Vortex meters for steam measurement
By Cheng Vue
Vortex flowmeters have been available on the market since the late 1960s, and the number of applications they are used for has steadily increased. Over that same time, reliability has also increased, as the technology continues to be refined. Where excess process noise was a problem in the past, for example, manufacturers are finding solutions to compensate for or mask process signal noise.
Over the years of vortex meter use, it has become clear that the vortex meter is ideally suited for many applications. In steam measurements in particular, vortex provides a high accuracy and steady meter performance even with the presence of two-phase droplets and condensate. (Traditionally, steam has been measured by a differential pressure [dP] meter, which requires impulse lines where droplets can freeze and plug the lines, causing challenges.) Such reliable steam measurement means vortex meters can help plant managers use and reuse water efficiently, potentially reducing expenses.
The vortex meter operates on the von Kármán effect, first documented by Hungarian-American physicist, aerospace engineer, and mathematician Theodore von Kármán. He observed the formation of vortices behind a bluff body or shedder bar placed in the path of a fluid or gas. The sensor in the vortex meter counts the number of swirls or vortices and how fast they are moving to determine the speed at which the fluid passes through the meter.
A small sensing element in the vortex meter body oscillates back and forth at a specific frequency caused by the vortices. This sensed movement is converted into an electrical signal through the vortex sensor, and from there passes through a circuit to go to the transmitter, which reads it as frequency. Calculating the volumetric rate is done by taking the frequency and dividing that by the vortex meter’s unique K factor, which is the calibration factor.
Steam can be challenging to work with. Despite its many advantages, there are also inherent dangers involved in using steam, particularly when there is the danger of leaks. Superheated or saturated steam can escape from the system and injure a worker or damage equipment. However, steam is used in many industries due to its unique physical properties. The industries where steam is used range from chemical to refining, pharmaceutical, food and beverage, and power generation.
There are two types of steam commonly used in industrial applications, saturated and superheated. Each of these requires highly accurate and reliable measurements to ensure that the steam stays at the correct flow rate, temperature, and pressure throughout the process it is used for.
Saturated steam is created when water is heated to a gaseous state but kept just below the boiling point for its pressure. Two common applications for saturated steam are heating and sterilization. In these applications, either pressure or temperature can be used to accurately compensate for changes in density in the steam.
Superheated steam is saturated steam heated above the boiling point for its respective pressure. It might seem surprising to learn that superheated steam is not very well suited to heating or sterilizing applications. This is due to its lower heat transfer coefficient. It does provide considerable work potential because of its high internal energy properties and is used in energy production, particularly in turbine applications.
With superheated steam, it is important to make sure that the steam does not condense at any point in the process, because water droplets can cause both plant and personnel danger. Water droplets increase wear and tear on machinery parts, which, if left undetected over time, can cause machinery or piping to fail.
Obtaining measurement accuracy
In most steam applications, traditionally, the measurement tool used has been the dP flowmeter. A dP meter is made up of primary and secondary elements. The primary element is installed in the pipe and creates a pressure drop, so that the pressure transmitter can measure the differential pressure, which is proportional to the square root of flow. External impulse piping is needed to connect the secondary element, the transmitter, to the primary element in order to obtain the dP measurement.
Because a significant amount of energy is expended to generate steam, which is often under considerable pressure, it is important to have flexible flow metering technology. That technology must provide accurate, repeatable, and dependable measurements of mass steam flow, as that ensures better mass balance for utilities and energy management.
The challenges with a dP flowmeter revolve around the use of impulse lines, which can get clogged or can freeze if water droplets build up in them. The addition of heat tracing—adding an external heating element to the impulse line so that it does not freeze—is an option, but not always a reliable solution. A dP flowmeter is particularly unreliable if a steam application is used to heat a material flowing through a process.
In contrast, a vortex flowmeter is a more reliable way to get an accurate measurement, because it eliminates the need for impulse lines. The meter is fitted directly into the pipe, and the steam flows over the shedder bars, which create the measurable vortices. The data collected by the sensor is sent to the transmitter, which gives an operator instant insight into the flow characteristics.
Reuse and recycle
One of the benefits of a steam heating system is that it requires only water to run through the system. The water is heated in a boiler, and the steam is sent through the system. At the end point, it can be allowed to condense and return to the boiler as water that can be heated again. This creates a relatively efficient arrangement that does not require excessive water usage.
The only time water is lost is when, for some reason, steam is vented out of the system to relieve excess pressure. The need for venting is less likely when using a vortex meter, because the meter will indicate almost instantly when there are changes in the flow. An operator can make adjustments to bring measurements back in line with the process specifications.
Making a system more efficient and reducing costs, as well as ensuring safe operations, are the goals of every plant manager. Using and reusing water, coupled with using the right tools that alert the operator when the process is out of alignment, is one way to ensure efficiency and potentially reduce expenses. The vortex meter is one such tool able to provide this added assurance and efficiency.