Air Preheater Retrofit and Enhancement

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Air Preheater Retrofit and Enhancement(O&M Conference-2009)

O&M

Knowledge Management System

Boiler

Paper Presentation On Air Preheater Retrofit and Enhancement

(O&M Conference-2009)

Date &Venue: February 13-15, 2009 at PMI, NTPC, NOIDA

Key Words: Sealing System, Leakage Drift, Sector And Axial Sealing

Plates, Radial And Axial Seals, Double Sealing

Organized by: Corporate OS, NTPC LTD

02/05/2009

Uploaded by : T.V.RAO,SR.MANAGER(HRD),PMI

Date : 05/07/2009


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HOWDEN - AIR PREHEATERS RETROFIT AND ENHANCEMENT

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ABSTRACT

AIR PREHEATERS PLAY AN INTEGRAL PART IN THE EFFICIENT OPERATION OF POWER PLANTS.

AS THE CONDITIONS IN THE PLANT CHANGE, FOR EXAMPLE BY CHANGES IN FUELS OR

OPERATING CYCLES, DEMANDS ON THE AIR PREHEATERS ALSO CHANGE. WHEN THIS

HAPPENS, HOWDENS ENGINEERS AND SITE SPECIALISTS CAN PROVIDE A COMPLETE

ADVISORY SERVICE.

UPGRADING AIR PREHEATERS TO THE LATEST TECHNOLOGY STANDARDS

BRINGS A WIDE RANGE OF BENEFITS.

It reduces the power demands of the draught fan, and thus raises the saleable power output.

It lowers gas velocities, allowing the precipitator to operate more efficiently and thus reducing dust

emission from the stack.

It helps to eliminate the temperature dilution effect of air leakage, and so helps reduce corrosion

downstream of the air preheater.

By reducing leakage it makes more air available at the coal mills, ensuring an adequate supply of

pulverised fuel to the burners especially when the coal is wet and compensating for shortfalls in output.

Reduced leakage also lessens the flow through the forced draught and induced draught fans, and thus

eliminates the output shortfall that would be caused by overloading the fans.

A routinely cleaning of the elements with highly efficient sootblowers will ensure a steady heat transferand minimize pressure drop in the plant system.

If operating condition has changed then replacement elements can improve overall performance

significantly.

RETROFITS AND ENHANCEMENTS ARE COST EFFECTIVE WAYS OF IMPROVING BOILER

PERFORMANCE

Upgrading or enhancing an air preheater is usually one of the most cost-effective ways of improving boiler

performance. Dramatic results can be achieved by either increasing thermal performance or reducingleakage. Before retrofitting FGD plant downstream of the boiler, it is always worth investigating air

preheater leakage, which can create unnecessary demands elsewhere in the system. Improving the air

preheater sealing system can actually reduce the size of the FGD plant, with obvious cost savings.

In many cases, heat recovery can be increased by installing higher performance elements, or increasing

their overall depth, or both.

Selecting from our broad range of different profiles, we supply direct replacements of all element types

commonly used in air preheaters regardless of make. In many cases alternative profiles or arrangementscan improve performance significantly, particularly if the operating conditions have been changed since the

original elements were specified. Improving the use of space within the rotor can also make significant

contributions to overall boiler efficiency.


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In several designs of air preheater, leakage can significantly increase over time. Known as leakage drift,

this can impair boiler operation in several ways. It can increase the fans power demands, raise velocities

and thus reduce effectiveness in the precipitators, reduce the flow of hot air to the mills or greatly diminish

the operational margins of the induced draught fan. These problems can be vastly reduced, or even

eliminated, by fitting the advanced Howden VN sealing system to the air preheater.

Air preheaters on coal and oil fired plants are subject to some degree of corrosion and fouling caused by

the approach to either the sulphuric acid or water dew point. Acid enhanced fouling is dependent on the

sulphur content of the fuel and the amount of SO2 to SO3 conversion in the boiler and FGD plant.

Corrosion can be minimized by the use of a cold end layer of higher grade steel or enamel coating. We

have sophisticated software that can calculate the cold end temperature for each case, enabling us to

determine the most economic choice of higher-cost material and the precise depth of it required.

