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II500.00/7en CCI
Installation Guidelines Steam Conditioning Valves
Installation Guidelines Steam Conditioning Valves
II500.00/7en
1
Table of contents
1 General ....................................................................................................................... 3 1.1 Inlet pipe recommendations ................................................................................. 4 1.2 Straight pipe-run upstream recommendations ..................................................... 6 1.3 Distance to first bend ........................................................................................... 7
1.3.1 Distance to first bend for special cases ........................................................ 7 1.3.2 Dump to condenser with wet steam before dump device ............................. 8
1.4 Distance to the temperature sensor ..................................................................... 9 1.4.1 Minimum distance to temperature sensor ..................................................... 9 1.4.2 Distance to temperature sensor for special cases ........................................ 9 1.4.3 Minimum degree of superheat .................................................................... 11
1.5 Distance to flow dividers .................................................................................... 12 1.6 Distance to pressure sensor .............................................................................. 12 1.7 Downstream piping material .............................................................................. 12 1.8 Additional comments ......................................................................................... 13 1.9 Diffusers and plates ........................................................................................... 13 1.10 Drains / Vent to atmosphere .............................................................................. 13 1.11 Pipe pre-warming .............................................................................................. 16 1.12 Water valve location .......................................................................................... 22 1.13 Water piping connection to desuperheater connection ...................................... 22 1.14 Control system ................................................................................................... 23 1.15 Pressure control ................................................................................................ 24 1.16 Temperature control .......................................................................................... 24 1.17 Special for feed forward with dump to condenser .............................................. 29 1.18 Actuation ............................................................................................................ 29 1.19 Preheating arrangement of upstream piping ...................................................... 30
2 Fix points and supports ............................................................................................. 35 3 Accessibility .............................................................................................................. 38
Steam Conditioning Valves
Installation Guidelines
2 II500.00/7en
List of figures
Fig. 1 Pipe elbow upstream the valve ............................................................................ 4 Fig. 2 XYZ installation with T-piece ................................................................................ 4 Fig. 3 XY installation with T-piece .................................................................................. 5 Fig. 4 Installation with stop and control valve welded together ...................................... 6 Fig. 5 Straight pipe runs up- and downstream the valve ................................................ 6 Fig. 6a Downstream temperature vs. time in a dump to condenser or a similar
process application .............................................................................................. 8 Fig. 6b Downstream temperature vs. time in an HP to cold reheat or similar
process application .............................................................................................. 9 Fig. 7 Downstream temperature vs. time in a dump to condenser or similar
process application ............................................................................................ 11 Fig. 8 Protective shield ................................................................................................. 12 Fig. 9 Horizontal inlet/vertical outlet ............................................................................. 14 Fig. 10 Drain/preheat system ......................................................................................... 15 Fig. 11 Drainage system (D) .......................................................................................... 16 Fig. 12 Vertical inlet/horizontal outlet ............................................................................. 17 Fig. 13 Horizontal inlet/outlet ......................................................................................... 17 Fig. 14 Vertical inlet/horizontal outlet ............................................................................. 17 Fig. 15 Horizontal inlet, outlet upwards and actuator downwards .................................. 18 Fig. 16 Drain in downstream piping Drip leg (L) .......................................................... 19 Fig. 17 Valve in low installation ...................................................................................... 19 Fig. 18 Installation with bend welded direct to outlet ...................................................... 21 Fig. 19 Expansion welded direct to outlet or closer than 0.1 s x Vmax ............................ 21 Fig. 20 Installation with an expansion cone in the outlet ................................................ 22 Fig. 21 Feed forward control example. ........................................................................ 26 Fig. 22 Feedback control example .............................................................................. 27 Fig. 23 Feed forward when a dump device cannot be used as a flow meter.
Algorithm based on valve position with compensation for variations in inlet pressure and temperature. Pin and Tin are also used in the heat balance ......... 27
Fig. 24 Bypass to condenser. Recommended installation ............................................. 28 Fig. 25 When the valve is very large, the below installation helps simplifying
maintenance ...................................................................................................... 28 Fig. 26 Preheating arrangement utilizing the natural pressure drop in the steam pipe .. 31 Fig. 27 Preheating arrangement bypassing the control valve ........................................ 32 Fig. 28 Preheating arrangement utilizing a higher pressure level than the valve
inlet pressure .................................................................................................... 34 Fig. 29 Vertical installation ............................................................................................. 35 Fig. 30 Horizontal installation ......................................................................................... 36
Installation Guidelines Steam Conditioning Valves
II500.00/7en
3
System Design Considerations
1 General The steam conditioning valve is an important part of the system. However, also other parts and
parameters in the system can have a significant impact on the performance of the steam
conditioning valve. In the following we have described those factors and also given some
general guidelines on how you can achieve a proper system design.
