GE AirFiltration WorldCement Article-Jan2012

7/27/2019 GE AirFiltration WorldCement Article-Jan2012

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In 1999, officials at what is now the Argos USA plant decided to build a new cement line to help

meet the demand for more production at a lower cost. Over time, the 1960s technology that had

been installed was seeing an increase in maintenance costs, a decrease in reliability and a growing

number of issues with equipment, including dust collection.

While the team at the plant set out to build a more efficient line, there was an important lesson

learned the hard way. There may be ways to save money initially, but the most efficient and the least

expensive thing to do in the long run often requires consulting with experts and finding equipment

that is right for your plant.

Without a doubt, said Nick Trout, Maintenance Manager at the 1.5 million tpa plant, you have to

look at the entire system it cant be just individual components. Were big on partnering with suppliers.

It needs to be a win-win.

Andy Winston, GE Energy,

USA, together with insight

from Nick Trout, Argos USA,describes events and lessons

learned from the start up of

a cement plant to meeting

looming environmental

regulations.

DustCollectorOptimisation

Reprinted from [Jan 12] worldcement.com


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Production bottleneck, maintenancenightmare

As part of the overhaul, the Alabama plant decided to add

a rotary preheater/precalciner kiln, the fifth that had been

installed on site. It would have enough capacity to replace

four smaller kilns two there and two in Atlanta, Georgia

and also have capacity for an additional 100 000 tpa. The

company wanted a plant that was so efficient it would create

the additional capacity. They also wanted to be the lowest

cost producer.However, plans changed. The cost of construction soon

needed to be reduced by 30%, and the kiln/raw mill dust

collectors were among the targets for cost cutting. The

dust collector was resized for the absolute minimum air

volume, not taking into consideration in-leakage and false

air. Additional compartments (air volume) would then have

to be added at a later date if required. Sure enough, initial

operation of the plant found higher air inputs than design

and air in-leakage in the region of 5%.

We had to compromise, Trout said. We knew the

baghouse design was marginal. We just didnt know it would

cause the problems it did. We thought it would be more of

a production bottleneck. It ended up being a maintenancenightmare, almost from the get go.

Initial set of filter bagsThe initial set of filter bags and cages were supplied by

GE Energy. Within the first two years of operation, fears

that the baghouse would be too small were proven correct.

In production, air volumes were more than 20% over the

baghouse design surging to 520 000 acfm in a collector

sized for only 454 000. It pushed the air-to-cloth ratio to the

limit, prompting a dramatic increase in filter bag cleaning

cycles due to the falsely elevated differential pressure

resulting from the additional air volume. The plant also saw

abrasion of the filter bags because of the increased volume

and high velocity in the ductwork and the hoppers. The net

result was that after 18 months of operation, all of the bags

and cages were replaced due to premature failures.

Second set of filter bagsThe second set of filter bags and cages were supplied by an

alternate supplier based on price. The bags and cages were

replaced compartment by compartment, with the kiln still in

continuous operation, and it was quickly apparent that the

plant was experiencing problems of a different kind.

This stemmed from multiple issues, including:

l A lighter-weight filter bag as measured in ounces/yd2.

l Poorly chosen snapband materials and construction that

were insufficient to keep a proper seal, so they fell from

the tubesheet into the hopper.

l A poor bag-to-cage fit.

l

Cages tapered at the top and bottom and constructed ofa lighter gauge material.

The construction left a lot to be desired, Trout said.

Detail to very important specifications was not there.

Trout knew he needed to add to the existing kiln/raw

mill dust collector, but how would he make it fit? To help

find a solution, GE was selected to look at all the different

configurations, addressing duct velocity and abrasion issues.

GE carried out engineering studies and used computational

fluid dynamics modeling to arrive at the right solutions for

the myriad of problems associated with the original dust

collector design.

The analysis showed that many of the problemsstemmed from the original equipment purchased at the

start-up. Trout says today that the plant should have

received more feedback on the original design of the overall

dust collector system.

It was a big lesson learned, Trout said. Every area

where we compromised, we had to go back and revisit and

reengineer.

Ultimately, plant leaders chose to:

l Add four new compartments, expanding from 10 up to

14.

l Enlarge inlet and outlet plenums to accommodate a 27%

air volume increase.l Redesign the inlet plenum to evenly distribute airflow,

based on the Computational Fluid Dynamics analysis.

l Modify the inlet ductwork in each module and the

hopper baffling.

l Provide 9016 filter bags and cages.

The biggest thing, Trout said, was to have a complete

package. GE had better industry knowledge rather than just

dust collector knowledge. Because of their specific cement

expertise they were aware of the conditions their products

would see or be exposed to.

Theres more to a good filtering system than just bagsand cages, and GE helped us find the solution and also

continued to help monitor to make sure it was working.

Today, the plant has the right equipment, but including

the initial purchase, the net total was 50% higher than

necessary.

Filter bag optionsIt is important to remember that the filter bags and cages

are just a component of the kiln/raw mill dust collector.

They alone cannot make up for shortcomings in the

system, but they are critical for several factors that must be

considered, including:

l Efficiency.

l Durability.Raw mill/kiln baghouse.



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l Construction.

l Bag-to-cage fit.

l Cage construction.

Efficiency and durability work hand in hand. One

without the other will result in higher emissions, short filter

bag life and higher operating and maintenance costs.

