Dealing with challenging flow measurements

Past experiences can be examined to show how new technologies help improve flow measurement problems. The following examples are taken from the author’s experience from nearly two decades at two coal-fired power plants and another two years at a chemical plant. This article presents several of these applications ranging from measurement of large flow volumes to obtaining accurate readings in lines with little straight-run piping.

Cooling water monitoring

Notable examples of large-scale cooling water processes include:

Steam-generating power plants: Large cooling water flow rates are required for the condensers that convert turbine exhaust steam to condensate for return to the boilers. (Some newer plants have been equipped with air-cooled condensers to reduce water usage.)
Steel industry: At integrated mills, blast furnace cooling is a primary cooling application, but at these and other facilities, cooling for continuous casters and additional applications is required.
Refineries and petrochemical plants: These are sprawling complexes where cooling water (and steam) are needed for many different unit processes.

This list is only a partial sampling of the numerous industries where cooling is a vital process. Critically important in many cases is accurate flow rate measurement to ensure that heat exchangers, reaction vessels, pumps, etc., are receiving the proper feed. Some of a plant’s primary heat exchangers may require very large flow rates, perhaps up to several hundred-thousand gallons per minute. Pipe sizes for these large volumes may reach 96 inches in diameter.

In the past, measurement of such large and voluminous flow rates was not reliable. A case in point comes from the author’s direct experience with steam surface condenser performance monitoring at two coal-fired power plants. Apart from the steam generator, the turbine exhaust steam condenser is the second largest heat exchanger in conventional power plants and for the heat recovery steam generators (HRSGs) of combined-cycle plants. The condensation process improves the thermodynamic efficiency of the Rankine cycle by nearly 1/3 over what it would be if the steam were not condensed. Water-side scaling or fouling, or excess air in-leakage on the steam side, can significantly reduce heat transfer and add costs to the plant, particularly during summertime operation.

Accurate flow measurements are important for monitoring condenser performance as well as detection of upset conditions. Prior to development of reliable large-flow metering, plant technical personnel had to rely on the design curves of the cooling water pumps, even though the pumps may have been in place for decades. Age and wear typically degrade pump performance, so not only were the condenser performance calculations probably skewed, but also plant thermal efficiency was lowered due to reduced cooling water flow.

A solution to this problem and similar high-volume flow applications is technology that uses the property of electromagnetic flow induction, per principles first developed in the 19th century by the towering experimental physicist and chemist, Michael Faraday.

Electromagnetic coils installed inside 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. One, it does not require a long straight run of pipe either upstream or downstream — an issue that is often a bane with other flowmeters. Second, the instrument can be hot-tapped into existing piping such that an equipment shutdown or unit outage is not necessary for installation. Third, the instrument is enclosed in a high-strength stainless steel body for structural integrity. And, as is typical with most modern instrumentation, the instrument signals can be directly transmitted to the plant’s distributed control system (DCS) for a continuous display of real-time data and 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.¹ Accurate cooling water flow rates are a key component of performance monitoring programs.

Another application in which this type of meter is becoming more widely used is potable water measurement, particularly in the large pipelines emanating from water treatment plants. A distinct advantage of the instrument, especially at existing facilities, is the ability to install it without shutting down the process.

Smaller-scale applications

Another issue the author encountered on a number of occasions was obtaining accurate flow in closed cooling water (CCW) lines where limited straight runs were not available. CCW piping often makes many twists and turns from the primary heat exchanger to specific, remote heat exchangers. 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 risk to expensive and critical equipment, e.g., steam turbines. More importantly, accurate and continuous flow measurements can help plant personnel quickly spot leaks in unobservable locations, such as the tube bundle immersed in a lubricating oil vessel. Quick action can prevent much damage and repair work.

Another example of the importance of flow measurement involves the continuous casting process for steel. It is not from the author’s direct experience, but has potentially critical implications from a safety aspect, among others.As Figure 2 indicates, the molten steel flowing from the tundish is jacketed by a primary closed cooling water loop followed by secondary cooling sprays. A decrease in cooling flow to the jacket could lead to breakout of molten steel from the hardening shell of the cast, which can be extremely damaging and dangerous to personnel.

