate the pressure
and flow. The seal-oil pressure at the hydrogen seals is maintained generally about 15 psi
above the hydrogen pressure to stop hydrogen from leaking past the seals. The differential
pressure is maintained by a controller to ensure continuous and positive sealing at all times
when there is hydrogen in the generator

Hydrogen Seals Failure Mode Analysis

Hydrogen seals pitting is a common phenomenon caused by several possible factors. It is
important to note that seals damage could contribute to an increased seal oil flow and
potential oil ingress.
Based on power plant large generator experience, the common contributors to hydrogen seals
pitting are following:
1. Electrical Discharge:
· Insulation degradation on the seals and bearing: Insulation deterioration can lead to
circulating electric currents (caused by shaft voltage) resulting in pitting damage to seals,
shaft and bearing.

· Damage of IR measuring cable connected to the seal assembly.

· Issues in the shaft voltage grounding system.

Recommendations:

· During shutdown, check the insulation resistance of the seal assembly and ensure it is as
per specification. If resistance is below 10,000 ohms, remedial actions are recommended.

· Site can also check the integrity of the insulation at running speed and without
disassembly by performing a qualitative check with either a volt meter or an ammeter, rather
than measure resistance. Ensure proper condition of the IR measuring cable attached to the
seal assembly.

· Maintain shaft voltage grounding system, and verify that it is operating correctly to
mitigate the buildup of charges on the shaft that can lead to seal and shaft pitting.

2. Seals Clearances:
· Seals are distorted or have reduced clearance.

Recommendations:

· Maintain radial clearances (between seal ring and shaft) as well as axial clearances

(between seal ring and seal casing) within OEM specified tolerance.

· Ensure seals meet OEM dimension specifications.

· Ensure seal casings and seals are installed correctly.

3. Lube/Seal Oil:
· Lube/Seal oil blockage.

· Lube/Seal oil contamination.

· Moisture in lube/seal oil.

Recommendations:

· Ensure oil tank and piping are clean of any debris.

· Inspect Filters.

· Test the oil for presence of moisture. Inspect oil coolers for leakage.

Overheating of the seal metal due to high seal oil temperature can also cause not only a
failure of the seal but subsequent serious damage to other generator components. A hydrogen
seal failure is extremely dangerous since hydrogen is likely to escape from the generator
casing at the location of the failure. The seal will ignite not only because of the high
temperature at the failure point but also because hydrogen leaking from the pressure vessel
will self-ignite. Hydrogen seal metal temperatures are monitored by thermocouples
embedded in the seal ring material.

Generator’s H2 Sealing System - Condition Monitoring

There is an increasing trend towards condition based maintenance (CBM) rather than a fixed
time based planned maintenance approach .The latter approach is well suited to schedule
shutdowns of other equipment i.e. planned generator maintenance ,but can be rather
conservative .To maximize the equipment availability, the former approach of condition
based maintenance routines of associated equipment looks more suitable.

For “on-condition” approach of the H2 seals’ replacement on both sides of

generator sides, the final determination to be based on collector’s both sides H2 purity,
sealing oil consumption and purity of Hydrogen itself in Generator.

Following are mentioned useful helpful tools to carry out the assessment.

Monitoring of Generator’s H2-Sealing system

For meaningful monitoring of Generator’s H2 sealing, simple trending procedures of a basic
performance parameters is all that is required .The procedure is to trend the following
parameters and present the results in a simple graphical form for easy interpretation .The
recommended parameters are:-

1. LO/SO Tank Level %

2. Control Valve Seal Oil Differential Pressure (ΔP)

3. Liquid Level Detector %

4. Generator’s H2 Consumption

5. Generator Drive End - H2 Consumption

6. Generator Non Drive End H2 Consumption

7. Generator DE & NDE H2 Purity %

8. Gas H2 Pressure
H2 Gas & Seal Oil Operation
The GT/ST and generator utilizes a combined lube and seal console which provides
lubricating oil to bearings, couplings, hydraulic system and generator seals.

During normal operation the LO/SO pump takes oil from the reservoir(tank) and circulates it
through the system. Oil flows through the oil cooler and temperature controlling valve,
LO/SO filter and flows into the seal oil section.

