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VGB PowerTech 6 l 2015 Deaerator developments

Author

Kurzfassung

Entwicklungen bei Sprühentgaser-Anwendungen

Der Sprühentgaser weist eine sehr flexible Fahr-weise auf und entfernt gelösten Sauerstoff effek-tiv aus dem Wasser-Dampf-Kreislauf. Es lassen sich problemlos Sauerstoffkonzentrationen von 5 ppb (Teilchen pro Milliarde) oder weniger er-reichen, wodurch sich die Lebensdauer des Kes-sels erheblich verlängern lässt. Die Einzelbehäl-terausführung benötigt nur wenig Platz, lässt sich leicht steuern und minimiert die Anzahl der zu verwendenden Druckteilbehälter. Es las-sen sich verschiedene Ströme direkt im Entgaser sammeln, was zum Entgasungsprozess beiträgt.Der Entgaser lässt sich innerhalb des Kreislaufes flexibel positionieren und kann darüber hinaus auch als Kondensat-Vorwärmer verwendet wer-den. Der Sprühentgaser ist im Hinblick auf die Kreislauf-Anlaufzeiten schnell, da mehr oder weniger sofort entgastes Wasser zur Verfügung steht. Im Detail werden der Hintergrund der Entgasung aus einer theoretischen Perspektive, verschiedene Ausführungsüberlegungen sowie häufig eingesetzte Konfigurationen zur Integ-ration des Entgasers erläutert. Die Anwendung von Entgasern im Wasser-Dampf-Kreislauf di-rekt befeuerter Kessel, in Abhitzedampferzeu-gern (HRSG), GuD, geschlossenen Kreisläufen, Solaranlagen und Kesseln in Kernkraftwerken verschiedene Spezial anwendungen werden im Detail erläutert. l

Developments in spray-type deaerator applicationsBart Bramer

ir. Bart Bramer (M.Sc.)Stork – Process Equipment –- Department of Process Engineering Hengelo/The Netherlands

Introduction

The lifetime of steam-generating boilers and their layout can be improved signifi-cantly by reducing the degradation of their internals caused by corrosion. This can be achieved by reducing the amount of oxygen dissolved in the water-steam cycle, which is called deaeration. In general, deaeration is the removal of dissolved air from a gas or liquid. For reasons of regulation, efficiency and cost optimisation, thermal deaeration is currently the most frequently applied technique for deaeration in industrial boil-ers and is preferred above chemical deaer-ation techniques.

Thermal deaeration is accomplished with a so-called deaerator, i.e. a device where in-timate contact takes place between steam and water. The overall design of deaera-tors has not changed over the last few dec-ades. However, their field of application has expanded significantly because the application of energy efficiency technolo-gies and new energy sources has increased. The Stork spray-type deaerator has several advantages with respect to competitor designs, because it combines deaeration and additionally, condensate pre-heating and feedwater storage functions in a sin-gle vessel. The general arrangement of the spray-type deaerator is quite flexible,

because emphasising one particular func-tional aspect only has a minor influence on the design. Nowadays, Stork spray-type de-aerators are typically incorporated in heat recovery steam generators (HRSG), direct-ly fired power plants, solar energy plants, nuclear power plants and industrial steam production facilities. The deaerator is always situated in the low or medium pressure end of the water-steam cycle, in order to reduce investment costs and increase plant efficiency. F i g u r e 1 shows a deaerator (8) in a water-steam cy-cle that is typical of energy production ap-plications. Within such a cycle, condensate coming from the condenser has the high-est oxygen concentration and is fed into the deaerator (7 to 8). Steam required for deaeration can originate from a variety of sources, but originates in particular from a low or medium pressure end of the cycle. In the cycle of Figure 1, steam originates from a low-pressure tap of a steam turbine (4). Deaerated and preheated water from the deaerator is transported to the boiler feed-water pump (10) and then subsequently fed to the boiler (11). This article describes the background to deaeration from a theoretical perspective, and considers various designs as well as frequently applied deaeration integrations in a number of installations.

