Feed pumps, Feed Injectors, Feed Regulators, Feed heaters, Air Heaters and Steam Accumulators A. Ranganath

Feed pumps, Feed Injectors, Feed Regulators, Feed heaters, Air Heaters and Steam Accumulators

A. Ranganath

Contents

• Feed Pumps• Feed Injectors• Feed Regulators• Feed Heaters• Air heaters• Steam Accumulators

Dienstag, 18. April 2023 Fußzeilentext 2

FEED PUMPS


Boiler feed pump

• Pump is a device – which converts mechanical energy into pressure energy due to which the fluid moves from one point to another.

• A Boiler Feed Pump is a specific type of pump used to pump feed water into a steam boiler.

• The water may be freshly supplied or returning condensate produced as a result of the condensation of the steam produced by the boiler.

• These pumps are high pressure units that take suction from a condensate return system and can be of the centrifugal type or positive displacement type.

• The pump also supplies water for attemperation sprays that control the Superheat/Reheater steam temperature.


BFP Design & Considerations

Features of BFP:• Capital cost, running cost and maintenance costs to be optimised.• Single pump option generally not considered.• Satisfactory operation during start up and continuous operation.• Should have adequate margins on NPSH.• Should be capable of taking care of the pressure decay during turbine

load variations resulting in decay in de-aerator pressure.• Flow & head margins shall be provided.• Generally the optimum values are for flow margin 5% above the total

flow and Head margin of 3% above the total head.


BFP Design & Considerations

• Max. availability with a design life of 45,000 hrs• Rugged high speed machine• Designed for rapid replacement of complete shaft /rotor assembly• Substantially stiffened shaft with the number of stages, giving

improved rotor rigidity and lower shaft deflections• Use of balance drum to oppose axial hydraulic thrust with residual

unbalance being carried by external oil cooled thrust bearing.


PUMP CHARACTERISTICS :- It is the relationship between Capacity, Head, Power and

Efficiency.- The graphs, showing the inter-relationship between Capacity, Head, Power and Efficiency, are called Pump Characteristic Curves.

Capacity :- It is the quantity of fluid flowing through the Pump for a

given time of period.- It is expressed in m3/hr.- It is measured by weight method, volumetric method,

orifice plate or by weirs. Head :

- It is the measure of energy to move the fluid from one point to another.

- It is expressed in metres of liquid column.


Power :

- The horse power produced by the liquid is called as Water Horse Power (WHP) or Liquid Horse Power which is expressed as

WHP = ( Q H) / 75

where Q = m3/sec , H = mlc & = kg/m3

- The power required to drive the pump is called as Brake Horse Power (BHP) which is expressed as

BHP = ( Q H) / 75 where is the efficiency of Pump.

Efficiency :

- It is the measure of the Pump performance.

- It is the ratio of WHP to BHP.


Characteristics of a Pump


1000

1200

1400

1600

1800

HE

AD

(m

lc)

0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450SU C TIO N FLO W (cub. m . / hr.)

0

10

20

30

40

50

60

70

80

90

EF

FIC

IEN

CY

(%

)

0

5

10

15

20

NP

SH

R (

mlc

)

500

1000

1500

2000

PO

WE

R (

kW)

50 Hz52.5 Hz

47.5 Hz

Optimisation of sizing & design margins

Design margins are provided on equipment / systems to cater for ageing, wear & tear, uncertainties etc

Conservative designs with large margins ( e.g. on flow and head of pumps) and specifying suitability for abnormal operating conditions result in lower efficiency and higher auxiliary power consumption

Proper standby philosophy based on efficiency of operation, availability & reliability, like following are considered.

1x100% Working + 1x100% Standby or

1x100% Working + 1x30% Startup or

2x50% Working + 1x50% Standby etc.


Pumps parallel operation


Types of Boiler feed pumps

Based on the Casing splitting type, Pumps are classified as :


AXIALLY-SPLIT CASING TYPE

RADIALLY-SPLIT CASING TYPE


AXIALLY-SPLIT CASING : - It refers to a Casing split in a plane parallel to the axis of rotation.

- Both Suction & Discharge nozzles are located on bottom half of the Casing so that the top half may be removed for inspection & repair without disturbing the Pump proper and Suction & Discharge piping.

