Vortex shedding tutorial - Part 3

In Parts 1 and 2 of this article on vortex shedding flowmeters we looked in detail at how they work and how they are constructed. In Part 3 we look at what needs to be considered in the application of vortex flowmeters.

The vortex meter has a very simple construction, provides accuracy (1% or better) comparable to higher-priced or more maintenance-intensive techniques, and works equally well on liquids and gases. In addition, it is powered primarily by the fluid and lends itself more readily than other linear flow devices to 2-wire operation. But like any flowmeter, there are some things that need to be taken into consideration when using one.

Meter selection

From a safety viewpoint, it is essential that the vortex flowmeter satisfies the appropriate electrical safety requirements and be compatible with the process, that is, be able to withstand the temperature, pressure and chemical nature of the process fluid. From a mechanical viewpoint, it must have the proper end connections and, if required, have a sensor that can be replaced without shutting down the process. Meter size and measurement output signal type are also two very important selection factors.

Size

Contrary to what one might expect, the required meter size is not always the same as the nominal size of the piping in which it is to be installed. In some applications, selecting the size based on adjacent piping will not allow the low end of the required flow range to be measured. The appropriate criterion for selecting meter size is that the meter provides a reliable and accurate measurement over the entire required flow range. This may dictate a meter size that is less than the adjacent piping.

Pressure drop is a competing criterion to that above for sizing. This drop is given by:

where C is a constant dependent on meter design, and D is the bore diameter of the flowtube. The tendency is to pick a flowtube with the same nominal diameter as the adjacent piping to eliminate the extra pressure drop introduced by a smaller-sized meter. However, in the majority of cases this added drop is of little consequence.

The meter manufacturer can provide the needed information for making the proper selection. In some cases sizing programs from manufacturers are available on the internet in either an interactive or downloadable form.

Measurement output options

As mention in Part 2 of this article, three types of measurement outputs are in current use, a 4-20 mA analog signal, a pulse train, and a digital signal. Some vortex meters will provide all three of these outputs, but not always simultaneously. It is essential that the meter has the outputs required by the application.

Meter installation

The performance specifications of a vortex flowmeter are normally established under the following conditions:

1. The flowtube is installed in a pipeline running full with a single-phase fluid
2. The piping adjacent to the flowtube consists of straight sections of specified schedule pipe (normally Schedule 40), typically a minimum of 30 pipe diameters in length upstream and five pipe diameters downstream of the flowtube with no flow disturbing elements located within these sections
3. The meter is located in a vibration-free and electrical interference-free environment. As a consequence, certain constraints are placed on where and how the meter is installed in process piping if published performance specifications are to be achieved. These constraints are discussed below. Because meters from different suppliers differ in their sensitivity to the above influences, the statements made are of a qualitative nature. The specific manufacturer should be consulted for more quantitative information.

Location

The flowmeter should be located in a place where vibration and electrical interference levels are low. Both of these influences can decrease the signal-to-noise ratio at the input to the transmitter. This reduction can degrade the ability of the meter to measure low flows.

The meter should not be installed in a vertical line in which the fluid is flowing down, since there is a good possibility that the pipe will not be full.

Adjacent piping

Recommended practice is to mount the flowmeter in the process piping according to the manufacturer’s stated upstream and downstream minimum straight length piping requirements. These are typically 15-30 pipe diameters and five pipe diameters, respectively. Piping elements such as elbows or reducers upstream of the meter normally affect its K-factor but not its linearity. This allows a bias correction to be applied to the K-factor. Many manufacturers provide bias factors for common upstream piping arrangements. Some that offer intelligent flowmeters make the corrections internally, once the user has selected the appropriate configuration from a pick list. For piping elements and arrangements where the bias correction is not available, an in-situ calibration should be run if the manufacturer’s specified uncertainty is to be achieved. If this is not possible, calibration in a test facility with an identical configuration should be run.

The same situation as above applies if the pipe schedule adjacent to the meter differs from that under which the meter was calibrated.

To avoid disturbance to the flow, flange gaskets should never protrude into the process fluid.

The following recommendations apply if a control valve is to be situated near a vortex flowmeter. In liquid applications the control valve should be located a minimum of five pipe diameters downstream of the flowmeter. This not only prevents disturbance to the flow profile in the flowtube, but also aids in preventing flashing and cavitation (see below). In gas applications the control valve should be installed upstream of the meter, typically a minimum of 30 pipe diameters upstream of the meter to ensure an undisturbed flow profile. Having the pressure drop across the valve upstream of the meter results in a decreased density and subsequent increased velocity at the flowmeter. This helps in achieving good measurements at low flows. For condensable gases, such as steam, it also helps to reduce the amount of condensate that might otherwise be present at the flowmeter.

