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CHP – A Guide to Steam Conditioning
Throughout the world, companies rely on CCI to solve their severe service control valve problems. CCI has provided custom solutions for these and other industry applications for more than 80 years.
CCI World Headquarters— CaliforniaTelephone: (949) 858-1877Fax: (949) 858-187822591 Avenida EmpresaRancho Santa Margarita,California 92688 USA
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2 15Overview of CHP Desuperheater Technology
CHP (Combined Heat and Power) is an Efficient Technology for Generating Electricity and Heat Together
A CHP plant is an installation where there is simultaneous generation of
usable heat (usually steam and sometimes hot water) and power (usually
electricity) in a single process. CHP is sometimes referred to as cogeneration ,
energy centres and total energy. The basic elements of a CHP plant comprise
one or more prime movers usually driving electrical generators, where the
heat generated in the process is utilized via suitable heat recovery equipment
for a variety of purposes including: industrial processes, district heating and
space heating. Figure 1 shows a possible configuration for a CHP plant. For the
purposes of this document we will cover Large Industrial Users.
The heat source can be established from many different sources. Waste heat
from process (e.g. ethylene, ammonia plants), incineration of waste, and waste
heat from gas turbine (also electricity generator) by a heat recovery steam
generator (HRSG) and from fossil fired boilers.
Once the industry has established its need for heat, it then has to determine
if the investment for power generation is economically viable. A study of the
economical benefits typically includes:
• Cost of the added investment
• Cost of added maintenance and man power
• Economical benefits to secure supply of power in case of external supply
failure. (key benefit)
• Cost of produced power compared with purchased power
Figure 1: Typical simple CHP scheme with gas turbine, heat recovery steam
generator (HRSG) and steam turbine
Figure 26:DA-O variable area nozzle desuperheater
Application: Desuperheating for Extraction and Exhaust Steam
As mentioned earlier control of the extraction and exhaust can be difficult
owing to the following.
Figure 25: Multi-nozzle DAM desuperheater
• Low velocity/ water fall out
• Insufficient cross sectional coverage
• Large piping diameters don’t encourage mixing
• Set temperature close to saturation is required
• Desuperheaters subject to transient conditions
• Excess water fall out creating inef-
ficiency, erosion, water hammer etc.
Key Components for Desuperheating
• Small inside Diameter + High Velocity = Good Mixing
• Quality of atomization proportional velocity2 (steam)
• Hotter Water = smaller water droplet dia(function of surface tension forces)
• More DP = smaller water droplet diameter
• Smaller water droplet diameter = quicker atomization
• Even distribution (across the area of the steam) of the spraywater regardless of steam flow
• Good Control of downstream temperature
• Installation
Solutions
CCI have several innovative styles of desuperheaters, but for extraction &
exhaust solutions, review and advice of the system is necessary. Aspects such
as liners, control, reduced sections of piping, location of instrumentation and
installation are all aspects necessary to meet performance requirements.
There are 3 stages to desuperheating:
• Primary. The spraywater is admitted into the steam via the nozzle.
The desuperheating nozzle can be either of the mechanical type or the
pneumatic type. The pneumatic type in this instance refers to steam
atomising. Mechanical relies on DP to provide spray pattern through nozzle.
• Secondary. This is where the momentum of the steam accelerates the water
droplets and this action breaks up the water droplets. The higher the velocity
of the steam the better the secondary atomization.
• Tertiary. This is where the water droplets evaporate in the steam when being
transported. If the velocity is to low or the size of the water droplets too
large, there will be water fall out. Time is required to complete this process.
To achieve excellent primary desuperheating:
• Variable area nozzles are used
which maintain excellent spray
pattern and fine constant droplet
size regardless of water flow.
• A swirl chamber to improves the
coverage of the spray pattern.
• Even distribution over the total
cross section.
• Avoid multiple spray patterns
recombining to form larger droplets.
• Accurate control of spray water with
well selected water control valve.
