Vortex-shedding flowmeters lend themselves well to cooling water service applications.

In any process application, you cannot underestimate how important properly cooling your equipment is. For instance, if the bearings in the shaft of a rotating steel-making vessel in a basic oxygen furnace are not cooled sufficiently, the heat treater stands to lose millions due to lost production. And, the safety and environmental hazards of insufficient cooling in a nuclear power plant should be self-evident.

Of course, many cooling water applications have less dire potential consequences, but they are nonetheless still important to monitor. Typically, the cost of a cooling water flowmeter is less than the equipment damage or lost production that can occur if the equipment is cooled insufficiently.

The flow rate through a production flowmeter can vary widely. For example, a unit that normally operates at 30 to 80 percent capacity may also be required to start up at only five percent capacity. Therefore, the flowmeter requires a large turndown.

By contrast, cooling water flow rates typically vary little over time because equipment manufacturers specify a fixed minimum cooling water flow rate. In practice, the cooling water flow usually is adjusted to be higher than the minimum required flow rate. Further, the cooling water is allowed to flow whenever the plant is in operation, even at low capacity and idle.

Overall, vortex-shedding flowmeters lend themselves to monitoring the flow of cooling water. The desired cooling water flow rate is known, so vortex-shedding flowmeters can be sized to remain well above their minimum operating velocity (approximately 0.3 meters per second). In addition, vortex-shedding flowmeters have linear outputs, do not have any moving parts and can be less expensive than flowmeters using other technologies. They are relatively easy to apply in cooling water service because suppliers typically provide a straightforward table stating the minimum and maximum water flow rates for each size flowmeter.

Vortex-shedding flowmeters have an internal bluff body (shedder) that generates vortices in the flow stream as the cooling water passes through the flowmeter. Similar examples of vortices in nature include the waving of a flag in wind and the vortices that form downstream of a rock or branch in a flowing river. The number of vortices formed is proportional to the cooling water flow rate. Vortex-shedding flowmeters do not have moving parts and are not subject to plugging, but they do not measure low flow rates where vortices are not formed.

Vortex-shedding flowmeters for small pipes (under 2" [50 mm]) commonly have screwed connections and are made from materials of construction such as brass and plastic. Often, they have local indication and a 4 to 20 mA analog output. These flowmeters generally offer sufficient accuracy for many cooling water monitoring applications and sometimes are available with stainless steel or polymer-wetted parts. Some are offered with additional outputs such as process temperature, which can reduce cost and complexity by eliminating the need to purchase a separate temperature instrument and provide another pipe penetration.

Vortex-shedding flowmeters for medium-sized pipes (between 0.5 and 8" [12.5 and 200 mm]) typically use flanged or wafer-style connections. More accuracy and sophistication often is required because these flowmeters may be located in hazardous locations, protect more expensive equipment or require higher pressure/temperature operation. More accurate flow measurement also can be helpful to reduce the cooling water flow rate to near its minimum to reduce the cost of pumping unnecessary additional cooling water. In addition, some cooling water applications require may require Hart, Foundation Fieldbus, Profibus, Modbus or BACnet communications for integration into existing process control or monitoring systems.

Full-bore vortex-shedding flowmeters for liquid service are offered in sizes up to approximately 18" (500 mm). However, many suppliers limit their vortex shedder offerings to approximately 8" (200 mm) because the low shedding frequency adversely affects response time. The demand for large-size full-bore flowmeters is relatively small and the larger flowmeter bodies are expensive.

Despite these limitations, insertion vortex-shedding technology can be used for pipe sizes larger than approximately 4" (100 mm). In general, insertion flowmeters are relatively small sensors that are placed at a strategic location in the pipe where there is a known relationship between the measured velocity at that location and the average velocity in the pipe. The flow in the entire pipe can be determined by multiplying the average velocity by the cross-sectional area of the pipe.

Insertion vortex-shedding flowmeters can be used to measure flow in large pipes. Some insertion vortex-shedding flowmeters can be installed using “hot tap” techniques without taking the pipe out of service. However, the sensor must be located in the correct position, and the velocity profile of the fluid passing the sensor must be fully developed - not distorted - to measure flow in the entire pipe accurately. Stated differently, if the velocity profile is distorted - such as would occur downstream of an elbow or partially open valve - the (small) insertion sensor can measure higher or lower than it would with a correct velocity profile, causing an inaccurate measurement.

Links

• Universal Flow Monitors Inc.