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

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.

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.

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.

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

Seal damage or weakened by misalignment.

Poor start-ups.

A water induction incident will causes seal rubs and HP inner shell distortion.

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.

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

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.

TWO METHODS ARE THERE

1) Temperature Variance Method

2) Blow down method

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

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

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 F3) 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

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.

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.

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