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PROJECT: Ennore TPS Unit 3 - 110 MW

PROBLEM: 110 MW Vacuum System improvement

In Unit 3 - 110 MW of Ennore Thermal Power Station M/s. TNEB hadrecently replaced the full set of HP, IP, and LP Steam Turbine rotors

with new ones. During the shutdown several modifications were initiated

outside the main STG rotor system, with a view to improve STG

performance.

At earlier times, when the unit was running at 70-90 MW after

R&M, the vacuum was always poor and affecting the unit output. Acomprehensive effort was made with all round focus on the vacuum and

related systems. Listed below are the important measures taken:

1.LP Casing parting plane modification:Grooves were cut in the in LP Casing parting plane and a siliconecord was introduced for sealing against air ingress.

2.LP Casing metal stitchingMetal stitching was done in cracked sections of the LP Casing to

seal the steam space from air ingress.

3.Polymer cladding of condenserPolymer cladding of condenser CW water boxes and partingplanes was done to reduce the seawater corrosion of water box

material. CW parting plane corrosion has resulted in poor

condenser heat transfer because of CW short circuit.

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4.Introduction of priming ejector in CW linesA Priming ejector was introduced for air removal in condenser

CW lines and water boxes for achieving full CW flow.

5.Rerouting of LPHs 1&2 drains:LPHs 1&2 drains were modified in consultation with BHEL (HYD)

to route them to condensate pump suction as per original

scheme.

6.Segregation of turbine extraction and pipeline drains:The steam from high pressure MS, CRH and HRH drains, and

most other low pressure drains, were earlier connected to

Service Condensate tank ( SCT) with a provision to be connectedto condenser or to atmosphere. This resulted in additionalthermal loading of the condenser, and was an undetectable

source of air ingress. The SCT was isolated from the condenser

by blanking its vent and drain connections to vacuum system, andconverting it to a purely atmospheric pressure vessel for start-

ups. Thus high temperature steam has been diverted away from

the condenser to reduce thermal loading on it. At the sametime the extraction line drains were removed from SCT and

connected to HPH Flash Tank.

7.Rerouting of LPH and HPH extraction drainsHPH 7,8 and LPHs 3,4,5 extraction drains from pipelines,

downstream of NRVs were routed to HPH Flash tank inconsultation with BHEL (HYD). This was required to ensure the

drains in the vacuum system were segregated from the SCT.

8.Replacement of EjectorsNew Ejectors were installed with enhanced air handling capacity,

considering the age and corrosive atmosphere in these old units

at Ennore.

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9.LP Gland Steam pressure tapping modificationLP Gland Steam pressure tapping was relocated at the LP frontgland seal with NB 65 impulse pipe header for accurate

measurement for auto control.

10.HP Gland steam line introducedHP Gland steam line for HP gland packing was introduced from

11 ata PRDS header since it was not envisaged in earlierSKODA design. It was one of the major reasons for air ingress

from HP Gland seals in this steam turbine, that prevented

achieving of full vacuum, when there was no self sealing steamwhile STG on barring gear.

11.Condenser water tightness testThe Condenser tightness test was conducted by filling water

upto TG floor (7.9 meters). Water was retained in the

condenser for 36 hours and zero drops in level was achieved

for this period after attending all the leaks.

12.LP Gland Steam header drain modified.A drain was introduced in the LP Gland Steam header, with a

steam trap and trap bypass to condenser, since it caused

hammering in the sealing steam lines every time these lineswere charged during vacuum pulling.

With this, most of the air ingress to condenser, and its thermal

loading points have been taken care of. The drain modifications were

undertaken by TNEB on the request of PSSR, on the original SKODAsystem, with the support of BHEL Hyderabad T & C Engineering for

attaining full load and improved steady state behavior of the machine

at higher loads.

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The unit was restarted recently in March 2006. During the restart

when vacuum was pulled in this modified system, 0.95 ksc (approx 720mm Hg of mercury) vacuum was achieved with STG on barring gear.

715 mm Hg of vacuum was found to be maintaining stably under steam

dumping, rolling, synchronization, and load conditions.