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| | Pumping abrasive bottom ash slurry with no wear or repairs. |
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In the United States, coal-fired steam generation is the most common method of producing electricity. Part of the generation process involves the handling of bottom ash, one of the solid products of coal combustion.
At Northern Indiana Public Service Company (Nipsco), pumping of a bottom ash slurry had long plagued its 520 megawatt Unit 12 boiler, since three cantilever-type, boiler room sump-pumps failed frequently, resulting in high maintenance and repair bills and expensive downtime.
These pumping problems have now been solved easily and economically with three abrasion-resistant ITT Flygt slurry pumps in the 5500 series.
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The abrasive nature of bottom ash slurry
When it drains out of the boiler, bottom ash is a molten hot liquid that is channeled into a tank of water, where it crystallizes into chunks and small glass-like particles with sharp edges. After pulverizing in crushers, the material is sluiced into the slag line at the bottom of the tank and piped to the slag pond.
The slag line sluices away 99.5 percent of the slag in the system. The remaining 0.5 percent passes over weirs and flows by gravity through pipes into the boiler-room sump-pump pit, where 98 to 99 percent of the slag settles out, while 1 – 2 percent becomes suspended and is pumped out.
Resembling shattered glass in nature, bottom ash is extremely abrasive and wears out pipes, impellers, pump housings and seals at a very high rate. Handling this bottom ash slurry is a problem not only for Nipsco, but for nearly all coal-burning power plants.
Long history of problems with long-shaft pumps
From 1974 to 1982, long-shaft conventional pumps handled the bottom ash slurry. Dissatisfied with the rapid wear of these pumps, Nipsco replaced them in 1982 with another make of long-shaft pump. These also failed, with maintenance costs averaging about $20,000 per year.
The three double-volute vertical cantilever pumps in the pit caused the following chronic problems:
| Their narrow operating range, typically 3000 gallons per minute at 145 ft of head, was unsuitable for the radical and variable changes in flow to the pit. | | | | | The vertical cantilever pumps, specified to have a fairly high pumping capacity, rapidly depleted liquid in the pit, causing motors to cycle on and off frequently and then burn out at an average rate of two per month. Maintenance and repair costs were high. | | | | | The pit's abrasive slag wore the pump impellers unevenly, causing intense shaft vibration and wearing down bearings and other parts. | | | | | Prolonged downtime: pump disassembly, removal, reassembly and reinstallation took five shifts. The pumps were so large they had to be removed in three phases. The components were also cumbersome, making handling difficult and time-consuming.
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Submersibles are the solution
"Once it became obvious that the existing pumps in the pit were unsuitable for the application, we stopped spending time and money making adaptations to 'force' the pumps to work. Instead, we looked around for the latest developments in pump technology," states David L. Rogers, Nipsco's Manager of Technology Transfer.
Nipsco approached ITT Flygt and an HS 5550, 35 hp pump was proposed.
The HS 5550 features a bolt-together split volute permitting rapid changeout of the highchrome steel liner used to resist abrasion.
"Once we found the equipment that seemed suitable, we installed one of the new pumps in the pit and put it through its paces within a controlled field test environment," continues David L. Rogers.
The test was a success. After 1,000 hours of running time, the ITT Flygt HS 5500 showed no measurable wear.
"As soon as we were convinced that it was the right pump for the job, we retrofitted all the pumps in the pit without hesitation," adds David L. Rogers. Two HS 5550s were purchased to replace all of the conventional long-shaft pumps. The first was checked again after 4,000 hours of running time and still showed no measurable wear.
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Out-performing conventional pumps
When maintenance is required, the pump is simply hoisted out of the pit.
The HS 5550s are considerably smaller than the original longshaft units, and because the pumps are submerged, they are cooled by surrounding water. The close-coupled 35-hp motors run cooler than the overhead motors that drove the original pumps, allowing them to withstand 15 stop/start cycles per hour, compared to just five per hour for air-cooled motors in this service.
The new pumps also reduce cycling, however, because they have a lower pumping capacity and wider operating range. They rest unsecured on the floor of the 13ft-deep pit and are held in place by their own weight and a steel base-plate. Thick, flexible discharge hoses connect to pipes leading to the plant's fly ash pond. Slip-ring flanges at both ends of the hoses ease pump removal. When maintenance is required, the pump is simply hoisted out of the pit.
When maintenance is required, the pumps are simply hoisted out of the pit and serviced on the plant floor. A new volute liner can be installed in just a few hours.
The submersible pump's ability to lower the liquid level to a depth of about 1 ft is another benefit, since plant personnel can then see if slag is building up on the sump floor. The original pumps did not drain the sump to a point where the slag was clearly visible, and the buildup would sometimes block the suction area of the original long-shaft pumps.
After replacing its three cantilever-type pumps with ITT Flygt submersibles, the boiler-room sump problem is now solved and the danger of flooding is over. The generating station's boiler-room sump-pump pit is the master drainage sump for all residual flows from equipment, floor drains and pump discharges at higher elevations. Previous pit pump failures resulted in water flooding crusher motors in the slag handling system and the entire station was forced to shut down at times.
Profitable change
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Upper: The seal isolation zone is free of solids.
Lower: High chrome liner shows no sign of wear.
"The application of new technologies has paid off handsomely for Nipsco," states David L. Rogers. "Management's approach for solving the problems in the boiler room sump pump pit at Unit 12 is typical of its willingness to try new approaches."
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The change to ITT Flygt pumps was very profitable for the following reasons:
| compared to the long-shaft units with motor, the purchase price for the ITT Flygt pumps was lower, representing a savings of 64 percent per pump. | | | | | operation and maintenance costs were cut dramatically – based on the annual maintenance cost per pump, the ITT Flygt pumps will pay for themselves in less than one year on maintenance savings alone. Since they do not wear, the only maintenance required is routine oil inspection. | | | | | by changing to the submersible pumps, Nipsco is also realizing big energy savings. To pump the desired flow of 1000 gallons per minute per pump, each conventional pump required a brake horsepower of 110 hp, compared to only 23 hp for the ITT Flygt HS 5550, which is about one-fifth as much power required to obtain the same results, making the payback time even shorter. |
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"The problem of potential flooding was eliminated totally. The key to solving problems of this type should be a cooperative program between the user and the equipment manufacturer. We feel that the joint efforts by ITT Flygt and Nipsco made the solution possible," ends David L. Rogers.
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