Block 15 Desuperheating
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Basic Desuperheating Theory Module 15.1
SC-GCM-114 CM Issue 4 © Copyright 2007 Spirax-Sarco Limited
Block 15 Desuperheating
Module 15.1 Basic Desuperheating Theory
The Steam and Condensate Loop
15.1.1
Basic Desuperheating Theory Module 15.1
Block 15 Desuperheating
Desuperheating 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: o
Is higher than saturation temperature.
o
Contains more energy than saturated steam.
o
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. 300
Superheated steam
250
St
Temperature °C
200
atura eam s
tion li
ne
150
Water
100
50
0 0
5
10
15
25 20 Pressure bar g
30
35
40
45
Fig. 15.1.1 Steam saturation diagram
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:
15.1.2
o
Wet steam within a turbine would result in water droplets and erosion of the turbine blades, as well as increased friction.
o
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).
o
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 Steam and Condensate Loop
Basic Desuperheating Theory Module 15.1
Block 15 Desuperheating
The use of superheated steam has a number of disadvantages: o
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 Enthalpy of evaporation 66%
Enthalpy of superheat 9%
Enthalpy of water 25% Fig. 15.1.2 Enthalpy in superheated steam o
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.
o
Some processes (for example, distillation columns) perform less efficiently when supplied with superheated steam.
o
The higher temperatures of superheated steam may mean that higher rated, and hence more expensive equipment is required.
o
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.
The Steam and Condensate Loop
15.1.3
Basic Desuperheating Theory Module 15.1
Block 15 Desuperheating
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: o
Indirect contact type - The medium used to cool the superheated steam does not come into direct contact 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 passes 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.
o
Direct contact type - The medium used to cool the superheated steam comes into direct contact 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 contact 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. Temperature controller
Control valve
Cooling water
Non return valve Desuperheated steam
Superheated steam
Desuperheater Fig. 15.1.3 A typical direct contact desuperheating station
15.1.4
The Steam and Condensate Loop
Basic Desuperheating Theory Module 15.1
Block 15 Desuperheating
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:
!"#$%&'()!"*(&+*,-.. /( !"#$%&'(*0"(*1(&+*,-.. !
(# ! (2(
"
(#" /(
(#" (3(
"
(## /(
!
4#" 3(## 5( /(
!$
"
!
/(
"#
"
(## (2(
!
(## (
(## (3(
!
(# ! (
4## (3(# !5
4#" 3(## 5 4## (3(# !5 "
!"
# "!$
!"! # ""# $
Equation 15.1.1
Where: mcw = Mass flowrate of cooling water (kg / h) ms = Mass flowrate of superheated steam (kg / h) hs = Enthalpy at superheat condition (kJ / kg) hd = Enthalpy at desuperheated condition (kJ / kg) hcw = Enthalpy of cooling water at inlet connection (kJ / kg) Example 15.1.1 Determine the required cooling water flowrate for the conditions in the following Table: Steam supply
Cooling water supply Required steam conditions
The Steam and Condensate Loop
Pressure Temperature Mass flowrate Pressure Temperature Pressure Temperature
10 bar a 300°C 10 000 kg / h 15 bar a 150°C 10 bar a Saturation temperature + 5°C
15.1.5
Basic Desuperheating Theory Module 15.1
Block 15 Desuperheating
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. Table 15.1.1 Extract from steam tables Saturated water and steam P Ts vg uf ug hf hfg 3 bar a °C m / kg kJ / kg kJ / kg
hg
sf
sfg kJ / kg K
sg
9 10
175.4 179.9
0.214 9 0.194 4
742 762
2 581 2 584
743 763
2 031 2 015
2 774 2 778
2.094 2.138
4.529 4.448
6.623 6.586
11 12
184.1 188.0
0.177 4 0.163 2
780 797
2 586 2 588
781 798
2 000 1 986
2 781 2 784
2.179 2.216
4.375 4.307
6.554 6.523
400
450
500
600
0.341 0 2 959 3 266 7.515 0.306 5 2 957 3 264 7.464 0.202 9 2 952 3 526 7.268
0.367 4 3 041 3 372 7.667 0.330 3 3 040 3 370 7.617 0.219 1 3 035 3 364 7.423
0.393 7 3 126 3 480 7.811 0.354 0 3 124 3 476 7.761 0.235 1 3 120 3 473 7.569
0.445 8 3 298 3 699 8.077 0.401 0 3 297 3 698 8.028 0.266 7 3 294 3 694 7.838
Table 15.1.2 Extract from steam tables Superheated steam Pressure T (temperature) 200 250 300 350 °C bar a vg 0.214 9 v 0.230 5 0.259 7 0.287 4 0.314 4 9 ug 2 581 u 2 628 2 714 2 796 2 877 (175) hg 2 774 h 2 835 2 948 3 055 3 160 sg 6.623 s 6.753 6.980 7.176 7.352 vg 0.194 4 v 0.206 1 0.232 8 0.258 2 0.282 5 10 ug 2 584 u 2 623 2 711 2 794 2 875 (180) hg 2 778 h 2 829 2 944 3 052 3 158 sg 6.586 s 6.695 6.926 7.124 7.301 vg 1 317 v 0.132 4 0.152 0 0.169 7 0.186 5 15 ug 2 595 u 2 597 2 697 2 784 2 868 (198) hg 2 792 h 2 796 2 925 3 039 3 147 sg 6.445 s 6.452 6.711 6.919 7.102
The information required to satisfy Equation 15.1.1 is therefore: ms = 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:
' ( )**!$+$ ,) !)-$./ 0 .1$$%$) **!$./ 0 .12$ 3$$ !" #$%$ !& # )&& #$%$ !& # '
)**!$+$ " $3$" )&
!"#"$%#" &''! ( )*+,'-./-0 15.1.6
The Steam and Condensate Loop
Basic Desuperheating Theory Module 15.1
Block 15 Desuperheating
Finally, applying Equation 15.1.1: " ! "#"!! $
"#
!! "#"!"# $
Equation 15.1.1
!"!!!"#"$%!&'"("'")* + $'")* "("-%!+
!