Contents:
Inlet pipe recommendations
Straight pipe-run upstream recommendations
Distance to the first bend
Distance to the temperature sensor
Distance to flow dividers
Distance to pressure sensor
Downstream piping material
Additional comments
Drains
Control system
Pressure control
Temperature control
Preheating arrangement of upstream piping
Fix points and supports
Accessibility
First rule
All steam valves are designed for dry steam and exposing valves for wet steam or condensate will damage the valve and this is not covered by any warranties. Vent valves and drains are to take care of pipe pre-warming until those conditions are fulfilled.
Steam Conditioning Valves
Installation Guidelines
4 II500.00/7en
1.1 Inlet pipe recommendations
Steam conditioning valves require straight pipe-runs both upstream and downstream to
provide good performance.
The reason for having a straight pipe-run upstream is that a pipe bend (elbow) fig 1
creates a flow pattern that is non-uniform. Especially two or more pipe bends in 3
dimensions (x, y, z) fig 2 just before the steam conditioning valve is known to cause
very unstable flow, resulting in vibrations and other flow induced problems. We
normally use an inlet strainer that minimizes the risk but the risk must be considered.
L1 Elbow
Fig. 1 Pipe elbow upstream the valve
The orientation of the closest pipe bend vs. the valve orientation is also very important.
An installation such as in figure 1 (two dimensional x-y) is far better than an installation
like the one shown in figure 2.
Fig. 2 XYZ installation with T-piece
An installation like the one shown in figure 2 is known to create rotational forces in the
valve plug. If a valve for some reason must have a pipe bend oriented as in figure 2,
please inform CCI about this before the valve design specification is made.
Installation Guidelines Steam Conditioning Valves
II500.00/7en
5
A T-piece as shown in figure 3 is also known to cause vibration and other problems, and
should always be avoided. Upstream S-pipe bends should also be avoided.
Fig. 3 XY installation with T-piece
If a T-piece is used, a straight pipe length of at least 20 x pipe should be used before
the valve.
WARNING
Undersized stop valves with reduced bore upstream a bypass valve are
known to cause noise and vibrations due to high vena contracta
velocities and ununiform velocity distribution.
Undersized stop valves also give the bypass valve a nonlinear characteristic due to the
strong influence of the pressure drop on the flow through the stop valve.
Stop valves of this type must be installed as far upstream as is required to provide a
uniform flow pattern. It is the responsibility of the supplier of the stop valve to provide
information about necessary distance.
A stop valve of angle type and a control valve, specially designed, can be assembled as
one unit and be welded together without any distance piece. This is quite common for
dump to condenser applications.
Steam Conditioning Valves
Installation Guidelines
6 II500.00/7en
Fig. 4 Installation with stop and control valve welded together
1.2 Straight pipe-run upstream recommendations
Fig. 5 Straight pipe runs up- and downstream the valve
CCI recommends the upstream straight pipe to be
< 200 mm / 8" 1 m /3.28 ft. or longer
200 - 400 mm / 8 - 16" min 5 x
> 400 mm (>16") min 3 x
If R 5 x of the inlet, the elbow can be fitted directly to the valve inlet stud.
If multiple pipe bends are located upstream, the straight pipe length before the valve
must be increased, and that distance must be estimated for each case.
L1
RL3
L2
L4
PI
PT
TE
Installation Guidelines Steam Conditioning Valves
II500.00/7en
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The upstream instrumentation should always be a temperature sensor to make sure the
steam is dry before the valve opens. In case any kind of algorithm is used for the
control, a pressure sensor is required.
1.3 Distance to first bend
After the water injection, it will take a short while for most of the water drops to evaporate. To
avoid problems with free water hitting the pipe wall, causing erosion and free water following
the pipe wall, it is necessary to have a minimum downstream distance before the first pipe
bend.
To minimize this problem, the distance before the first pipe bend should be a minimum of 0.1 s
x maximum velocity in the pipe for all valves except VST-SE, for which we recommend a
minimum distance of 0.05 s x maximum steam velocity, before the first pipe bend. This
because of the internal principle which means proportioning of water into the steam flow.