SuperheaterEvaporaterEconomiserSteam turbine

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Fig. 1. Typical example of a deaerator in water-steam cycle.


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Deaerator developments VGB PowerTech 6 l 2015

Theoretical background

Material degradation mechanism Corrosion is expected to be the most critical degradation mechanism in terms of boiler lifetime. Wall thicknesses are reduced due to corrosion and particles may be released from the surface due to pitting corrosion, which can then prove catastrophic for downstream equipment. The three major factors determining the rate of corrosion are oxygen concentration, acidity and con-ductivity. High conductivities can be ob-served in the steam drum due to the accu-mulation of salts. A high conductivity will increase the rate of corrosion. The water temperature also influences the rate of cor-rosion, but is usually not a free variable and is prescribed by the boiler requirements. Oxygen concentrations required to extend service life sufficiently, are in the range of 10 ppb or less. An oxygen content of this level will result, together with an acid-ity of pH > 9, in the formation of a corro-sion- and erosion-resistant magnetite layer (Fe3O4). Oxygen concentrations and acid-ity levels out of this range result in much more voluminous iron oxide layer (Fe2O3) and eventually result in pitting corrosion. It must be noted that a stable magnetite layer requires a minor amount of oxygen. The spray-type deaerator is a very effective device for controlling this parameter.

The physical background of deaerationAccording to [1], the equilibrium between a gas dissolved in water and a gas dis-solved in steam is described by Henry’s law which, amongst other gases, is valid for oxygen and carbon dioxide. According to Henry’s law at a given temperature, the gas concentration in water is proportional to the partial gas pressure in steam. Here the partial pressure is the pressure exerted by a particular gas component in a mixture of gases. Deviating from these equilibrium conditions results in gas transport because

a new equilibrium state tries to establish itself. Deaeration of water is achieved by reducing the partial gas pressure of steam or increasing the water temperature, both of which change the equilibrium. Such a change creates a driving force, transport-ing the dissolved oxygen from the water into the steam.The transport of gas molecules in water is much slower than the transport of gas molecules in steam, therefore the rate of deaeration due to diffusion is almost fully determined by the water phase. The rate of deaeration can be increased, by having an optimal contact surface between the water and steam and by a dynamic movement of these phases. When the water temperature is increased to the boiling point at the cor-responding pressure in the deaerator, the total pressure equals the water-vapour pressure, implying a gas partial pressure equal to zero, which makes complete de-aeration possible. The removal of CO2 from the water phase is analogous to the removal of O2. However the CO2 concentration in the water phase is already low due to the high pH value of the water. Consequently, also for CO2 removal good results can be obtained.

Processes taking place in a deaeratorThe deaeration processes in the Stork single vessel deaerator are presented in F i g u r e 2. The deaeration process takes place in two steps: pre-deaeration (step 1), in which water droplets are sprayed in a steam space; final-deaeration (step 2), in which steam bubbles are injected in a large volume of water, travelling upwards and coming into close contact with water. Condensate with a temperature below the deaerator operating temperature must al-ways enter the deaerator through a spring loaded injection nozzle, i.e. the spray noz-zle (1). The spray nozzle produces a radial spray of fine droplets, which are heated dur-ing their flight to saturation temperature