,
!
, "'! ."/0 1 2
Note that the desuperheated steam is supplied at a rate of: 10 000 + 1 208 kg / h = 11 208 kg / h Had the requirement been for 10 000 kg / h of the desuperheated steam, the initial superheated steam flowrate can be determined using a simple proportional method:
!"!!! "#!$
The Steam and Condensate Loop
!"!!!
!"#$%"&'()
15.1.7
Basic Desuperheating Theory Module 15.1
Block 15 Desuperheating
Questions 1. Which of the following are properties of superheated steam? a| The specific enthalpy of superheated steam is greater than that of saturated steam
¨
b| It has a greater specific volume than saturated steam
¨
c| It is at a higher temperature than saturated steam
¨
d| All of the above
¨
2. What is the main disadvantage of using superheated steam in heat exchanger applications? a| The superheated steam contains less heat than saturated steam
¨
b| The coefficient of heat transfer is lower than that of saturated steam
¨
c| Superheated steam has a high dryness fraction, typically unity
¨
d| Superheated steam has a low Rankine efficiency
¨
3. Using steam tables (Table 15.1.1 and Table 15.1.2) determine the additional amount of energy contained in superheated steam at 9 bar a, with 74.6°C of superheat, compared with saturated steam at the same pressure. a| 1%
¨
b| 6%
¨
c| 11%
¨
d| 24%
¨
4. What is the primary function of a desuperheater? a| To reduce the pressure of the steam
¨
b| To increase the specific volume of the steam
¨
c| To condense the steam into water so that it can be disposed of
¨
d| To reduce the temperature of the steam
¨
5. Determine, by interpolating from steam tables, the enthalpy of desuperheated steam at 9 bar a, 8°C above saturation temperature (TS + 8°C). a| 2 795 kJ / kg
¨
b| 2 806 kJ / kg
¨
c| 2 810 kJ / kg
¨
d| 2 815 kJ / kg
¨
6.
A boiler in a power station produces 108 kg / s of superheated steam. Given that the steam downstream of the turbine is at 110 bar a and has a temperature of 500°C, how much cooling water (at 200°C) would be required to desuperheat the steam by 150°C?
a| 5 kg / s
¨
b| 15 kg / s
¨
c| 25 kg / s
¨
d| 30 kg / s
Answers
¨
1: d, 2: b, 3: b, 4: d, 5: a, 6: c
15.1.8
The Steam and Condensate Loop
Basic Desuperheater Types Module 15.2
SC-GCM-115 CM Issue 2 © Copyright 2005 Spirax-Sarco Limited
Block 15 Desuperheating
Module 15.2 Basic Desuperheater Types
The Steam and Condensate Loop
15.2.1
Basic Desuperheater Types Module 15.2
Block 15 Desuperheating
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.
Fig. 15.2.1 Typical multi-nozzle spray desuperheater
15.2.2
The Steam and Condensate Loop
Basic Desuperheater Types Module 15.2
Block 15 Desuperheating
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: o
Turndown ratio - turndown is used to describe the range of flowrates over which the desuperheater will operate, as shown in Equation 4.2.1. 7XUQGRZQ 0D[LPXPIORZ 0LQLPXPIORZ
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:
#$
K K K K !
"
=
!
Equation 15.1.1
#$
Where: mcw ms hi hd hcw
= = = = =
Mass flowrate of cooling water (kg / h) Mass flowrate of superheated steam (kg / h) Enthalpy at superheat condition (kJ / kg) Enthalpy at desuperheated condition (kJ / kg) Enthalpy of cooling water at inlet connection (kJ / kg)
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. Mathematically: !" ∝ N K K :KHUH N = K K !
!
"#
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. o
Operating pressures and temperatures.
o
Steam and water flowrate.
o
Amount of superheat before, and amount of desuperheated steam required after, the process.
o
The water pressure available (a booster pump may be required).
o
The required accuracy of the final temperature.
o
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. The Steam and Condensate Loop
15.2.3
Basic Desuperheater Types Module 15.2
Block 15 Desuperheating
Indirect contact 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 passes through the floating head and will accumulate in the outlet side of the shell. It then passes through the open end of the shell where it is mixed with the desuperheated steam. Pressure and temperature sensors
Safety valve
Superheated steam
Saturated steam
Floating head
Normal water level Float trap as overflow
Cooling water at saturation pressure and temperature
Water level control system Cooling water supply (with pump if pressure is insufficient)
Drain
Float trap Fig. 15.2.2 A tube and bundle type desuperheater
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. 15.2.4
The Steam and Condensate Loop
Basic Desuperheater Types Module 15.2
Block 15 Desuperheating
Direct contact desuperheaters Water bath type desuperheater This is the simplest form of direct contact 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. Saturated steam Pressure controller
Pressure gauge Superheated steam
Pressure sensor
non-return valve
Steam offtake
Safety valve
Steam drying space
Pressure control valve Float trap as overflow
Water bath
Level control system Cooling water supply with feedwater pump
Drain Cooling water supply with feedwater pump, if feedwater is underpressure Fig. 15.2.3 Water bath type desuperheater (schematic)
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. 2. 3. 4.
Simple Steam is produced at saturation temperature. Steam with a dryness fraction of 0.98 can be produced. 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. The Steam and Condensate Loop
15.2.5
Basic Desuperheater Types Module 15.2
Block 15 Desuperheating
Water spray desuperheating This type of desuperheating represents the vast majority of desuperheating applications. In water spray desuperheaters, superheated steam is passed 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: o
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.
o
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.
o
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: T = &$ JK
Equation 4.2.11
Where: qv = Cooling water volumetric flowrate (m³ /s) C = Coefficient of discharge for the nozzle A = Area of the nozzle (m²) g = Gravitational constant (9.81 m /s²) h = Pressure drop over the orifice (m head) 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, qv: Changing the pressure drop over the orifice (nozzle), h - Expressing flowrate as a function of pressure drop over the nozzle: 9 ∝ K
This means that if, for example, flow is increased by a factor of 5, the available pressure must increase by a factor of 52 = 25. The effect of this relationship is to severely hamper the turndown ratio. 15.2.6
The Steam and Condensate Loop
Basic Desuperheater Types Module 15.2
Block 15 Desuperheating
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: 9 ∝ $
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 passing the coolant. Cooling water flow Valve head movement
Seat
Nozzle Valve head annulus Fig. 15.2.4 Variable area orifice o
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. Cooling water
Thermal sleeve
Superheated steam
Fig. 15.2.5 A thermal sleeve inserted in an in-line spray desuperheater
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. The Steam and Condensate Loop
15.2.7
Basic Desuperheater Types Module 15.2
Block 15 Desuperheating
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.