Figure 2. General schematic of the continuous casting process for steel.

Cooling water flow monitoring, along with inlet and outlet water temperature measurements, helps ensure that the cooling coils are operating properly and are not losing heat transfer capabilities due to fouling or scaling. High-quality water with properly maintained and monitored chemical treatment is an absolute requirement for cooling systems such as these.²

For these applications and many others, and especially where straight runs may be limited, Coriolis flowmeters have provided good performance. However, the V-Cone device is another impressive, yet less expensive, technology.

The V-Cone design works on the principle of differential pressure — with high pressure measured through a port in the pipe before the cone, and low pressure measured through a port at the back end of the cone. It is based on Bernoulli’s theorem for the conservation of energy in a closed pipe. The presence of the V-shaped cone in the pipeline reshapes the velocity profile upstream of the cone, neutralizing the effects of turbulence as the flow approaches and passes around the V-Cone inside the pipe.

This style of instrument can also be valuable for steam flow monitoring. Consider a refinery with its numerous processes; many of which use low-pressure steam. These may include reboilers, steam strippers to remove light hydrocarbons from heavier materials, steam-jet air ejectors for vacuum distillation towers, and so forth.

Also, consider chemical plants, where steam is required for heat exchanger tubes, plates or jackets, and for evaporators and crystallizers. In the author’s experience, low-pressure steam served as both indirect and direct heating of the chemical process. The steam plant was located a significant distance from the production facilities. Between the long length of piping and the spaghetti-like network of steam lines feeding the process equipment, numerous sources for leaks were possible. In fact, the plant had been inadequately mothballed for several years, so cooling water, process water and process steam leaks were common for a long time after startup.

Another important but often-overlooked industrial measurement is that of service water flow. Much water may be lost at a plant due to decaying infrastructure or from other issues that cause a failure or failures in the service water system. The author once assisted a company that was losing several million gallons of service water per month due to a failure in a system main. The leak was located in underground piping near a river and was visually difficult to detect. Primary metering data gave the first indication that a problem had arisen, which was then pinpointed with aid of a portable, ultrasonic flowmeter.

Conclusion

Cutting-edge technology has greatly improved the ability of personnel at a wide variety of industries to monitor process flows. This article touched upon some modern technology for cooling water and process water/steam measurements — measurements that are often critical for establishing and maintaining plant reliability and efficiency. And, given the tremendous advances in computer technology, the accurate and reliable measurements from modern flowmeters and other devices can be incorporated into algorithms and computer programs that allow finely tuned adjustments to process variables.

Lastly, it is important to note that this discussion represents good engineering principles developed over many years of research and practical experience. However, it is the responsibility of the plant owners to develop the correct monitoring systems based on consultation with industry experts. Many additional details go into the design and subsequent operation of industrial monitoring systems that were not outlined in this article.

Acknowledgment: The author wishes to thank Melvin Smith of McCrometer for supplying technical information for this article.

References

B. Buecker, “Condenser Chemistry and Performance Monitoring: A Critical Necessity for Reliable Steam Plant Operation,” from the proceedings of the 60th Annual International Water Conference, October 18-20, 1999, Pittsburgh, Pennsylvania.
Buecker, B., Post, R., Leitze, J., and M. Bush, “Advances in Cooling Water Treatment for the Steel Industry,” Iron & Steel Technology, May 2020.

Brad Buecker is senior technical publicist with ChemTreat. He has many years of experience in or affiliated with the power industry, much of it in steam generation chemistry, water treatment and air quality control, as well as results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s La Cygne, Kansas, station. Buecker also spent two years as acting water/wastewater supervisor at a chemical plant. He 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 ACS, AIChE, AIST, ASME, CTI, NACE, the Electric Utility Chemistry Workshop planning committee, the Power-Gen International planning committee and the Advisory Council for the International Water Conference.