by the surrounding steam. Since this heat-ing process is very efficient, the spray noz-zle acts as a perfect condensate preheater. The droplet size ensures a short diffusion path for the oxygen within the droplet to its surface. The droplets impinge upon a cir-cular splash baffle (2) located around the spray nozzle, breaking the small droplets into smaller ones. This results in a maxim-ised surface- to-volume ratio, providing a large surface area for oxygen diffusion. The complete deaeration process in the droplet occurs in milliseconds. The residence time in the pre-deaeration step is nearly always too short for complete deaeration. Lower oxygen concentrations will be obtained in an additional final-deaeration step. After completion of the primarily deaeration step, the water droplets fall into the water reservoir. In the final deaeration step, the remain-ing dissolved oxygen is removed from the condensate, by injecting steam in a large volume of water (3). Steam bubbles are created which provide effective contact be-tween water and steam, allowing the oxy-gen to be transported from the water to the steam bubbles. This steam enters the vessel through a steam sparger (4), ensuring an even distribution of steam bubbles in the water and enforcing homogeneous condi-tions in the deaerator. Simultaneously, the water is intensively circulated by these mov-ing bubbles. The gas transport takes place more rapidly due to the circulation and continuously ensures a large difference in oxygen concentration at the steam-bubble/water interface. This enhances the rate of deaeration. The remaining steam leaves the water through the surface with an in-creased oxygen concentration and travels into the steam zone of the vessel (8) towards the primary deaeration zone. As the steam condenses on the water, the concentration of oxygen in the direct vicinity of the spray nozzle is relatively high, making it possible to vent a small amount of steam with an in-creased oxygen concentration (9). To obtain maximum deaeration perfor-mance, residence time can be increased by using a baffle (5). This baffle is placed in the water reservoir and divides the vessel into multiple sections. Another baffle near the outlet of the deaerator (6) prevents the water following a by-pass route to the outlet. Finally, the deaerated water leaves the deaerator at the bottom of the vessel via a vortex breaker (7) or similar device. The size of the vessel depends mainly on two factors, namely the residence time required to achieve the deaeration target and the required maximum amount of wa-ter to be stored in the vessel. The latter is normally defined by the entire water-steam cycle and is often specified by the system designer.Summarising, the functional features of the Stork spay-type deaerator in the water-steam cycle are:

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Fig. 2. Flow mechanisms in a Stork spray-type deaerator.


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– the removal of dissolved oxygen from water to extend boiler service life,

– water storage, required for start-up and shutdown of the deaerator and allowing for the collection of various excess con-densate streams and

– an efficient direct contact condensate preheater.

The emphasis on the individual design as-pects of the deaerator can differ to a great extent and depend on the specific applica-tion.

Design considerations

Most spray-type deaerators are installed horizontally. This horizontal configuration has a low centre of gravity and requires only a minimal support structure. It distrib-utes the steam perfectly and allows a small number of by-pass streams to the deaera-tor outlet. Careful attention must be paid to the layout of the deaerator and its immedi-ate surroundings, the feedwater pumps and the routing of pipework. Nevertheless this is very flexible. Most of the important design considerations for spray-type de-aerators are given in this section.

Steam and water sourcesThis section focuses on the sources re-quired to operate the deaerator, i.e. water and steam as shown in Figure 2. Steam is required to achieve deaeration and for heating up cold condensate and can be supplied from a large variety of sources, e.g. a boiler, a steam turbine or an external production facility. The deaerator can op-erate using saturated, superheated or wet steam. Saturated steam can originate from a low-pressure steam drum, an evapora-tor section of the boiler or a steam turbine. Wet steam often originates from a hot con-densate source from an economiser, which operates at a higher pressure. Superheated steam is usually obtained from a low- or medium-pressure tap at a steam turbine. The steam enters the deaerator through a steam sparger equipped with loading pipes, located below the water level. The steam sparger is available in several de-signs, largely dependent on the operating modes and the deaerator start-up philoso-phy. Condensate flows with a temperature lower than the deaerator operating temper-ature (cold) should be fed into the deaera-tor using the spray nozzle. On the other hand, condensate flows with a temperature higher than the deaerator operating tem-perature (hot) should be injected below the water level where they will flash into a water-steam mixture. Hot condensate in-jection must take place through a sparger or perforated pipe, which evenly distrib-utes the water-steam mixture in the water reservoir. The sizing of the deaerator is predominantly determined by the required holdup volume, in order to safely feed the boiler feedwater pump in all conditions. The deaerated condensate is discharged us-

ing the outlet device, situated at the bottom of the deaerator in a direction opposite to the direction of the spray nozzle.