Advantages:•The pump internal can be inspected by simply removing the top case; no need to remove its rotor•It is relatively inexpensive than a radial split case pump.Disadvantages:Typically limited to 204 deg C operating temperature due to thermal expansion considerationsTypically limited to 248 bar maximum working pressure due to the difficulty in bolting with a flat, unconfined, and irregular case gasket, and due to the non-symmetrical volute and suction areas between the upper half and lower half casing.

RADIALLY-SPLIT CASING :- It refers to a Casing split in a plane perpendicular to the axis of rotation.

- It contains two Casings, the inner casing encloses the rotating parts of Pumps and the outer casing encloses the inner casing.

- Suction & Discharge nozzles are an integral part of outer casing and the internal pump assembly can be removed without disturbing the piping connections.


Advantages:•Typically suitable for operating at very high temperature of up to 426 deg C as the centerline support design ensures equal case thermal expansion in radial direction•The case and cover design is suitable for higher working pressure than an axial split case pump due to its smaller bolting area, symmetrical bolting pattern.• Full cartridge pull out for rapid changeoverDisadvantages:The pump internal cannot be inspected without removing its rotor assembly from inside the casing.In some multistage pumps the rotor assembly cannot be removed from the casing without removing the driver to clear the way for the rotor assembly.It is very expensive as the pump will have to be of double barrel construction.


BFP TRAIN WITH COMMON FOUNDATION FRAME :

General Problems/Fault finding of BFP


Problem Possible cause

Pump fail to start Motor problem / Seizure of pump set / De-aerator level low low

Pump performance low Problem in motor, suction strainer choked, position of suction valve, excessive wear of pump internals, etc.

Bearings overheating Defective lube oil system, bearings worn or misaligned, misalignment of pump

Mechanical seals overheating Insufficient cooling water, mechanical seal damaged

Excessive noise and / or vibration Misalignment of pump, bearing misalignment, excessive clearance of pump internals, rotating assembly out of balance

FEED INJECTOR


Feed Injector

Why an Injector• They’re a simple device that uses no extra power source. • The design is such that, live steam can inject water into the boiler that

supplies the steam. They are used to replace mechanical driven pumps, as injectors are very reliable and very efficient.

Where it is used• Since very early 1900’s the primary water source for putting water

into locomotive boilers.

What are Injectors• They are a device which is used by locomotive crew to take water

from the water tender and combine it with live boiler steam and inject it into the boiler via a check valve.

• It is achieved by opening the steam and water valves in their correct order, they will pick up and inject the water into the boiler.Dienstag, 18. April 2023 Fußzeilentext 18

Feed Injector

How does the Injector work• A injector has several pipe connections, live steam supply, water

supply, overflow and delivery to the boiler. Inside, it has several conical shaped cones being, steam, combining and delivery.

• Steam injector works on the principle of steam nozzle.• It utilises the kinetic energy of a steam jet for increasing the pressure

and velocity of feed water.• It is used for forcing the feed water into steam boiler under pressure.


http://www.dvr.com.au/all-about-us/rs_images/injector1.gif

Feed Injector Operating Principle

• The explanation is based on an injector operating at 12.41 bar,• When the steam valve is opened, steam flows through the steam

cone. It is here where the cone reduces in size and this in turn, throttles the steam until it reaches a speed of approximately 1856 kph.

• At this speed it is admitted to the water space or combining cone. Where the steam is condensed and carries forward by the force of it’s momentum about twelve times it’s own weight of water at speed of about 144 kph.

• The speed attained is sufficient to carry the combined jet across the space to the delivery tube, and through the check valve into the boiler.

• When the steam mixes with the water and condenses, this is when it forms the vacuum. This then allows atmospheric pressure to push the water from the tender up into injector (a very important action).


Feed Injector- Types

Lifting type• They are capable of lifting water from a lower into the injector then forcing it

past the check valve into the boiler. They can be placed above the water supply on the locomotive.

• It works when the steam is turned on first, this allows for a vacuum to be formed causing the water to fill the void, under the influence of atmospheric pressure.

Non-lifting type• Very similar to lifting type, the main difference is that they cannot lift water

into the injector. That means that the water supply must be above the injector so that the water flows through the injector freely.

