Basic Desuperheater Types 2m174j

Basic Desuperheater Types Desuperheaters The simplest type of desuperheater is an unlagged section of pipe, where heat can be radiated to the environment. However, apart from the obvious risk of injury to personnel from such a hot item of plant, and the expensive energy wastage, this approach does not adjust to compensate for changes in the environmental conditions, steam temperature or steam flowrate.

Several designs of desuperheater are available and it is recommended that the following properties be considered when sizing and selecting a suitable station for a given application: • Turndown ratio - ‘turndown’ is used to describe the range of flowrates over which the desuperheater will operate, as shown in Equation 4.2.1.

This is an important parameter, as any variation in inlet pressure, temperature or flowrate will cause a variation in the requirement of cooling liquid. In general, the two turndown values may be specified for a particular desuperheater: Steam turndown ratio - This reflects the range of steam flowrates that the device can effectively desuperheat. Cooling water turndown ratio - This reflects the range of cooling flowrates that can be used. Although this directly affects the steam turndown ratio, the relationship depends on the temperatures of the superheated steam, the cooling water and the resulting desuperheated steam. Equation 15.1.1 is the mass/heat balance equation for this application:

It should be noted that the steam and water flowrates are directly proportional to each other; the constant of proportionality ‘k’ depends on the enthalpies of the superheated steam, the cooling water and the required desuperheated steam.

If the required turndown cannot be achieved using a single desuperheater, two desuperheaters can be installed in parallel, with operation switching from one to another; or both can be in operation depending on steam demand. It should be noted that the desuperheater itself is only one part of a desuperheating station, which will include the necessary control system for correct operation. • Operating pressures and temperatures. • Steam and water flowrate. • Amount of superheat before, and amount of desuperheated steam required after, the process. • The water pressure available (a booster pump may be required). • The required accuracy of the final temperature. • In the case of in-line desuperheaters, the distance travelled by the steam before complete desuperheating has occurred is also an important consideration. This is referred to as the absorption length. The following Sections include descriptions of the common types of desuperheater available, their limitations and typical applications.

Indirect desuperheaters Tube bundle type desuperheaters This type of desuperheater (Figure 15.2.2) consists of a heat exchanger, typically a shell and tube, with superheated steam on one side, and the cooling medium on the other. The shell of the first heat exchanger (containing the cooling water) is fixed at both ends on the inlet side, whereas on the outlet side, it is fixed at the bottom and open at the top. The floating head allows the pressure in the two sections of the shell to equalise. The cooling medium is water at saturation temperature and pressure. As superheated steam enters the first and then the second set of tubes, it gives up heat to the water, some of which will be evaporated by this addition of energy. Any evaporated cooling water es through the floating head and will accumulate in the outlet side of the shell. It then es through the open end of the shell where it is mixed with the desuperheated steam.

Advantages: 1. Turndown is only limited by the controls that are fitted. 2. This design is capable of producing desuperheated steam to within 5°C of the saturation temperature. 3. High maximum operating temperatures and pressures, typically around 60 bar and 450°C. 4. Fast response.

Disadvantages: 1. Bulky - because there are now a number of in-line devices available, they have been largely superseded. 2. Cost. 3. An important concern with this type of desuperheater is the efficiency of the heat exchange process. The build up of air or scale films on the heat exchange surface can act as an extremely effective barrier to heat transfer.

Applications: 1. Those applications that experience wide variations in load.

Direct desuperheaters Water bath type desuperheater This is the simplest form of direct desuperheater. The superheated steam is injected into a bath of water. This additional heat will cause saturated steam to evaporate from the surface of the bath. A pressure controller maintains a constant pressure in the vessel, and hence the temperature and pressure of the saturated steam in the downstream pipe.

Since the superheated steam has more energy per unit mass than the saturated steam, more steam will be evaporated than actually enters the desuperheater. Consequently, the water level will fall and therefore provision must be made to maintain this level. This usually requires a pump of similar design to a boiler feedwater pump, as the water must be pumped against the vessel pressure. A good non-return valve is required in the superheated steam supply to avoid any water from the bath being drawn into the superheated steam system should the pressure in the superheated main drop.

Advantages:

1. Simple 2. Steam is produced at saturation temperature. 3. Steam with a dryness fraction of 0.98 can be produced. 4. Turndown is only limited by the controls that are fitted.

