water steam chemistry

8/2/2019 Water Steam Chemistry

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Water Steam Chemistry

General

The water quality in the water - steam cycle is an essential element for thelifetime of the condenser, feed water heaters, boiler, the connecting pipesystems and also the turbine. The water quality needs to be controlled inorder to:

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-walltemperature.

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 notdetected. As long as the quantity of the impurity in the water/steam is below acertain level the substance will be dissolved, (often in dependence of pH -

value, temperature and pressure), without causing any harm. If the saturationstate 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:

All Volatile treatment (AVT), with or without oxygen scavengersConsists primarily of ammonia injection into the feedwater to controlpH.

Oxygenated treatment (OT)Here oxygen and ammonia are added to the feedwater. The use of oxygen as a corrosion inhibitor allows satisfactory operation over awide 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, toreduce the effects of corrosive impurities and additionally to protect theLP 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 riskof cycle contamination (condenser leaks, condensate returns) such that all-volatile and oxygenated treatments cannot be used. PC and CT cannot beused 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 avoidintroduction of sodium hydroxide to the turbine. Caustic treatment (CT) ismost applicable in instances where phosphate continuum has been evaluatedand found to be unsatisfactory, or where severe FAC has been found in thelow-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-ferrousmetallurgy downstream of the condenser. Oxygenated Treatment (OT) isused in special cases with high purity water and all-ferrous feedwater systems; cation conductivity should be


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Volatile treatment or more specifically ammonia (or other amines) has twoimportant characteristics that must be considered when establishing the cyclechemistry program. As the name implies, the treatment chemicals must bevolatile and when water is completely evaporated no solid chemical residuemust be present (i.e. zero solids treatment). This characteristic allows the useof feedwater for desuperheating (water is sprayed into the superheater to

control steam temperatures). When the boiler water is also on "AVT", it meansthat the ammonia, which enters with the feedwater, controls the boiler water pH. Since some portion of the ammonia will volatilize, the boiler water pH willbe 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 ableto measure the local pH at high temperature, the pH would decrease as thefluid temperature increases. These are the reasons that ammonia or similar amines are not good buffers or considered effective neutralizing agents athigher temperatures. For example, strong alkaline buffers such as sodiumphosphate and sodium hydroxide do not exhibit this relationship with

temperature. The pH remains constant. Therefore, with AVT, relatively smallamounts of salts or other acid producing substances (organics) can have asignificant effect on pH in higher temperature regions. It should be noted thatthis relationship is taken into account when formulating the cycle water chemistry regime. However, operators must also realize that the AVTtreatment is less "conservative" and requires immediate attention should thefeedwater/boiler water become contaminated by either acid or causticproducing salts or substances.Therefore, in many units, strong alkaline substances such as trisodiumphosphate 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 itswater treatment plant:

Optimization of conditioning mode:Review of the existing system and elaboration of an improved conceptbearing in mind the operation mode of the plant.

Water bottles:Water samples taken by the power plant staff ac-companied by aquestionnaire to give additional system information. Alstom labanalyzes the samples. The report shows results, conclusions andrecommendations.

Complete review (Step W):Complete review of the chemistry related issues of the water - steamcycle i.e. water treatment plant (WTP), condensate polishing plant(CPP) 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 stageblades is endangered.

Preservation of power plantsDuring stand still power plant components are subject to corrosion inthe watery phase in the presence of oxygen, salts or acids. Anoptimized preservation plan helps to reduce maintenance costs on a

long term.

water steam chemistry

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