• The non-lifting injector must have the water on first. Then seeing the water flowing from the overflow, turn on the steam valve and it will work.

• Turning off the injectors is the same for both. Turn off the steam first, then the water.


Feed Injectors- Advantages and Disadvantages

Advantages:• No extra power source required• Very much suitable for small boilers like steam-driven locomotive

boilers.• The addition of heat to the flow of water lessens the effect of the

injected water in cooler the water in the boiler compared to the case of cold water injected via a mechanical feed pump.

• They are thermally efficient, as most of the heat energy in the condensed steam is returned to the boiler increasing the thermal efficiency of the process.

• No moving parts as in case of pump.

Disadvantages:• Limited to very small boilers


FEED REGULATORS


Feed Regulators

• A boiler feed water regulator valve used in many power plants is required to transition from feedpump recirculation to operation of the unit.

• Not only is the valve used to initially fill the steam drum, it is also used to control flow during normal operation when the steam drum is under pressure. This valve, therefore, must address cavitation during initial operation and provide adequate rangeability to address the entire feedwater requirements.


Feed Regulators

• The regulator valve will begin to transition the flow from the recirculation valve and will open as the recirculation valve closes.

• The valve must have adequate cavitation protection during initial filling of the drum and then transition to flow control mode.


Feed Regulators

• Feed regulators are used to control the drum level in power plant.• It is critical within power plant operation in drum boilers (or flow in

once-through boilers) to maintain the quantity of feed water to drum to match with steam generation.

• Drum or boiler level control is crucial at plant start-up, when the pressure differential between the BFP and boiler is very high and control is difficult.

• Boiler feed water control valves must achieve a smooth start-up and maintain required drum level for safe, reliable and efficient plant operation.

• The high pressure differential at start-up/low-load, and sensitive control requirement, requires a high-performance severe service control valve.


Feedwater Control Valve Requirements

During Start-up and Low-load Operation• Operate at high pressure differentials of up to 240 bar (Drum Boilers),

without damaging the trim components, and maintaining good control• Smooth and quick transition from start-up to normal operation.• Consistent and reliable operation.• Tight shut-off to prevent leaks and subsequent valve erosion

During Normal Operation• A valve with a high capacity is required at normal operation to minimize

friction losses in the system to minimise Boiler feed pump power requirements.

During Load Change (assuming fixed speed boiler feed pump)• Load changes are often experienced and this will result in a lower steam

pressure and drum pressure, but feed water pressure will remain similar, resulting in a higher pressure differential. So the control valve used must be able to meet a wide range of capacity requirements to provide full flexibility to the plant.Dienstag, 18. April 2023 Fußzeilentext 27

Feedwater Control Valve Problems

Erosion damage: Caused by:• Insufficient number of trim stages, creating excessive trim velocities• Poor seat design and insufficient seat force

Plug or stem breakage: Typically caused by high trim velocities, and subsequent trim vibration and fatigue failure

Vibration and noise: Caused by cavitation and excessive internal velocities.


Consequences of Feedwater CV Problems

Cavitation/flashing: Insufficient pressure reducing stages will cause high velocity flows, leading to valve/trim damage owing to cavitation/flashing.

Lost Production: Poor control at low flows can lead to plant trips and/or an extended start-up process.

High maintenance costs: Frequent replacement and repair of valve components adds to maintenance costs.


Problems caused by applying the wrong Boiler Feedwater Control Valve


FEED HEATERS


Feedwater heaters

• A Regeneration process in steam power plants is accomplished by extracting steam from the turbine at various points. This steam, which could have produced more work by expanding further in the turbine, is used to heat the feed water instead.

• The device where the feedwater heated by regeneration is called a Regenerator or a Feedwater Heater (FWH).

• A feedwater heater is basically a heat exchanger where heat is transferred from the steam to the feedwater either by mixing the two streams (open feedwater heaters) or without mixing them (closed feedwater heaters).


Why feed water heating?


Above figure shows the basic Rankine cycle with water heated only in the boiler. The carnot cycle diagram for the same steam conditions is superimposed and indicates the maximum efficiency, (that is the greatest area of useful work) that can be achieved in any power plant operating between the temperature T1 and T2. But practically this cannot be achieved.