• High water turndown capability
(steam turndown is a function of
several other factors.)
CCI will provide the correct total system solution for the application.
CHP – A Guide to Steam Conditioning
Overview of CHP 2
Benefits of CCI CHP Technology 3
The Critical Role of Steam 4
Unique Requirements for CHP Steam 5
CHP Application Examples
Extraction/Exhaust 6
Bypass to Exhaust and Others 7
Condensing 8
Desuperheater 9
Vents, Startups & Silencers 10
Key Products for Severe Service Applications
VST-SE 11
VLB 12
DRAG® 13
Desuperheating 15
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14 3Benefits of CCI CHP Technology
Figure 2: Conventional fossil fuelled power station
Figure 3: For combined power plant CCPP
Figure 4: Conventional power plant with CHP
Figure 5: CCPP with CHP
Benefits and Examples of Industry Utilizing CHP Technology
CHP provides a secure and highly efficient method of generating electricity
and heat at the point of use. Due to the utilization of heat from electricity
generation and the avoidance of transmission losses because electricity
is generated on site, CHP typically achieves a 35% increase in efficiency
compared with power stations and heat only boilers. This can allow economic
savings where there is a suitable balance between the heat and power loads.
Figure 2 shows typical percentage gains and losses for conventional fossil
fuelled power station. Figure 3 similarly indicates typical figures for combined
cycle power plant (CCPP) incorporating electricity generated from the gas
turbine and a steam turbine. Figure 4 shows the immediate benefits in useful
energy in CHP when the steam turbine exhaust/extraction steam is utilised
as heat energy. Figure 5 indicates CHP plant with CCPP where electricity is
generated from the gas turbine and steam turbine and the exhaust heat energy
from the steam turbine is used for the process. Note that owing to the gas
turbine the proportion of useful electrical energy on Figure 5 is higher than
that in Figure 4.
The current mix of CHP installations achieves a reduction of over 30 percent in
CO2 emissions in comparison with generation from coal-fired power stations,
and over 10 percent in comparison with gas fired combined cycle gas turbines.
The newest installations achieve a reduction of over 50 percent compared with
generation from coal-fired power stations.
With this in mind both the EU and US have optimistic goals of increasing the
percentage of electrical generation by 2010 to approximately double the current
level. The USCHPA mission is to double the contribution of CHP to the nation’s
power supply (46GW in 1998 to 92GW by 2010.)
Examples of industry utilizing the CHP technology:
• Ammonia/fertilizer plants
• Incineration plants
• Chemical plants
• Pharmaceutical plants
• Pulp & Paper
• Sugar/food
• Power and desalination
• District heating
• Universities/hospitals
Typical industries that require hot water:
• District/Community heating
• Fish farming
• HVAC, heating, ventilating, and air conditioning
• Universities/hospitals
Velocity Control Technology
Applications for DRAG® Valves on CHP
The DRAG® trim can be installed in several body styles and can even
incorporate steam conditioning as a total system solution. Applications for
DRAG® in CHP in general are where service is particularly severe, for example
very high differential pressure, high risk of cavitation and especially when
there are strict low noise requirements. The DRAG® valve can be utilized for the
following example applications.
• Bypass to condenser (low noise)
• Vent valves
• Vent resistor
• Dump tube (low noise to air cooled condenser)
• Combined startup and feedwater control valves
• Boiler feedpump minimum flow recirculation control valves
• Startup valves
• Spraywater control valves
CCI DRAG® Benefits
• Low noise: depending on application, noise levels of 85 dBA or lower at
1 m are possible even with large flow and high DP. Working with CCI can
provide reduced total system noise rather than just individual product.
• Reliable operation: by controlling velocity.
• Longer valve life: controlling velocity and pressure head, preventing
damaging conditions such as cavitation.
• High performance: disk stack can be custom characterized to suit
particular application, such as boiler level control valve (feedwater control
valve.)