1.3.1 Distance to first bend for special cases
1. High pressure by-pass to cold reheat or equal process application
The distance to the first bend can be reduced to 0.067 s x maximum
velocity if the following conditions are fulfilled:
Downstream pressure 15-60 bar / 217 870 psi (lower value normally only occurs during sliding pressure mode or start-up).
Degree of superheat 100C or higher.
Water temperature 140C / 284F or higher.
2. Hot reheat to condenser, HP to condenser or equal process application
The distance to the first bend should in this application be increased to 0.12 s x
maximum velocity if the following conditions are fulfilled:
Typically 3-6 bar / 43-87 psi outlet pressure before dump device at full load.
Degree of superheat 30C or lower
Water temperature 60C / 140 F or less
Water to steam ratio > 0.25
Feed forward is always recommended for this type of application. See the separate
document Dumping into Condenser.
The figures on the following page show the downstream temperature as a function of
time.
Steam Conditioning Valves
Installation Guidelines
8 II500.00/7en
1.3.2 Dump to condenser with wet steam before dump device
When the steam/water mixture has an enthalpy below saturation, bends are not allowed
at all before the dump device since this leads to separation of steam and water resulting
in risks for noise and vibration in downstream piping and worst of all blocking of holes
in the dump device with free water that can cause damages to dump device and inside
condenser.
For dump to condenser 0.05 s x Maximum velocity is the recommended distance from
water injection to condenser wall. The dump device drilling starts normally directly on
the inside of condenser wall.
If the installation does not allow for the required straight distance, both the dump device
and condenser may be damaged.
Fig. 6a Downstream temperature vs. time in a dump to condenser or a similar process application
This diagram shows the typical temperature downstream of water injection of a steam
conditioning valve in a dump to condenser application as a function of time when steam is
minimum 10C superheated.
0 0,05 0,10 0,15 0,20 0,25 0,30 0,35 [s]
500
C[ ]
400
300
200
160
4 bar143,6
Installation Guidelines Steam Conditioning Valves
II500.00/7en
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Fig. 6b Downstream temperature vs. time in an HP to cold reheat or similar process application
This diagram shows the typical temperature downstream of water injection of a steam
conditioning valve in an HP to cold reheat application as a function of time.
1.4 Distance to the temperature sensor
The recommended distance before the temperature sensor is 0.2 s x maximum steam
velocity for a ratio 15% of spray water / steam flow and 0.3 s x maximum steam
velocity for a ratio >15% spray water / steam flow for all PRDS valves except for VST-
SE. This valve has an integrated steam atomization and water proportional to steam flow
can be installed at a distance of 0.2 s x maximum steam velocity.
The values are based on a set-point of approx. 10C / 18F above saturation for steam
and 90C / 194F for water and a steam pressure bar / 28 psi.
Lower degree of superheat gives a longer distance and higher degree of superheat gives
a shorter distance.
Exact time to sensor is normally finalized when all parameters are known, but the
general rules shall normally be followed.
A higher water temperature reduces the evaporation time and a lower water temperature
increases the required minimum distance to the temperature sensor. For dump to
condenser, see Dump tube philosophy, paper no. P1010.04en.
1.4.1 Minimum distance to temperature sensor
In valves with low outlet velocity ( m/s, 100 ft/s), the required distance calculated
as time can be longer than usual and minimum distance to the temperature sensor should
therefore never be shorter than 12 m / 39 ft) for process applications.
1.4.2 Distance to temperature sensor for special cases
1. High pressure by-pass to cold reheat or equal process application
0 0,02 0,04 0,06 0,08 0,10 0,12 [s]
500
C[ ]
400
250,3
420
440
460
480
380
360
340
Steam Conditioning Valves
Installation Guidelines
10 II500.00/7en
The distance to the temperature sensor can be reduced to 0.15 s x maximum velocity if
the following conditions are fulfilled:
Downstream pressure 15-60 bar / 217 870 psi (lower value normally only occurs during sliding pressure mode or start-up).
Degree of superheat 100C / 212F or higher.
Water temperature 140C / 284F or higher.
2. Hot reheat to condenser, HP to condenser or equal process application
The distance to the temperature sensor should in this application be minimum 0.3 s x
maximum velocity.
Typically 3-6 bar / 43-87 psi outlet pressure before dump device at full load
Degree of superheat 30C / 86F or lower
Water temperature 60C / 140F or less
Water to steam ratio > 0.25
Feed forward is always recommended for this type of application. See the separate
document Dumping into Condenser (from the Applications Handbook).