Deaerator controlThe deaerator plays a major role under almost every power plant operating condi-tion; therefore a proper design of its con-trol system is important and can be criti-cal for the entire power plant. Deaerators generally operate in a sliding- or constant pressure regime which is determined by the steam source. A deaerator obtaining steam from a steam turbine tap is operat-ing in a sliding pressure regime, in which superheated steam enters the deaerator directly without control. The deaerator operating pressure will vary dependent on the load changes in the steam turbine. The water level in the deaerator is controlled by monitoring the incoming condensate flow and must be kept at the desired level. The incoming condensate flow needs to be bal-anced with the amount of water discharged from the deaerator. This is achieved by an external control loop. The water level con-trol is the only control system for a sliding pressure operated deaerator. It can also regulate a separate, relatively small uncon-trolled steam source as long as it does not interfere with the normal control system. For a deaerator operating at constant pres-sure, an additional pressure control system is required in addition to the water level control. Pressure control for spray-type deaerators is relatively simple because the steam is injected into the water reservoir, making use of the huge buffer. This en-sures smooth continuous pressure control. To ensure proper control of the deaerator pressure, the steam flow must always be sufficiently high.

Competing deaerator types require an ad-ditional feed forward control from the steam inlet, which makes the spray-type deaerator effective in terms of control.

Start-up proceduresDeaerator start-up procedures are often de-signed in close cooperation with the plant designer. During a cold start-up, the water level in the deaerator should be at least the lowest water level allowed (LLWL). During the start-up period, the deaerator is heated and pressurised with steam, until normal operating conditions are achieved. During start-up, steam typically originates from a start-up boiler or other steam supply. Air is removed using start-up vents, while the temperature and pressure are rising. When normal operating conditions are achieved (temperature and pressure), the spray noz-zle is used to increase the amount of water to the normal level (NWL). Subsequently, the steam feed can be switched to its regu-lar source, setting the deaerator to normal operation mode. Effectively, the deaerator delivers deaerated water right from the start.

Steam turbine tripIn sliding pressure mode the deaerator is connected to a turbine tap and variations in the steam turbine flow pass directly into the deaerator. When a steam turbine trips, the boiler steam flow is halted within a few seconds and redirected through a by-pass loop. During this period, the deaerator steam flow is halted resulting in a pressure and temperature decay in the deaerator. Due to the opening time of the by-pass valve and condensation of steam in the pip-ing, the steam from the by-pass enters the deaerator after approximately one minute. Simultaneously, the temperature of the incoming condensate falls drastically. The deaerator will be fed with steam from this high-pressure by-pass and in parallel the pressure will drop. During these pressure reductions, the saturation temperature in the deaerator decreases and some portion of the water reservoir gets converted into steam. Consequently, the steam volume in the deaerator increases and pushes up the water column (swell). A spray-type deaera-tor can easily handle the sudden flow vari-ations caused by a turbine trip because all the flows are balanced against each other in the water reservoir.

Boiler feedwater pumpReliable performance of the boiler feed-water pump is required under all circum-stances. The pressure at the pump inlet is an important factor related to its lifetime in terms of pump cavitation. If the pressure at the pump inlet is close to the local vapour pressure, vapour bubbles could arise which then have a destructive effect on the inter-nal components of the pump when they implode. A parameter expressing the safety margin against pump cavitation is the net positive suction head (NPSH), which is de-fined as the difference between the static pressure at the pump inlet and the local vapour pressure. Here, the static pressure at the pump inlet depends on the deaerator operating pressure, the hydraulic head cre-ated by the water column and the pressure loss in the piping, valves, outlet nozzle, vortex breaker and other equipment. The minimum NPSH (R) required for a pump depends on its design and is specified by the pump supplier. To overcome pump cav-itation and achieve maximum NPSH, the deaerator needs to be installed at a level significantly above the level of the pump, in order to obtain sufficient hydraulic head.