Why feed water heating?


No. Of feed heaters in a regenerative cycle








It is clearly seen that the efficiency improves with each additional heater but the incremental gain with each becomes progressively smaller.

Types of Feedwater Heaters

Open Feedwater Heaters• An open (or direct-contact) feedwater heater is basically a mixing

chamber, where the steam extracted from the turbine mixes with the feedwater in a chamber. Ideally the mixture leaves the heater as a saturated liquid at the heater pressure.

Eg: Deaerator• The advantages of open heater are simplicity, lower cost, and high heat

transfer capacity. • The disadvantage is the necessity of a pump at each heater to handle

the large feedwater stream.


Types of Feedwater Heaters

Closed Feedwater Heaters• In closed feedwater heater, the heat is transferred from the extracted

steam to the feedwater without mixing taking place. • The feedwater flows through the tubes in the heater and extracted

steam condenses on the outside of the tubes in the shell. The heat released from the condensation is transferred to the feedwater through the walls of the tubes. The condensate (also called as drip) formed passes to the next lower pressure heater. This, to some extent, reduces the steam required by lower pressure heater.


Temperature rise of feed water in heaters


Feed water heater


Feed water heater performance

• Feed water temperature rise: is the difference between the feed water outlet temperature and feed water inlet temperature.

• Terminal Temperature Difference (TTD): provides feedback on the feedwater heater’s performance relative to heat transfer and is defined as the saturation temperature of the extraction steam minus the feedwater outlet temperature. An increase in TTD indicates a reduction in heat transfer while a decrease a improvement.

• Drain Cooler Approach (DCA): is a method used to infer feedwater heater levels based on the temperature difference between the drain cooler outlet and the feedwater inlet. An increase in DCA temperature indicates the level is decreasing; whereas, a decreasing DCA indicates rise in level.


AIR HEATER


Air Heater

Requirement of Air Pre-heater• Pre Heating Combustion air using Heat in out going Flue gas• Flue gas leaves Economiser at a temperature of approx. 3800C• Every 400C drop in Flue gas Temp. improves Boiler Efficiency by 2

to 3%

• Air pre-heater is an heat recovery unit used in the last stage in boiler.

• It absorbs heat from exit flue gas in boiler and transfers the heat to the incoming cold air.

• In utility boilers it is used to heat the air required for combustion purpose as well as dry and transport coal


Advantages of using Air heater

• BOILER EFFICIENCY IS INCREASED.• ENABLES EFFICIENT BURNING OF LOWER GRADE FUELS.• SAVINGS ON FUEL COSTS.• MORE STABLE AND EFFICIENT COMBUSTION OF FUEL.• PREHEATS AIR FOR COAL DRYING AND TRANSPORTING THE

PULVERISED COAL TO BURNERS.• REDUCED FLUE GAS VOLUME LEADS TO SIZE REDUCTION

IN POLLUTION CONTROL EQUIPMENT.





Air Heater types

• Based on Operating Principle

Recuperative and

Regenerative


Recuperators

• IN RECUPERATORS GAS AND AIR ARE ALLOWED TO FLOW IN SEPARATE CHANNELS AND HEAT IS CONTINUOUSLY TRANSFERRED FROM GAS TO AIR THROUGH THE WALLS OF THE FLOW CHANNELS.

• TUBULAR AIR PREHEATER• PLATE TYPE AIR PREHEATER • STEAM COIL AIR PREHEATER


Tubular Air Heater


Plate type Air Heater


Steam Coil Air Pre-Heater (SCAPH)


steam

COLD AIR

COLD AIR

HOT AIR

Advantages of Recuperative Air Heaters

Advantages• No moving parts• No possibility of Fly ash carry over by air

Disadvantages• Occupies more area• Tube puncture results in air mixing with Flue gas• Soot deposits reduce heat transfer• Less effective cross flow heat transfer• More material cost• High pressure drop• Severely affected by cold end corrosion


Regenarators

• IN REGENERATOR TYPE GAS AND AIR ALLOWED TO FLOW ALTERNATIVELY IN THE SAME CHANNELS TO STORE AND RETRIEVE HEAT RESPECTIVELY.