• Reduced maintenance cost & downtime: provide repeatable tight shutoff
utilizing high shutoff capability MSS-SP61 shutoff with 1000 pounds per
linear inch utilizing pressurized seat design.
• Reduced installation cost: custom designed including inlet/outlet
connections to suit application.
Figure 23: Steamjet® for high pressure drop/low noise applications
Figure 22: Low noise DRAG® for turbine bypass
Figure 24: Low noise DRAG® dump
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4The Critical Role of Steam
Why is Steam Required and at What Degree of Superheat?
Steam is normally required at a condition close to saturation owing to the
excellent heat transfer properties of saturated steam. The industrial process
will require heat at a particular temperature and the steam conditions will
reflect this. Examples of the use of steam are as follows:
• Paper making. Typically 3.5 bar a at 145 C
• Steam used for evaporating the juices in sugar making process are
normally at about 3.5 bar a at saturation.
• Steam used at varying pressure and temperatures for distillation and
cracking in a refinery.
For heat transfer purposes, it is important to provide the steam to the
process at a temperature as close to saturation as possible. If there is
too much superheat in the steam, then there the heat transfer at the
process will be inefficient. If the temperature is too high, then the steam
consumer’s equipment can be damaged or the paper run can be ruined.
Steam is therefore normally available from the steam turbine or its bypass
valve or a combination of both. It should be noted, the requirement of
the power plant is primarily to provide steam for the process (industry)
and generating electricity is merely a benefit as the electrical needs can be
imported if necessary, but if there is no steam then there may be not be
any production. Steam supply at the correct pressure and temperature is
therefore of the utmost importance to the relevant industry.
Steam Provided by Steam Turbine
Steam from the steam generator (HRSG), normally high pressure and
superheated, will pass through the steam turbine. The steam to the
relevant process can be taken from extraction or the exhaust of a steam
turbine if the turbine is of the backpressure design (see Figure 1.) The
temperature of this steam will vary depending on flow and will therefore
require temperature reduction (desuperheating) before being supplied to
the process.
Steam Provided by the Bypass Valve
If the steam turbine is not available, then the bypass valves is utilized to
condition the steam to the exact conditions required for the process. In
some cases, the steam flow through the turbine does not meet the process
demand and in this case the bypass valve must respond to make up the
difference between the process demand and that being supplied by the
steam turbine. Availability of turbine bypass valve is therefore critical to
overall profit of the associated industry.
13
Figure 6: CHP in pulp and paper application using VST-SE steam conditioning valve.
Figure 7: VST-SE providing low pressure steam for paper making.
DRAG® — Velocity Control Technology
How to Solve Severe Service Valve Problems
Uncontrolled flowing velocity—erosion—a control valve’s worst enemy. High
velocity fluid or gases as a result of high pressure drop or large change in
pressure ratio creates velocity, which if to high causes cavitation and or erosion
resulting in valve failure (refer Figure 19.)
Even today, despite widespread attempts to copy the CCI DRAG® solution is
unique in solving this, utilizing multi flow paths and introducing the required
number of pressure reducing stages. Refer to CCI DRAG® brochure.
Taming Velocity
Fortunately, the solution is found in basic engineering principles.
The fluid in a valve reaches its maximum velocity just slightly downstream of
the valve trim’s vena contracta or minimum flow area. This high velocity in a
single path or multi-path design can produce cavitation, erosion and abrasion
— all of which can quickly destroy the valve. Even before damaging the valve,
the symptoms of excessive noise, severe vibration, poor process control and
product degradation may be observed.
DRAG® velocity control valves from CCI solved the problem a generation ago.
DRAG® valves prevent the development of high fluid velocities at all valve
settings. At the same time, they satisfy the true purpose of a final control
element: to effectively control system pressure over the valve’s full stroke.
