New low-pressure bottom ash pump removed for inspection.
Ash Pump Retrofit Reduces Maintenance Costs and Improves Reliability
By Joe W. Shaw,
Gainesville Regional Utilities
Gainesville Regional Utilities (GRU) has resolved a problem with the frequent
maintenance of the five low-pressure sump pumps on Deerhaven generating
station's Unit 2 bottom ash handling system. The old sump pumps were subjected
to abrasion from pumping water entrained with hard, crystalline bottom ash.
Recently, three of the pumps were replaced with submersible pumps that have
abrasion-resistant components and replacement liners.
|
New low-pressure bottom ash pump removed for inspection. |
As the first zero-discharge facility east of the Mississippi River,
Deerhaven's 235 MW Unit 2 helped pioneer plants that operate with recycled
water. The plant burns 500,000 tons of low-sulfur coal annually. The water for
the boiler and the bottom ash sump is supplied from two, 10 million-gallon
settling ponds that the plant calls ash cells. These 365 ft x 365 ft x 15 ft,
bentonite-lined settling ponds are recharged by recycled process water, water
drawn from two adjacent lime settling ponds, and by rainfall captured in the
stormwater run-off system.
Operating Problems
The two ash cells supply makeup water for the boilers and cooling towers. However, before it can be used as makeup, the water must first pass through either a brine concentrator system, which produces demineralized boiler water, or a front-end treatment system, where calcium and other minerals are removed before supplying the cooling towers. Water drawn from the cells also recharges the 12-ft deep, trough-like sump of the bottom ash handling system. At full capacity the refractory-lined hopper and sluice system can transfer 30 tons/day of bottom ash slurry to high-pressure pump intakes.
The sump water, pumped by low-pressure pumps, is used for flushing the unit's three bottom ash hoppers and for purging the bottom ash sump. These pumps are also used to enhance the operation of the high-pressure pumps. Twice per day the high-pressure pumps pump the bottom ash slurry out to the settling ponds through an 8-inch, basalt-lined pipe. Some additional water with a relatively high concentration of the gritty bottom ash reaches the sump from the overflow of the bottom ash hoppers. Since only about 60 percent of this abrasive ash normally settles out in the sump, the sluice water delivered from the sump back to the hoppers has a fairly high amount of suspended fines.
Over time, the fines suspended in the sump water caused wear on the impellers, sleeve bearings and line shafts of the low-pressure pumps. As a result, this deterioration caused a significant shaft imbalance and eventual failure of the entire rotating assembly.
The situation was further aggravated when the hopper system underwent a reconfiguration from a wall cooling system to a notched weir system. This enabled some of the overflow from the hoppers that had high concentrations of bottom ash to reach the sump. As a result the sump required periodic cleaning to remove the ash. In addition, the residual ash also damaged and dictated repairs to the five low-pressure pumps every three to four months.
 One of the settling ponds at the 235 MW Deerhaven power plant. |
The five low-pressure pumps required complete shaft replacements every third
or fourth rebuild. On average the pump shafts needed replacing once every year.
Just to pull one of the pumps was a daylong ordeal and then only yielded three
to four months of service from a completely refurbished unit.
Accurate budgeting for this type of recurring maintenance became prohibitive because there were so many variables, including the cost of replacing the sleeves and seals, which needed replacing every time a pump was removed for maintenance. In addition, the impellers very often needed extensive repairs and machining. OEM replacement parts and some non-OEM fabricated parts were also required.
Solution
Deerhaven Unit 2's bottom ash system requires 2,200 gpm of water at 50 psi. Five low-pressure pumps served the system previously. However, due to their inefficiency, at least three of the pumps had to operate in tandem to provide marginally enough total dynamic head (TDH).
As repair and maintenance costs steadily increased for the low-pressure pumps, the plant's management studied alternative pumps. A subsequent engineering study recommended replacing them with submersible pumps that had already proven to be reliable in GRU's wastewater treatment systems. The retrofit could also be achieved without any major changes to the base of the sump.
 |
The new pumps were mounted on vertical guide rails and a base elbow at the
bottom of the sump. By using the guide rails, proper re-alignment was possible
whenever the units were lifted out for inspection and/or maintenance. The only
structural modifications necessary to install the submersibles involved a slight
enlargement of the original pump mounting in the sump.
Even so, adopting a smaller submersible unit designed for wastewater applications for the power plant sump proved to be a mistake in the sump application. The first submersible pumps soon became susceptible to lower end bearing failures and impeller and casing damage from the abrasive ash water. After the first year of operation these pumps also began failing.
As the expense to repair them approached the replacement costs, the plant decided to replace two of them with wear-resistant submersible pumps. In April 2001 larger wear-resistant ITT Flygt Model 5560 submersible pumps were installed.
At $10,000 to $15,000 per pump in repair costs, the annual maintenance for the original low-pressure pumps would run as high as $75,000 per year. Today, the new submersible pumps are expected to reduce pump maintenance costs to $20,000, for an annual savings of approximately $50,000.
The new larger pumps have chromium impellers, replaceable chrome alloyed cast iron case liners and an improved lower seal design. In addition, the pumps have a bolted, split volute design that makes exchanging the replaceable casing liners easier.
Since being put into service the pumps have operated trouble free. GRU is among the earliest utilities in the U.S. to adopt the new abrasion-resistant pumps for a retrofit. Based on experience to date, the goal to acquire a more reliable pump has been fully met by the utility.
Author
Joe W. Shaw is the interim plant manager at Gainesville Regional Utilities' Deerhaven Station. He has progressed through the plant's craft training program, holding various positions including power plant mechanic, maintenance supervisor, and maintenance manager prior to being named interim plant manager in June 2001.