Disadvantages: 1. Bulky. 2. Not practical for high temperatures.

Applications: 1. Wide variations in the flowrate. 2. Where no residual superheat can be tolerated.

Water spray desuperheating This type of desuperheating represents the vast majority of desuperheating applications. In water spray desuperheaters, superheated steam is ed through a section of pipe fitted with one or more spray nozzles. These inject a fine spray of cooling water into the superheated steam, which causes the water to be converted into steam, reducing the quantity of superheat. The cooling water may be introduced into the superheated steam in a number of ways; consequently, there are a number of different types of water spray desuperheater. Despite this, most water spray desuperheaters are affected by the following factors: • Particle size - The smaller the water particle size, the greater the ratio of surface area to mass, and the higher the rates of heat transfer. Since the water is being directly injected into the moving superheated steam, the smaller the particle size, the shorter the distance required for heat exchange to take place. The water is broken into small particles using either a mechanical device (such as a variable or fixed orifice nozzle) or steam atomising nozzles. • Turbulence - As the flow within the pipeline becomes more turbulent, the individual entrained water particles reside longer in the desuperheater, allowing for greater heat transfer. In addition, turbulence encourages the mixing of the cooling water and the superheated steam. Increased turbulence results in a shorter distance being required for complete desuperheating to occur. Turbulence can be created in two ways: Pressure drop across the nozzle - Subjecting the cooling water to a higher pressure drop will increase its velocity and induce greater turbulence. Velocity - By increasing the overall velocity of the water and steam mixture, the amount of turbulence is inherently increased. The increase in velocity is usually achieved by creating a restriction in the steam path, which further generates turbulence by vortex shedding. In addition to these high velocities, if poor piping design practices are used, the speed of the superheated steam could in theory approach Mach 1. At such speeds a number of problems would occur (including the generation of shock waves). However, this would be far in excess of the velocities used in good piping design. Typical velocities of steam entering a desuperheater should be around 40 to 60 m/s. • Cooling water flowrate - The rate at which cooling water can be added to the superheated steam is affected by a number of factors, which are related by Equation 4.2.11:

Bearing in mind that C and g are constants, reviewing Equation 4.2.11 shows that only two factors can be manipulated to alter the cooling water flowrate, q v: Changing the pressure drop over the orifice (nozzle), h - Expressing flowrate as a function of pressure drop over the nozzle: This means that if, for example, flow is increased by a factor of 5, the available pressure must increase by a factor of 5 2 = 25. The effect of this relationship is to severely hamper the turndown ratio. In addition to affecting the cooling water flowrate, there are two other important considerations when determining the required cooling water pressure: 1. The cooling water pressure must be greater than the superheated steam pressure at the point of injection. 2. The greater the pressure drop across the nozzle, the better the atomisation of the cooling water. Changing the area of the orifice, A - Expressing flowrate as a function of the area of the orifice: V A This direct relationship means that if, for example, flow is to be increased by a factor of 5, the available area must also increase by a factor of 5. This change may simply be achieved by an orifice, which has the ability to change in area (see Figure 15.2.4), or alternatively by altering the number of orifices ing the coolant.

• Thermal sleeves - Careful control of the spray is required to ensure that the water does not fall out of suspension as this can result in thermal stresses being generated in the pipeline and cracking may occur. However, in some cases, an inner thermal sleeve can be used to provide protection from this.

The thermal sleeve also allows the circulation of superheated steam around the annular area between the sleeve and the inside diameter of the pipe. This provides a hot surface upon which the injected water can evaporate, as opposed to the walls of the desuperheater, which are inevitably cooler.

Water spray type desuperheaters Single point radial injection spray desuperheaters The simplest method of injecting cooling water is to introduce a nozzle through the pipe wall.

The cooling water particles are sprayed across the flow of the superheated steam. The quantity of cooling water injected is controlled by varying the position of the valve in the centre of the nozzle. Advantages: 1. Simple in operation. 2. Cost effective. 3. Minimum steam pressure drop.

Disadvantages: 1. Low turndown ratio, typically a maximum of 3:1 on both steam and cooling water flow. 2. Desuperheated steam temperature can only be reduced to 10°C above saturation temperature. 3. Longer absorption length than the steam atomising type. 4. Most prone to cause erosion damage to the internal pipework. This can be overcome by the use of a thermal sleeve. 5. Limited pipe sizes. Applications: 1. Constant steam load. 2. Constant steam temperature. 3. Constant coolant temperature. All of which mean a relatively constant cooling water requirement. Multiple point radial injection spray desuperheaters This is a progression of the single point radial injection spray desuperheater. Cooling water is sprayed in from a number of orifices around the perimeter of the pipe.