Here’s how the DRAG® valve accomplishes what the others can only approach:
• The DRAG® trim divides flow into many parallel multi-path streams
(Figure 21.) Each flow passage consists of a specific number of right angle
turns—a tortuous path where each turn reduces the pressure of the flowing
medium. By increasing the number of turns, damaging velocity can be
controlled while an increased pressure drop across the control valve can be
successfully handled.
• The number of turns, N, needed to dissipate the maximum expected
differential pressure across the trim is determined by limiting the velocity
to an acceptable level, then changing element = 2gh/N and solving for
N. Applying this principle to the DRAG® valve’s disk stack and plug as
shown in Figure 20 means that velocity is fully controlled in each passage
on every disk in the stack and that the valve can operate at a controlled,
predetermined velocity over its full service range.
• In the DRAG® trim, the resistance, number and area of the individual flow
passages is custom matched to the specific application and exit velocities
are kept low to eliminate cavitation of liquids and erosion, vibration and
noise in gas service.
Velocity Control Technology
V2
V1
V2
V2 V1
= 2gh
>
VenaContracta
Figure 19: Uncontrolled velocity – a control valve’s worst enemy.
Figure 20: Single-stage and single-path pressure reduction.
Figure 21: DRAG® disk multi-trim multi flow path
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Unique Requirements for CHP Steam512
What are the differences in the requirements for steam conditioning equipment in a CHP plant compared with a conventional fossil fuel power station?
In a power station the reason for using steam conditioning (steam turbine
bypass) equipment is to allow quick and easy start and stop and to protect the
equipment in case of turbine trip etc. These demands are also applicable to a
CHP plant, with the addition that there is a requirement for tightly controlled
Parameters to meet the downstream process requirements. Here are some
examples of the erroneous conditions that steam conditioning Equipment has
to handle on a CHP plant.
• While the steam conditioning valve (bypass) on a power plant must
open sufficiently quick to prevent safety valves from opening, the steam
conditioning valve on a CHP must additionally open quickly enough to
prevent pressure fluctuations in the process header, this can be sometimes
less than 1 second.
• Downstream temperature control on a CHP plant is far more critical
than on conventional power plants. Typically the temperature should be
within parameters acceptable for the condenser or reheater, while in a CHP
application it has to be close to the set point.
• The CHP steam conditioning valve will operate more frequently and can
depend on several factors.
a) Steam turbine not in operation.
b) Export of electricity to the grid may or may not be required and the
bypass will provide the required flexibility in operation.
c) It has to make up the shortfall of steam supply from the steam
turbine compared to system demand.
d) Sometimes the steam conditioning valve can operate almost
constantly.
• Extreme turndown with respect to control of steam flow to the correct
temperature at can be expected. If for example the steam turbine available
supplies to a process is 39T/hr and demand is 40 T/hr, the bypass will
have to supplement with 1T/hr, thereby requiring 40 to 1 turndown.
If the bypass valve can only achieve 5 to 1 turndown, it would have to
supplement a minimum of 8T/hr and the steam turbine would have to
back down to 32T/hr, meaning that there is 7T/hr not going through
the steam turbine resulting in lost revenue, due to decreased electrical
production. Assuming inlet steam conditions to be 80 bar a at 520 C and a
back pressure of 4 bar a, this 7T/hr would equate to a power loss of 1.4MW.
Every CHP plant is unique and requires system understanding to provide not
only the correct equipment, but also knowledge and experience regarding
aspects such as installation and control. Consult with CCI, who have more
than 80 years of expert knowledge and experience, to establish best practice
and operational performance for your CHP plant.
