REDUCTION IN DISSOLVED OXYGEN IN CONDENSATE WATER OF STAGE-1 UNITS (6X210MW) OF NTPC/VINDHYACHAL (O&M CONFERENCE-2009)

O&M

Knowledge Management System

Turbine (O&M)

Paper Presentation On –“Reduction In Dissolved Oxygen In Condensate Water Of Stage-1 Units (6x210mw) Of NTPC/Vindhyachal”

Date &Venue: February 13-15, 2009 at PMI, NTPC, NOIDA Key Words: DM Make Up, Condensate DO, CEP Glands, Dissolved Gasses

Organized by: Corporate OS, NTPC LTD Uploaded by : T.V.RAO, SR.MANAGER(HRD),PMI, 9868398747, tvrao02@ntpc.co.in Date : 03/07/2009

REDUCTION IN DISSOLVED OXYGEN IN CONDENSATE WATER OF STAGE-1 UNITS (6X210MW) OF NTPC/VINDHYACHAL

By

V.A Sharma, DGM (MM-TMD)/NTPC/Vindhyachal Mukul Rai, Sr.Supdt (MM-TMD)/NTPC/Vindhyachal A Sengupta, Sr.Supdt (MM-TMD)/NTPC/Vindhyachal Dr R PDas, Sr.Mgr (Chemistry)/NTPC/Vindhyachal

INTRODUCTION

Acceptable level of dissolved oxygen (D.O.) in condensate is required to be maintained to contain the copper pick up from low pressure heater and heat exchangers where the tubes of heat exchangers are made of copper alloys. Mixed metallurgy system containing copper based alloy requires reducing condition to minimize the extent of copper corrosion. Such corrosion prone components in power plants are cupro-nickel tubes of LP heaters, GSC, etc. Under reducing condition the surface oxide (Cu2O) is in the form of compact & even / smooth protective layer on cupro-nickel tube material surface at below 100

0

C that provides protection with respect to metal pick up process. If high oxygen level be there in such zones for longer period then oxidizing atmosphere prevails that promotes conversion of Cu2O to CuO. Under such oxidizing condition the surface oxide contains increasing amount of CuO depending on exposure time (10 hours approx.) and copper release rate becomes approximately 3 times higher than under reducing condition. It also increases surface roughness and contributes to flow accelerated corrosion process.

Copper corrosion not only results tube failures of LP heaters or GSC, but also generates corrosion product in feed water that finally gets deposited on the boiler tubes especially on water wall tubes. Moreover, the volatility of copper oxides is responsible for it’s deposition on super heater tubes also These deposits cause under deposit corrosion, corrosion fatigue and overheating failure of the boiler tubes. Long term deposits on boiler tubes require lengthy process of chemical cleaning of the boiler to wash out copper deposits

This paper deals with the Root cause analysis of high DO in condensate and remedial action taken to contain it.

OBSERVATION & ROOT CAUSE ANALYSIS

In the Vindhyachal Stage-I Units, right from the beginning, the condensate dissolved oxygen was maintaining on higher side (well above acceptable limit of 20 PPB). Root cause analysis of some of the Boiler water walls tubes failures showed that overheating of these tubes are due to deposits on the internal surfaces of the tubes. Further analysis showed that the copper pick has been caused due to high condensate D.O as one of the reason. After these observation efforts has been intensified to contain Condensate D.O levels in Vindhyachal Stage-I Units.

After thorough and detailed analysis of the high condensate D.O problem, it has been concluded that the single main contributing factor for this is the high D.O. level in DM make up water to the hot well which is taken from condensate storage tank (CST) and the other reasons being air ingress in the condensate (being in vacuum) through CEP glands, suction strainer / valves flanges etc.

The DM make up water which is stored in CST is saturated with oxygen and the D.O level in the water is in the range of 6500-7000 PPB. When this DM water is taken into the hot well through DM make up lines, the D.O. value suddenly rises and remains in the condensate. The intake pipe in the condenser is almost submerged in water when the hotwell level is normal. The rise in D.O. value is due to the direct mixing of DM make up water with condensate present in the hot well as no deaeration is occurring.

