By: Jerrod Hunt EIT, Mississippi Power and John J. Beca CBC, ITT Industrial Products Group
To meet projected demand, Mississippi Power recently installed two 525 MW combined-cycle units at the existing Daniel coal burning plant. During peak summer demand periods, natural gas burners in the HRSGs are used to increase steam production and boost electric output from the combustion turbines. However, before the plant could fully utilize power augmentation of the gas turbines, the plant needed to upgrade its water treatment facility.
Mississippi Power Company’s Daniel power plant. Photograph courtesy of ITT Industrial Products Group.
Four skid-mounted reverse osmosis water treatment units, each capable of producing up to 220 gpm water, originally used horizontal submersible turbine driven pumps. Unfortunately, because they were installed horizontally, they suffered from cavitation and frequent seal problems that ultimately led to catastrophic pump failure. Invariably, when a pump failed, the plant’s output was reduced 25 to 30 MW.
Mississippi Power experienced three pump failures during the first year of operation. During the one-year startup the pumps ran constantly during peak periods and about half the time during non-peak periods. In addition to the lost generating capacity, the pump failures were costly to repair.
When a seal failure occurred the pumps had to be totally dismantled and in some cases the motor required replacing. The first pump that failed was sent off-site for repair. However, the cost of repairing the pump was only $400 less than the cost of buying a new pump.
To find a reliable solution for the frequent pump failures before the 2002 peak energy season, Mississippi Power contacted Brownlee-Morrow Engineering Company, a Goulds Pumps distributor. The plant originally considered a single-stage pump, but was unable to get the desired pump performance needed for the reverse osmosis system.
Multistage pumps replaced old submersible pumps feeding reverse osmosis systems. Photograph courtesy of ITT Industrial Products Group.
After analyzing the different options, the plant decided to purchase a Goulds 3355 MultiStage Pump with a PumpSmart control system. The Model 3355 is an end suction, low profile industrial pump with various porting options. The 3355 pump enabled Mississippi Power to retrofit the pump into the system simply by removing a portion of the piping and setting the pump in place.
The Goulds PumpSmart control system uses a standard centrifugal pump in conjunction with a smart controller and proprietary pump control software. The software, which resides on the controller microprocessor chip, is the “brains” of the system, allowing the pump to monitor and react to any system condition.
Originally the old style submersible pumps operated with a soft start controller and control valve to maintain a constant flow. In operation the pump operated at full rpm and the valve was adjusted for constant flow. Installation of the new control systems has eliminated the need for a throttling valve because the new control system is able to slowly ramp up the pump output.
When a pump is first started there is a large increase in the current. At the Daniel plant, the existing transformer did not have the capacity to support the additional starting load. Nonetheless, because the new control system is able to provide soft starting of the pumps and thus bring them up to full capacity slowly, the plant was able to keep the old transformer.
While waiting for the first Goulds pump to be delivered, the plant suffered two other pump failures. One of the pump failures was caused by water traveling up the conduit and damaging the motor and its controls. After these two failures, Mississippi Power decided to replace all four pumps. The first pump was installed in March 2002 and the other three pumps were installed in the middle of 2002. Since being put into service the plant has experienced no major pump failures.
Even though the motor size is rated at 100 HP, an increase from the original pump’s 75 HP, the rpm of the new pump is slower. The new pumps run at 2000 rpm instead of 3600 rpm and thus reduce the energy requirements of the pumps. Based on the power consumption savings alone, the new pumps are expected to have a four-year payback. Besides reducing energy requirements the new pumps are also quieter in operation and can be maintained by the plant’s own staff.
Jerrod Hunt (EIT) joined Mississippi Power in 1998 as part of a cooperative education program. He has a BS in mechanical engineering from Mississippi State University and is maintenance team leader for the combined-cycle units at the Daniel plant.
John J. Beca has served in various marketing positions within the pump industry. Beca began his career with Worthington Pumps. For the last 25 years he has directed marketing communications for ITT Industries companies.