• CHANNELS ARE MADE IN MATRIX FORM TO ACHIEVE THE HEAT TRANSFER.

• TYPES• Rotating matrix (Ljungstrom RAPH)• Stationary matrix (Rothemuhle RAPH)




Ljungstorm Air Heater Working Principle


• As the rotor revolves, waste heat is absorbed from the hot exhaust gas passing through one-half of the rotor.

• This accumulated heat is released to the incoming air as the same surfaces pass through the other half of the rotor.

• The heat transfer cycle is continuous as the surfaces are alternately exposed to the outgoing gas and incoming air streams.

Advantages, Disadvantages and Frequently encountered problems of RAPH

Advantages:

• Compact and hence save space and structure cost.

• Can be effectively cleaned when in service

• Economically suitable for high capacity boiler. As the boiler size increases heat transfer area required in air heater also increases and hence Regenerator is better in comparison with Recuperative as it assumes a greater size.

Disadvantages:

• Moving parts need operator’s attention.

Frequently encountered problems

• Fouling, plugging and corrosion

• Erosion (normally encountered in tubular air heaters)


Air heater fire

• All types of air heaters are a potential fire hazard particularly at start-up of the boiler and shut-down especially if the heater is having a thick deposit of soot.

• Finely divided particles of combustible matter is deposited on the low temperature air heater surface when the combustion is poor in the furnace due to various reasons.

• If the ignition temperature at combustible matter is reached and sufficient oxygen is available, fire occurs and may sometime destroy the whole air heater, duct etc., if not noticed earlier and put off.

• The outlet gas and air temperatures from the air heater will rise above normal in case of fire and is the best indication to detect fore and to take necessary step, for fighting.

• Use of on-load cleaning at frequent intervals during boiler starting, at low load or during shutting down periods will reduce the hazard to a great extent.

• Cutting out of fuel automatically on fire out, and automatic combustion monitoring are essential features of modern boiler which may eliminate this hazard.


Significance of Acid Dew Temperature



STEAM ACCUMULATORS


Steam Accumulators

• Generally used in pulp and paper industry, chemical plants or breweries, combined heat and power generation process, etc.

• It can be noted that, in the processes, the usage of steam sometimes is very rapid and may cross the limits of boiler capacity and this leads into rapid depressurisation of boiler.

• A rapidly depressurised boiler can suffer poor steam quality and nuisance of trips on high/low water levels. And bringing back the boiler into service takes its own time.

• To mitigate these consequences of high steam demand, the engineers must oversize the boiler, design in back pressure regulators to control depressurisation or use a steam accumulator.


Steam Accumulators

• Generally, oversized of boiler means additional cost, and back pressure regulation will starve the steam-using equipment, causing high cycle times.

• One solution to this challenge is to incorporate steam accumulation equipment into the steam system design.


Steam Accumulators


• To meet these instantaneous steam load demands, usage of a dry accumulator or usage of a wet accumulator are preferred.

• Both design enhancements increase the mass of stored steam. Either type of steam accumulation will not create steam, but rather, create a means to store steam.

• Only adding fuel energy to a boiler will create more steam.


Dry Accumulator


This extra amount of steam will slow down the depressurisation rate of the boiler and help mitigate water carryover from the boiler

Wet Accumulator

• If more instantaneous steam is required than a dry accumulator can supply, then a wet accumulator can be used.

• A wet accumulator is a pressurised vessel in line with the boiler steam line. This vessel is pressurised to the boiler operating pressure and will discharge stored steam when the header is depressurised.

• Once depressurised, the boiler will recharge the accumulator when the load equipment no longer requires steam.

• Therefore, during idle periods of the steam use cycle, the accumulator can be fully recharged and be readied for the next cycle.

• The amount of stored steam is proportional to the water volume and change in pressure (based on the flash steam charts).

• This wet accumulator will store significantly more steam than the same size dry accumulator.




References

• Google• Central Electricity Generating Board Manuals• BHEL Manuals• Steam Accumulators and Steam Boiler Response to Load

Changes, by C. Merritt, Fulton Thermal Corp.• Friends and Colleagues



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Documents

Feed pumps, Feed Injectors, Feed Regulators, Feed heaters, Air Heaters and Steam Accumulators A. Ranganath