To maintain the optimum performance of air preheaters it is vital to ensure that the elements are routinely

and effectively cleaned. Failure to do so will quickly lead to fouling, an increase in pressure drop and a

reduction in heat transfer. We supply a wide range of highly efficient sootblowers, which combine steam or

air blowing with high and low pressure water washing as appropriate.

A good sealing system had to be one that solved the problem of leakage drift

Air preheater leakage was recognized as a generic problem and to overcome the performance issues

listed above a joint operator/supplier co-operative developed a package of modifications which would

improve the situation. In view of the above, the objective of a good sealing system had to be one that

solved the problem of leakage drift thus providing sustainable performance in the time period between

outages. This ability to keep leakage levels constant over time is becoming even more important today

with the period between major overhaul outages increasing.

In the development of the sealing system, the direct causes of leakage drift were fully investigated on a

number of different operational preheater designs at various power plants throughout Great Britain. It was

found that a typical design aspect of air preheaters most commonly used on these power plants was the

adjustable sector plate. This design of air preheater, shown in Fig 1, is fundamentally very similar to many

preheaters supplied worldwide in the 70s and 80s.

Whilst the concept of using adjustable sealing surfaces was aimed at the outset to control leakage, the

design nevertheless suffered a number of factors that contributed to leakage drift.

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Cold Ho t

AdjusterActuated

AdjusterManual

AdjusterActuated

AdjusterManual


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Fig.1 Fig.1 Design of air preheater in the 70s and 80s.

These factors are:

- Leakage past primary seals between adjustable sector plate and casing due to corrosion and

erosion attack as illustrated in Fig 2.

Fig.2 Adjustable design

Two leakage paths

Adjustable design

- Leakage past primary seals between adjustable axial seal plates and casing.

- With sensor control systems any malfunction in the measuring device leads to lack of confidence

in the system and operation with sector plates in retract.

- Many preheaters had either 12 sector rotors with 30 sector plates or 24 sector rotors with 15

sector plates which meant that only one radial seal was in the sealing zone at any one time.

Damage or corrosion and erosion of these seals dramatically increased the leakage.

- In the event of heavy seal rubs it was customary for operators to make adjustments to external

adjusters and linkages to withdraw plates increasing running tolerances and leakage levels.

Adjustments of this nature were generally unrecorded leading ultimately to a lack of knowledge of

plate alignment.

The features and benefits of the new sealing system developed are described in this paper with particular

reference to retrofit applications.

Sealing System Developments

Over a decade ago a sealing system was developed by Howden in conjunction with the then C.E.G.B.

which substantially overcame the problems described above.

This sealing system was retro-fitted to existing air preheaters on coal and oil fired boilers with considerable

success. The sealing system developed offers unique advantages not only in its simplicity of construction,

but more importantly its ability to maintain design leakage levels for long periods of time.

Sector and Axial Sealing Plates

The top and bottom sector plate primary seals were a sliding arrangement to allow movement of the

sealing surfaces, and were prone to wear as well as erosion by particles in the gas streams. These sealsshowed good characteristics initially but after years of operation contributed to the leakage drift by allowing

an additional passage of air around the sector plates. As a direct consequence of this, the depositing of

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ash particles behind the sector plates caused the sector plates to have reduced travel and finally jam in

position.

Similar problems were found on the axial seal plates, and the best way to eradicate this was found to be

the complete removal of the adjustment to the sealing surfaces. During a preheater retrofit at site, the

sector and axial sealing plates are set to accommodate all clearance gaps for all operational duties and

welded in position, thereafter requiring no further adjustment or maintenance.

Radial & Axial Seals

Single leaf sealing strips are fitted to the radial division plates and axial seal holding bars to provide a

sharp edge to flow and allow fine adjustment for consistent seal setting.

Early experiences showed that two-leaf type seals showed better initial sealing characteristics with

adjustable mechanisms, but because of this allowance of adjustment they were prone to wear. When put

under laboratory tests, it was shown that once worn these two-leaf seals were no longer any better than an

equivalent single leaf seal. Moreover, with the modifications to fixing the sealing surfaces, the single leaf

seal was more effective as it had the characteristic of a sharp edge rather than a smooth curve.