Cooling water actuator and control valve
Cooling water inlet
Superheated steam
Desuperheated steam
Fig. 15.2.6 Single point radial injection spray desuperheater
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. 15.2.8
The Steam and Condensate Loop
Basic Desuperheater Types Module 15.2
Block 15 Desuperheating
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.
Superheated steam
Desuperheated steam
Radial spray injector
Cooling water Fig. 15.2.7 Multiple radial injection point desuperheater
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. Cooling water
Injector
Desuperheated steam
Superheated steam
Fig. 15.2.8 Axial injection spray desuperheater The Steam and Condensate Loop
15.2.9
Basic Desuperheater Types Module 15.2
Block 15 Desuperheating
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.
Flow Vertical bluff body Fig. 15.2.9 Vortex shedding around the cooling water delivery pipe
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. Filtered water supply inlet
Superheated steam flow
Desuperheated steam out
Mixing module Water spray nozzle Fig. 15.2.10 Reverse flow type axial desuperheater
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.
15.2.10
The Steam and Condensate Loop
Basic Desuperheater Types Module 15.2
Block 15 Desuperheating
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. Pneumatic actuator
Cooling water inlet
Cooling water control valve
Multiple nozzles
Superheated steam
Fig.15.2.11 A multiple nozzle desuperheater
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. Movement of stem by an actuator
Stem
Cooling water inlet
Water jacket
Superheated steam
Disc Fig.15.2.12 A variable area type multiple nozzle desuperheater The Steam and Condensate Loop
15.2.11
Block 15 Desuperheating
Basic Desuperheater Types Module 15.2
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.
15.2.12
The Steam and Condensate Loop
Basic Desuperheater Types Module 15.2
Block 15 Desuperheating
Questions 1. Which type of desuperheater should be used if it is essential that steam at saturation temperature be produced?
¨ ¨ ¨ ¨
a| Spring assisted type b| Reverse flow axial type c| Single nozzle radial injection type d| Water bath type
2. Which of the following parameters can be altered in order to increase the rate at which cooling water is injected into superheated steam, using a spray type desuperheater? i. Pressure drop over the orifice ii. Area of the orifice iii. Cooling water temperature
¨ ¨ ¨ ¨
a| i only b| ii only c| i and ii d| i, ii and iii
3. What advantage does a multiple point radial injection spray desuperheater have over a single point desuperheater? a| Improved approach to saturation temperature b| Increased turndown ratio c| Shorter absorption length d| It can be used on smaller pipe sizes
¨ ¨ ¨ ¨
4. What is the primary function of a thermal sleeve? a| To reduce the temperature of the superheated steam b| To prevent erosion of pipework downstream of the desuperheater c| To reduce the energy losses from the pipework d| To reduce the amount of cooling water required
¨ ¨ ¨ ¨
5. What advantage do variable area axial type desuperheaters have over fixed area types? a| Improved approach to saturation temperature b| Increased turndown ratio c| Shorter absorption length d| It can be used on smaller pipe sizes
¨ ¨ ¨ ¨
6. What are the main applications of spray type desuperheaters? a| Applications with fairly constant cooling water requirements b| Applications with high turndown requirements c| Applications where the steam flowrates will vary widely d| Applications requiring the elimination of all superheat
¨ ¨ ¨ ¨
Answers
1: d, 2: c, 3: c, 4: b, 5: b, 6: a The Steam and Condensate Loop
15.2.13
Block 15 Desuperheating
15.2.14
Basic Desuperheater Types Module 15.2
The Steam and Condensate Loop
Other Types of Desuperheaters Module 15.3
SC-GCM-116 CM Issue 2 © Copyright 2005 Spirax-Sarco Limited
Block 15 Desuperheating
Module 15.3 Other Types of Desuperheaters
The Steam and Condensate Loop
15.3.1
Block 15 Desuperheating
Other Types of Desuperheaters Module 15.3
Desuperheaters Venturi type desuperheaters The venturi type desuperheater employs a restriction in the superheated steam pipeline to create a region of high velocity and turbulence where the cooling water is injected. This helps to establish intimate contact between the steam and the cooling water, improving the efficiency of the desuperheating process. Cooling water
Shell Superheated steam flow
Internal Internal diffuser nozzle
Main diffuser Fig. 15.3.1 Venturi type desuperheater
The desuperheating process is carried out in two separate phases: 1. The first stage of desuperheating occurs in the internal diffuser. A portion of the steam is accelerated in the internal nozzle and the velocity is used to atomise the incoming water. The cooling water is injected into the diffuser through a number of small jets, which helps to further atomise the water. 2. In the second stage of desuperheating, a saturated mist or fog emerges from the internal diffuser into the main diffuser where it mixes with the remainder of the steam. The main diffuser itself creates a restriction to the remainder of the steam thereby increasing its velocity in this region. Thus, there is a region of turbulence where the second stage of desuperheating occurs. This mechanism minimises cooling water contact with the sidewalls, combining maximum desuperheating effectiveness with minimum pipe wear. The steam flow turndown ratio does vary depending on the actual conditions, but 4:1 is typical. In applications where there is a dedicated pressure reducing station upstream of the desuperheater, the available steam turndown can be improved to over 5:1. The cooling water turndown is usually satisfactory for most plant applications, with 20:1 possible depending on the actual operating conditions. At cooling water turndowns beyond 20:1, the need for a cooling water booster pump also increases. Venturi type desuperheaters can be installed either horizontally or vertically with the steam flow upwards. When installed vertically, better mixing occurs which can result in an improved turndown ratio of over 5:1. The main problem with this is ensuring that there is enough vertical space to install the desuperheater, as it will be more than several metres long. A modification to the standard venturi type desuperheater is the attemperator desuperheater. This essentially uses the same method of injecting the coolant into the superheated steam, but does not utilise the venturi shaped mixing section. Attemperator desuperheaters are used in place of the venturi type where there is sufficient space available to install a long absorption pipe, especially where slightly higher turndown is required, but where the additional cost of a steam atomising type cannot be justified. The term attemperator is also generally used to refer to a desuperheater that is installed after a boiler or superheater to give accurate control over temperature and pressure. 15.3.2
The Steam and Condensate Loop
Other Types of Desuperheaters Module 15.3
Block 15 Desuperheating
Advantages: 1. Steam turndown ratios of up to 5:1 and cooling water turndowns of over 20:1. 2. Simple operating principle (although more complex than the spray type). 3. No moving parts. 4. Accurate control of desuperheated steam temperature; typically within 3°C of the saturation temperature. 5. Suitable for operation under steady or variable steam conditions. 6. There is reduced wear in the downstream pipework compared to a spray type desuperheater, as the cooling water emerges as a mist rather than as a spray.
