539 Chp a Guide to Stream Conditioing Desuperheater CCI

8/10/2019 539 Chp a Guide to Stream Conditioing Desuperheater CCI

1/16

CHP A Guide

to Steam

Conditioning


2/16

CHPAGuidetoSteam

Conditioning

2 Overview of CHP

CHP (Combined Heat and Power) is an Efficient Technology forGenerating 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 dif ferent 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

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


3/16

CHPAGuidetoSteam

Conditioning

3Benefits of CCI CHP Technology

Figure 2: Conventional fossil fuelledpower station

Figure 3:For combined power plant CCPP

Figure 4:Conventional power plantwith 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 electricitygeneration 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

CO2emissions 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 optimist ic 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 nations

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


4/16


5/16

CHPAGuidetoSteam

Conditioning

Unique Requirements for CHP Steam5

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.

Ext reme 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 backdown 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.


6/16

CHPAGuidetoSteam

Conditioning

6CHP 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 ext raction 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 exper ience and knowledge can provide installation

guidelines and recommendations in conjunction with the correct product

selection for the optimum system solution.Figure 8: Turbine extraction/exhaustdesuperheating

Why Severe Service Solutions Recommended Input Data for Selection

Large diameter process piping

Desuperheater should provide good crosssectional 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 orhydraulic

Failure mode

Use multi-nozzle configuration

High rangeability of steam flowtending to water fall out at lowflow

Installation should incorporate smallerdiameter piping

Partial or full steam atomization

Control set point temperatureclose to saturation

Use hottest water available fordesuperheating

Utilize enthalpy control

Consider steam atomization

Increase velocity at point ofdesuperheating

Application: Exhaust or Extraction Steam to Process Desuperheating


7/16

CHPAGuidetoSteam

Conditioning

7

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


Noise and vibration

Control of inlet and outlet velocity by providing

connections to suit application/pipingSteam 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


8/16

CHPAGuidetoSteam

Conditioning

8CHP Application Examples Condensing

Condensing Steam Turbine with Extraction

Depending on the proportion of output energy with respect to elect rical versusheat, 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 electric ity 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



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 theoptimum system solution.

Figure 10: Condensing steam turbine withextraction


Noise and vibration

Control of inlet and outlet velocity by providingconnections to suit application/piping

Steam fow rate, max, min normal


Upstream steam temperature at theapplicable steam flow rate

Condenser pressure

Required enthalpy of steam tocondenser

Pipe diameter and schedule inlet

Water pressure available

Water temperature available

Design pressure of upstream anddownstream steam.

Design temperature of upstreamsteam

Design pressure of water

Design temperature of water

Actuating speed

Type of actuation (pneumatic orhydraulic)

Failure mode (normally closed)

Noise requirements

Multiple pressure reduction stages, introduce sufficientstages to meet noise and vibration requirements

Consideration of dump tube, single stage or resistor

Thermal shock, up to 300 C inless than 2-3 seconds

Forged circular section body machined on inside andoutside to provide even material distribution

Provide adequate drains and preheating


High rangeability of steam flow

Multiple variable area orifice desuperheaterscircumferentially mounted

Modified linear characteristic

Double acting pneumatic actuators or hydraulicactuators

Control of >30% of water tosteam without vibration anddamage 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 tocondenser device

Careful installation to ducting for air cooled condenser

Application: Turbine Bypass to Condensor


9/16


10/16

CHPAGuidetoSteam

Conditioning

10CHP Application Examples Vents, Startups

and Silencers

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:DRAGvent resistor with shroud

Other severe service valves and desuperheaters which are covered in other CCI

literature. They include:


Noise and vibration

Control of inlet and outlet velocity by providingconnections to suit application/piping Steam flow rate, max, min normal


Upstream steam temperature at theapplicable steam flow rate

Pipe diameter and schedule inlet

Design pressure of upstream anddownstream steam.

Design temperature of upstream steam

Actuating speed

Type of actuation (pneumatic electric orhydraulic)

Failure mode (normally closed)

Noise requirements

Multiple pressure reduction stages

Consideration of silencer, or resistor

Thermal shock, up to 300 C inless than 2-3 seconds

Forged circular section body machined on insideand outside to provide even material distribution

Angle pattern body to provide integrity againstthermal transients

Provide adequate drains and preheating


High rangeability of steam flowModified 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



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


11/16

CHPAGuidetoSteam

Conditioning

11Steam 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 steamflow 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 easi ly 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 isproportioned 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


12/16

CHPAGuidetoSteam

Conditioning

12 Steam Conditioning Technology

Variable areanozzles

Yes qty as required

In-Line Repair Yes

Material ofConstruction To suit application

Shut-off Class III , IV or V, MSS SP 61

Plug Size 28-400 mm/1.1 16

Characteristic Modified Linear

Stem Guiding, 2Positions

Yes

EquivalentRating

To Cl 2500 (PN420)

MaxTemperature

Up to 600 C

PressureReducing 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 (


13/16

CHPAGuidetoSteam

Conditioning

13

DRAG Velocity Control Technology

How to Solve Severe Service Valve Problems

Uncontrolled flowing velocityerosiona control valves 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 trims 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.

DRAGvelocity control valves from CCI solved the problem a generation ago.

DRAGvalves 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 valves full stroke.

Heres how the DRAGvalve accomplishes what the others can only approach:

The DRAGtrim divides flow into many parallel multi-path streams

(Figure 21.) Each flow passage consists of a specific number of right angleturnsa 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 DRAGvalves 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 DRAGtrim, 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

>

Vena

Contracta

Figure 19: Uncontrolled velocity acontrol valves worst enemy.

Figure 20: Single-stage and single-pathpressure reduction.

Figure 21: DRAGdisk multi-trim multiflow path


14/16

CHPAGuidetoSteam

Conditioning

14 Velocity Control Technology

Applications for DRAGValves on CHP

The DRAGtrim can be installed in several body styles and can even

incorporate steam conditioning as a total system solution. Applications for

DRAGin 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 DRAGvalve 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 star tup and feedwater control valves

Boiler feedpump minimum flow recirculation control valves

Startup valves

Spraywater control valves

CCI DRAGBenefits

Low noise: depending on application, noise levels of 85 dBA or lower at

1 m are possible even with large flow and high P. 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.

More revenue owing to higher electrical production. Properly applied,

will reduce or eliminate maintenance activity or process shut down

owing to equipment failure.

High performance and stable control: 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.

Maximize plant flexibility.

Reduced installation cost. Valve custom designed including inlet/outlet

connections to suit application.

Figure 23:Steamjetfor high pressure drop/low noise applications

Figure 22:Low noise DRAGfor turbine bypass

Figure 24:Low noise DRAGdump


15/16

CHPAGuidetoSteam

Conditioning

15 Desuperheater Technology

Figure 26:DA-O variable area nozzledesuperheater

Application: Desuperheating for Extraction and Exhaust Steam

As mentioned earlier control of the extract ion and exhaust can be difficult

owing to the following.

Figure 25: Multi-nozzle DAMdesuperheater

Low velocity/ water fall out

Insufficient cross sectional

coverage

Large piping diameters dont

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 P = smaller water

droplet diameter

Smaller water droplet diameter =

quicker atomization

Even distr ibution (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 P 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 correcttotal system solution for

the application.


16/16

Date post:	02-Jun-2018
Category:	Documents
Upload:	cecep-atmega
View:	241 times
Download:	0 times

539 Chp a Guide to Stream Conditioing Desuperheater CCI

Documents