Steam Conditioning Technology
Variable area nozzles
Yes qty as required
In-Line Repair YesMaterial of
ConstructionTo suit application
Shut-off Class III, IV or V, MSS SP 61Plug Size 28-400 mm/1.1” – 16”
Characteristic Modified LinearStem Guiding, 2
PositionsYes
Equivalent Rating
To Cl 2500 (PN420)
Max Temperature
Up to 600 C
Pressure Reducing Stages
Up to 8
Valve Specifications and FeaturesApplication: Bypass to Condensor
The VLB was designed as a steam turbine bypass valve and is widely used for
bypass or dump to condenser. The requirements are:
• Allow independent operation of
the steam turbine and the
H.R.S.G. during startup
• Bypass the turbine in the event
of a turbine trip (<2 seconds)
• Stabilize steam header in the
event of island operation
• Allow flexible plant operation
• In the event of short-term
process trip, The bypass valve
will stabilize system.
The bypass system with VLB will benefit from:
• Reliable operation: suitable for
up to 300 C thermal shock.
• High performance and stable
control: system stability despite
pressure, flow and temperature
transients with CCI total system
understanding implemented.
• Reduced maintenance cost &
downtime: provide repeatable
tight shutoff despite exposure to
thermal shock.
• Accurate control of final steam
conditions to condenser,
preventing condenser damage
owing to overspray and
vibration.
• Low noise (DRAG® dump tube
used if noise requirements are
onerous.)
• Custom design of bypass valve
inlet/outlet connections to suit
application.
The Solution
The VLB design in conjunction with a CCI dump tube can simply and easily
cater for these requirements as the valve was designed to meet these arduous
requirements.
• Thermal shock: forged fully machined valve body both outside and inside
to handle thermal fatigue which is critical for reliable service.
• Flexible seat and excellent guiding. Thermal change can cause crushing
of the seat as the body contracts. The special two piece seat prevents
crushing of the seat. Good guiding ensure that the valve can be installed
horizontally or vertically without risk of sticking.
• Multi nozzle variable area desuperheating. Variable area nozzles provide
excellent atomization regardless of water flow and are easily exchangeable.
The nozzles are positioned around the circumference to provide an even
distribution and to best utilize the energy of the high velocity steam
exiting the diffusers which further atomize the desuperheating spraywater.
The nozzles are after pressure reduction and situated so that cool
spraywater will not impinge on components or piping. The combination
results in reliable and accurate control of outlet conditions.
• Pressure sealed bonnet: maintains tightness regardless of temperature
transients.
Figure 17: Bypass to water cooled condensor
Figure 18: VLB desuperheating features
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6 11CHP Application Examples – Extraction/Exhaust
Turbine Extraction/Exhaust
The outlet steam temperature from extraction or exhaust varies depending
on the steam going through the steam turbine. For example, considering
exhaust steam only, as the steam flow through the turbine decreases, the
outlet temperature increases. Depending on the exhaust flow in general as
the extraction flow reduces, the extraction steam temperature increases. This
means to obtain a constant set temperature downstream, the proportion of
spraywater required at low flow is higher than compared to at full flow where
the requirement will be small if any at all.
On most CHP plants, the exhaust line can be of a large diameter and in
view of the conditions detailed above combined with the large diameter and
potentially low flow, providing good temperature control to the process close to
saturation can be extremely difficult and needs special consideration.
CCI with extensive experience and knowledge can provide installation
guidelines and recommendations in conjunction with the correct product
selection for the optimum system solution.Figure 8: Turbine extraction/exhaust desuperheating
Why Severe Service Solutions Recommended Input Data for Selection
Large diameter process piping
Desuperheater should provide good cross sectional coverage
Steam flow rate, max, min normal
Steam pressure
Upstream steam temperature
Required downstream temperature
Pipe diameter and schedule
Water pressure and temperature available
Atomizing steam if applicable
Design pressure of steam
Design temperature of steam
Design pressure and temperature of water
Type of actuation pneumatic, electric or hydraulic
Failure mode
Use multi-nozzle configuration
High rangeability of steam flow tending to water fall out at low flow
Installation should incorporate smaller diameter piping
Partial or full steam atomization
Control set point temperature close to saturation
Use hottest water available for desuperheating
Utilize enthalpy control
Consider steam atomization
Increase velocity at point of desuperheating
Application: Exhaust or Extraction Steam to Process Desuperheating
Steam Conditioning Technology
Application: Bypass to Process
The VST-SE was designed as a steam turbine bypass to process conditioning
valve. The requirements are to open and close very quickly (refer to application
examples) in response to a turbine trip, startup or to provide additional steam
flow to the process. This means that the system will benefit from:
Benefits
• Reliable operation: suitable for up to 300 C thermal shock
• More revenue owing to higher electrical production. This is achieved by
providing high turndown capability with regard to steam flow.