Advantages: 1. The pressure of the cooling liquid is less than that in the single point version; therefore, it is not necessary to use a thermal sleeve. 2. The absorption length is shorter compared with that of the single point version due to better mixing of the water and the superheated steam. The absorption length is still significantly longer than other types of water spray desuperheater. Other advantages, disadvantages and applications are similar to those of single point radial injection spray desuperheaters.

Axial injection spray desuperheaters This is also a simple in-line injection spray desuperheater, but the point of injection is moved to the axis of the pipeline. The cooling water is injected into the steam flow via one or more atomising nozzles (see Figure 15.2.8). The unit usually employs a thermal sleeve.

Axial injection of the cooling water improves the mixing of the water and the superheated steam by two methods: 1. As the water is injected along the centre of the pipeline, it will be more evenly distributed throughout the superheated steam. 2. The cooling water delivery pipe that is inserted in the pipeline acts as an obstruction, creating additional turbulence at the point of water injection due to vortex shedding.

A modification of this basic arrangement involves turning the nozzle so that the cooling water is sprayed upstream, against the steam flow. The high velocity of the superheated steam reverses the spray water flow pattern and sends it back through a mixing chamber. This achieves more efficient mixing of the water and steam over a short absorption length.

Advantages: 1. Simple in operation. 2. No moving parts. 3. Cost effective across the entire range of sizes. 4. Minimal steam pressure drop.

Disadvantages:

1. Low turndown ratio, typically a maximum of 3:1 on both steam and cooling water flow. 2. Desuperheated steam temperature can only be reduced to 10°C above saturation temperature. 3. Longer absorption length than the steam atomising type, but less than the radial type desuperheaters. 4. Most prone to cause erosion damage to the internal pipework. This can be overcome by the use of a thermal sleeve.

Applications: 1. Constant steam load. 2. Constant steam temperature. 3. Constant coolant temperature. All of which mean a relatively constant cooling water requirement.

Multiple nozzle axial injection desuperheaters Rather than a single nozzle, the multiple nozzle axial injection desuperheater provides a number of nozzles across the flow of superheated steam. This gives good dispersion of the water droplets. There are three main types of multiple nozzle axial injection desuperheater: 1. Fixed area type - All the nozzles are open when the desuperheater is operating, and the cooling water is regulated by a spray water control valve.

2. Variable spray type - The downstream temperature determines the number of exposed nozzles. Cooling water enters the desuperheater through the water jacket to the sealing area above the disc (see Figure 15.2.12). When an increase in the downstream steam temperature is detected by the associated temperature

control system, the actuator moves the stem down, progressively exposing more nozzles. When the demand for the cooling water changes, the stem and disc arrangement moves up and down as required. This has the effect of changing the overall orifice area.

3. Spring-assisted type - This is essentially a combination of the two previous types. Instead of the stem and disc arrangement being controlled by an actuator, the spring-assisted type contains a spring-loaded flow plug, which moves in response to a change in the differential pressure between the coolant and the superheated steam. The moving plug changes the number of open nozzles, thereby adjusting the flow into the main pipeline. In addition, the cooling water is regulated by a spray water control valve. Being able to control both the pressure and flow of the cooling water enables accurate control over the amount of water injected into the superheated steam. This type does, however, require a high cooling water pressure.

Advantages: 1. Turndown ratios of up to 8:1 are possible with the fixed area type, up to 9:1 with the spring assisted type and 12:1 for the variable area type. 2. Better dispersion of the water droplets means that the absorption length is less than that of single nozzle devices. 3. Minimal steam pressure drop.

Disadvantages: 1. The desuperheated steam temperature can only be reduced to 8°C above saturation temperature. 2. Longer absorption length than the steam atomising type. 3. Most prone to cause erosion damage to the internal pipework, if a thermal sleeve is not used. 4. Not suitable for small pipe sizes. 5. Requires high pressure cooling water (particularly true of the spring assisted type). 6. Variable area and spring assisted types can be expensive.