When designing the pump layout, the fol-lowing operational aspects should be taken into account. For a sliding pressure oper-ated deaerator, variations in the deaerator pressure travel at the local speed of sound to the suction side of the pump. Accompa-nying variations in the temperature travel significantly more slowly to the pump and depend on the residence time of a water particle in the suction pipe. Sudden re-ductions in the deaerator pressure can be


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critical in respect of cavitation, because the pump inlet temperatures can become very close to the local saturation temperature. The available NPSH (A) can fall considera-bly during such an event. To reduce the risk of cavitation, flow velocities in the suction pipe need to be high to increase the tem-perature propagation speed. On the other hand, the flow velocity needs to be limited, in order to reduce the pressure losses. The designer must compromise between those aspects. Significant pressure losses close to the deaerator level should be limited as much as possible, because here the local vapour pressure can be reached relatively quickly and consequently, evaporation will occur inside the piping. Pressure losses due to the flow entering the suction pipe depend largely on the shape of the inlet of this pipe and can be a significant fac-tor giving rise to cavitation. Consequently, streamlined suction pipe inlets (nozzle) and the vortex breaker are required to minimise pressure loss. Horizontal pip-ing parts and valves at the deaerator level should be omitted and should be placed at a level close to the feedwater pump where the NPSH is maximal.

Deaerator applications

Deaeration, preheating and water/steam storage can be easily combined in the spray-type deaerator. The emphasis of a functional aspect of the deaerator depends on the application. Commonly applied de-aerator integrations are summarised in this section.

Power generation from fossil fuelsA common application for deaerators is found in fossil fuel powered boilers or heat recovery steam generators (HRSG). Deaerators are situated in the low- or me-dium pressure end of the water-steam cy-cle, in order to reduce investment costs and increase plant efficiency. Efficiency losses due to deaeration are minimised by con-suming the lowest possible steam quality for deaeration.

In conventional, directly fired power plants, fossil fuel is combusted in a steam generating boiler which powers a steam turbine. The efficiency of such plants can be improved by preheating the fresh air used for combustion. The air preheating takes place by using heat from the exhaust gasses. Returning condensate streams from the air pre-heater and the primary HP condensate preheater are typically mixed in the deaerator. The deaerator operating conditions are often determined by the steam turbine tap serving as a steam sup-ply for the deaerator.

Another efficient manner to generate elec-trical power from fossil fuel is by coupling a gas turbine to HRSG. Here, hot exhaust gasses from the gas turbine exchange heat with the water-steam cycle of the HRSG

which in its turn powers a steam turbine. In such plants, deaerators are often operated at a constant pressure, slightly above at-mospheric pressure or into vacuum. Steam is often supplied from an LP drum or econ-omiser section. In an HRSG arrangement, efficiency cannot be increased by pre-heat-ing the inlet air of the gas turbine, but is increased by pre-heating the condensate. Preheating the condensate is performed by cooling the flue gasses. This takes place at the coldest end of the HRSG using a con-densate pre-heater. For the deaerator feed, the lowest enthalpy heat source must be used. Due to their versatility, spray-type deaerators can be integrated relatively eas-ily within the HRSG cycle.

Often, the operating pressure of the de-aerator is determined by the critical dew point of the flue gas which depends on the combusted fuel. To avoid external tube cor-rosion, stack temperatures must not fall be-low the local water or acid dew point. The most critical section of the boiler in respect of flue gas condensation is located at the coldest end, where feedwater is supplied. Critical flue gas dew points are typically around 60 and 130 °C for gas and oil op-eration, respectively. The boiler feedwater temperature is often determined by the deaerator conditions and should have a safety margin relative to the actual dew point. Due to its versatility, the same spray-type deaerator can easily fulfil vacuum or higher pressure operating requirements and can be easily adapted to meet oil and gas fired power plant requirements.

