Guidelines RELATED TO GENERATOR AIR TIGHTNESS TEST
CRITERIA FOR THE ACCEPTANCE OF TEST:
·
The test pressure drop in the system should be monitored
for a period of 24 hours. No break is allowed.
·
The gas system can be considered sufficiently tight when
the pressure drop during a 24 hours test with compressed air does not exceed
0.15 bar or the loss of air is below 1.5m³/24 hrs (s.t.p).
·
If air loss is higher than 0.15 bar / 24 hrs, a search
for leakages must be made.
· The test pressure should correspond to the rated gas pressure of the generator (i.e. 3.5 bar) in normal cases, otherwise 4.0 Ksc if rotor is threaded out.
·
The loss of air is determined as follows;
·
V= [(0.2694×24)/z] X VG {[(P1+ Pb1)/
(273+ t1)] - [(P2+ Pb2) / (273+ t2)]}
o
0.2694 = (273ºK / 1013.25mbar).
o
Where
o
V = loss of air in m³ (s.t.p) / 24 hrs periods
o
Z = duration of leakage test in hours
o
VG = Generator volume in m³
o
P1 = gauge pressure within the system at beginning of the
leakage test in mbar
o
P2 = gauge pressure within the system at end of the leakage
test in mbar
o
Pb1 = Barometer reading at beginning of the leakage test
in mbar
o
Pb2 = Barometer reading at end of the leakage test in
mbar
o
t1 = temperature of the gaseous atmosphere at beginning
of leakage test in ºC.
o
t2 = temperature of the gaseous atmosphere at end of
leakage test in ºC.
·
Suspected areas should be brushed with Diprol or other
foaming solution. The formation of bubbles (foam) indicates a leak. The likely
location of leaks is flange connections, joints, screw couplings, welds and
bushings, Valve glands etc.
·
If the test is not successful then the reason of higher
pressure drop is to be ensured & the test is to be re-conducted.
·
It is better to carry out a sample calculation half way
to ensure the test is going well.
PRE TEST CONDITION OF THE SYSTEM:
·
Unit is under shutdown.
·
All pipe connections of the gas system are restored back
& necessary clearance is given by maintenance departments for conducting
the air tightness test.
·
Check that the stator housing is complete with the
followings: -
·
End covers at DE / NDE, Cooler pockets 4nos, Terminal box
/ bushing chamber manhole, Rotor shaft seals at DE / NDE and stator end ring
manholes at DE / NDE 4nos.
PREPARATION FOR THE TEST:
·
Check, all LLDs drain line valves are closed and H2 drier
is isolated.
·
Ensure generator instrumentation is complete, seal oil
system is in operation and all parameters are OK. Seal Oil DP > 1.5 ksc. H2
side seal oil pump will come into operation after the AS Seal oil Pressure >
4.5 ksc. Lube oil system, Jacking oil system is in service.
·
Ensure that the H2 coolers are filled with / with out
water but important is no circulation of water takes place through the coolers.
Either inlet & outlet valves of all 4 hydrogen coolers are closed, or cold
gas temperature control valve is closed.
·
Ensure the availability of instrument air and bleed the
instrument air sufficiently to ensure the air is free of moisture.
TEST PROCEDURE: -
·
Purge the instrument air in the rack at ‘0’ meter by
disconnecting flexible hose until required dew point is reached.
·
Close the waste gas (vent) valves (purge valve).
·
Connect flexible air hose with the gas plant through the
coupling and ensure no air leakage is there.
·
The filling is to be done by opening the valves in the
normal H2 filling line where as the valve from H2 rack side will remain closed.
·
While pressurizing the LLD / Seal oil DP / Seal oil
parameters are to be recorded.
·
The generator casing is to be pressurized with instrument
air up to pressure of 3.5 ksc.
·
If any problem is experienced in maintaining the DP or
DPR is haunting then TMD is to be informed.
·
Record the time when the pressure has reached at 3.5 ksc.
·
During the filling procedure, the valves at the gas valve
rack (except for the waste gas valves) should be open continuously to ensure that
the air components are included in the leakage test.
·
Stop the air filling and shut off the valve when the test
pressure has been reached (3.5bar) and then disconnect the air hose from the
system to avoid further pressurization of the system.
COMMENCEMENT OF TEST: -
·
Once the rated pressure has been attained, the test can
be commenced.
·
The test is in 2 parts i.e. stabilization period 24 Hrs
& test period 24 hrs. But depending upon the requirement it can be reduced.
·
Once the test is commenced the test record data at hourly
basis for continuous 24 hrs. No break is allowed.
·
During test ensure the LLD‘s are not getting oil.
·
Monitor the pressure drop in the system for a period of
24 hours.
·
Once the test is completed the test results are to be
evaluated for pressure drop.
·
If an air loss higher than 2.4m³ (s.t.p) / 24 hrs, a
search for leakages must be made.
·
Suspected areas should be brushed with Diprol or other
foaming solution. The formation of bubbles (foam) indicates a leak. The likely
locations of leaks are flange connections, joints, screw couplings, welds and
bushings, Valve glands etc.
·
Attend the leakages and then repeat the test until
satisfactory results are obtained.
·
If a satisfactory tightness is established the generator
may be filled with CO2 and subsequently with H2 as per normal operation
practice.
NORMALISATION OF THE SYSTEM: -
·
After the test is declared complete, the air is to be
depressurized and the system is to be normalized.
·
If immediate filling of H2 is not programmed,
depressurize the system by opening waste gas line valve up to 2.0 bar.
·
After completion of the test, the electrical purity meter
system should be calibrated with pure CO2 and H2.
·
Normalize the all-4 hydrogen coolers.
·
Open the main gas and measuring gas pipes at the
generator.
·
Then the generator may be filled with CO2 and
subsequently with H2 as per normal procedure
Prepared
by:
Pankaj Rana (Sr. Manager– Operation)
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