Control valve of the seal oil’s differential pressure (Regulator) located in the drain piping
from the drain enlargement tank controls the pressure of seal oil between 5 till 7 psi higher
than H2 pressure . This prevents the H2 gas from entering the bearing chambers. The mixture
of H2 gas/Seal oil (Contaminated) is applied to the lower part of the regulator diaphragm
while seal oil is applied to the upper side .An action of diaphragm, which works in
conjunction with mixed H2 gas/seal oil (Contaminated) is resulting in providing the required
differential pressure (ΔP) at seals of Generator.

The seal oil trap system permits drainage of seal oil from the H2 gas side of the seal without
loss of H2 gas pressure. H2 gas and seal oil are being separated by two traps installed in the
seal oil drain lines at the generator .The traps are controlled by floaters which open valves
when oil level in the trap rise and close before the traps are completely empty. Seal oil flows
from the traps to the reservoir and the H2 gas back into the drain enlargement.

Alarm and shutdown switches are used to sound an alarm in case of adverse operating
conditions such as seal oil inside the generator or high level in liquid level detector .

The H2 gas is provided to the generator from the hydrogen plant with pressure 4 barg to
generator casing after regulating and with purity 99.5% and maintaining purity of 95%-98%
for cooling the winding and rotor of Generator. Part of the H2 gas quantity admits to the
sealing chamber between the oil deflector and inner ring seal in both turbine end (DE) &
collector end (NDE) not less than 85% of the purity and scavenging to atmosphere.
H2-Sealing system Monitoring Procedure
Most operators or condition monitor technicians have to know the exact condition of the
generator sealing system, but want to know the changes in condition with time. Accurate
condition monitoring would require an unrealistic level and calibration of instrumentation,
therefore, this procedure enables sufficient information to be obtained to enable a reasonable
assessment of sealing condition monitoring to be produced.
The procedure is to produce a footprint of all of the above parameters immediately following
installation of either a new H2 seal or reused H2 seal after planned or unplanned inspection.
This data should then be monitoring the conditions and plotted against time .The readings of
each parameter should be taken at regular intervals and the changes compared with the
previous records, which could enable to get an accurate picture of the H2 seals’ health.

Using condition monitoring to measure physical parameters like seal oil tank level , seal oil
differential pressure (ΔP) of the control valve, liquid detector & float trap levels and gas H2
pressure ,purity & consumption can help to determine which combination provides the best
indication of machine health.

Periodic or continuous monitoring readings are taken on the machinery .If a measurement
exceeds its alarm limit, the system automatically detects the exception and produces plots and
reports that help analyze the problem.

As the problem is likely detected early in its failure stage, the analyst has time to schedule the
most efficient and effective repair prior to component failure. This allows maintenance
personnel to get time to order parts in advance, schedule manpower, and plan multiple repairs
during a scheduled downtime that best fits the plants schedule.

Interpretation of Performance Results

1. Seal Oil Tank Level

Seal oil tank level is a useful parameter to monitor the generator H2 seal condition. Drop in
the seal oil level can indicate the seal oil ingress into generator and drain enlargement
through the inner ring.

2. Liquid Level Detector Vs Control Valve (ΔP)

Plotting liquid level detector against the control valve differential pressure (ΔP) will give a
good indication of the efficiency of the H2 seal condition. An increase of the contaminated
oil level in the liquid level detector is indicative that H2 seal’s inner ring and also the gas oil
deflector might be worn out. If H2 seals are slightly worn out, the problem would be
compensated by djustment/compensation of the control differential pressure (ΔP) valve to
minimize the seal oil passing , whereas H2 seal worn out can be only recovered by renewal
the seal.

3. H2 Gas Purity
H2 gas purity is an excellent reference against which subsequent operating parameters can be
compared & evaluated in the plotting of the purity percentage (%) on the DE & NDE of
Generator against the period (Monthly) which can be given good indication of the H2 seal
condition. If H2 gas purity percentage on both sides is decreasing (drops) very
significantly in short period, then the H2 seal & inner seal deflector deterioration could be
suspected.
4. H2 Gas Consumption
The general relationship between H2 gas consumption around one cylinder (m liter /hr) in
daily bases and scavenging of the generator ( e.g; 3000 m liter/ hr) and the turbine
end(e.g; 500 m liter/ hr) & collector end(e.g; 500 m liter/ hr) of the generator against the
period (Daily) which can be given good indication of the H2 seal condition. High H2 gas
consumption can be indictor of deterioration the H2 inner seal & inner H2 seal deflector.

Common Tests for Hydrogen Seal assessment

· The main tests done on the hydrogen seals are liquid penetrant inspection (LPI) for
cracks and other surface damage, and ultrasonic (UT) for babbit bonding to the seal ring
components.

