Water Steam Chemistry 1g531w

Water Steam Chemistry General The water quality in the water - steam cycle is an essential element for the lifetime of the condenser, feed water heaters, boiler, the connecting pipe systems and also the turbine. The water quality needs to be controlled in order to: • •

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Avoid corrosion of the water or steam touched surfaces of the water steam cycle. Avoid deposits on the tube surfaces: Deposit becomes an insulator that will increase the mid-wall temperature. Avoid deposits on the steam turbine.

Above listed issues can be avoided by; • •

Using high purity water Chemical conditioning of the water to reduce the corrosive effect: o Keep the water moderately caustic (not acidic) o Reduce the oxygen content

Often leakages and contamination are not in the focus and therefore are not detected. As long as the quantity of the impurity in the water/steam is below a certain level the substance will be dissolved, (often in dependence of pH value, temperature and pressure), without causing any harm. If the saturation state is exceeded, the substance will hide out leading to contamination. Possible damages caused by poor water quality: • • • •

Magnetite deposition at control valves Copper deposition at HP - steam turbine Caustic embrittlement at IP - /LP - steam turbines Pitting corrosion followed by LCF cracks at LP - steam turbines

There are five possible choices for drum boiler water treatment: •

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All Volatile treatment (AVT), with or without oxygen scavengers Consists primarily of ammonia injection into the feedwater to control pH. Oxygenated treatment (OT) Here oxygen and ammonia are added to the feedwater. The use of oxygen as a corrosion inhibitor allows satisfactory operation over a wide pH range Phosphate Continium (PC) The addition of a solid alkalizing agent to the HP evaporator water, such as tri-sodium phosphate (TSP) and/or sodium hydroxide, to reduce the effects of corrosive impurities and additionally to protect the LP evaporator against two-phase flow-accelerated corrosion. Equilibrium Phosphate treatment (EPT)

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Caustic treatment (CT)

PC or CT should be considered in applications where there is a significant risk of cycle contamination (condenser leaks, condensate returns) such that allvolatile and oxygenated treatments cannot be used. PC and CT cannot be used in once-through units and care must be taken to avoid dry out in drum units. Carryover from all drums must be monitored carefully; this is to avoid introduction of sodium hydroxide to the turbine. Caustic treatment (CT) is most applicable in instances where phosphate continuum has been evaluated and found to be unsatisfactory, or where severe FAC has been found in the low-pressure evaporator. EPT should not be used for CC power plants, because of the risk of the possibility of hideout and hydrogen damage. In contrast with AVT, OT can be applied only in plant cycles with all-ferrous metallurgy downstream of the condenser. Oxygenated Treatment (OT) is used in special cases with high purity water and all-ferrous feedwater systems; cation conductivity should be <0.15 μS/cm for feedwater. There are three possible choices for feedwater treatment: •

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All Volatile treatment with reducing chemistry (AVT(R)) , which uses ammonia and a reducing agent. Here the oxidationreduction potential, ORP, should be in the range --300 to --350mV [Ag/AgCl/sat, KCl], which is necessary to protect mixed-metallurgy feedwater systems. All Volatile treatment with oxidizing chemistry (AVT(O)) where the reducing agent has been eliminated. Here the ORP will be around 0 mV and could be positive. This treatment has to be used in cycles with mixed-metallurgy feed waters systems (copper alloy tubing). Oxygenated treatment (OT) where oxygen and ammonia are used. Here the ORP will be around +100 to +150mV.

The chosen treatments need to match the unit, unit materials (particularly feedwater), cooling water, and possible contaminant ingress.

All Volatile treatment The All Volatile Treatment (AVT) in combined cycle power plants consists primarily of ammonia injection into the feedwater to control pH. Chemicals such as hydrazine and other oxygen scavengers are normally restricted if the feedwater oxygen concentration is maintained below 10 ppb by mechanical deaeration. Hydrazine or hydrazine substitutes are reducing agents which will lower the Oxygen Reduction Potential (ORP) and increase the FAC potential in carbon steel components where the fluid temperatures are in the range of approximately 170°F(77°C) to 450°F (232°C). On the other hand, studies indicate that as little as several ppb of oxygen can lower the rate or even arrest FAC. However, if oxygen is zero and hydrazine is present, then the ORP is driven to very negative potentials and could substantially increase the corrosion rate. It has also been shown that FAC is arrested when the pH is maintained above 9.2.

Volatile treatment or more specifically ammonia (or other amines) has two important characteristics that must be considered when establishing the cycle chemistry program. As the name implies, the treatment chemicals must be volatile and when water is completely evaporated no solid chemical residue must be present (i.e. zero solids treatment). This characteristic allows the use of feedwater for desuperheating (water is sprayed into the superheater to control steam temperatures). When the boiler water is also on "AVT", it means that the ammonia, which enters with the feedwater, controls the boiler water pH. Since some portion of the ammonia will volatilize, the boiler water pH will be slightly lower than that of the feedwater. The other inherent characteristic of ammonia (or other amines) is its lower ionic dissociation rate at higher temperatures. In other words, if one were able to measure the local pH at high temperature, the pH would decrease as the fluid temperature increases. These are the reasons that ammonia or similar amines are not good buffers or considered effective neutralizing agents at higher temperatures. For example, strong alkaline buffers such as sodium phosphate and sodium hydroxide do not exhibit this relationship with temperature. The pH remains constant. Therefore, with AVT, relatively small amounts of salts or other acid producing substances (organics) can have a significant effect on pH in higher temperature regions. It should be noted that this relationship is taken into when formulating the cycle water chemistry regime. However, operators must also realize that the AVT treatment is less "conservative" and requires immediate attention should the feedwater/boiler water become contaminated by either acid or caustic producing salts or substances. Therefore, in many units, strong alkaline substances such as trisodium phosphate are used for boiler water treatment which provides a high pH, unaffected by temperature, and thus good buffering capabilities. In this case, the potential for FAC in the LP evaporator is negligible.

Service packages Alstom service packages to optimize the complete water-steam cycle with its water treatment plant: •

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Optimization of conditioning mode: Review of the existing system and elaboration of an improved concept bearing in mind the operation mode of the plant. Water bottles: Water samples taken by the power plant staff ac-companied by a questionnaire to give additional system information. Alstom lab analyzes the samples. The report shows results, conclusions and recommendations. Complete review (Step W): Complete review of the chemistry related issues of the water - steam cycle i.e. water treatment plant (WTP), condensate polishing plant (P) or conditioning system (CondS). Cold end corrosion diagnostics (CECD) CECD enables the plant owner to monitor temperature and humidity inside the LP - turbine during operation and shut - down to avoid

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corrosion. Especially the blade attachment area of the last stage blades is endangered. Preservation of power plants During stand still power plant components are subject to corrosion in the watery phase in the presence of oxygen, salts or acids. An optimized preservation plan helps to reduce maintenance costs on a long term.