Installation Guidelines Steam Conditioning Valves
II500.00/7en
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1.4.3 Minimum degree of superheat
Fig. 7 Downstream temperature vs. time in a dump to condenser or similar process application
As can be seen from the diagram above, the temperature will decrease very quickly
when water is injected and will then decrease slower and slower.
The reason is that the droplets after a very short time after they have been injected into
the flow will be exactly at saturation temperature. The heat transfer mechanism is heat
transfer from the surrounding steam to the droplets with temperature differences as the
driving force. The closer to saturation temperature the slower the process, thus allowing
more time for the droplets to fall out.
This typically begins to be a problem when the degree of superheat is 20-30 and
become quite difficult at 10 or lower.
Another problem associated with this is the effect of one droplet or more hitting the
sensor. If one (or more) droplet hitting the sensor will cause a misreading, which is
unpredictable and even not possible to calculate.
During transient when the temperature swings around the set-point, this problem can
increase beyond stability and the system will not be possible to control with severe
water fall outs and temperature swings as a result.
0 0,05 0,10 0,15 0,20 0,25 0,30 0,35 [s]
500
C[ ]
400
300
200
160
4 bar143,6
Steam Conditioning Valves
Installation Guidelines
12 II500.00/7en
Rules
A protective shield should be used for 10 superheat or less.
Fig. 8 Protective shield
To avoid misreading, a protective pocket must be used for 5-7C / 9-11F superheat to
minimize the effect of water hitting the sensor.
Below 5 superheat is not acceptable and no warranties will be given. Exceptions are,
however, possible in certain cases. Therefore please contact the factory. Problems like
this are always minimized with a valve like VST-SE with built in mechanical water
proportioning. For less than 7C / 11F special control and extra instrumentation often
must be used and the factory must always be contacted in such cases.
For dump to condenser applications where cold water, typically less than 50C /122F is
used for desuperheating, the degree of superheat should be at least 20C / 68F since
evaporation time otherwise can be very long. CCI strongly recommend you to avoid
feedback control for dump to condenser applications due to big risks for thermal fatigue
damages related to difficulties in control. CCI always recommend feed forward control
for dump to condenser applications.
1.5 Distance to flow dividers
The outlet flow from a valve must never be divided by a T-piece, Y-piece or any other
configuration before the outlet temperature can be properly controlled.
1.6 Distance to pressure sensor
To receive an acceptable and stable signal, it is necessary to have a relatively uniform
velocity gradient. The distance L4 should be at least 5 x outlet .
1.7 Downstream piping material
We recommend 5 m / 16 ft. of downstream piping in low alloy material when the steam
temperature before cooling is > 425C / 800F. The reason is that the evaporation is not
instant after water injection.
Installation Guidelines Steam Conditioning Valves
II500.00/7en
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1.8 Additional comments
For a well performing temperature loop it is also very important to consider
Response time for the temperature sensor during temperature gradient (T66).
Response and sampling time for the controller / DCS.
Response and sampling time for the actuator.
Resolution and control characteristic of the spray water control valve.
Pipe size; large pipes (approx. diameter 0.8 m/ 32) downstream of the valve, particularly with low velocity, very frequently have a non-uniform temperature
distribution and should therefore have three temperature sensors installed
perpendicular to the pipe.
Velocity at minimum load. If the minimum steam velocity at the water injection
point is below 8 (12 bar) 12 (4 bar) m / s (26-40 ft / s) depending on pipe
size, steam-assisted steam conditioning valves (Steam Jet, VST-SE, VLB-SE)
should be used, unless a dump device is used downstream (see Dump Tube
Philosophy, P1010.04en).
Protective pockets, quick response type should always be used.
Stop valves and particularly drain studs upstream of the valve have proven to be a cause of high noise and vibrations.
Drains must always be sized to handle maximum fallout caused by transients, heat losses or minimum 3-5% of maximum water injection.
Temperature sensors in vertical pipes should always be avoided if technically possible due to the risk of water hitting pipe wall which may affect the
measurement correctness.
1.9 Diffusers and plates
Diffusers and plates can for certain applications be used downstream of the water
injection, but with limited service life. Always consult the factory for this type of
applications that always require feed forward control without exception.
1.10 Drains / Vent to atmosphere
It is essential to keep free water out of the steam system. The main sources for free
water are:
Condensate
Spray water that has not evaporated in the steam system.
Vent / drain system upstream of the valve is undersized and cannot handle condensate from the pipe warming when the plant is started from cold. This is
the most common reason for damage of valves.
Note! Free water in the steam system causes noise, mechanical damage and makes temperature measurement difficult.
