Coal, Plant Optimization, Water Treatment

Solutions to Challenging Power Plant Flow Measurements

Water flow in a steam generating power plant may range from the small such as process samples to the very large, i.e., condenser cooling water. From direct experience, the author has observed that monitoring some of these flows is quite challenging. Most notable are accurate measurements of flow in large pipes, or piping that has many elbows and fittings without sufficient straight-run sections to place a conventional meter. This article examines some modern developments in flow meter technology to alleviate these difficulties.

Flow Measurement Challenges and Solutions

A particularly challenging issue has been measurement of high volume flows in large piping. This is of significant importance in the power industry, and in heavy industrial plants that generate power or have large process heat exchangers, where cooling water piping may be several feet in diameter.

In the power industry, the largest flow is cooling water to the steam surface condenser, with cooling water flow rates of 200,000 gallons per minute or greater at large fossil plants is not uncommon. Accurate flow measurements are important to monitor condenser performance, as fouling or scaling can quickly develop, which may be overlooked without reliable flow and temperature data.  For a long time, accuracy was a difficult prospect. Thus, for condenser performance monitoring plant personnel often had to rely on the design pump curves, even though the pumps may have been in place for decades. Age and wear degrade pump performance, so condenser performance calculations were frequently skewed.  Also, accurate flow measurements are necessary for timely pump repair and replacement.

A solution to this problem is technology that utilizes the property of electromagnetic flow induction, per principles first developed in the 19th Century by the towering experimental physicist and chemist, Michael Faraday.

Figure 1.  McCrometer FPI Mag Flow Meter sensor

Electromagnetic coils installed inside of the sensor produce magnetic fields that are influenced by flowing water.  Stainless steel electrode pairs on the outside of the sensor collect the induced voltage generated by the water flow. Each voltage signal is transmitted to a converter that calculates an average flow velocity. The converter then multiplies this average velocity by the pipe cross-sectional area to produce a volumetric flow rate.

The design and mode of operation of the instrument shown in Figure 1 offer several distinct advantages, including that the instrument does not require a long straight run of pipe either upstream or downstream of the tap; an issue that is often problematic with other flow meters. Secondly, the instrument can be hot-tapped into existing piping, such that an equipment shutdown or unit outage is not necessary for installation. Thirdly, the instrument is enclosed in a high-strength stainless steel body for structural integrity. And, as is typical with most modern instrumentation, the signals can be directly transmitted to the plant’s distributed control system (DCS) for a continuous display of real-time data, and also for direct incorporation into heat exchanger efficiency algorithms.

Accurate cooling water flow measurements are important to detect process upsets.  A case in point is sudden failure in a steam surface condenser shell that allows excess air to enter the condenser. These failures may occur without warning, whereupon the excess air coats condenser tubes and dramatically reduces heat transfer.

Another cooling water issue, indeed an issue with power plant wastewater discharge in general, is emerging in which mag flow meter technology can be of great value. Permits for many new plants are becoming increasingly restrictive with regard to makeup consumption and discharge quality, and often quantity. These factors must be considered for proper cooling tower sizing and operation.

Figure 2.  Basic cooling tower flow schematic

The majority of heat transfer in a cooling tower occurs due to evaporation of a small portion of the circulating water. But evaporation increases the dissolved solids concentration in the circulating water, which, even with the best chemistry treatment, will cause some constituents to reach the saturation level and induce scale formation. Thus, either periodically or continuously, some water must be blown down to keep the solids concentration below a certain limit.  The level of dissolved solids in the circulating water as compared to the makeup is known as the cycles of concentration, and may range from near level 1 for towers with seawater as makeup to perhaps higher than 10 for towers in arid locations. While the mathematics to precisely calculate overall cooling tower water and heat balances can be somewhat complex, the relationship of blowdown (BD), evaporation (E), and cycles of concentration (COC) is outlined in the following fundamental equation:

            BD = E/(COC – 1)

As is evident, increasing the cycles of concentration in the cooling tower lowers the blowdown rate, thus reducing water consumption (although evaporation is usually by far the largest water consumer in a cooling tower and discharge volume. But, higher COC increases the dissolved solids concentration in the blowdown, which then may run up against discharge regulations.  Precise monitoring of both makeup and discharge flow rates may be necessary to operate the tower within the required chemistry parameters, and at the same time comply with permit guidelines.

Yet another cooling water application, closed cooling water, can benefit from accurate flow measurement, if not via mag meters, perhaps with a V-Cone instrument or some other insertion type meter. Many plants have numerous auxiliary heat exchangers that are part of closed cooling water systems. While chemical treatment of closed cooling water is often more straightforward than open systems, flow measurements are still important.  Gradual loss of flow due to pump wear and aging will result in decreased heat exchanger efficiency. This in turn can place auxiliary systems in peril. For example, many closed heat exchangers cool lubricating oil for rotating machinery. Degradation of cooling water flow results in higher oil temperatures that in turn can increase the damage risk to expensive and critical equipment.

A common heat exchanger design for closed cooling water systems is the plate-and-frame type.  

Figure 3.  A plate-and-frame heat exchanger

Such exchangers provide a large surface area between plates to maximize heat transfer, but the close spacing offers many locations for fouling and flow restriction.  Accurate process monitoring, including flow and temperature measurements, is necessary to maintain reliable equipment performance.

Another important but often-overlooked industrial measurement is that of service water flow.  Of course, technologies such as mag meters can be quite effective for the large flows at municipal water treatment and wastewater treatment plants, but service water monitoring at individual industrial facilities should not be neglected.  Much water may be lost at the plant due to decaying infrastructure or from other issues that cause a failure or failures in the service water system.  The author once assisted with a project at a power plant that was losing several million gallons of water per month due to a failure in a service water system main.  The leak was in underground piping near a river, and visually was difficult to detect.  Only flow data indicated that a problem existed.

Conclusion

Cutting-edge technology has greatly improved the ability of personnel at power plants and other industries to monitor process flows.  This article touched upon several important applications with regard to cooling and process water measurements; measurements that are valuable for establishing and maintaining plant reliability and efficiency.

About the author: Brad Buecker is Senior Technical Publicist with ChemTreat.  He has 35 years of experience in or affiliated with the power industry, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s La Cygne, Kansas station.  He also spent two years as acting water/wastewater supervisor at a chemical plant.  Most recently he was a technical specialist with Kiewit Engineering Group Inc.  Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry.  He is a member of the American Chemical Society, American Institute of Chemical Engineers, American Society of Mechanical Engineers, Association of Iron and Steel Technology, Cooling Technology Institute (via corporate membership), National Association of Corrosion Engineers, the Electric Utility Chemistry Workshop planning committee, the EPRI-sponsored Power Plant & Environmental Chemistry Committee, and the Power-Gen International planning committee.  Buecker has authored many articles and three books on power plant and water/steam chemistry topics.  He may be reached at [email protected].

(A previous version of this article appeared in the May/June 2018 issue of Industrial WaterWorld magazine.)