Closed loop cycleIn a closed loop cycle, condensate from the condenser will be directly fed to the boiler feedwater pump without passing through the deaerator. In this case, the deaerator is installed in a bypass loop and operates only on demand, i.e. in situations where extra make-up water is supplied, oxygen leaks occur or during start-up. Typically the steam supply originates from a low-pres-sure steam drum and cold condensate orig-inates from the condenser with condensate preheater. Under normal operating condi-tions, the deaerator is maintained in hot standby mode. The water storage function-ality of the deaerator can be shifted to the condenser hot well, leading to a compact deaerator size. If the deaerator volume is small, it can also be oriented vertically, re-ducing its footprint. This makes its location at the plant highly flexible.

Solar power plantsIn concentrated solar power (CSP) plants a large area of sunlight is focused into a concentrated beam, which is used as heat source for the power generation process. This heat is often primarily absorbed in a sodium salt cycle, which acts as heat source for a parallel operating cycle with water and steam containing a deaerator.

Examples of such applications are: para-bolic trough plants and solar power tow-ers. During peak hours thermal energy is extracted from the cycle. Sunlight is not available during the night; therefore much attention must be paid to the insulation of the CSP cycles. A typical deaerator vessel has a relatively low surface-to-volume ratio and can be insulated very effectively. Typi-cal values for overnight cooling are 2 °C or less. The deaerator operates in hot-standby mode during the night with automatically operating purge vents kept closed to avoid unwanted steam losses. Thermal segrega-tion during a long standby mode period can always be avoided because, if neces-sary, the medium can be mixed by injecting a small amount of steam which maintains homogeneous conditions. No delay is re-quired for deaerating the plant first thing in the morning because the water in the reservoir is adequately homogenised and up to its start-up level. Likewise pressure and temperature correspond to the start-up conditions.

Nuclear power plantsWater steam-cycles of nuclear and con-ventional facilities are quite similar, but differ typically in the maximum conditions allowed for temperature and pressure. Consequently, the flow rates of steam and condensate need to be significantly high-er, in order to generate a similar amount of electrical power. For such high condensate flows in the nuclear cycle (up to 6,000 t/h), the preheating function of the deaerator is much more in demand, often resulting in multiple spray nozzles. Oxy-gen concentrations of condensate flows brought into the deaerator are typically less than 50 ppb, because leakage and make-up flow quantities are relatively small. In spite of the high flow rates pre-sent, the water storage capacity can be kept limited, because residence time re-quirements for nuclear power plants are relatively small (2 to 5 minutes) compared to conventional power plants (5 to 15 min-utes). Single vessel spray-type deaerators are often applied in nuclear power plants, because they combine pre-heating, de-aeration and water storage aspects by ef-ficiently using the available space.

In a nuclear power plant all emissions are stringently limited; therefore the water-steam cycle needs to be closed. Vent flows from the deaerator need to be redirected to the condenser where the oxygen and other non-condensables are removed. Risk mitigation is of highest priority in nuclear plants, therefore extra attention must be paid to non-destructive testing, detailed load analysis and proper operation of the feedwater pump under all conditions.

Internal drum deaeratorsIn a standard spray-type deaerator, differ-ent streams can be easily mixed. It offers


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a water storage capability and can effec-tively preheat condensate. If these aspects are not of primary concern, deaeration in an encapsulated box placed inside the LP drum could be an effective solution. Inter-nal drum deaeration (IDD) can be applied for condensate with low oxygen concen-tration, typically below 100 ppb. A typi-cal example of an IDD system is shown in F i g u r e 3.Condensate enters an IDD through a regu-lar spray nozzle (1), running through a primary deaeration zone and pre-heating step. Subsequently, the condensate drops onto an inclined plate (2) and enters a water reservoir at the lower section of the IDD (3). The water travels to the exit (4), while steam is injected into the water reservoir using a series of steam chargers

(5), i.e. the final deaeration step. A baffle plate (6) fixes the water level and holds up the water reservoir. A small portion of the steam is vented at the top of the IDD (10), discharging steam with a high oxygen concentration. The excess water leaves the IDD by the discharge pipes (7) and drops into the drum. The IDD operates at a pressure slightly lower than the drum pressure, therefore steam can enter the deaerator (9) directly from the drum and external steam supply is not required. The IDD requires no additional control and can handle condensate capacities up to 500 t/h, making it a simple and economic de-aeration device. A schematic overview of the IDD arrangement in an HRSG is shown in F i g u r e 4.