Disadvantages: 1. Pressure drop is incurred (although this is generally small and within acceptable limits). 2. The absorption length is still longer than the steam atomising type; so more space is required for installation. 3. A minimum cooling water flow rate is required.
Applications: 1. Suitable for most general plant applications, except where high turndowns on steam flowrate are required.
Steam atomising desuperheaters Steam atomising desuperheaters employ a high-pressure auxiliary steam supply to atomise the incoming cooling water. Atomising steam
Cooling water
Internal nozzle
Superheated steam flow
Seals
Internal diffuser
Fig. 15.3.2 Steam atomising desuperheater
The desuperheating process occurs in two stages: 1. The first stage occurs in the diffuser, where the cooling water is atomised by the high velocity atomising steam. The auxiliary steam pressure needs to be at least 1.5 times the desuperheater inlet pressure, typically with a minimum pressure of 4 bar a. The flowrate of atomising steam is normally between 2% and 5% of the mainline flow. The use of atomising steam means that the cooling water can be introduced into the diffuser at lower pressures. In general, the only requirement is that the pressure must be greater than that of the superheated steam. 2. In the second stage, a wet mist or fog emerges from the diffuser where it mixes with the mainline steam in the pipeline. Evaporation occurs in the pipework immediately downstream of the desuperheater, where the remaining water droplets remain suspended in the steam and gradually evaporate. Using steam to atomise the cooling water produces finely atomised water particles, which ensures efficient heat transfer and evaporation.
The Steam and Condensate Loop
15.3.3
Block 15 Desuperheating
Other Types of Desuperheaters Module 15.3
This arrangement allows for high steam turndown ratios; ratios of up to 50:1 are possible. It should however be noted that at turndowns greater than 20:1, low pipeline velocities may result in the settling out of water, caused by the decreasing momentum of the water droplets. In this case, a drainage and recycle arrangement is required (see Figure 15.3.3). If such a recycle arrangement cannot be fitted, the turndown ratio will be reduced. The typical installation of a steam atomising desuperheater is illustrated in Figure 15.3.3.
5 Air supply
7.5 m
Maximum 7 taper
Superheated steam flow
6
9
7
8
1 2
Desuperheated steam flow
3m
2m
Cooling water re-cycle loop Atomising steam loop
Non return valve 3 4
Modulating control valve
Cooling water 1. Steam atomising desuperheater 2. Steam flow control valve 3. Cooling water control valve 4. Automatic steam on / off valve 5. Pressure controller
6. Temperature controller 7. Temperature sensor 8. Pressure sensor 9. Air filter / regulator
Fig. 15.3.3 Typical steam atomising type desuperheater installation
Advantages: 1. Good turndown steam turndown of up to 50:1 is possible, but operation and control is most efficient for a turndown of around 20:1. 2. Very compact with a short absorption length relative to the other types. 3. Pressure drop is negligible. 4. The cooling water used can be cold, as the atomising steam will preheat it. 5. Low approach to saturation temperature typically to within 6°C of saturation temperature.
Disadvantages: 1. Auxiliary high pressure steam is required. 2. The amount of extra equipment required and the additional pipework is relatively expensive.
Applications: 1. Suitable for applications where the steam flowrates will vary widely, for example in combined pressure reducing and desuperheating stations. 15.3.4
The Steam and Condensate Loop
Other Types of Desuperheaters Module 15.3
Block 15 Desuperheating
Variable orifice desuperheater The variable orifice desuperheater controls the flow of cooling water into the mainline by a free-floating plug placed in the flow. Saturated steam outlet connection
Cage with travel stop Plug
Spring loaded plunger Cooling water
Cooling water annulus Seat
Superheated steam flow Fig. 15.3.4 Variable orifice desuperheater
The variable orifice desuperheater consists of a plug that moves up and down in a cage. This movement is limited by a travel stop incorporated into the top of the cage. Its position within the cage depends on the flow of superheated steam in the mainline. Under no-flow conditions, the plug rests on a seat ring, surrounded by an annulus of cooling water. When superheated steam starts to flow through the desuperheater, the plug is forced off the seat by the steam pressure. As the flow increases, the plug is lifted further away from the seat, thereby creating a variable orifice between the plug and the seat. The increase in velocity between the plug and the seat creates a pressure drop across the annulus, drawing water into the superheated steam flow. The low pressure drawing the water into the pipeline also helps to atomise the water into a fine mist. The turbulence associated with the change in velocity and direction of the steam assists in mixing the coolant and the steam. Vortices created immediately upstream of the plug ensure that the coolant is completely mixed with the steam. The efficient mixing of the coolant and the superheated steam within the desuperheater body means that the absorption length is relatively short, and the temperature sensing element may be installed within 4 or 5 metres of the desuperheater body.