The comparison of performance shown in Fig 6 shows conclusively that for seal gaps larger than 3mm the

leaf seal is superior to the channel seal. For the major regions of a fixed sealing surface air preheater

where the gap varies between 3 and 30mm a reduction in radial seal leakage approaching 6% is

achievable from this change of seal type.

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12

13

0 2 4 6 8 1 1 1 1 1 2 2 2 2 2 3 3 3

Seal Gap

Standard Channel

Worn Channel

Datum 100% - Leaf

L

a%oese

Channel / Worn Channel / Leaf Seal Comparison at seal differential

10mm

25m

Seal

Gap

Channel

Seal Worn LeafSeal

5m

25mm

Fig 3. Reduction in radial seal leakage approaching 6%

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The tests on worn channel seals confirmed the drift tendency of this seal type. The results indicated a loss

of performance which was sustained at around 7% for gaps of 3mm to 20mm and which had decreased to

zero by the time the gap had been increased to 30mm. The amount of the wear has an influence on this

loss and tests on seals with the recess worn flat indicated even greater drift.


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Double Sealing

Traditional rotor designs have only one radial sealing strip passing under the sector plate at any one time.

Theory shows that leakage is directly proportional to the square root of the driving pressure differential:

dPL

and so it follows that if you double the number of seals under the sector plate, effectively halving the

pressure differential acting on each sealing surface, you will decrease the direct radial leakage by

approximately 30%:

dPLdPLdP

L

7071.02

1

2

Rig tests were performed where the intermediate seal pressure was measured and the pressure drop

across the first seal was found to be 50% of the overall pressure drop.

Various arrangements of seal gap sizes and spacing between seals were tested and it was found that as

the seal gap size increased, the effectiveness of double sealing decreased, but at the normal operational

gap sizes of 3mm the expected leakage reductions of 30% were obtained.

However, the effectiveness of double sealing increased as the spacing increased, particularly so on larger

gap sizes. Double sealing effectiveness was found to be influenced by the pressure differential, gap size

and seal spacing and so tests were carried out to simulate the following sets of criteria :

1. Average pressure differential & large gaps (main heaters)

2. High pressure differential & medium gaps (mill heaters)

For fixed sealing surface main air preheaters with pressure differentials of around 4kPa there was a

progressive fall off in hot end double sealing effectiveness as the heater radius increased (and the seal

gap increased). This fall off actually starts within the radius of a size 28, and at a radius of a size 35 the

effectiveness of the hot end double sealing has fallen to about 16%. As the seal gap size in the cold end is

fixed at the optimum operational settings and does not increase, the double sealing effectiveness is

maintained at the full 30%.

The loss of effectiveness in the hot end has the effect of reducing the overall double sealing effectiveness

of larger heaters, typically to around 24% on a size 35 heater.

The hot end effectiveness of higher pressure differential mill air preheaters is not significantly lower than

that for main air preheaters, and this reduction in effectiveness only accounts for a difference of 0.4% of

mass flow.

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onclusions

em described can be retrofitted to existing air preheaters to providea reduction in existing

Reduction in FD, ID and PA fan power for the same generated load.

gher performance heatingsurface during the retrofit.

from efficiency improvements and due to improvedefficiency of the precipitator brought about by increased residence time.

emsassociated with adjustable sector plate designs.

nt absorbers and boosterfans can be reduced.

rmance without leakage drift improves the plants abilityto obtain production targets during periods between major outages.

costs.

C

The sealing syst

leakage levels and to eliminate the inherent problem of leakage increase with time. The benefits of this

are:

Increased unit output by using the saved fan power.

Improvement in boiler efficiency by the adoption of hi

Reduction in stack emission

Considerable reduction in maintenance on seals and electrical control syst

If FGD plant is being considered, size of FGD pla

Maintainability of perfo

Reduced operating costs.

Reduction in on load monitoring.

Reduction in outage maintenance

Documents

Air Preheater Retrofit and Enhancement