• High performance and stable control: system stability despite severe
transients with respect to pressure and flow. Solved by integral
water proportioning.
• Reduced maintenance cost & downtime: provide repeatable tight
shutoff despite exposure to thermal shock.
• Maximize plant flexibility: the VST-SE provides modulating steam
atomization. Generally standard systems provide on/off atomization
requiring at least 5% steam for atomization.
The Solution
The VST-SE design is unique as it can simply and easily cater for these
requirements as the valve was designed to provide solutions to these
requirements.
• Thermal shock: forged fully machined valve body both inside and
outside to handle thermal fatigue, critical for reliable service.
• Steam atomized desuperheating: Steam is bled through the central stem
to atomize the spraywater. From 0-15% of stroke, (0-5% of steam flow)
the control of steam is only through this channel and is controlled by
the positioning of the main plug and which uncovers sequential holes
leading to the atomizing channel. Above 15% (5% flow), then the main
cage proper opens and the steam flow modulates normally through
control section providing a linear characteristic. The total characteristic
will therefore be modified linear providing excellent control at low flow.
With steam atomization the VST-SE will achieve turndown with respect to
desuperheated steam flow of greater than 50 to 1.
• Water proportioning: As steam flow modulates, the spraywater flow is
proportioned mechanically by a unique system linked to the main steam
plug. This minimizes temperature spikes and enhances system stability
regarding temperature control.
• Flexible seat and excellent guiding. Thermal change can cause crushing
of the seat as the body contracts. The special two piece seat prevents
crushing of the seat. Good guiding ensure that the valve can be installed
horizontally or vertically without risk of sticking.
Figure 14: VST-SE valve
Figure 15: VST-SE features
Figure 16: VST-SE mini valve
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710
CHP Application Examples – Vents, Startups and Silencers
Steam Turbine Bypass to Extraction/Exhaust for Back Pressure Turbines
The steam turbine bypass is used to reduce the pressure and temperature of the
steam to match the appropriate extraction/exhaust conditions. These valves are
used during startup, in the event of a turbine trip, non availability of the steam
turbine or to supplement steam to process that may not be available from the
extraction or exhaust from the steam turbine.
The bypass valve should:
• Be suitable for severe thermal shock (up to 300 C)
• Modulate in 2-3 seconds or less. Snap action in this time is not
acceptable as the boiler will trip and the system will be unstable.
• Have high range ability to maximize turndown
• Provide repeatable tight shutoff
• Inline repairability
• Be of low noise design
Reliability of this valve is of the utmost importance. Non availability of this
valve can often mean loss of production. CCI with extensive experience
and knowledge can provide installation guidelines and recommendations
in conjunction with the correct product selection for the optimum system
solution.
Figure 9: Steam turbine bypass to extraction/exhaust for back pressure turbine
Exhaust
Extraction
Why Severe Service Solutions Recommended Input Data for Selection
Noise and vibration
Control of inlet and outlet velocity by providing connections to suit application/piping Steam flow rate, max, min normal
Upstream steam pressure
Upstream steam temperature at the applicable steam flow rate
Required downstream pressure
Required downstream temperature
Pipe diameter and schedule, inlet and outlet
Water pressure available
Water temperature available
Design pressure of upstream and downstream steam.
