Installation of a fabric filter for the “El Alto” plant

At the beginning of 2007, Cementos Portland Valderrivas signed a contract with FLSmidth for replacement of the existing electrostatic precipitator with a new fabric filter to process exhaust gases from the kiln and raw mill of Line VII of the “El Alto” plant, located in Morata de Tajuña (Comunidad de Madrid).

The existing precipitator (Fig. 1), installed by FLSmidth in 1991, was no longer compliant with the dust emissions requirements for the plant and needed to be replaced. In addition to the those singularities inherent in the design of a filter meeting the required specifications, in terms of the characteristics of the gases to be treated and the required operating guarantees, the project presented two major challenges:

– The first of these was to design the layout of the installation. This design required, within the limited space in the designated area the installation of a filter with a greater floor area than the previous one (Fig. 2), in addition to the new system of inlet and outlet ducts, the new I.D. fan with an outflow to the chimney and the necessary supports for all of these components.

– The other challenge was to ensure that all tasks, from dismantling of the existing equipment to assembly of the new components, were carried out within the very limited downtime available before the resumption of kiln operation.

The new installation (Figs. 3 and 4) is a jet-pulse “Fabric Clean™” filter model, with 8 modules with corresponding inlet dampers and 16 clean air compartments, each with an outlet damper. The clean air compartments are made from stainless steel.

The central inlet manifold (with one part for dirty air inflow and another for clean air outflow) is a variable section and is made from AVESTA 2205 type stainless steel, a material with expansion characteristics similar to those of carbon steel. This standard of quality is needed to prevent damage that could occur as a result of differences between the expansion ratios of the different steel types.

Each clean air compartment includes a compressed air tank for filter cleaning, and in total there are 3456 filter bags with a filtration area of 8294 m². The filters selected for this project are made from glass fibre with an e-PTFE membrane and are 6 m long. The cages are made of stainless steel.

During the raw mill operation, the filter is designed to deal with an effective gas flow rate of 520 000 m³/hr at a temperature of 95 °C. For operation when the mill is offline, kiln gases are normally water-cooled in the condition tower before entering the fabric filter. The installation has been designed for a gas flow rate of 490 000 m³/hr at a temperature of 170 °C.

For safety reasons, however, the glass fibre filter bags can operate at a temperature of up to 260 °C at the entrance to the filter. To protect the filter against higher gas temperatures (for example: in the event of malfunction of the cooling tower), the new installation includes a valve for dilution of kiln gases with ambient air prior to entry into the filter. This valve begins to open when the temperature at the entrance to the filter reaches 220 °C and its opening is adjusted automatically to maintain this temperature.

The filter compartments enable online maintenance to be carried out, which means that one dirty air module and its two corresponding clean air chambers can be isolated so that they can be opened and the filters replaced, while the filter con-tinues to operate with its “normal” gas rate flowing though the other modules.

The limited space available in the plant necessitated the design of a very compact system that would allow all of the components (filter, ducts, fan, supports and access points) to be accommodated in the designated area. There was also a need to ensure access for people and vehicles to the area once the installation was complete.

In addition to the conditions previously described, the supporting concrete structure of the old precipitator had to be retained and the position of the existing chimney supported on the ­cyclone tower of the kiln.

The fan for the existing electrostatic precipitator filter was installed on the ground just below the entrance to the precipitator. The fan did not provide sufficient capacity (pressure) for the new fabric filter, which meant it had to be replaced. In addition, the filter was required to operate under negative pressure, which meant installing a fan between the filter outflow and the chimney.

As for the concrete structure, even taking into account the advantage of its supposedly having such a wide support upon which to place the new filter, this presented the difficulty of adapting the new components to the existing dimensions. Also this structure had to be lengthened using a steel structure upon which to rest the weight of the first filter chambers and the end part of the filter inflow conduits (Fig. 5). Given these circumstances, the new fan was located just below the fabric filter, inside the above-mentioned concrete structure. Locating the fan here meant that the design of two of the filters was not easy:

– This first affected the duct running from the fabric filter to the fan (Fig. 6). This duct runs out and then doubles back on itself beneath the filter, to connect with the fan entrance.

– Located at the fan exit is the duct which runs to the chimney (Fig. 7). Its route takes it first through a 90 ° turn to the left, then runs in an uninterrupted vertical line, after which it connects with the chimney, the joint being 36 m above ground level. One restriction, which also applied to all of the components previously mentioned, was that the use of new floor areas was to be avoided, which is why the duct route indicated above was located in the narrow space between the concrete supporting structure and the cyclone tower, thereby not blocking any of the plant’s access routes in any way.