Steam Conditioning Valves
Installation Guidelines
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The steam conditioning valve performs an important function in the steam system. It is
therefore essential to protect the valve from damage that might occur if water enters the
valve. It is equally important to protect the downstream system from damage caused by
a malfunctioning temperature control system (see separate section). It is, therefore, nec-
essary to have drains both upstream and downstream of the valve.
Maximum condensate normally occurs during cold start-up. This must be considered
already at the design stage for sizing of the drainage system. Condensate volume that
must be removed during start up should of course be calculated each time based on real
data but as an estimation, a mass of typically 7-14 % of the upstream piping weight
need to be removed through the condensate system. There should always be a
temperature sensor to confirm that the upstream piping is dry before the valve is opened
to prevent damages.
The following shows examples of drain arrangements for different valve
positions, etc.
Note! In case of a valve position according to fig. 10 and 11, the position of the drain connection must be exactly defined.
Fig. 9 Horizontal inlet/vertical outlet
D
Installation Guidelines Steam Conditioning Valves
II500.00/7en
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Flash tank
To hotwell/condensate recovery system
From DCS on/off From DCS on/off
CCI
Customer
L1
L2 LIC
Y On when contaminated condensate Drain
On when clean condensate
Drain of level control type
From DCS on/off
Preheat flow
To handle start up condensate removal
Manual or on/off from DCS
Fig. 10 Drain/preheat system
Steam Conditioning Valves
Installation Guidelines
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Fig. 11 Drainage system (D)
The main reason for installing a bypass to drain is normally to give extra capacity during start
up to get rid of the large volume of condensate formed during start-up with cold piping.
Specially combined cycle plants are difficult since the degree of superheat from HRSG is very
low at low load of GT. That results in steam reaching saturation after a short travel through the
cold piping.
A recommended location of the drain is 2/3 of the distance to the temperature sensor.
Level control type drains are recommended for pressure above 20 bar / 290 psi.
Displacement type condensate pumps are recommended for low pressure < 3 bar / 43 psi
process lines.
1.11 Pipe pre-warming
Modern fully machined forged symmetrical valves do not normally need pre-warming but
when the valve is in standby mode, the upstream piping must be preheated to avoid condensate
formation. This small flow, typically 50-200 kg / h / 134-535 lbs/h, should be piped to a steam
consumer as dearator or similar. The connection point can be the same as a drain stud at the
valve body if the valve is installed in a low position and should be in high position point if the
valve is installed higher than the piping. Without this small flow, it is also very difficult to
confirm that steam always is superheated in the valve inlet before the valve is opened.
Manual/motorized by-pass for start-up
Steam trap
Installation Guidelines Steam Conditioning Valves
II500.00/7en
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The inlet steam pipe shall have a drain connection at a low point close to the valve
Fig. 12 Vertical inlet/horizontal outlet
Fig. 13 Horizontal inlet/outlet
Valves shall have a drain stud at the bottom of the valve body
Fig. 14 Vertical inlet/horizontal outlet
D
D
D
Steam Conditioning Valves
Installation Guidelines
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Fig. 15 Horizontal inlet, outlet upwards and actuator downwards
Note: Generally not preferred but possible in certain applications
please contact CCI.
Installation Guidelines Steam Conditioning Valves
II500.00/7en
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Fig. 16 Drain in downstream piping Drip leg (L)
Fig. 17 Valve in low installation
Min 1 x /O
Steam Conditioning Valves
Installation Guidelines
20 II500.00/7en
Locate the drain (D) on the downstream side, at the lowest point after the valve. Weld
a drip leg (L) to the pipe, and connect the drain to the bottom of the drip leg.
The drip leg shall have a diameter of 0.5 x pipe-diameter. The depth of the drip leg shall
be between 300 and 600 mm / 1-2 ft.
If possible, always avoid an installation where the valve outlet is at a low point. If the
drain is undersized or not working, water can accumulate and cause many serious
problems.
WARNING
CCI is only responsible for problems related to the equipment included
in the CCI scope of supply.
The weight of the accumulated water can seriously damage piping and supports.
Free water at the bottom of the piping can cause very serious vibrations and pressure transients in the piping.
Downstream temperature sensor can be hit by non-evaporated water causing severe control problems.
Minimum slope to drain at a low point should never be less than 100:1.
Water hammers.
A simple closed temperature control loop is in most cases too slow for this type of installation. A feed forward system is always required to minimize the effect
of transients recommended for this installation.
Transient analysis is necessary and must always be performed before designing the control system.