Orientation

In general, meter orientation is not an issue for vortex flowmeters, particularly for vertical pipe installations. However, for meters having electronics at the flowtube, it is recommended in high-temperature horizontal pipe applications that the flowtube be oriented with the electronics beneath the meter. Although vortex flowmeters are not recommended for multiphase applications they do operate with somewhat degraded performance with dirty fluids (small amounts of gas bubbles in liquid, solid particles in liquid, or liquid droplets in gas). The degree of degradation in horizontal pipe applications depends to some extent on the specific meter design. Orienting the flowtube according to manufacturer’s recommendations for the dirty fluid in question, if given, may help to alleviate this problem.

Pressure and temperature taps

The placement of pressure and temperature taps for determining gas densities, if required, is also an important consideration. Recommendations for location of these taps vary depending on manufacturer. The temperature probe is inserted typically six pipe diameters downstream of the flowtube. This prevents any flow disturbance in the meter and, at the same time, gets the probe as close to the meter as possible. The pressure tap is made typically four pipe diameters downstream of the meter. Although a pressure tap does not significantly affect the flow, its placement is critical for achieving an accurate density measurement.

Process conditions

Flashing and cavitation can occur in a liquid application just downstream of the shedder if the pressure drop across the meter results in the downstream pressure being below the vapour pressure of the liquid. These phenomena lead to undefined measurement errors and possibly to structural damage, and hence should be avoided. This is usually accomplished by increasing the inlet pressure or inserting a back-pressure valve downstream of the meter. To avoid flashing and cavitation, the downstream pressure after recovery (approximately five pipe diameters downstream) must be equal to or greater than P_{d min}:

where:

P_{d min} = minimum downstream pressure after recovery
P_vap = vapour pressure of the liquid at the flowing temperature
ΔP = overall pressure drop
C₁,C₂ = empirical constants for a specific meter (normally available from the meter manufacturer)

Pulsating flow can also, in some circumstances, lead to measurement errors. It is best to avoid placing the meter in process lines where noticeable pulsation exists.

Meter configuration

It is important when installing an analog or intelligent vortex flowmeter that it be configured for the specific application. This is often done by the supplier prior to shipping if the user supplies the relevant information at the time the order is placed. If this is not the case, the user must carry out the configuration procedures provided by the manufacturer.

Recent developments

Recent efforts have been made to make the vortex flowmeter into a real-time mass flow measurement device. As was demonstrated in Part 1, the output of the meter, based on the frequency of vortex shedding, is related to actual volumetric flow. In intelligent transmitters the flowing density (the density at flowing conditions) and the base density can be entered into the transmitter’s database. Based on these values mass flow or standard volumetric flow can be computed. This procedure is valid if the flowing density does not vary in time. If this is not the case, an online real-time measure of the density must be provided. Two different approaches have been used. One employs sensors in addition to the vortex sensor, the other relies on additional information being extracted from the vortex shedding signal.

Multi-sensor

In this method, temperature and pressure measurements are made in addition to the vortex frequency. This approach is similar to that used in orifice-differential pressure mass flowmetering, in which case temperature and pressure ports are located in the pipe normally downstream of the orifice plate. However, in this case, the temperature and pressure sensors are incorporated into the flowmeter rather than located in the adjacent piping. Using these two additional measurements, the flowing density is calculated from the equation of state for the process fluid.

Single-sensor

This approach takes advantage of the fact that, in principle, for a force- or pressure-based vortex shedding sensor, the amplitude of the vortex shedding signal is directly proportional to the density times the square of the fluid velocity:

The fluid velocity can be determined from the vortex frequency:

Hence,

This approach, in principle, is independent of the process fluid, and requires no additional sensors.

 

By Wade Mattar and James Vignos, PhD, Foxboro, Invensys

 

Additional reading RW Miller, Flow Measurement Engineering Handbook, 3rd edition, McGraw-Hill, 1996, chapter 14. WC Gotthardt, ‘Oscillatory Flowmeters’, Practical Guides for Measurement and Control: Flow Measurement, editor DW Spitzer, Instrument Society of America, 1991, chapter 12. JP DeCarlo, Fundamentals of Flow Measurement, Instrument Society of America, 1984, chapter 8. American Society of Mechanical Engineers, Measurement of Fluid Flow in Closed Conduits Using Vortex Flowmeters, 1998.