In the area where the gas enters the fabric filter, it was necessary to configure the new ducts so that the installation should be as simple as possible, with optimal characteristics in terms of the space to be occupied in the plant by its supporting structures.

This design (Fig. 8) was successfully achieved in the following way:starting from the existing structure for the gas ducts (coming from the kiln and the raw mill respectively), at which point the first support was located, the new ducts rise at an angle of 60 ° from the horizontal, and then descend to the filter at a 45 ° angle. Over the course of this descent path, the two ducts join to form one, and instead of employing a support point in the middle area, the support was successfully positioned further along where the fabric filter structure became wider. This meant that the total weight of the structures to be installed was substantially reduced, while the number of components at ground level that could have blocked access routes through the area was kept to a minimum.

The greatest restriction regarding the project in terms of ­scheduling was the downtime of the plant. This was limited to 34 days, which meant that an exact schedule was needed, which began with the engineering phases and was followed by manufacturing, materials transportation and component preassembly, and ended with disassembly of the old installations and assembly of the new system, in order to complete the project by the specified deadline.

Engineering, manufacturing and transportation to the plant were carried out starting in the first few months of 2007, as per schedule. Then in the second half of August, these were followed by preassembly and preparation of the components before shutting down the line.

During the preassembly phase the filter was assembled on the ground, in four main sections (Fig. 9). The two largest sections each comprised four dirty air chambers, with their respective clean air chambers in the upper section. This left the central inlet duct and inlet and outlet dampers to assemble, once the installation was in its final position. The two remaining sections both comprised four conjoined material hoppers.

In addition to preassembling the fabric filter, the I.D. fan was placed in its final position (Fig. 10), while the concrete structure of the existing filter was widened with a new steel structure as previously described. Once this was widened, the support base of the new filter was consolidated with a number of lattices placed over the supporting structure.

With regard to the system of ducts described in previous sections, these were preassembled on the ground (Fig. 11) in the longest lengths possible and were then erected , taking special care to maintain the respective angles between each of the sections so that there were no subsequent adjustment problems on bringing them into their final positions.

All of the stated components: the filter chambers divided into two sections, the two groups of hoppers, the filter support lattices and the conduit systems, were preassembled in an area close to the final installation site (Fig. 12). However, despite being so close, each component still had to be lifted by crane and moved to the filter location site, and once there, to be installed in their final position.

Once all of the previously described components were prepared, the line was shut-down on the date designated by the plant.

Shut-down began with an unexpected occurrence, since the crane selected for the disassembly and subsequent assembly phase was not available. The crane had run behind schedule in another project but the scarcity of market supply for cranes of this size (800 t capacity) meant that it was impossible to immediately obtain another crane with the same characteristics.

The decision was taken to agree with the mechanical assembly company and the crane company regarding the provision of a smaller crane (400 t), to begin with those disassembly tasks that were feasible, and to reorganize the workflow schedule until the crane initially chosen became available. After six days of working very intensively to compensate for the lower capacity of the crane, the crane of choice became available and it was then possible to begin disassembly of the main parts of the electrostatic precipitator.

From this point onwards, disassembly was carried out in large blocks weighing between 60 and 70 tonnes each (Figs. 13 and 14). To obtain these sections, the precipitator was literally sliced carefully into blocks of similar weight and size. For removal from the position of the electro-filter to the area where components were taken away for flattening, the crane used a caterpillar traction system.

In addition to the precipitator, the old gas ducts in front had to be dismantled (Fig. 15), a task that was carried out when only the first crane was available, the same applied to the dust transportation lines. As for the old fan, this was dismantled once the new filter was finally in place, since this was not located in an area where it would have interfered with the installation to be assembled. The total combined weight of the disassembled components was approximately 310 t.

Once the components described were disassembled and the base area for the new filter had been completely cleaned, the various parts with which the new system had been prepared during the preassembly phase began to be lifted into their respective raised positions, in accordance with the pre-determined sequence. The first parts to be installed were some of the hopper access platforms and the four screw conveyors for transportation of dust beneath the new filter.

Then the support lattices for the filters were hoisted up (Fig. 16) which were placed over the concrete structure retained from the old installation.

Following this, the stretch of duct carrying gases from the filter to the fan was installed (Fig. 17) which, owing to the bends in its course had to be fitted prior to placing the hopper groups into the concrete structure.

After this, the two groups of four hoppers were hoisted into positions (Fig. 18), which were already fitted with rotary locks through which they were connected to the previously installed screw conveyors. The support points for the hoppers over the support structure were the lattices already mentioned. To install the hoppers, the 19 m long modules had to be moved, each one weighing approximately 15 t.