Design temperature of upstream steam
Design pressure of water
Design temperature of water
Actuating speed
Type of actuation (pneumatic hydraulic)
Noise requirements
Failure mode
Multiple pressure reduction stages
Thermal shock, up to 300 C in less than 2-3 seconds
Forged circular section body machined on inside and outside to provide even material distribution
Pressure seal bonnet
High rangeability of steam flow
Steam atomizing to avoid water fall out at low flow
Modified linear characteristic, typically from opening, 15% stroke = 5% capacity
Piston double acting pneumatic actuators or hydraulic actuators
Control set point temperature close to saturation
Use hottest water available for desuperheating, typically above 100 C if possible
Proportion water flow with steam flow to prevent temperature spikes and water fall out
Steam atomization
Consider steam atomization
Increase velocity by reducing pipe diameter at point of desuperheating
Application: Bypass to Exhaust or Extraction Process Line
CHP Application Examples – Bypass to Exhaust
CHP Vent Valves, Startup valves and Silencers
Startup vents are used to warm up piping in the various header. In the case
of the HP header, the steam should be superheated to preset pressure and
temperatures before steam can be admitted to the turbine. The vent can also
be used in the process headers for warming up the long length of piping.
Furthermore if for example the process shuts down for a short time and there is
a need to keep the gas turbine generating electricity, then it may be necessary to
vent the steam (assuming there is no dump condenser.)
Requirements of vent valves and silencers:
Figure 12: CHP venting and startup systems
Figure 13:DRAG® vent resistor with shroud
Other severe service valves and desuperheaters which are covered in other CCI
literature. They include:
Why Severe Service Solutions Recommended Input Data for Selection
Noise and vibration
Control of inlet and outlet velocity by providing connections to suit application/piping Steam flow rate, max, min normal
Upstream steam pressure
Upstream steam temperature at the applicable steam flow rate
Pipe diameter and schedule inlet
Design pressure of upstream and downstream steam.
Design temperature of upstream steam
Actuating speed
Type of actuation (pneumatic electric or hydraulic)
Failure mode (normally closed)
Noise requirements
Multiple pressure reduction stages
Consideration of silencer, or resistor
Thermal shock, up to 300 C in less than 2-3 seconds
Forged circular section body machined on inside and outside to provide even material distribution
Angle pattern body to provide integrity against thermal transients
Provide adequate drains and preheating
Pressure seal bonnet
High rangeability of steam flow Modified linear characteristic
Double acting pneumatic or hydraulic actuators
Leakage Class V repeatable tight shutoff
• If required to quick open
should be capable of handling
severe thermal shock.
• If required for operational
purposes, then valve and
silencer should be designed
for low noise.
• Be suitable for severe thermal
shock (up to 300 C)
• Modulate in 2-3 seconds or less
• Provide repeatable tight shutoff
• Inline repairability
• Main and booster feed-pump
recirculation
• Startup and main feedwater
regulation
• Deaerator level control
• Spraywater control
• High level heater drains
• Auxiliary steam PRDS
• Blow-down, continuous &
intermittent
• Boiler attemperators, final and
inter-stage
• HP/LP heater bypass
• Economizer mixer valve
(3 way valve)
Application: CHP Venting and Startup Systems
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CHP Application Examples – Desuperheater
Desuperheaters for Controlling Final Temperature
On some CHP plants, the process that requires the steam can be quite a
distance from the CHP plant and to prevent excessive water fall out and
wetness in the steam close to the plant, the steam exiting the CHP plant can
have a higher than required temperature to allow for the temperature drop
owing to the transportation time. The drop in temperature is a function of
distance, ambient conditions and flow rate at the time.
As the inlet temperature will fluctuate to the process (higher the flow, hotter the
steam owing to less time for transportation loss) then there is a need to have
a final control of the temperature close to process using the steam. It should
also be noted that there may be more than one process demanding the steam
and each one may require a separate desuperheating station to control the
temperature.