Applications: 1. Applications with a requirement for a higher turndown ratio than that offered by single nozzle devices, but where the expense of more sophisticated devices is not justified. 2. Constant steam load. 3. Constant steam temperature. 4. Constant coolant temperature. All of which require a relatively constant desuperheating load

Superheated steam has important advantages on certain applications, for example, when used in power stations to drive turbines. For efficient use on heating applications however, the steam must be desuperheated. This tutorial considers basic desuperheating theory and calculations.

Basic Desuperheating Theory Superheated steam is steam that is at a temperature higher than the saturation temperature for the steam pressure. For example, steam at a pressure of 3 bar g has a saturation temperature of 143.762°C. If further heat were to be added to this steam and the pressure remained at 3 bar g, it would become superheated. This extra heat results in steam which: • Is higher than saturation temperature. • Contains more energy than saturated steam. • Has a greater specific volume than saturated steam. The relationships between these three properties are well documented and can be found in most texts relating to the thermodynamic properties of steam.

Superheated steam is principally used in power generation plants as the driving force for turbines. A review of the Rankine gas cycle will demonstrate that, for driving turbines, superheated steam is more thermally efficient than saturated steam.

Superheating the steam has further important advantages: • Wet steam within a turbine would result in water droplets and erosion of the turbine blades, as well as increased friction. • Higher pipeline velocities (up to 100 m/s) can be used. This means that smaller distribution pipelines can be used (provided that the pressure drop is not excessive). • For continuously running plants, superheated steam means there is no condensation in the pipework, therefore, there is only a requirement for steam trapping during start-up.

The use of superheated steam has a number of disadvantages: • Although superheated steam contains a large amount of heat energy, this energy is in three forms; enthalpy of water, enthalpy of evaporation (latent heat) and enthalpy of superheat. The bulk of the energy is in the enthalpy of evaporation, and the energy in the superheat represents a smaller proportion. For example, take superheated steam at 10 bar a and 300°C, then: Enthalpy of water = 763 kJ/kg Enthalpy of evaporation = 2 015 kJ/kg

Enthalpy of superheat = 274 kJ/kg

• The coefficient of heat transfer when using superheated steam as the heating medium is variable, low and difficult to quantify accurately. This makes accurate sizing and control of heat transfer equipment difficult, and will also result in a larger and more expensive heat exchanger. Once the superheated steam is cooled to saturation temperature, the heat transfer coefficient increases dramatically, and the temperature at which the steam condenses back into water is constant. This greatly assists accurate sizing and control of heat transfer equipment. The presence of high heat transfer coefficients associated with saturated steam leads to smaller and cheaper heat exchangers than those which utilise superheated steam. • Some processes (for example, distillation columns) perform less efficiently when supplied with superheated steam. • The higher temperatures of superheated steam may mean that higher rated, and hence more expensive equipment is required. • The higher temperature of superheated steam may damage sensitive equipment. These disadvantages mean that superheated steam is generally undesirable for thermal process applications. However, sites exist where superheated steam is raised for power generation, and it makes economic sense to desuperheat some of this steam from some point in the power generation cycle, and then use it for process applications. (More information on superheated steam can be found in Module 2.3). Sites also exist where large quantities of waste are used as fuel for the boiler. If the quantity of waste is sufficiently large, then superheated steam may be produced for power generation. Examples of this type of plant can be found in the papermaking and sugar refining industries. In plants that have superheated steam available for process use, it makes sense to distribute the superheated steam to remote points in the plant, as this will ensure that the steam remains dry.

This becomes significant if there are long lengths of pipe separating the point of generation and the point of use.