The above phenomena is evident from the Fig no 1 & 2 of Condensate DO reading with respect to DM Water make up in the hotwell taken in stage-1 units during our studies (before modification).

Fig-1

Line scheme Actual photograph

Existing Scheme (before modification)

Fig-3 CORRECTIVE ACTION TAKEN

There are several methods are available for removal of dissolved oxygen form DM Make up water stored in CST(Condensate storage tank). The most popular method is GTM (Gas transfer membrane) technology. GTM technology utilizes a hollow fiber configuration with internal baffles. The internal baffles activate turbulent flow in and around hollow fibers while tube interior operates under vacuum or nitrogen gas or both in combination.

The dissolved gasses are liberated due to lowered partial pressure. The hollow fibre membrane is hydrophobic in nature and thus allowing only liberated gasses to pass. However this being a very costly option it was not considered and other option was explored.

The best option considered was to raise the DM make up line inside the hotwell well above normal hotwell level and extending it above the tube nest before it mixes with hot well condensate. Sufficient more numbers of suitably sized holes were drilled on the exit pipe and the direction of exit was kept in the horizontal direction so that water exits in a fine spray and also ensuring that the spray does not impinge on the LP Turbine blades. In this process the dissolved oxygen of make water is liberated due to deaeration. The deaeration process is further accelerated as DM Water spray falls on condenser tube nest which increases surface area of DM Water. The liberated air from the DM make up water is taken out by Main Ejectors and deoxygenated make up water mixes with condensate in the hot well.

The modification of DM Make up line is carried away with following precautions.

1 Sufficient structural integrity of raised make up line is maintained. For this tie rods were welded at suitable places on the raised DM make up pipeline. 2 The make up water spray direction was so maintained that it strikes the inclined surface of upper part of condenser sidewall and reflected water comes only downwards (towards condenser tube nest) and never reaches to LP Turbine blades in any condition of make up flow. 3 Most of the pipe fabrication work is done outside the condenser to maintain the condenser cleanliness. The line sketch of the modification and actual photographs are shown in Fig-4

After Modification

Fig-4 After the modification the reduction in condensate DO has been observed which is evident from following graphs taken in various stage-1 units. It is clear from the Fig5 that there is no change in Condensate DO with respect to DM make up water flow.

Fig-5

SCOPE FOR FURTHER IMPROVEMENT

The other secondary problems (air ingress in the condensate in the hot well) is solved by installing specially designed mechanical seals in the CEP glands (already installed in one unit and ordered for other five units), adopting better maintenance practices for sealing of flanges, procurement of glandless valves for root valves and thorough

attending of condenser

flood test defects which is carried out during turbine

overhauls.

6

DM Make up water flow in the Condenser after modification (Actual)

Fig-6 CONCLUSION

Thorough study and root cause analysis helped to contain the high condensate DO problem in NTPC/Vindhyachal stage -1 units. At present the DO has come down to acceptable limit of less than 20 PPB. The best value achieved for condensate D.O. is less than 10 PPB in Unit-4. Based on our feedback, Kahalgaon has also carried out similar modification and successfully contained the high Condensate DO problem.

Reference 1 Modern power station practice, 3

rd

Edition, Volume-E, Central Electricity Generating Board, U.K. 2 R.B. Dooley, A. Aschoff, K.J. ShieldsB.C. Syrett, Guidelines for copper in fossil plants; TR-1000457, Electric Power Research Institute, CA, November-2000 Authors:

V.A. Sharma Graduated in BE(Mech Engg) from SVNIT , Surat in 1986 and joined NTPC as XI th batch EET.

Mukul Rai Graduated in BE (Mech Engg) from MANIT, Bhopal in 1989 and joined NTPC as XIV Th batch EET.

A. Sengupta Acquired AMIE, BOE and joined NTPC as Asst Engr. in 1988.

Dr. R.P. Das Done M.Sc (Chemistry) from Burdwan University & PhD from Jadhavpur University, joined NTPC as ET (Chemistry) in 1990