Hydrogen Purging

Pre-Purging checklist:

1. The generator may be on barring gear or at standstill. Preferably be on barring speed of around 50-80rpm (without condenser vacuum).
 At least 60 CO2 filled cylinders should be available in the unit where purging is to be done.
 Arrangement for filling hot water on CO2 cylinder / lines (to avoid freezing) is to be ready.

 Non sparking tools for opening and connecting CO2 cylinders should be available.
 Ensure availability of CO2 flash evaporator.
 Check all shut-off valves at H2 cylinders rack manifold and H2 cylinders are intact and closed.
 Ensure the availability of Seal oil system along with necessary protections & interlocks.
 Keep bearing chamber exhauster fan in service, as long as H2 is there inside the generator.
 For normal purging, take clearance from EMD, TMD and C&I before starting of purging operation.
 Select CO2 / H2 purity transmitter to measure 0 to100 % CO2 in H2 with the help of C&I.
 Inform EMD to deploy their maintenance group for attending leakage from any valve/line immediately.

H2 purging out with CO2:
1. H2, Co2 Valve in Vent Condition
2. Open Casing Vent and reduce Pressure from 3.5 ksc to 0.5 ksc maintaining SO DP
3. Now Co2 valve in filling condition , H2 in Vent Condition
4. Switch on SLC CO2 Evaporator,Wait until the temperature reaches 150 deg C
5. Now open the shut off valve after CO2 flash evaporator and shut off valve for high pressure CO2
6 Open shut off valves at CO2 cylinder and CO2 cylinder rack manifold.
7. Hot water should be continually poured over the cylinder and CO2 line to keep them warm.(Replace empty CO2 cylinders by full cylinders (Approx CO2 cylinders consumption is 35 nos)
8 When casing pressure rises to 1ksc, open the vent valve and gradually bring down the pressure to 0.5ksc again.
9 Continue the process until at least 2 consecutive samples has purity of 98 % (CO2 in H2)
10 After getting purity of 98 %, purge the LLDs (liquid level detectors).
11 Run the blower for both H2 driers and blow off the accumulated gases inside the drier.
12 When CO2 in H2 reaches 98%, cross check with chemist and get clearance from chemist for Stopping CO2 filling and for starting CO2 purging with dry air. Then filling can be stopped and CO2 pressure inside the generator is to be maintained around 0.5 ksc.
13 Switch off the SLC of CO2flash evaporator. Isolate its power supply.
14 Close the valves in CO2 supply line.
15 Disconnect all the CO2 cylinders from the filling header.

PROCEDURE FOR REPLACING / PURGING OUT CO2 WITH AIR
1. Prior to commencing air filling in generator casing, the instrument air connection provided for this purpose is to be Kept open for about half hour to drain off any moisture or oil in the line.
2. Keep three way valve MKG25AA518 (CO2 filling side) connected to the vent line and
MKG25AA519 (there Way valve in H2 filling side) closed (handle in horizontal position).
3. Connect the instrument air supply line between compressed air filter MKG25BT001 and shutoffValve MKG25AA 501 with the hose provided for the purpose. Keep the manual valves MKG25AA501 and MKG25AA509 Open. Adjust the casing vent valve MKG25AA502 to maintain a casing pressure of about 0.5-1.0 ksc.
4. After about 2 hours chemist may be asked to take sample for finding % CO2 in air and result to be compared with Online reading.
5. When CO2 in air reaches 0%, the CO2 has been driven out of the generator. Cross check with chemist and get Clearance for declaring completion.
6. Run the blower for H2 drier and blow out the CO2 inside the drier. purge LLDs
7. Close shut off valves MKG25AA501 and MKG25AA509 in compressed air line.
8. Disconnect hose connection between compressed air filter MKG25BT001 & shut off valve
MKG25AA 501
9. Then keep continues air filling of generator and venting at 0.5 kg/cm² pr for about 12 hrs.
Subsequently keep open generator manholes for about 24hrs.