Steam Bypass

II500.00/7en CCI

Installation Guidelines Steam Conditioning Valves


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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

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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


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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.



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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.


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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.



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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


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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.



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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


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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



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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).


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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



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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.


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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.



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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


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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



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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


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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



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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.


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Fig. 16 Drain in downstream piping Drip leg (L)

Fig. 17 Valve in low installation

Min 1 x /O



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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.


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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



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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


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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.



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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).


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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.



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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.

Documents

Steam Bypass