Plug Cooling water
Plug Orifice
Fully opened orifice
Seat ring
Superheated steam Light-load Full-load No-load The orifice is fully opened. The plug lifts and opens the The plug is seated on the There is equal pressure drop and orifice slightly. Cooling water is seat ring. There is no flow instantly and thoroughly mixed high turbulence at all loads of superheated steam and and over the full range. with the superheated steam. no flow of cooling water. Fig. 15.3.5 Operation of variable orifice desuperheater installation
The Steam and Condensate Loop
15.3.5
Block 15 Desuperheating
Other Types of Desuperheaters Module 15.3
The rate at which cooling water enters the annulus is varied by a control valve that is regulated as a function of the downstream temperature. The plug is typically fitted with a spring-loaded plunger, which increases the friction between the plug and the cage, effectively damping the plugs movement. Given a fixed pressure drop across the valve this effectively enables the amount of cooling water to be varied when mixing with the flow of superheated steam. The plunger also provides stability under light load conditions. The fact that the coolant is not sprayed into the desuperheater, and that virtually all the desuperheating occurs in the body of the device, means that there is little wear of associated pipework or the desuperheater itself. Therefore, thermal sleeves are unnecessary. A typical installation of a variable orifice desuperheater is illustrated in Figure 15.3.6 Temperature sensor Temperature controller
Modulating control valve
Saturated steam
+ Long radius bend
Cooling water
Variable orifice desuperheater
Superheated steam
Fig. 15.3.6 Typical variable orifice type desuperheater installation
Advantages: 1. The turndown is only limited by the cooling water control valve, and steam turndown ratios of up to 100:1 can be readily achieved. 2. Low approach to saturation temperature typically to within 2.5°C of saturation temperature. 3. Short absorption length. 4. The cooling water pressure need only be 0.4 bar superior to the superheated stem pressure. 5. Superheated steam velocities may be very low.
Disadvantages: 1. Significant pressure drop across the desuperheater. 2. Relatively higher cost. 3. The desuperheater has to be installed vertically. If a bend is located immediately after the outlet, it must have a long radius.
Applications: 1. Suitable for applications where the steam flowrate will vary widely and a relatively high pressure drop is not critical. 2. Where the steam velocity is likely to be very low. 15.3.6
The Steam and Condensate Loop
Other Types of Desuperheaters Module 15.3
Block 15 Desuperheating
Combined pressure control valve and desuperheater In some instances, it is convenient to integrate the pressure control valve and the desuperheater into one unit. Actuator spindle
Superheated steam inlet
Pressure control valve assembly
Cooling water inlet
Mixing chamber
Desuperheated steam outlet Fig. 15.3.7 Combined pressure control valve and desuperheater (cut section)
The pressure reducing aspect is similar to a standard pressure reducing valve. Although a number of different designs of pressure reducing valve could be used, angle or globe configurations are most commonly used. In addition, the valve is typically of the balanced type (with either a balancing plug or a balanced bellows arrangement) to reduce the required actuator force. As accurate pressure control is usually important in desuperheater applications, pneumatic actuation of the valve is virtually universal, and so is the use of positioners. In addition, because quite substantial pressure drops may be involved, the manufacturer will often offer a noise reduction trim for the pressure control valve (see Figure 15.3.8).
Fig. 15.3.8 Typical valve noise reduction trim
The desuperheating aspect will also vary depending on the application, but it is common for a multiple point radial injection type to be used. The mixing of the coolant and the steam is improved due to the high velocity of the superheated steam after the pressure-reducing valve. Radial injection type desuperheaters have the advantage that they can be easily combined with the pressure reducing valve to produce a single unit. In some combined pressure control and desuperheating stations, there are a number of baffle plates installed immediately after the desuperheating station. These plates induce further pressure drop and improve mixing of the steam and coolant. Combined pressure control valve and desuperheating stations are commonly used in turbine bypasses, where the valve dumps the flow directly to the condenser or to cold reheat. The Steam and Condensate Loop
15.3.7
Block 15 Desuperheating
Other Types of Desuperheaters Module 15.3
Comparison of types of desuperheater Table 15.3.1 compares the typical performance and installation characteristics of the different desuperheater types. It should be noted that these properties may vary between different manufacturers, and indeed, they may depend on the particular operating conditions of the system. Table 15.3.1 Comparison of desuperheater types
Type
Tube bundle
Water bath
Steam turndown ratio Depends on coolant control valve Depends on coolant control valve
Minimum Minimum Velocity at temperature cooling water minimum above Ts pressure* flow (°C) (bar) (m/s)
Pipeline sizes (mm) Minimum
Maximum
5.0
-
-
-
-
0
-
-
-
-
Multiple point radial injection
3:1
10.0
1.0
6.0
20
600
Single nozzle axial injection
3:1
10.0
0.5
6.0
50
1200
8:1
8.0
4.0
6.0
150
1500
9:1
8.0
15.5
9.0
150
600
12:1
8.0
3.5
9.0
150
1500
5:1
3.0
1.0
6.0
50
1270
50:1
6.0
Greater than steam pressure
1.5
100
1500
coolant
2.5
0.4
3.0
80
800
Multiple nozzle axial injection (fixed area) Multiple nozzle axial injection (spring assisted)
Multiple nozzle axial injection (variable area) Venturi Steam atomising Depends on Variable orifice control valve
* Typical minimum cooling water pressure above the superheated steam pressure
15.3.8
The Steam and Condensate Loop
Other Types of Desuperheaters Module 15.3
Block 15 Desuperheating
Questions 1. What is the main function of the venturi restriction in a venturi type desuperheater? a | To create high velocity and turbulence b| To increase the pressure of the resulting desuperheated steam c | To contain the expansion of the superheated steam when cooling water is added d| To reduce the required pressure of the cooling water
¨ ¨ ¨ ¨
2. How does the cooling water system in the venturi type desuperheater differ from that in the steam atomising type? a | Cooling water in the venturi type desuperheater is injected directly into the main flow of superheated steam ¨ b| The venturi type uses an external source of saturated steam to atomise the water ¨ c | The steam atomising type uses an external source of saturated steam to atomise the water ¨ d| The steam atomising type uses superheated steam to atomise the water ¨ 3. In comparison with most other types of desuperheaters, what is the main disadvantage of spring assisted type desuperheaters? a | Very low turndown ratio b| High pressure cooling water is required c | A close approach to the saturation temperature d| All of the above