Performance hinges largely on the ePTFE membrane,

which handles efficiency, and the backing fabric, which

plays a key role in both efficiency and durability. The

correct weight for the application and the proper bonding

of the ePTFE membrane to the backing fabric is vital. One

of the most frequently selected fabric bags for US cement

kiln/raw mill dust collectors has been a 22 oz./yd2 woven

fibreglass laminated with GE Energys Preveil expanded

PTFE membrane, formulated for this application. During

the life of the filter bag, it is under great stress from online

operation and cleaning. The bond of the membrane to

the backing fabric (in this case fibreglass), must maintain

its integrity under this stress and not separate or crack.This plant has had several sets of filter bags installed from

different companies, but with the last one, which was

furnished by GE Energy, the stack tests for the three years

have been below the new particulate standards that will take

hold in 2013. The fourth-year results were slightly above,

but the results were affected by rust scale from the clean air

plenum, not leaks in the filter bags.

Construction can make or break filter bag quality, and,

unfortunately, this is an area where many manufacturers cut

corners. Snap band materials must match the application,

from stainless steel band material to the proper gasket

materials. Proper sewing thread is also key. It should match

the backing fabric, in this case fibreglass.Bag-to-cage fit is another area greatly ignored. It is

challenging and takes accurate manufacturing to maintain

the fit from the first filter bag and cage to the last, but

the relationship is very important to filter bag cleaning

efficiency. If too tight, the filter bag cannot flex and will not

break off the dust cake. If too loose, the bag over flexes and

leads to filter bag failure due to flex fatigue. Manufacturing

both components in the same manufacturing plant is

extremely helpful, and GE Energy is the only supplier that

has this capability. At a minimum, at least one cage must be

on-hand when the filter bags are manufactured to check fit

during the production run. In addition, the inside diameter

of the cage top must fit properly with the outside diameter

of the filter bag, as well as a snap band top to keep the filter

bags and cages from swaying during operation. This was a

huge problem in this dust collector with the second set of

filter bags, before the plant switched to bags provided by

GE.

Cage construction causes several areas of concern,

starting with the gauge of wire. Most kiln/raw mill dust

collectors use 11 gauge wire, but this plant decided on

9 gauge. The 9 gauge gives a little more support and stands

up better to handling and shipping than the 11 gauge.

The number of wires is also important when it comes to

supporting the filter bag properly in this case, a minimumof 20 vertical wires and the horizontal rings at a minimum

of 8 in. spacing to accomplish the proper support. The

cages should be the same diameter top to bottom without

any tapering. All welds should be to the inside, not in any

filter bags contact area, and there should be no sharp areas

that make filter bag contact.

CorrosionThe third set of filter bags provided by GE worked well at

the Alabama plant, helping to resolve the production and

environmental issues.

In time, though, new issues with corrosion emerged that

tied back to operating procedures.Todays kiln/raw mill kiln dust collectors, in the raw mill

on condition, operate at much lower dust collector inlet

Large amounts of rust scale on the tubesheet.

Corrosion on the tubesheet.

Corrosion on the compartment walls and ceiling.



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temperatures. In this plant, the inlet temperature to the

dust collector is below 300 F (150 C), meaning the dust

collector often operates in or near the acid dew point. In

addition, if a good start-up procedure is not followed, the

dew point condition only worsens.

At the Alabama plant, sulfur was present in the

gas stream and, with temperatures below 300 F,

condensation ensued. The filter bags were well-

protected, but the metal surface areas made of carbon

steel were not. Excessive corrosion caused rust to flakeoff the walls, and clumps landed inside the filter bags.

What this plant now normally does to help during

warmup, when the full volume of the dust collector

is not needed, is have two compartments online at a

time. With the reduced volume, the velocities through

the dust collector are also reduced. If the entire dust

collector is online, it takes quite a while for the collector

to come up to temperature, and the compartments

operate for a considerable amount of time in the dew

point.

The plant now starts with two compartments online

and the cleaning system off. As the differential pressure

reaches 6 in. (150 mm) across the dust collector andthe inlet temperature reaches 300 F (150 C), the next

two compartments get added. This procedure continues

until all compartments are online, and the cleaning cycle

does not start until feed is put to the process equipment.

The new procedure brings the temperature in the

compartments up rapidly, passing through dew point

quickly.

The fourth year stack test that was slightly above

0.04 lb/t of clinker was elevated due to the rust scale going

out the stack (see below). Early in the fourth year, filter bags

started to fail from the rust being pulsed down into them.

The dust particles came to rest between the cage and the

clean side of the filter bag, creating an abrasive condition

where the bags contacted the cage wires and eventually wore

holes in the woven fibreglass. Over time, this has created

more and more failures. It is important to note that the

rust scale and dust was the initial mode of failure. Had itnot been for this condition, the filter bags would likely have

lasted an additional two to three years.

Immediate corrective actionGoing forward, the Alabama plant will sandblast and apply

a corrosion resistant coating, replace some roof sections and

add a fourth set of bags and cages from GE. It should be

finished early this year in time to meet the regulations that

take hold next year.

There are plans to monitor all aspects of the dust

collector operation, and one advantage of the new

monitoring system is that it will help with corrective action.

When something happens out of the norm, it needs to beaddressed promptly. Operators need to anticipate what

might lead to bag failure. After all, each compartment has

644 filter bags.

We dont necessarily have to call GE, Trout said. They

call us to check whats happening. Its an extra set of eyes to

analyse whats happening and make sure things are running

smoothly.


GE AirFiltration WorldCement Article-Jan2012

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