1 January 2007
A multiple path to ultrasonic optimization
By Harvey Klaer and Terry Burch
Cross-planed and single-planed flowmeter yields surprising power plant efficiencies, dollar savings
A mere 1% change in the thermal operating efficiency of a large modern turbine generator is equivalent to about $15-20 million when capitalized over its working life.
This is the result of increased fuel costs and deregulated markets within the thermal power industry.
Power plant efficiency has become an even more important focus for power generation companies.
Advantages exist in many of the nooks and crannies of the process. Here is a description and discussion of implementing a multiple-path ultrasonic transit-time flowmeter for monitoring the circulating cooling water system at La Cygne Generating Station (south of Kansas City, Kans.).
Unique flexibility in providing installation alternatives and superior flow-rate measurement capability associated with this flow-metering technology provided significant advantages for this type of power plant application.
Specific benefits for improving overall power generation efficiency and reducing plant maintenance requirements are the principal focus. This is real-world experience from the challenging competitive environment in which steam-generated power providers must operate today.
Kansas City Power & Light Company (KCP&L) is an investor-owned generating utility. KCP&L's La Cygne Generating Station consists of two coal-fired units.
Unit 1 is a 750-Megawatt Cyclone Fired once through super-critical unit. An 11-ft diameter underground circulating water tunnel provides cooling water supplied from a combination of three large, low-RPM 166,000-GPM pumps.
Unit 2 is a 700-Megawatt conventional Radiant Boiler type unit. A 10-ft diameter underground circulating water tunnel provides cooling water supplied from two large, low-RPM 167,500-GPM pumps.
Operational engineers at La Cygne Generating Station knew the plant needed a water-flow measurement system that would provide accurate flow data for the circulating water system to the Unit 1 condenser.
The condenser tube sheets would occasionally plug up with debris from the plant's source water lake, resulting in reduced overall efficiency of power generation.
Consequently, they desired a system with the capability to detect condenser fouling before it developed into a load limiting condition and to identify condition-based maintenance actions that could go into service during winter months prior to the higher load demands common in the summer.
Plugged tubes in the condenser resulting in even a 10% reduction in unit efficiency could raise overall plant fuel rates considerably.
Another costly facet of the plant's operation was performing maintenance on each of three circulating water pumps on a strict periodic schedule, typically every 3-4 years, even if no obvious plant performance degradations were registering.
Engineering serviced the pumps on a regular schedule, pulling them form their deep wells, disassembling them, and shipping them outside the plant for rebuilding. Because there were no reliable methods for detecting specific problems, this costly pump maintenance routine occurred on a calendar basis rather than on condition-based criteria.
They needed some kind of predictive maintenance monitoring.
Identifying a solution
The level of flow-rate measurement accuracy necessary for detecting potential condenser fouling precluded any flowmeter system using conventional pitot tube arrangements.
When KCP&L investigated multiple-path ultrasonic transit-time flowmeter technology, the utility recognized this approach could well meet their requirements for a cooling water system flow monitor.
Some of the potential advantages using this flowmeter approach included:
- Flexibility for metering location within the circulating cooling water system
- High-accuracy flow measurement even with perturbed pipe flow profiles
- Flowmeter cost independent of pipe size
- No "flow profile" calibration or recalibration requirements
- Superior long-term flowmeter performance
Ultrasonic technology tenets
The multi-path transit-time flowmeters discussed here are online worldwide for high-accuracy flow measurement in over 2000 large pipes, open channels, and other types of flow conduits. They have been working in hydroelectric and water system applications since the 1960s for high accuracy and reliable flow measurement.
Because these flowmeters use relatively high-power ultrasonic pulses for flow velocity measurement, they are capable of operating in large pipes with water having relatively high concentrations of suspended sediments.
The ultrasonic transit-time technique uses the principle that an acoustic pulse traveling at an angle across a pipe will travel faster in the downstream direction, carried with the water flow, and will arrive at a receiving transducer in less time than an acoustic pulse traveling along the same acoustic path in the reversed upstream direction.