¨ ¨ ¨ ¨
4. What is the function of the spring loaded plunger in a variable orifice type desuperheater? a | To alter the amount of cooling water added to the superheated steam for a given flowrate ¨ b| To improve stability under high loads ¨ c | To limit the movement of the plug ¨ d| To act as a bluff body, increasing the amount of turbulence ¨ 5. Which of the following are advantages of the steam atomising type desuperheater? a | High turndown ratio ¨ b| Short absorption length ¨ c | The cooling water does not have to be pre-heated ¨ d| All of the above 6. Why is noise reduction trim commonly incorporated in combined pressure reduction and desuperheating stations? a | To increase the velocity and turbulence of the superheated steam ¨ b| To improve the mixing of the coolant and the steam ¨ c | Due to the pressure drop across the valve being typically large ¨ d| None of the above ¨
Answers 1: a, 2: c, 3: b, 4: a, 5: d, 6: c The Steam and Condensate Loop
15.3.9
Block 15 Desuperheating
15.3.10
Other Types of Desuperheaters Module 15.3
The Steam and Condensate Loop
SC-GCM-117 CM Issue 3 © Copyright 2007 Spirax-Sarco Limited
Block 15 Desuperheating
Typical Desuperheater Installations Module 15.4
Module 15.4 Typical Desuperheater Installations
The Steam and Condensate Loop
15.4.1
Block 15 Desuperheating
Typical Desuperheater Installations Module 15.4
Typical Desuperheater Installations Installation There are a number of important considerations to take into account when installing a desuperheater, namely: o
The properties of the cooling water.
o
The installation of the desuperheater itself.
o
The ancillary components required.
o
The control valves used on the cooling water line and the superheated steam line.
A generalised installation of an in-line desuperheater is shown in Figure 15.4.1. Pressure controller Pressure regulating control valve
Safety valve
Desuperheater
Superheated steam supply Temperature regulating control valve
Temperature controller
Non return valve
Cooling water
Fig. 15.4.1 A typical in-line desuperheater installation
Properties of the cooling water: o
Temperature - The most effective desuperheating will be achieved using cooling water that is hot, preferably as close to the saturation temperature as possible. However, cooling water temperatures as low as 5°C could be used if absolutely necessary. The use of hot water has the following advantages: - It minimises the time period for which water particles are suspended in the steam. - It evaporates more quickly. - It minimises the amount of water falling to the inside walls of the pipework. There are however, two disadvantages to using high temperature cooling water: 1. The higher the temperature of the cooling water, the greater the required flowrate due to its lower cooling effect. 2. Unless a supply of the water at the required temperature is available, additional heating mechanisms may have to be incorporated. Due to the benefits of using hot water, it is logical to insulate the hot water supply pipes to minimise heat loss, and to protect personnel.
15.4.2
The Steam and Condensate Loop
Block 15 Desuperheating
o
Typical Desuperheater Installations Module 15.4
Quality - The quality of the injected water is important. The Total Dissolved Solids (TDS) content of the injection water should be as low as possible, as any solids that come out of solution will be deposited on: - The faces of valves. - The small orifices in the desuperheater nozzles. - The inner side of the piping downstream of the desuperheater. In addition to reducing the TDS levels, all cooling water should be passed through a suitable strainer installed before the water control valve.
o
Pressure and flowrate - As mentioned in Module 15.2, the pressure of the cooling water, along with the area of the nozzles, determines the flow of cooling water into the desuperheater. Table 15.3.1 shows the typical minimum pressures (above the superheated steam pressure) required for each type of desuperheater. It should be noted that these might vary between manufacturers and for different steam pressures. If a booster pump is used, a spill back line will be required to ensure that there is always sufficient flow through the pump at times of low cooling water demand.
o
Control - A pressure drop will inevitably be required over the water control valve. When using cooling water close to saturation temperature, care is needed to ensure that the pressure drop is not large enough to cause the water to flash into steam. An equal percentage characteristic plug in the water control valve may be selected, which will usually complement the pump characteristic.
o
Source - The availability of water at high pressure and temperature may be difficult. There are a number of possible sources of the cooling water; and options include: - Water from the pressure side of the boiler feedpump (providing the boiler uses modulating level control). - De-mineralised water. - Condensate. - Town water. This however may require treatment to improve the quality, otherwise salts may be deposited on the inside of the desuperheater downstream pipework.
Desuperheater installation The total installed length of a desuperheater station will vary with size and type, but it is typically about 7.5 m. Most desuperheaters can be installed in any direction (the variable orifice type is a notable exception), but if installed vertically, the flow should be upwards. The venturi type is best installed in a vertical pipe with the flow in the upward direction, as this aids mixing of the water and the steam. However, such installations are not usually possible due to the vertical space required.
Superheated steam pressure control Although it is possible to design desuperheater installations to operate with varying upstream pressures, it is much simpler if a constant supply pressure is maintained. The amount of cooling water added is controlled by the temperature of the steam after the desuperheater. The higher the temperature, the more the control valve will open, and the greater the amount of water added. The target is to reduce the steam temperature to within a small margin of the design discharge temperature. If the superheated steam supply pressure is increased, the saturation temperature will also increase. However, the set value on the coolant controller will not change, and an excessive amount of water will be added, resulting in wet steam. Pressure sensors used in the control of the superheated steam pressure should ideally be located at the point of use, so that the pressure control valve can compensate for any line loss between the desuperheater and the point of use. The Steam and Condensate Loop
15.4.3
Block 15 Desuperheating
Typical Desuperheater Installations Module 15.4
Temperature sensor positioning The minimum distance from the point of water injection to the temperature sensing point is critical: o
If the sensor is too close to the water injection point, the mixing of the steam and the water will not have been completed, and the temperature sensor will give a false output.
o
If the sensor is too far away, it will make the installation unnecessarily long.