Installation Guidelines Steam Conditioning Valves
II500.00/7en
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WARNING
This type of installation with bend direct to outlet is very
dangerous and is therefore always forbidden.
Fig. 18 Installation with bend welded direct to outlet
WARNING
This type of installation will in most cases not work or cause
severe water fall out
Fig. 19 Expansion welded direct to outlet or closer than 0.1 s x Vmax
Expansion welded direct to outlet or closer than 0.1 s x Vmax
Steam Conditioning Valves
Installation Guidelines
22 II500.00/7en
Exception
In certain cases, the design below can be used after written confirmation from the
factory.
Fig. 20 Installation with an expansion cone in the outlet
1.12 Water valve location
Water valves are always to be installed below the lowest point of the spray connection and
recommended distance is 4-6 m between water valve and inlet of the connection point is on the
bypass valve.
Any pressure drop between water valve outlet and connection point on the bypass valve must
always be specified in order to include this pressure drop when sizing.
WARNING
Not providing correct information may lead to capacity / control problems for which CCI cannot take the responsibility.
1.13 Water piping connection to desuperheater connection
The water pipe routing (isometric) must be done in such a way that the forces originating from
thermal expansion are not transferred to the connection point. Note! Deviations from this point must be clearly stated before ordering, as they can be critical for the design of the product
Installation Guidelines Steam Conditioning Valves
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1.14 Control system
Steam conditioning valves combine two functions:
Modulated pressure control
Steam desuperheating within a limited space
The reason for having this combination of functions is that the conditions for accurate
temperature control in a piping system with geometrical limitations by far is superior to
the use of a separate pressure reducing station and a desuperheater.
When designing a control system it is important to understand that the steam
conditioning valve and the spray water control valve are the tools that execute the
actual pressure reduction and the water injection for the temperature reduction.
The pressure and temperature controllers give the input to the valve on how much to
open or close. The valves only do what they are told to do, i.e. the pressure and
temperature control loops must operate correctly in order to help the valves achieve
desired pressure and temperature reductions.
Balance in all systems is always advisable when designing control systems.
It is always advisable to use a feed forward system to position the water valve to a
predicted correct position during a transient to minimize deviations in the water flow.
Excessive over or under spraying can cause severe problems of thermal cycling, impact
damage, erosion and unstable actuator downstream of the valve.
Note! Always open the steam valve slightly before the water valve and close the water valve slightly before the steam valve. To minimize the risks, it should be hardwired to the system so that spray water valve should not be able to open before the steam valve. When the equipment reaches minimum specified flow, the steam valve and the spray water valve shall close simultaneously and quickly.
When steam atomized nozzles are used in combination with spring-loaded variable area
nozzles two separate water valves must be used. For steam atomized nozzles a separate
on/off valve is used to supply atomizing steam that must be interlocked to open only
after the steam valve has opened. Water valve associated with steam atomized
desuperheating system must open after atomizing steam valve.
Remember that water hammer can destroy all kinds of piping and valves and opening or
closing should therefore never be faster than what the process requires. A spray water
valve may never open quicker than a bypass valve.
Inexactness is normally a by-product of too short actuating times and incorrect PID
settings.
Steam Conditioning Valves
Installation Guidelines
24 II500.00/7en
WARNING
Systems with low degree of superheat, especially at low pressure,
do often require enthalpy-based feed forward control to be able to
operate correctly.
Recommended distance to temperature sensors is based on a well tuned PID loop.
The instrumentation is often the weakest point for exact control and must also be
discussed with the factory for applications with large pipes, low pressure and high
rangeability.
Note! Distance to a temperature sensor must be enough to allow not only for complete evaporation at steady state conditions, but also for transients when a steam valve opens or closes quickly and therefore causes a quick change in required water injection.
1.15 Pressure control
The pressure control system is normally quite uncomplicated and causes very few
problems. Pressure control is always a closed loop system with feedback.
To be considered
Valve stroke time.
System response time
Start-up condition
Stability, especially in the actuator
Boosters because they can create instability
Response on small signal variations
1.16 Temperature control
A successful temperature control system is a result of several considerations:
A closed loop control system shall be used, only when the downstream temperature can be accurately measured and used for feedback.
Degree of superheat. The higher degree of superheat, the easier the control.
A feed forward control system shall be used when accurate temperature measurements cannot be made. Feed forward systems require more exact
instruments and also correct flow measurement over the full range especially at
the water side.
Note! With feed forward control system we here mean the control system for the spray water control. It shall not be mixed up with our description of feed forward control in other CCI literature, e.g. for steam conditioning valve type VST (mechanical link between steam and water flow).