Basic steam desuperheating Desuperheating is the process by which superheated steam is restored to its saturated state, or the superheat temperature is reduced. Most desuperheaters used to restore the saturated state produce discharge temperatures approaching saturation (typically to within 3°C of the saturation temperature as a minimum). Designs for discharge temperatures in excess of 3°C above saturation are also possible and often used. There are basically two broad types of desuperheater: • Indirect type - The medium used to cool the superheated steam does not come into direct with it. A cooler liquid or gas may be employed as the cooling medium, for example, the surrounding air. Examples of this type of desuperheater are shell and tube heat exchangers. Here the superheated steam is supplied to one side of the heat exchanger and a cooler medium is supplied to the other side. As the superheated steam es through the heat exchanger, heat is lost from the steam, and gained by the cooling medium. The temperature of the desuperheated steam could be controlled by either the inlet superheated steam pressure or the flowrate of the cooling water. Control of the superheated steam flow for this purpose is not normally practical and most systems adjust the flow of the cooling medium. • Direct type - The medium used to cool the superheated steam comes into direct with it. In most cases, the cooling medium is the same fluid as the vapour to be desuperheated, but in the liquid state. For example, in the case of steam desuperheaters, water is used. A typical direct desuperheating station is shown in Figure 15.1.3. When the desuperheater is operational, a measured amount of water is added to the superheated steam via a mixing arrangement within the desuperheater. As it enters the desuperheater, the cooling water evaporates by absorbing heat from the superheated steam. Consequently, the temperature of the steam is reduced. Control of the amount of water to be added is usually achieved by measuring the temperature of the steam downstream of the desuperheater. The set temperature of the desuperheated steam would typically be 3°C above that at saturation. Therefore, in such arrangements the inlet pressure of the superheated steam should be kept constant.

Desuperheating calculations The amount of water added must be sufficient to cool the steam to the desired temperature; too little water and the steam will not have been cooled enough, too much and wet saturated steam will be produced which will require drying through a separator. Using Equation 15.1.1, which is based on the conservation of energy, the cooling liquid requirement can be easily and quickly determined:

Example 15.1.1 Determine the required cooling water flowrate for the conditions in the following Table:

Solution: The necessary information can be obtained or interpolated from hard copy steam tables; the relevant extracts are shown in Table 15.1.1 and Table 15.1.2. Alternatively, the Spirax Sarco online steam tables can be used.

The information required to satisfy Equation 15.1.1 is therefore: s

= Mass flowrate of superheated steam = 10 000 kg/h

hs = Enthalpy at superheat condition (From steam tables 300°C at 10 bar a) = 3 052 kJ/kg hcw = Enthalpy of the cooling liquid = 4.2 kJ/kg°C x 150°C =630 kJ/kg Determining the enthalpy at the desuperheated condition, hd: From steam tables, the saturation temperature (Ts) at 10 bar a is 180°C, therefore at the required desuperheated condition, the temperature will be: Ts + 5°C = 185°C Interpolating between the enthalpy of steam at 10 bar a and its saturation temperature, and at 10 bar a and 200°C: Enthalpy at 10 bar a, Ts (saturated steam tables) = 2 778 kJ/kg

Enthalpy at 10 bar a, 200°C (superheated steam tables) = 2 829 kJ/kg Interpolating for enthalpy at 10 bar a and 185°C:

Finally, applying Equation 15.1.1:

Note that the desuperheated steam is supplied at a rate of: 10 000 + 1 208 kg/h = 11 208 kg/h supplied at a rate of: 10 000 + 1 208 Had the requirement been for 10 000 kg/h of the desuperheated steam, the initial superheated steam flowrate can be determined using a simple proportional method:

Deg HRSG desuperheaters for performance and reliability 03/15/2006 | Dr. Sanjay V. Sherikar, PE, Control Components Inc., and Peter Borzsony, CCI International Ltd.

Desuperheaters—often called attemperators—in heat-recovery steam generators (HRSGs) are located between the primary and secondary superheaters and reheaters, and sometimes after the final stage of superheating (Figure 1). They are responsible for controlling steam temperature in accordance with start-up and steam-turbine-inlet requirements. The attemperators also prevent thermal damage to superheater and reheater tubes, and to outlet steam piping and downstream equipment.

1. Where you’ll find them. Attemperators for heat-recovery steam generators (HRSGs) are located between the primary and secondary superheaters and reheaters, and sometimes after the final stage of superheating. Courtesy: CCI International Ltd. Interstage attemperators for superheaters typically see pressures up to 1,900 psig, temperatures to 1,020F, and steam flow rates to 660,000 lb/hr. Reheater attemperators experience similar temperatures and flow rates, but pressures normally go to only 450 psig.

Problems with attemperators The addition of unwanted water to the steam line due to improper operation of an attemperator, or to the inability of its control element to remain leak-tight, is a major concern of operators. Failure of the attemperator to control the injection of water into interstage lines often produces thermal-shock damage to hardware and piping. In severe cases, the unatomized water will even erode piping. In most cases, the usual consequences are a forced outage and an expensive repair bill. Another problem seen with interstage attemperators, though not as catastrophic, is their inability to control final steam temperature within specified limits. This occurs when the attemperator lacks sufficient turndown, or if installation is poor, or when the leakage across the attemperator’s control element exceeds the demand for spray water. Such failures reduce the steam turbine’s efficiency and electrical output. Root causes of these problems include poor design of the attemperator and/or spray-water control element, poor installation, and improper control instrumentation.