The minimum installation distance will vary between different types of desuperheater and with different manufacturers. It is usually specified as a function of the temperature difference between the required outlet temperature and either the inlet temperature or the coolant temperature. Figure 15.4.2 shows a typical manufacturers sensor positioning chart. Difference between saturation and outlet temperatures (°C) 125
100
75
50
25
0
24 22 20
Distance to sensing point (m)
18
Single nozzle axial injection type
16 14 12 10 8
Variable orifice type
6 4 2 0 0
100 50 150 200 Difference between coolant and outlet temperatures (°C)
250
Fig. 15.4.2 Positioning of the temperature sensor
Separator station Efficient drainage of the pipework following the desuperheater is essential. To ensure that water cannot accumulate at any point, the pipe should be arranged to fall approximately 20 mm per metre in the direction of flow, and should be provided with a separator station. The steam trap used to drain the separator should be carefully selected to prevent air binding, and the discharge pipe from the steam trap should have ample capacity to deal with the drainage and it should be fixed as near to vertical as possible. In addition, there must be sufficient space in the drainage pipe for the water to flow down and air to pass up the pipe. The steam trap must also be able to withstand superheat conditions. On critical applications, for example, prior to a turbine, a separator is even more important; the separator station will remove entrained water in the case of control failure, and prevent too much water being added to the steam. 15.4.4
The Steam and Condensate Loop
Block 15 Desuperheating
Typical Desuperheater Installations Module 15.4
Isolation valves To allow maintenance to be safely carried out, isolation valves are recommended upstream of: o
The superheated steam pressure control valve.
o
The desuperheater.
o
The cooling water supply.
Typically, these should be installed approximately, but no less than 10 pipe diameters from the item they are isolating.
Safety valve A safety valve may be required to protect equipment downstream of the desuperheating station from overpressure, in the event of failure of the pressure control station. It is necessary to ensure discharge pipework from the safety valve is led away to a safe area. This is of particular importance as high temperature superheated steam may be discharged.
Temperature and pressure ratings Most equipment to be used on steam systems is designed with saturated steam in mind. It is therefore important that all equipment used in a desuperheater station will tolerate both the maximum temperature and pressure of the superheated steam. Most equipment will have specified pressure and temperature limitations that are based on the nominal pressure (PN) rating of the material and the specific design of the device. By definition, the PN rating is the maximum pressure that a material can withstand at 120°C. For example, a PN16 rating means that the material will withstand a pressure of 16 bar g at 120°C. At higher temperatures, the maximum pressure will decrease, however, the exact relationship varies and depends on the material. Figure 15.4.3 depicts typical pressure / temerature gradients for PN16, PN25 and PN40 rated products on a non-specific material. It is important to note that different materials will, by specification, produce variations in the temperature gradient.
Temperature (°C)
In addition, components such as gaskets, fasteners and internal components may have a further limiting effect on the maximum temperature and pressure.
!&'
!
%$!"#
120 100
0
25
16
40
Pressure (bar g) Fig. 15.4.3 Typical PN pressure/temperature gradients
Controls The selection and installation of the control devices to be used in a desuperheater station are an important consideration, as they can affect the overall turndown of the desuperheater. If the controls installed have a lower turndown ratio than the desuperheater itself, the turndown of the desuperheater station will be reduced (refer to Module 15.2) Further information on basic control theory and practice can be found in Blocks 5 to 8 inclusive.
The Steam and Condensate Loop
15.4.5
Block 15 Desuperheating
Typical Desuperheater Installations Module 15.4
Selection When selecting a suitable type of desuperheater for a particular application, the following factors need to be considered: o
Separator station - This is probably one of the most important considerations, as the different types of desuperheater vary significantly in the range of superheated steam flowrates that can be effectively desuperheated. It is important to note here that, although ensuring that the device will have sufficient turndown for the flow likely to be encountered, it is important not to specify more turndown capability than is really needed. This predominantly affects cost, but it can also lead to poor system performance. Poor performance is often aggravated by the fact that most desuperheaters tend to perform better at the higher end of the specified flowrates and a system designer would tend to allow for increases in capacity due to expansion. As an extreme example, if the maximum flow specified was ten times the current requirement (in order to take into account future growth), the desuperheater will operate between 1 and 10% of its full flowrate instead of the 10% to 100% it is designed for.
o
Desuperheated steam temperature - As seen in the previous Module, different types of desuperheater are capable of reducing the steam temperature to within several degrees of the saturation temperature. For example, if desuperheated steam temperatures within 5°C of the saturation temperature (TS) were required, a venturi or variable orifice type desuperheater would be selected (see Table 15.3.1). Generally, where some degree of residual superheat can be tolerated, the desuperheated steam temperature should be as high above saturation as possible. This is beneficial for several reasons: 1. Cost a close approximation to saturation temperature is generally only achievable with the more costly types of desuperheaters. 2. Controller sensitivity this may be a problem where the desuperheated steam temperature is required to be close to the saturation temperature. Limited controller sensitivity is one of the reasons why most desuperheaters are limited in their approach to saturation temperature. For example, if a controller had a sensitivity of ±5°C, it would not be able to distinguish between saturation temperature and 5°C above. If such a controller interpreted the steam temperature at 5°C above TS, and the steam were actually at TS, it would increase the flow of cooling water. But since the temperature of the saturated steam will not decrease (due to the latent heat of evaporation), the controller will add increasingly more coolant as it would still believe the system to be at 5°C above TS. This will result in very wet steam flooding the steam main after the desuperheater station. 3. It becomes increasingly difficult to evaporate the cooling water as the superheated steam temperature drops towards saturation, due to the reduced temperature difference between the two. 4. The lower temperature difference also reduces the heat transfer rate between the water and the steam, and therefore the water droplets have to stay in suspension for longer to be evaporated. This increases the likelihood that the water particles will fall out of suspension in the pipe. In order to prevent this from occurring, as the temperature approaches T S, the velocity of the steam needs to be increased in order to create more turbulence.
o
Available coolant supply pressure - The choice of desuperheater type will also depend on the availability of cooling water at the necessary pressure. It would provide a cost advantage to use cooling water that is already available, for example, from the pressure side of a boiler feedwater pump. If the available pressure were not sufficient for a particular type of desuperheater, additional pumping arrangements would have to be made.
