Steam turbine HP to IP packing leakage method and calculation

2. High pressure and High temperature steam
leakage form HP to IP due to seals
damaged or weakened by
misalignment, poor start-ups, or multiple
temperature excursions will cause
leakage

3.  Increases in calculated heat rate.
 Increases in calculated IP efficiency.
 Decreases in calculated LP efficiency.
 Decreases the mass flow rate through
the HP turbine down stream.

4.  LOSING LOAD-
1) A turbine’s output and reliability can be affect
by high internal leakage. An enlarging internal
leak will initially increases the unit’s capacity in
a manner similar to reheat spray. The cycle
flow restriction in the first few stages of the HP
turbine will be by passed. Eventually, the
effects of reduced boiler re-heater flow will
cause overheating of the re-heater tubes and
more tube leaks. Load may have to be
curtailed to avoid overheating the re-heater.

5. 2) Nut and bolt aside, the thrust balance can
also be affected by a change in internal
flow distribution . It may not be possible to
achieve full load following such a change if
it triggers a thrust bearing alarm.
3) Trouble controlling reheat temperature- this
situation could involve in to one where the
flow capacity of reheat spray is “topped
out”. At this point, the only alternatives for
control would be reduce load or to lower
superheat temperature.

6. 4) TURBINE PRESSURE CHANGES AT VALVE WIDE
OPENING- The main steam flow calculated
from the first stage curve will decreases,
whereas the main steam flow determined by
the feed water flow (plus superheat spray flow,
if applicable) will increases.
5) TURBINE SHELL TEMPERATURE DIFFERENCE-
Verified difference of over 100 degrees F
between the upper and lower shell metal and
steam temperature could be a sign that an
internal leak is coming an upper or lower
section

7.  Seal damage or weakened by
misalignment.
 Poor start-ups.
 A water induction incident will causes
seal rubs and HP inner shell distortion.

8.  Upper and lower main steam inlet snout rings
clearance increases.
 The N2 packing head’s horizontal joint and how to fits
in to inner shell.
 The HP inner shell horizontal joint (if the shell distorts or
the joint develops a loose bolt).
 The turbine blow down pipe’s snout/piston rings.
 The first stage pressure flanged probe and how to fits
in the lower inner cylinder.

9.  Replace N2 packing seals if they have
excessive clearance or broken teeth.
 Proper alignment and a controlled start-up
after the turbine outage are critical to
maintaining the clearance.
 Replacing snout rings (For main steam and
the N2 packing blow down pipe) that have
excessive clearance, taper, or erosion.
 The snout pipe themselves may be eroded
enough to require refurbishment

10.  Weld build up and machining the HP
inner shell horizontal joint surface,
including an evaluation of its studs and
shell threads, leakage can actually flow
up through shell holes, eroding the studs.
 The stud nuts should be “sounded” with
a hammer to determine if any loose prior
to dismantling the unit.
 It is very important to know the both stud
material and the nut tightening spacs.

11. TWO METHODS ARE THERE
1) Temperature Variance Method
2) Blow down method

12. The temperature variance method uses the
difference between the enthalpy at the first
stage of the HP turbine and the enthalpy at
the IP turbine, upstream of the intercept
valve, to estimate N2 leakage.
Because of lower enthalpy of the N2 leakage
form first stage, there is cooling effect on
the steam at the IP turbine inlet, which
carries on the down cross over .
This effect is maximized by decreasing
throttle temperature, and minimized by
decreasing hot reheat temperature

13. To run the temperature variance test
1) The hot reheat and super heat temperature
are set to a temperature differential of
approximately 75 degree F for example, set
hot reheat to 1000 degree F and superheat
925 degree F. test data is collected and IP
efficiency is calculated using assumed
value of N2 leakage from 0 to 10 percent of
first stage flow and result of this calculations
plotted as an IP efficiency Vs leakage flow
trend
HRH- 1000 degree F MS – 925 degree F

14. 2) Next the unit is set up with reheat and super heat
temperature are set to a temperature differential of
approximately 75 degree F for example, set hot
reheat to 925 degree F and superheat 1000 degree
F. Another set of test data is collected and IP
efficiency is calculated using the same assumed
value of N2 leakage and result of this calculations
plotted on the same graph.
HRH- 925 degree F MS – 1000 degree F
3) Calculate IP efficiency at same temperature and
plot on the same graph.
The intersection of these trends indicates the true HP to
IP leakage and the true IP efficiency and

15. The blow down method uses the
emergency blow down valve to divert
the N2 leakage from IP turbine inlet to
the condenser.
The emergency blow down valve Is safety
mechanism design to prevent turbine
over speed by removing HP turbine
leakage steam and passing it directly to
the condenser, bypassing the entire IP
and LP turbine section.

16. According to GE,
The blow down test should be run below 50 percent load,
with the blow down valve open for no longer than 30
minutes and . This allows approximately 15 minutes for the
unit to stabilize and 15 minutes to data collect
1) Bring unit load to 50 percent load.
2) Allow unit to stabilize at design throttle and hot reheat
condition.
3) Begin data acquisition and collect 30 to 60 minutes of
performance test data.
4) After checking unit stability, begin data acquisition for
blow down test.
5) Open blow down valve.
6) After data is collected for 30 minutes, close blow down
valve.

17. 1) Advantage of the blow down test is that a
true IP turbine efficiency is calculated at
the tested load, allowing N2 leakage to be
calculated directly.
2) In addition , only 60 minutes of test data is
required at one load.
How ever , these are only
advantages if the blow down system is
capable of passing the entire N2 flow
before attempting to run a blow down test,
the flow passing capability of the blow
down valve should be calculated