· Megger checks of the seal insulation to ground are done to ensure the rotor shaft is not
grounded through the hydrogen seals. This is usually done at only 500 V dc.

Hydrogen seal-oil experience

GE’s Dhruv Bhatnagar returned to the podium to address the challenges associated with
seal-oil systems and how to mitigate them. Challenges include the following:

 The seal rings themselves. Damage, contamination, improper assembly, and

cocked seals all can lead to operational issues—including oil ingress.
 Float traps require manual bypass during every start-up/shutdown. Improper
procedures are conducive to seal-oil ingress.
 Oil contamination of the hydrogen control panel.
 Seal-oil-system mechanisms and effects was the next topic. Mechanisms include
cocked seals, loss of seal oil, damaged anti-rotation pin, contaminants, damaged
seals, generator pressurization, clogged drain lines, improper assembly, and mis
 operation. Resulting effects include higher total and hydrogen-side seal-oil flow,
improper liquid-level detector alarms, high float-trap oil level.

Checklists for disassembly and reassembly followed:

Disassembly. Measure rotor position from the outboard oil deflector fit to the shaft,
measure the distance between the hydrogen seal casing and the rotor shaft, determine
“as-found” seal clearances, inspect seals, and ensure seals are not out-of-round.

Reassembly. Inspect seals and ensure they are not out-of-round, check for any foreign
material between the inner oil deflector and hydrogen seal casing, check vertical face of
the end shield between the upper half and lower half for any steps across the horizontal
joint, perform blue check and ensure 100% contact, check for any RTV that may have
squeezed from between the upper half and lower half of the end shield, remove any RTV
material that has come onto the horizontal joint of the lower-half casing, ensure seal-oil
inlet feed and gas-side seal-oil drain in the end shield are clear.

The presentation closed with a case study of a unit that was offline, but pressurized and
with seal-oil system in operation, when a blackout occurred. The DC system came online,
but the site lost seal-oil differential pressure (DP). By the time DP was restored, the unit
had dropped 10 psi in hydrogen pressure. The decrease in seal-oil DP allowed oil ingress.

The operator received multiple liquid-level detector alarms, and low and low-low lube-oil
alarms. Site personnel tried to start up the unit next day but were unable to build lube-
oil header pressure. Personnel purged and inspected the generator, which was flooded
with lube oil. Air-side seals and shaft surfaces were found to have rub marks (Figs E8 and
E9); seals were out of round.

https://www.ccj-online.com/generators-lessons-learned-best-practices-shared-at-gug-2017-
part-3/
1. Hydrogen seal rings are used to prevent _______ losses.

Hydrogen seal rings are used to prevent pressure losses. These rings seal the
cylinders so that no pressure and temperature loss take place.

2. _________ oil flow can be controlled by using hydrogen rings.

Hydrogen rings control excessive oil flow. Excessive oil flow consumes more heat
and increases the heat loss.

3. Hydrogen rings are used to control _______ consumption in gas turbine.

Hydrogen rings are used to control hydrogen consumption in gas turbine.

Hydrogen is used as coolant in gas turbine, Excessive usage of coolant leads to
low efficiency.

4. When hydrogen consumption is __________ its efficiency is decreased in turbo

generators.

Hydrogen consumption is inversely proportional to efficiency in turbo generators.

As hydrogen is used as the coolant and when more amount of coolant is used
most heat is lost and hence efficiency is decreased.

5. _________ is used as coolant in turbo generators.

Hydrogen is used as coolant in turbo generators. Compared to the other coolants

available for gas turbines hydrogen is much cheaper.

6. Hydrogen is combustible below _______% purity limit.

Hydrogen is combustible below 74% purity limit.

7. Hydrogen rings are made at _____ degree segments.

Hydrogen rings are made at 180 degree segments. The total ring circumference
must be 360 degree comprising two parts ( i.e Hydrogen rings) each of 180
degree.

8. Hydrogen rings are of two parts upper case and lower case.

Hydrogen rings are of two parts upper case and lower case.

9.Hydrogen is used because of its ______ specific heat.

Hydrogen is used because of its low specific heat.

10. For hydrogen to use as coolant its purity must be more than _______

For hydrogen to use as coolant its purity must be more than 74%. Below 74%
hydrogen is more combustible in nature.

https://www.sanfoundry.com/steam-turbines-questions-answers-types/