Design considerations The service requirements for interstage desuperheating are extremely demanding. As the HRSG cycles, attemperator hardware can remain for extended periods at elevated temperatures without spray water flowing through it. Adding insult to injury, the hardware then is quenched instantaneously by relatively cool spray water. Attemperator designs with flow-control elements in the steam path—the multinozzle probe style shown in Figure 2, for example—are particularly susceptible to such damage. Cycling causes fatigue and thermal cracks in critical components, including the nozzle holder, individual nozzles, the lower body, and piston rings.

2. Vulnerable to damage. Multinozzle, probe-type attemperators generally are not recommended when the difference in temperature between the steam and spray water exceeds 450F. Courtesy: CCI International Ltd.

Multinozzle designs also are prone to internal flashing, which can occur when the flow of spraywater is extremely low and the water is allowed to heat up to saturation temperature before exiting the nozzles. Flashing fosters erosion of nozzles and the nozzle holder. Galling of piston rings and related components also is a possibility when temperature swings are large. This design also has the control element within the hot steam, making it subject to wide temperature variations. Probe-style attemperators of any type are prone to vibration created by vortex shedding and the high-velocity head (kinetic energy) of the steam ing the probe assembly. The vibration induced by the vortices, in combination with the high temperature, can cause cracking of the weld t between the probe’s mounting flange and its lower body. Thermal cycling can initiate cracking of the seal welds connecting the lower probe body with the nozzle head, thereby loosening the nozzle head and changing spray-angle orientation. The turndown required for attemperation is quite high and often underestimated. Note that a 20:1 turndown in attemperation water flow does not necessarily equate to a 20:1 turndown in capacity for the flow-control element, or Cv. The spraywater control-element turndown requirement is influenced by variations in supply water pressure, steam pressure, and nozzle backpressure, which in combined-cycle power plants can be extreme. The last varies with flow demand. In some cases, the difference between supply-water pressure and interstage steam pressure (Dp) at low flow is much higher than at high steam flow. In other cases, constant-speed boiler feedpumps provide spray water at relatively constant pressure, but interstage steam pressure slides during start-up—

particularly when multiple HRSGs serve one steam turbine. For both situations, the variation in differential pressure across the operating range may require a spraywater flow-control element with extremely high turndown capability. The turndown requirement of the spray-water flow-control element also is influenced by variations in pressure drop across the attemperator’s spray nozzles. This influence is much less pronounced in attemperators with spring-loaded nozzles than in those with fixed-area nozzles. In addition to providing high turndown, the spray-water control element may experience high Dp at low flow and a low Dp at high flow and, therefore, must be able to handle these conditions. Repeatable, tight shut-off of the control element is necessary. This calls for a high plug-to-seat thrust. Specify a trim exit velocity of less than 100 ft/sec to prevent cavitation and erosion damage. In short, s should look for a control valve that offers equal-percentage-characterized trim to maximize resolution during low Cv requirements, thereby ensuring tight control of spraywater and of outlet temperature. Of course, proper installation is important to the success of every attemperator. Here are three cardinal rules to : Provide a straight run of pipe upstream of the attemperator of no less than three diameters. Installation of a liner in the inlet piping is recommended to ensure uniform geometry of the steam flow at the point of spraywater injection.  Provide a straight run of pipe downstream of the attemperator. Insufficient distance between the attemperator and the first downstream elbow can cause the agglomeration of water droplets along the elbow wall, a phenomenon conducive to water fallout, thermal shock, inaccurate from instrumentation to the flow-control element, and erosion.  Install the temperature sensor downstream of the attemperator at a point where all the spraywater has been evaporated to avoid false readings and inaccurate to the flow-control element. Figure 3 indicates the recommended installation distances when attemperator spray—in conjunction with appropriate liner—is perpendicular to steam flow and nozzles are circumferentially mounted. 

3. Measure carefully. The recommended installation distance is quite specific when the attemperator spray is perpendicular to steam flow. Courtesy: CCI International Ltd.