The two largest components to be moved were the sections in which the filter modules had been preassembled (Figs. 19 and 20). These were raised into position above the hoppers with the thermal insulation already fitted. The movement of these sections, given the enormous weight of each of them (approximately 60 t), had to be carried out with the greatest care, on the one hand because of the sheer weight to be moved and on the other because of the limited space available to perform the maneuvers. (Fig. 21), in which the slightest collision could have damaged the filter itself or the installations located around it. As the filter itself was being assembled, the remaining inlet and outlet ducts were placed in position.

Added to the outflow ducts (the section running from the filter to the fan having already been installed) were the sections running from the fan to the chimney. The join between the conduit and the chimney was particularly difficult to execute, since its position was very high and the join area was difficult to access. The inlet sections, which had been assembled into four sections during the preassembly stage, were not installed until the filter had been placed in its final position.

Two of the four sections were joined with those coming from the kiln and the raw mill and a third was installed at the inlet to the filter. This left the final central section, which had to be installed with extreme precision, since this would create the join in mid-air between three sections (those previously described) which had completely different angles of cut on totally different planes. Under these circumstances there was a risk that unless the planes and the preassembly work were completely accurate, one of the conduits would have required modification, which would have meant a failure to meet the agreed deadlines. Nevertheless, the section was correctly placed in position (Fig. 22) with hardly any adjustments to be made, so the task was carried out exactly as planned.

In the section containing the filter the inlet and outlet valves still had to be installed, in addition to the central duct. These components were progressively installed in coordination with the duct systems, leaving two very important phases for mechanical finalization of the assembly.

One of these was the installation of the 3,456 filter bags and their respective cages, (Fig. 23), a task that was carried out very routinely, though with level of care that this essential component of the installation required.

The other was the installation of the climate cover, which although not very heavy, had a huge handling volume, since it was twice the volume of the components moved with the filter hull. This made it very easy for any one of the surrounding installations to be damaged by just one false move during elevation or as a result of the wind. These difficulties were overcome without any great problems, and the mechanical assembly phase was completed according to schedule. The approximate weight of the components elevated was the 400 tonnes, in addition to the thermal insulation, which put the total weight at 435 tonnes.

As the final phase of mechanical engineering was being carried out, electrical assembly began, consisting of the installation of the FLSmidth‘s Smart-Pulse SPC 800 filter control system, along with all connections from the control system to the compressed air valves, the compressed air manifold pressure detectors, the differential pressure gauge, etc. This assembly also included integration of the system into the plant’s electrical system, so that it could be operated and controlled from the central control room. The execution of this installation was very easy; the advantage being that all of the connections to the compressed air valves and pressure detectors were supplied already wired and only needed to be mounted into their final positions.

In addition to the mechanical and electrical installations, and fully synchronized with the rest of the work, was completion of the thermal insulation of components such as the filter, fan and duct system. This task required a high degree of coordination with the mechanical works, from which it followed on, both in the preassembly and assembly phases.

Following execution of the assembly work described, and still within the 34 day downtime limit, the modules were tested for air tightness. Also, the filter inlet and outlet dampers were checked for correct functioning, as was the new fresh air valve, which stops gases ­
reaching the filter at a temperature exceeding that which is specified for the filter bag material.

Once these checks had been carried out, together with those for the fan, the air locks, the new screw conveyors and the SPC 800 controller (both from the filter and from the central control room), the production line could be started up.

Since being installed, the new filter has been operating effectively, with the predicted load loss and a dust emission substantially below the guaranteed level: 20 mg/Nm³ of dry gas. Table 1 compares the operating levels recorded during the first months of operation with the guaranteed levels.

Dust emissions, gas flow rates and chimney temperatures, are the average  values from a total of 17 measurements made by two accredited companies (14 of them with the raw mill operational). During the measuring period, the filter controller was adjusted to start the cleaning cycle of the filter bags at a differential ­pressure of 15 mbar, and to stop the cycle with a differential pressure of 14 mbar. With this adjustment, the calculation of compressed air recorded to clean the filter bags is surprisingly low. Currently, more than one year since the filter began operating, dust emission remains at its initial level and not one of the filter bags has needed changing.

Managing to carry out the necessary disassembly and assembly operations within the established time frame of 34 days would not have been possible without the high degree of coordination between the various departments involved, both within Cementos Portland Valderrivas and FLSmidth. However, the work conducted by the mechanical assembly, thermal insulation, and electrical installation companies and crane operators, etc. should not be forgotten, since together they ensured that all of the tasks of the various different specializations were carried out with minimal interruption, enabling the unforeseen difficulties that undeniably arose to be resolved onsite.

The Health and Safety at Work program conducted during assembly also deserves a special mention, since thanks to implementation of the Safety Plan and its recommendations for the workers of all of the companies involved, no accidents were reported during execution of the project.