15.4.6
The Steam and Condensate Loop
Block 15 Desuperheating
Typical Desuperheater Installations Module 15.4
A typical manufacturers selection chart is shown in Figure 15.4.4. It is based on the typical performance and installation characteristics, which can be found in Table 15.3.1. The method used to size a desuperheater will vary depending on the particular manufacturer and the type of desuperheater, and therefore it is outside the scope of this publication. Is required steam flow turndown Ts + 3°C
Yes
No
No
Yes
Is coolant available with superior pressure >0!5 bar
Yes
Is coolant available with superior pressure >1 bar
Single nozzle, axial injection No
Yes
Venturi type
No
No
No
No
Is required steam flow turndown Ts + 8°C
Is coolant available with superior pressure >4 bar
Multiple nozzle, axial injection (fixed area)
Yes
Yes
Yes
No
No
No
No
Is required steam flow turndown Ts + 8°C
Is coolant available with superior pressure >3!5 bar
Multiple nozzle, axial injection (variable area)
Yes
Yes
Yes
No
No
No
No
Is required steam flow turndown Ts + 6°C
Is coolant available with superior pressure >steam pressure
Is atomising steam available >4 bar
Yes
No
No
Is required steam flow turndown >50:1
Yes
Yes
Is final temperature >Ts + 2!5°C
No
Yes
Is coolant available with superior pressure >0!4 bar
Yes
No
Yes
Steam atomising type
Yes
Variable office type Fig. 15.4.4 Desuperheater selection chart
The Steam and Condensate Loop
15.4.7
Block 15 Desuperheating
Typical Desuperheater Installations Module 15.4
Typical applications Desuperheaters are mainly applied in two areas: 1. Power generation - Desuperheaters are mainly used to reduce the temperature of steam emitted from turbine bypass systems to an efficient level for operation on other parts of the plant requiring saturated steam for heat transfer purposes. 2. Process industry - In process industries, desuperheaters are used as part of a system for reducing the temperature and pressure of steam from boilers to economical levels of operation. Table 15.4.1 shows some common application examples in particular industries. Table 15.4.1 Typical applications for desuperheaters in process industries Industry
Applications
Paper and board
Paper drying machines.
Food
Steam cooking kettles. Evaporator heat exchangers.
Textiles
Fabric finishing autoclaves.
Tobacco
Tobacco leaf drying plants. Reactor heater jackets and coils. Distillation plants.
Chemical and pharmaceutical
Methanol plants. Sulphur plants. Polymerisation plants. Chemical. Vacuum distillation start-up heaters. Glycol and ethylene plants. Aromatics recovery. Vinyl chloride plants. Thermal and catalytic crackers. Vacuum distillation.
Oil and petrochemical
Polymerisation plants. Sulphur plants. De-salting. Isomerisation processes.
Refineries Brewing and distilling Boiler and turbine installations
15.4.8
Thermal and catalytic crackers. Vacuum distillation. Sulphur plants. Steam heating systems. Power generation. Shipbuilding.
The Steam and Condensate Loop
Block 15 Desuperheating
Typical Desuperheater Installations Module 15.4
Questions 1. Which of the following are advantages of using hot, instead of cold water as the cooling fluid in a desuperheater? i.
Water particles spend less time suspended in the steam.
ii.
Less water impinges on the inside walls of downstream pipework.
iii. It reduces the absorption length as the water is evaporated more quickly. a| i only
¨
b| ii only
¨
c| i and ii
¨
d| i, ii and iii
¨
2. Why is tap water not an ideal source for cooling water for most types of desuperheater? a| It is generally available at low pressures
¨
b| It is at room temperature
¨
c| High TDS levels can result in solids falling out of suspension
¨
d| All of the above
¨
3. Why do most desuperheaters not reduce the temperature of the superheated steam to saturation temperature? i.
The decrease in temperature difference between the cooling water and the steam makes this difficult.
ii.
The sensitivity of the cooling water flowrate controller will cause flooding of the pipework at temperatures close to saturation.
iii. Producing steam at saturation temperature is not beneficial. a| i only
¨
b| ii only
¨
c| i and ii
¨
d| i, ii and iii
¨
4. Why is it important not to over specify the turndown ratio of a desuperheater? a| It will increase the cost of the desuperheater station
¨
b| It will increase the risk of the downstream pipework flooding
¨
c| It will increase the amount of cooling water required for a given superheated steam flowrate ¨ d| The statement is false and all desuperheaters should be specified with a significantly larger turndown to allow for expansion
The Steam and Condensate Loop
¨
15.4.9
Block 15 Desuperheating
Typical Desuperheater Installations Module 15.4
5. Using the selection chart (figure 15.4.4), which type of desuperheater would be most suitable for use in a sugar refinery for the following conditions? The site already has a boiler producing 10 bar g saturated steam. Coolant is available at 14 bar g. Maximum superheated steam flow 16 000 kg / h Minimum superheated steam flow 2 000 kg / h Final temperature TS + 7°C a | Tube bundle type
¨
b | Multiple nozzle, axial injection type
¨
c | Steam atomising type
¨
d | Variable orifice type
¨
6. Which of the following considerations have to be made when selecting a type of desuperheater? a | Turndown ratio
¨
b | The required final temperature
¨
c | The availability of cooling water
¨
d | All of the above
¨
Answers 1: d, 2: d, 3: c, 4: a, 5: c, 6: d
15.4.10
The Steam and Condensate Loop
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