Importance of proper atomization Proper atomization and evaporation of the spraywater supplied by an attemperation system is necessary both for good temperature control and to prevent water carryover. The complete integration of injected water into superheated steam involves three steps: primary atomization, secondary atomization, and evaporation. Primary atomization is the breakdown of water into droplets by the attemperator’s nozzles. The goal is to create as small a droplet as possible, regardless of the water spray flow rate. Variable-area nozzles offer this capability. They can provide good primary atomization at flows down to less than 220 lb/hr, and they are self-cleaning. Fixed-orifice nozzles, by contrast, are sized for the maximum flow rate and do a progressively poorer job of atomization as water flow decreases. The reason is the inherent reduction in Dp characteristic of reduced flows through fixed-area nozzles. They typically are limited to a 3:1 turndown. Secondary atomization refers to the breakup of large droplets by the dynamic force of the steam flow. However, for secondary atomization to occur, the dynamic forces acting on a droplet must be greater than the viscous forces holding the droplet together. This is a function of the Weber number (We), which is equal to the dynamic force divided by surface tension. The equation is: Surface tension (stabilizing) force s where D is the droplet diameter, U the density of steam, r the characteristic velocity, and s the surface tension (which depends on water temperature). At Weber numbers greater than 12, the aerodynamic or destabilizing force will overcome surface tension—the stabilizing force—and the droplet will break up. To achieve good secondary atomization, design engineers suggest injecting the water spray perpendicular to the steam flow (Figure 4) rather than parallel to it, as in Figure 2. In HRSG main and reheat attemperator applications, steam velocity is sufficiently high to maximize relative velocity and ensure efficient secondary atomization under all operating conditions.

4. The right angle. Injection of spray water perpendicular to steam flow improves atomization and offers other benefits as well. Courtesy: CCI International Ltd.

The small droplets produced by secondary atomization boil and evaporate. The time needed to complete the evaporative process depends on the total surface area of the water volume and is proportional to the square of the droplet diameter. Any droplets that do not evaporate before reaching the temperature sensor may wet the sensor and make it difficult to control steam temperature as intended. Inadequate design of the attemperation system can result in a combination of poor control and poor atomization during transients that would permit carryover of water into the secondary superheater, damaging headers and tubes.

Design considerations

Attemperators for HRSG service operate during cold, warm, and hot restarts, as well as during load transients. During steady-state operation, however, desuperheating should not be necessary. This means that the attemperator assembly is exposed for extended periods to rated steam temperature without cooling by injection water. When injection water is required, attemperators are quenched instantaneously from operating steam temperature to the temperature of the spraywater—a difference of 630F to 810F at most plants. Attention must be paid to thermal shock at the design stage, because attemperating systems for HRSGs will experience many quenching cycles in their lifetimes, particularly in cycling power plants. Attemperators with integral control valves, such as the multinozzle unit shown in Figure 2, should be analyzed carefully before writing your specification. Here’s why:  Because the nozzle head is downstream of the water control/isolation point, it can be at steam temperature when cool water is itted.  Thermal fatigue has been experienced at some plants that have been cycled fewer than 500 times.  The flow-control element is prone to cracking, sticking, and leaking. In addition to being vulnerable to thermal shock, probe-style attemperators are susceptible to bending moments created by the flow of steam and to flowinduced vibration as well. If the vibration frequency matches the natural frequency of the probe, there is risk of catastrophic damage. Multinozzle heads are even more susceptible to the effects of thermal shock than simple, fixed-nozzle probe arrangements because of their greater mass. You can avoid thermal fatigue issues by specifying a desuperheating system that has a separate spray water flow-control element located outside the hot steam environment. Extensive analysis and testing indicate that attemperators with integral control-valve elements should not be specified when the difference between the temperatures of the steam and spray water is greater than 450F. Another recommendation: Specify a thermal barrier to separate the hot and cold working elements. It will mitigate the intensity of the thermal cycles experienced by critical components (Figure 5).

5. Provide protection. A thermal barrier mitigates the intensity of thermal cycles experienced by critical components. The liner protects steam piping from thermal shock and helps improve secondary atomization. Courtesy: CCI International Ltd.

There’s more to deg a liner than just providing more metal to protect steam piping against thermal shock. A properly engineered liner is also capable of:    

Increasing steam velocity to improve secondary atomization. Creating vortices that improve atomization and enhance mixing. Assisting with heat transfer and evaporation. Controlling the penetration of the spray pattern by flow profiling.