Installation Guidelines Steam Conditioning Valves
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Temperature measurements must only be made at a location where all injected water has evaporated, and the steam is absolutely dry.
The temperature at the measuring point shall be at least 5 C / 9 F above saturation temperature to guarantee that the steam is dry. This can be
achieved under steady state conditions with very careful instrumentation,
installation and control and must be discussed prior to order.
Turndown requirement and capability of steam conditioning and spray water valves.
Available spray water pressure and temperature.
The piping arrangement must allow for required straight run and distance to the temperature sensor.
Piping arrangement must allow for proper draining in case of malfunction of the temperature control system.
An interlock shall be used, and the spray water valve will therefore automatically close if and when the steam conditioning valve closes.
If possible avoid a separate pressure reducing valve upstream of the spray water valve. If you must use it take great care in designing the control system
and consider the response times in different control loops. This often leads to
pressure transients far above specified inlet pressure, resulting in increased
maintenance and wear of the last spray water valve. This is a design that always
should be avoided if possible, since there today are excellent water valves that
can take the full pressure drop.
Prevent particles in the water from damaging the valve or nozzles by installing a strainer upstream of the spray water control valve in the spray water supply line.
Spray water valves must always be Class V tight to prevent water from being collected in the system, thus causing others problems.
Note! Max acceptable particle size is 100 - 200 microns, depending on type of spray water nozzle used.
Steam Conditioning Valves
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26 II500.00/7en
To achieve good performance the control loop itself must also be analyzed to find out, within
which exactness the water flow can be controlled by the complete control loop. The most
important parameters are the following:
Sensor response time
Controller response time and exactness
Positioner exactness
Valve exactness
Valve characteristic
Valve position exactness
Dead band
Maximum transients
Actuator exactness and stiffness
This analysis also provides the answer regarding sizing of the drainage system
downstream of a desuperheater or steam conditioning valve.
Fig. 21 Feed forward control example.
The algorithm is based on the dump device used as a flow meter and a heat
balance using inlet pressure and temperature
PT
TT
Steam
Spray water
PT DCS
FT
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Fig. 22 Feedback control example
Fig. 23 Feed forward when a dump device cannot be used as a flow meter. Algorithm based on valve position with compensation for variations in inlet pressure and temperature. Pin and T in are also used in the heat balance
PT
TT
Steam
Spray water
DCS
FT
PS
DCS or PLC with heat balancing ability
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Fig. 24 Bypass to condenser. Recommended installation
Fig. 25 When the valve is very large, the below installation helps simplifying maintenance
Steam Isolation Valve(Quick Closing)
Dump Tube Condenser
Desuperheater
Dump Tube Condenser
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1.17 Special for feed forward with dump to condenser
Note! Bends downstream can only be used if the enthalpy of the mixture is at least 10C / 18F above saturation in the inlet of the dump device since all bends create separation between steam and water.
Feed forward control is superior to temperature control since all transients that may damage the
pipe and condenser are avoided. Damages to valve outlet, piping, dump device, condenser, etc.
are very rare. Every year there are damages reported where the temperature control is used
especially when there is a long distance (more than 20 m / 65 ft) between water injection to
condenser.
A recommended set point for this application is to have approx 20C / 36F superheat after the
dump tube to avoid the risk of having free water after the dump tube. An enthalpy of 2650
Kj/kg is normally recommended for most applications.
For this application a water flow meter must be used in the water line for each water valve in
case of more than one water valve. There should also be a pressure sensor upstream and
downstream of the water valve to allow for good estimation of the water flow below the
effective range of the water flow meters. Those pressure sensors can also be used for back up
of the flow meter if it fails and as preventive maintenance measurement since deviations from
calculated values indicate the true cause of the problem before consequences as damages etc.
can occur.
1.18 Actuation
Actuator type is very important for the performance, especially for a control which requires
stable temperature close to saturation since wet steam hitting the temperature sensor during
transients are heavily amplified if the actuator makes any overshoot.
Electrical, hydraulic or double-acting piston actuators with a high performance smart positioner
must be used.
To avoid problems the actuator must be slower than the steam valve as stability is much more
important than speed.
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1.19 Preheating arrangement of upstream piping
Upstream piping must be done in such a way that pockets of condensate always are
drained away.
Note! It is absolutely necessary in all conditions to have superheated steam in the inlet of the downstream valve.
Note! Additional preheating of CCI valves is not required.
Arrange for preheating, in those cases when the control valve normally is closed under
operation e.g. in a turbine bypass application.
