By Teresa Hansen, Associate Editor
Dairyland Power Cooperative of La Crosse, Wis., meets the energy needs of more than half a million people through 25 electric distribution cooperatives and 20 municipal utilities. Formed in 1941, Dairyland currently services 62 counties over 44,500 square miles in Wisconsin, Minnesota, Iowa and Illinois. The cooperative’s generating stations (coal, hydro, natural gas, landfill gas and animal waste-to-energy) have a capacity of more than 1,100 MW.
Dairyland’s generating facilities use mostly steam turbine generator units. To operate these systems efficiently, a low-pressure vacuum must be maintained in the condenser component of each system. Although efficiency had always been important in the power cooperative’s 64-year history, as the plants aged, air in-leakage in the low pressure condenser and ancillary components became a growing concern. Any air ingress originating from seals, joints or broken lines increases the risk of thermal efficiency loss. This results from an increase in condenser pressure or vacuum loss. Addressing and managing this area of concern were priorities at Dairyland Power when Leif Tolokken, performance technician, met with Intek engineers to discuss the RheoVac system.
The RheoVac instrument detects air in-leakage and measures the performance of the exhausters that remove noncondensables and maintain low pressure in the condenser. Dairyland Power purchased a RheoVac 950 system for the John P. Madgett Station (JPM), a 400 MW generating unit that burns low-sulfur coal. JPM operates in a load following mode; the plant maintains continuous power to meet Dairyland Power’s system needs. Efficiency and economy are optimized by continuously generating power.
The RheoVac 950 instrument measures 10 diagnostic parameters on a real-time basis, providing power plant personnel with the information to distinguish between the possible causes of excess back pressure. The plant is alerted to specific problems so that corrective action can be taken promptly. Without the RheoVac 950 system, it is difficult to know whether high air in-leakage, degraded exhauster capacity or fouled tubes – all contributors to increased turbine back pressure – is undermining plant thermal efficiency. When air in-leakage is present, a multi-probe instrument system will indicate which side of the condenser is affected by the air in-leakage or whether the leak is above or below the water line. Troubleshooting time is significantly decreased because plant personnel have logical places to begin searching for the source of the problem.
Knowledge is Power
Vacuum pumps are used to remove the air that inevitably infiltrates the low-pressure side of a generating unit. When the air in-leakage does not contribute to excess condenser pressure on the turbine exhaust pressure, one pump is usually sufficient to maintain vacuum conditions. However, as air in-leakage increases and overwhelms a single pump, excess condenser pressure results. Additional pumps must be brought online to remove this excess pressure, otherwise plant thermal efficiency will be adversely affected. The intrusion of air is also accompanied by harmful gases, which can be dissolved in the condensate and become the source of corrosion for many generating components. Corrosion by-product deposition hinders heat transfer in the heat exchangers and may lead to equipment failure (particularly in the boilers), resulting in forced outages and lost revenue. In addition, the decreased thermal efficiency, caused by air in-leakage creates the need for a greater amount of fuel (coal) to produce the same amount of power.
Air in-leaks can be detected at their onset by monitoring the RheoVac 950 instrument’s measured water vapor to air mass ratio (W/A). This term, as defined by the RheoVac technology, is a direct measurement of vacuum quality. When the W/A value falls to three or below, air in-leakage is adversely affecting condenser performance. Leak repairs should be made promptly or additional pumps brought on-line before performance is seriously compromised. The increased energy needed to run an additional exhauster is generally minor when compared with the cost of operating with excess turbine back pressure.
Dairyland Power adopts the common practice of alternating its two vacuum pumps (same model). Tolokken had suspicions that one of the pumps was not working correctly, but had no specific data to evaluate his concern until the RheoVac system was installed. Almost immediately, a check of the RheoVac stored data for total mass flow rate and volume displacement (ACFM) confirmed that the capacity data corresponding to the suspect pump were lower than the values for the other pump. The capacity deterioration was clearly quantified by the newly available RheoVac data (Figure 1). Pump 61, operating alone, pumped about 1,800-1,900 ACFM while Pump 62, operating alone, pumped only 600-700 ACFM. Both pumps, operating together, pumped approximately 2,500 ACFM. Also, a significant decrease in the water to air mass ratio was observed when pump 62 operated independently.
“The RheoVac technology enabled our plant teams to put facts, graphs and figures on paper demonstrating differences in readings between the two pumps,” said Tolokken. “The readings proved to all of us that one of the pumps was malfunctioning, and to what degree.”
Moving Toward Resolution
Armed with this knowledge and data, Dairyland personnel removed the malfunctioning pump for repair and cleaning. Today, the once defective pump is operating at an optimal level. “Without the RheoVac 950, we could not have proven without reasonable doubt that this pump was malfunctioning,” offers Tolokken. “We had to be very sure before allocating the considerable investment required to remove and repair this equipment.”
Dairyland personnel were supported in their effort to use the RheoVac data to troubleshoot their condenser system by attending Intek’s annual Condenser Operations and Management Workshop. The seminar offers instruction on condenser dynamics and performance. Participants learn to analyze RheoVac data and use the information to make practical decisions that have an immediate economic benefit on plant operations.
After completing the program, Dairyland personnel returned with unique insight to share with others at the power plant. “It was easy for me to explain the technology, its benefits and operations, to the rest of our performance team,” said Tolokken. “With its user-friendly nature, the RheoVac 950 looks much like an interactive video game. The monitored data pops up on your computer. The data, once charted, is easy to interpret and use.”
Dairyland’s analysis of the baseline data, measured with the RheoVac 950 system, indicated that repair of one defective pump was necessary. “Without the new technology to identify the defective pump, this malfunction could still be causing efficiency losses to the plant,” said Tolokken. The RheoVac system saved Dairyland Power up to $36,000 per year – allowing the company to recoup the total cost of ownership (TCO) within one year.
Dairyland’s experience with the RheoVac system at the JPM station convinced them of the value of the instrument. Management is looking into outfitting all of its generating units with the RheoVac system in the coming years.