Deaerator as steam accumulatorBesides feedwater deaeration, water stor-age and preheating applications, a spray-type deaerator can also be used for steam accumulation. A steam accumulator is able to release steam when the demand from the plant is greater than the boiler’s ability to supply at that time, and accepts steam when the demand is low. When the boiler generates more steam than required for the process, the surplus of steam can be injected into the deaerator/accumula-tor and stored under pressure. Loading the accumulator takes places by injecting steam through a steam sparger, perma-nently maintaining the accumulator under saturated conditions and preventing un-desirable vibrations. When the net steam flow into the accumulator is positive, its pressure will rise. Saturated steam can be released from the accumulator by reduc-ing its pressure. A steam-accumulator can be pressurised and depressurised relatively quickly, therefore cyclic loading must be taken into account by structurally evaluat-ing the design. In particular, the construc-tion material used, wall thickness transi-tions and nozzle connection designs must be carefully considered to prevent exces-sive stresses and fatigue problems.

Typical deaerator integrations

Due to its high flexibility, the spray-type deaerator can be adapted to a large variety of plant set-ups. In general, the efficiency of the plant is increased by allowing small temperature differences between the in-coming condensate and the deaerator op-erating conditions. Deaerator integrations commonly occurring are described in this section.

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Fig. 3. Schematic representation of an internal drum deaerator.

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Fig. 4. Typical integration of an internal drum deaerator (IDD) in an HRSG.


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Figure 1 shows a deaerator integration commonly observed for directly-fired power plants. Here, condensate originates directly from the steam condenser and is distributed to the deaerator using the spray nozzle. Steam for the deaeration originates from a low-pressure steam turbine tap and is directly fed into the deaerator without control. The deaerator operates at saturat-ed conditions and should operate at a tem-perature at least 10 ˚C above the incoming condensate temperature to safeguard de-aeration quality. If the steam turbine trips, the HP steam produced by the boiler is dumped into the condenser and partly into the deaerator.A deaerator setup in an HRSG which re-quires a minor level of safeguarding is a deaerator operating under vacuum con-ditions, as shown in F i g u r e 5. In such cases, cold condensate from the condenser of about 30 to 50 °C is brought directly into the deaerator (blue line), which typically operates at 0.2 bar. As the deaerator is sub-

ject to vacuum conditions, extra stiffness is required. Condensate discharged from this deaerator (black line) is at around 60 °C and can be directly fed into the economis-er. As a steam source for the deaerator, hot condensate which flashes into low quality steam, can be used. Condensate flashing takes place in the water reservoir similar to the deaeration with steam and contributes to the deaeration process. Hot condensate is typically obtained from an economiser. In parallel, a small portion of steam (up to 2 %) from a low pressure steam drum can be fed through the steam spargers to maintain the pressure. During oil firing, the deaerator operating pressure must be increased to approximately 4 bar to pro-vide a safety margin relative to the critical dew point. Under these oil firing operating conditions, the LP drum water enthalpy is insufficient to provide proper steam distri-bution in the deaerator. Consequently, LP steam is used for deaeration by use of the steam sparger.

An alternative deaerator integration for an HRSG is shown in Figure 5b, making use of an external heat exchanger to pre-heat the condensate. In this case, the deaerator op-erates slightly above atmospheric pressure. The setup of figure 5b is observed in many HRSG’s while the setup of figure 5a has several benefits and should be preferred. It has less components, a high availability, an easy control philosophy and is very cost effective. This deaerator setup is applied in an HRSG and is typically fed with flashing hot condensate and low-pressure steam (purple line). The cold condensate flow from the condenser (blue line) extracts heat from the relatively hot deaerator out-let flow (red line), in order to obtain an economiser feedwater flow of around 60 °C (black line) which is safely removed from the critical dew point.Condensate can also be pre-heated inside the HRSG (F i g u r e 6). The condensate pre-heater is located at the coldest end of the boiler. Its inlet condition is determined