Preheating of the piping upstream of the valve is not necessary when the valve is located
above the live steam line and the pipe slopes down to the main steam line.
When the valve has to be installed lower than the live steam header there are a number
of solutions for the preheating arrangement. The main objectives, when selecting the
arrangements are to:
Create an effective system so that the piping will be sufficiently preheated and drained.
Minimize the energy losses due to preheating steam.
Figures 18 20 show a few arrangements that can be used. The piping layout will
determine the preheating arrangement that will be the most effective for a specific
application.
Note! CCI can give advice on the best solutions for different types of plants; such as conventional reheaters, combined cycle, cogeneration plants and others.
Typical pipe dimensions for the preheating line is in the range of 25 to 50 mm / 1 to 2
inches. The preheating line must be equipped with an isolation valve, which also can be
used for manual flow control of the steam flow for preheating.
The preheating sizing is based on:
- Length of pipe between valve and main line
- t between surface temperature outside the insulation and the environment.
- Indoor or outdoor installation
A heat balance based on this information will provide the necessary preheating flow.
Additional preheating of CCI valves is not required.
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Fig. 26 Preheating arrangement utilizing the natural pressure drop in the steam pipe
The method above is the most energy effective, but it also requires a suitable system
design.
When the steam flows from (1) to (2) there will be a pressure drop p in the line. With
proper sizing of the preheater piping, (1) to (3) it is possible to have a sufficiently large
flow to keep the piping (3) to (2) free of water. Warning! Required pressure drop is
often practically difficult to achieve and requires often both a big pre-warming
pipe and restriction in the main line. Reasonable pressure drop is 0.2-0.5 bar.
Preheating flow
Preheating line
Steam condition valve
Bypass line
Main steam line1 2
3
A
B
Bypass line
Preheating flow
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Fig. 27a/2/3 Preheating arrangement bypassing the control valve
Fig. 27b Preheating arrangement bypassing the control valve
A
B
Bypass line
Preheating flow
TIC TT
PTTo DCS
Set fromDCS
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Bypass
line
Preheating flow
TIC TT
PTTo DCS
Set fromDCS
Min 0,5 m
B
A
Fig. 27c Preheating arrangement bypassing the control valve
The method shown in fig. 27 is the easiest and most common way of preheating upstream
piping (valve). Here you simply connect the inlet with the outlet and use a restriction to limit
the steam flow.
The steam flow used for preheating shall be moderated according to required pipe / valve
temperature, to reduce energy loss. In most cases normal preheating flow is 50-200 kg/h /
134-535 lb/h.
This preheating with connecting inlet to outlet can cause very high temperature downstream
and special springs made of Nimonic or similar must be used. The factory must also be
informed prior to manufacturing if this method is used. Normally the downstream connecting
point must be used after the valve to prevent hotspots that may cause deformation of the valve
outlet.
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Fig. 28 Preheating arrangement utilizing a higher pressure level than the valve inlet pressure
The solution shown in above figure is energy effective but can sometimes require a long
preheating line.
Preheating flow
Preheating line
Bypass line
Main steam line
Hot reheat line
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2 Fix points and supports The piping system must be so designed that the valve(s) will not be used as a fix point.
Supports are often required for auxiliary equipment and valves, and must be properly
designed. See figs. below for examples.
Fig. 29 Vertical installation
SteamInlet
DumpTube
Duct
Cranefor l ifting
Nozzle
L1
L 2
Platform
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Inlet pipe from above Inlet pipe from below
Inlet pipe from below Inlet pipe from above
Fig. 30 Horizontal installation
Spring hanger
Spring hanger
Vibrationabsorber
Slidingsupport
Slidingsupport
Vibrationabsorber
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2.1 Actuators with springs must always supported due to the weight to avoid excessive forces on yoke and mounting details. Especially when the actuator is mounted
horizontally this must be considered and spring hangers are always recommended.
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3 Accessibility
Space must be provided for service and maintenance of the valve. If the valve is
installed in a pipe rack or any other location which is difficult to reach, you need to
provide a platform around the valve as well as a safe route to it. The platform shall be
sufficiently large to accept a minimum of two persons and temporary storage of valve
internals.
Note! Consider also transportation of heavy spare parts.
A bracket or other arrangement for a lifting device shall also be available. The capacity
shall be at least 5 tonnes.
To facilitate maintenance within scheduled time, lifting equipment as well as a working
platform must be provided.
Planning for future maintenance is a very good investment and should always be taken
into consideration.
CCI reserves the right to make technical improvements.