By mounting transducers on the pipe walls to define a path crossing the pipe at an angle to the flow axis and measuring the difference in acoustic transit times in the upstream and downstream directions, it is possible to calculate an average flow velocity at the elevation of the acoustic path across the pipe using this formula.
V = ((T1–T2) / (T1×T2)) × (L / 2cosΘ)
V = Average fluid velocity at the level of the path
T1 = Acoustic transit time in the upstream direction
T2 = Acoustic transit time in the downstream direction
L = Acoustic path length between transducers
Θ = Acoustic path angle relative to flow axis
In the multiple-parallel-path method, we measure average velocity nearly simultaneously at several elevations across the flow (typically four elevations for measurement of full-pipe flow rates).
These measured velocities define a velocity profile throughout the flow cross-section for use in calculating an integrated flow rate. This is in contrast with the single-point or single-path velocity to estimate the average velocity throughout the cross-section.
The use of multiple simultaneous velocities also makes the method responsive to changing flow profiles associated with quickly changing flow regimes.
Flow rate measurement accuracies better than ±1.0% of true flow are typically achieved using a four-path flow measurement configuration. A wide variety of independent field and laboratory tests has verified this.
Such accuracy is possible even in the presence of upstream piping bends close to the metering section by including symmetrically crossed acoustic paths at each of the four measurement elevations spanning the pipe.
The crossed-path approach compensates for any errors due to off-axis flow vectors (cross-flow components) through the measurement section caused by upstream flow disturbances.
An additional advantage of the ultrasonic transit-time technique is the system is "dry calibrated," based on measurement of as-built path lengths and path angles, taken at the time of transducer installation.
Because the transducers are permanent, once the path lengths and angles are known and programmed into the flowmeter system, there is no need to recalibrate the system over time.
The multiple-path method also eliminates the need for flow profile calibrations that are required for single-point or single-path flowmeters.
We installed one of these devices on Unit 1's circulating water tunnel (132-inch-diameter) in order to detect reduced pump flow rates and thus indicate when maintenance was required to maintain optimum cooling water flow through the condenser.
Internal-mount transducers, originally designed for use at hydroelectric plants with buried penstocks, allowed for ready installation of the flowmeter ultrasonic transducers in the buried circulating water tunnel at the La Cygne facility while providing the needed flow rate accuracy.
In addition, pump flow analysis on each of three 166,000-GPM pumps was conducted to assess overall operational efficiency of the cooling water system.
Lowering heat rate
In utilizing the flow rate measurement data collected and recorded with the flowmeter, flow-trend analysis provided clear indications when any reduction in cooling water flow levels occurred, alerting operators to potential condenser fouling, possible debris on the tube sheets, or degraded pump performance.
The accumulated database of flow-rate information helped to determine specific corrective actions to implement during low power load periods for the plant, thus resulting in more cost-effective maintenance for the units.
Overall unit condenser efficiency increases were a result of this program, lowering the heat rate and the amount of fuel used at the plant.
There were beneficial heat rate decreases in the range from 0.5% to 1.0% when the condition-based condenser cleaning took over.
In addition to the benefits from improved condenser performance and heat rate efficiency, pump flow analysis determined the lift on one of the three pumps needed adjustment to improve performance.
Adjusting the lift, a problem that might otherwise have gone undetected, increased flow rates by as much as 10,000 gallons per minute, improving the overall performance of the Unit 1 condenser and decreasing heat rate by another 0.5%.
Monitoring the circulating water pump flow on a daily basis enables KCP&L to remove pumps for rebuilding based on specific need rather than a calendar schedule and to make necessary adjustments between refurbishment.
Thus, there are no longer unnecessary and costly pump overhauls. We installed a second flowmeter for Unit 2 cooling water tunnel at the La Cygne plant to further improve plant condenser performance and lower fuel costs.
The multiple-path ultrasonic transit-time flowmeter technology has given KCP&L the ability to accurately foresee and anticipate potential maintenance issues in advance.
This foresight is critical to maintaining efficient operations and lowering fuel costs for the plant since even small decreases in operational efficiency can significantly drive up fuel costs.
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