HP section

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TT

TC

TCP 30 to 50 °C

0 % (-2 %)80 %

60 °C135 °C

0.2 bar 1.2 bar

100 °C

4 bar

105 °C

30 to 50 °C60 °C

A B

Super-heater

Eva-porator

Economiser

LPdrum

Fig. 5. Typical integration of a deaerator in an HRSG operating under a vacuum (a) and under approximately atmospheric conditions (b).

Supplementaryfiring


HP section

LP section 30 to 50 °C

Superheater Evaporator Economiser Condensatepreheater

TT

TT

TC

TC

Drum

1

2

3

4

5

Fig. 6. Typical integration of a deaerator in an HRSG equipped with condensate preheater.


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by the critical dew point, while its outlet condition is determined by the tempera-ture required to obtain proper deaeration. The condensate inlet temperature (1) should remain 6 to 8 °C above the critical dew point. This could be achieved by mix-ing preheated water from the deaerator (2) with cold condensate (3). The latter could result in recirculation rates of 30 to 300 % depending on the combustion fuel. Many HRSG’s are provided with a sup-plementary firing possibility to increase steam production, efficiency or separate power and process steam requirements. HRSG operation with supplementary firing changes the critical flue gas dew point and the preheater performance. This will influ-ence the deaerator operating conditions and requires another operating philoso-phy. Due to the versatility of the spray-type deaerator, combining multiple operating philosophies can easily be achieved with the same device.

Conclusions

Boiler life span can be improved by apply-ing a deaerator in the water-steam cycle because it reduces the amount of dissolved

oxygen and reduces the rate of corrosion. Spray-type deaerators are highly flex-ible and compact. They combine deaer-ating, preheating and feedwater storage functionalities in the same device. Due to their versatility, they can be adapted to a wide range of plant setups and operating requirements. The investment costs of a spray-type deaerator mainly depend on the amount of condensate which needs to be produced as well on the required hold up time of the boiler. The deaerator should operate at a temperature at least 10 °C higher than the incoming conden-sate temperature to safeguard deaeration performance. Superheated- or saturated steam or flashing hot condensate can be used as deaeration sources. With the spray-type deaerator, oxygen contents of 5 ppb or less can easily be obtained. Application of a spray-type deaerator costs a negligible amount of thermal efficiency and oxygen scavenger chemicals are not required. The most common applications of spray-type deaerators are to be found in directly fired plants, HRSG’s for power generation and in solar- and nuclear power plants. The spray-type deaerator design can also be used for steam storage, by operation as a

steam-accumulator. To optimise the de-aerator design, Stork, in conjunction with its major partners, is currently employing standardisation and “configure to order” programmes. This will reduce design cost, project and quotation lead times.

References[1] Coulson, J.M., Richardson, J.F., and Sinnott,

R.K.: Chemical Engineering Volume 6, An In-troduction to Chemical Engineering Design, 5th edition, Pergamon Press, Oxford ed. 1983.

[2] Bird, R.B., Stewart, W.E., and Ligthfoot, E.N.: Transport Phenomena, 2nd edition, John Wiley & Sons, Inc, 2002.

[3] Batchelor, G.K.: An Introduction to Fluid Dy-namics, Cambridge University Press, 1967.

[4] Japikse, D., and Baines, N.C.: An Introduc-tion to Turbomachinery, Concepts ETI, Inc, 1994.

[5] Budinski, K.G., and Budinski, M.R.: Mate-riaalkunde voor Technici, Prentice Hall, 1999.

[6] Groen, Ir.: Theoretical Aspects of Physical Deaeration, Stork, 2006.

[7] Albrink, W.: The Feedwater Deaerator in the Water-Steam System, Stork Thermeq B.V. 2010. l

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