As the process will benefit from a temperature close to saturation, and with the
little superheat available in the steam, the energy available for atomising the
spraywater is low and this application sometimes requires special consideration
particularly in large diameter piping and high turndown requirements.
Figure 11: Desuperheater for trimming process temperature
Why Severe Service Solutions Recommended Input Data for Selection
Large diameter process piping
Desuperheater should provide good cross sectional coverage in steam flow
Steam flow rate, max, min normal
Steam pressure
Upstream steam temperature at the applicable steam flow rate
Required downstream temperature
Pipe diameter and schedule
Water pressure available
Water temperature available
Atomizing steam if applicable design pressure of steam
Design pressure and temperature of steam
Design pressure and temperature of water
Type of actuation (pneumatic electric or hydraulic)
Failure mode (desuperheater to close or open)
Variable area nozzle to provide good evaporation of water regardless of flow
High range ability of steam flow tending to water fall out at low flow
Installation should incorporate smaller diameter piping to increase velocity at point of desuperheating
Partial or full steam atomization
Control set point temperature close to saturation
Use hottest water available for deuperheating
Utilize enthalpy control with and trim with temperature feedback & CCI guidance
Consider steam atomization
Increase velocity by reducing pipe diameter at point of desuperheating
Application: Desuperheaters for Trimming Process Temperature
CHP Application Examples – Condensing
Condensing Steam Turbine with Extraction
Depending on the proportion of output energy with respect to electrical versus
heat, sometimes a CHP plant, will have a higher proportion of electricity
output. To facilitate this, the turbine will exhaust to condenser and extract
steam to process. Sometimes when electricity price is at a premium, the gas
turbine will continue to generate electricity even in the event of non availibility
of steam turbine, as the waste heat needs to be removed from the heat recovery
steam generator (HRSG.) On some occassions, the process may be stopped for
short periods and excess steam can be dumped to the condenser to keep the
system stable and when the process start again the condenser bypass will close
and the steam will continue to process.
The bypass valve to condenser should:
• Be suitable for severe thermal shock (up to 300 C)
• Modulate in 2-3 seconds or less. Snap action in this time is not
acceptable as the boiler will trip and the system will be unstable.
• Have high rangeability to maximize turndown
• Provide repeatable tight shutoff
• Inline repairability
• Be of low noise design
Reliability of this valve provides optimum plant flexibility. CCI with
extensive experience and knowledge can provide installation guidelines and
recommendations in conjunction with the correct product selection for the
optimum system solution.
Figure 10: Condensing steam turbine with extraction
Why Severe Service Solutions Recommended Input Data for Selection
Noise and vibration
Control of inlet and outlet velocity by providing connections to suit application/piping
Steam fow rate, max, min normal
Upstream steam pressure
Upstream steam temperature at the applicable steam flow rate
Condenser pressure
Required enthalpy of steam to condenser
Pipe diameter and schedule inlet
Water pressure available
Water temperature available
Design pressure of upstream and downstream steam.
Design temperature of upstream steam
Design pressure of water
Design temperature of water
Actuating speed
Type of actuation (pneumatic or hydraulic)
Failure mode (normally closed)
Noise requirements
Multiple pressure reduction stages, introduce sufficient stages to meet noise and vibration requirements
Consideration of dump tube, single stage or resistor
Thermal shock, up to 300 C in less than 2-3 seconds
Forged circular section body machined on inside and outside to provide even material distribution
Provide adequate drains and preheating
Pressure seal bonnet
High rangeability of steam flow
Multiple variable area orifice desuperheaters circumferentially mounted
Modified linear characteristic
Double acting pneumatic actuators or hydraulic actuators
Control of >30% of water to steam without vibration and damage to condenser
Utilizing CCI enthalpy control algorythm
Proportion water flow
Water cooled condenser use single stage dump tube
Air cooled condenser < 90 dBA use resistor type dump to condenser device
Careful installation to ducting for air cooled condenser
Application: Turbine Bypass to Condensor