EPC School

Ultrasonic flow meters can be used on wide range of fluids, i.e. liquids, gases and steam. The “transit –time” or “time-of-flight” meters are basically “time-difference” meters, as they are based on time difference measurement of acoustic waves that are transmitted in opposite directions through the flow. The time difference is directly proportional to flow rate.

To generate sound, transducers equipped with high frequency piezoelectric crystals are normally used. To measure the time difference, transducers A/B of an interrogation path will alternatively be switched into transmitting and receiving mode respectively. Sometimes ultrasound is launched simultaneously in both directions. Depending upon the application the working frequency will be adapted to optimise the acoustic coupling and penetrates through the fluid depending upon the interrogation path length and viscosity of the fluid.

Since particles or bubbles will scatter the sound beam, this principle can be basically be successful in single phase and clean fluids.

Ultra sound meters are volume flow meters. Since volume flow is in most cases not relevant for plant operations purposes, their output is correlated to mass flow assuming a fixed actual density (reference density) under operating conditions. Deviations in actual density will cause a misreading in mass flow which inversely proportional to the deviation compared with the reference density.

Orifice meters are actual volume flow meters as well, but by correlating their output to mass flow, the effect of variations in actual density as compared with the reference density is smaller since they are not linearly dependent on actual density but on the square root of the actual density.

Apart from measuring the actual volume flow they will measure the propagation velocity of the ultrasound of the fluid, often referred to as Velocity Of Sound (VOS). Since the VOS is a characteristic property of a fluid, the VOS output signal in conjunction with the temperature and pressure of the fluid can be used as a measure of:

Also, a deviation in signal strength could indicate viscosity changes, an increase or decrease n level of solids in fluid.

Ultrasonic meter measures the averaged velocity over the interrogation path of a fluid through a pipe and calculates the actual volume flow rate by multiplying that velocity by the cross-sectional area of the pipe. As fluid properties influence the velocity profile, there are indeed some effects. Also, temperature gradients and velocity profiles bend the sound waves. This too influences the accuracy.

This principle measures the difference in transit time between two ultrasound beams transmitted in the flow direction and counter direction. In operation, a set of two ultrasonic transducers transmit and receive alternately ultrasonic sound at an angle across the pipe. The angle which can be chosen to create a longer transmission path can vary between 20 and 70 degrees.

The fluid flow adds velocity to the signal transmitted in the flow direction (downstream signal) and subtracts velocity from the signal transmitted in the counter direction of the flow (upstream signal). The resulting time difference between transmission and receipt of the ultrasound signal in the upstream and downstream direction is directly proportional to flow rate.

Equation for upstream transit time: Co – V * Cos θ = L/t1

Equation of downstream transit time: Co+ V * Cos θ = L/t2

Subtracting, we get: V= L/ 2*Cosθ*t1*t2 *Δt

V= L² / 2*Lp*t1*t2 * Δt

The minimum measurable velocity is directly related to the resolution of the clock, which in modern commercial meters lies in nanoseconds range. As a consequence thereof, meter is independent of fluid properties, as long as ultrasound with sufficient strength can be unambiguously be detected in the receiving mode of the transducer.  Meter sizes range in 0.125 to 120 in. (3 mm to 3 m) diameter.

In-line ultrasonic flow meters will be calibrated by the supplier on a flow rig using water or air. By calibrating the meter, its relationship is determined between input and output and is expressed as volume per time. (m3/h etc.)

It should be noted that off-line calibration of a meter on a flow rig using water or air will give limited information about the ultrasonic sound parameter under operating conditions. Normally, one has to rely on the experience of the Manufacturer for similar cases.

One of the advantages of two interrogation paths at mid radius is that they will compensate for velocity profile asymmetry.

Electronics Part

The electronic part can be integral part of ultrasonic flow meter or a separate part to be mounted at a short distance away from the spool piece.

Though dependent upon the make, in general electronics part contains-

Most makes have two signal outputs, one for flow signal and an auxiliary output used for the measured Velocity Of Sound (VOC).

Note- Electronics require a separate supply connection (230 VAC or 24 VDC)

In order to minimize the signal interference most makes will make use of coded pulse train and correlation technique to recognise the pulse train at the receiver. Also, time window is used, only signals falling in time window will be considered a valid signals. Signals falling outside of this window are ignored.

Ultrasonic in-line flow meters with their buffer rods or window flush mounted with the pipe wall are non-intrusive meters. Consequently, the pressure drop and permanent pressure drop and permanent pressure loss for line size meters will be minimum. If the size of the spool piece is smaller than the line size, the pressure loss will slightly increase.

Applications

A) Clean liquids with little or no solids or bubbles, gases.

B) Slurries with solids (0.2 to 60% concentration, depending upon particle size), liquids that are

    aerated or contain bubbles, gases with sound reflecting particles; single phase turbulent clean

    liquid.

C) Open-channel flow measurement based on upstream level in front of flumes or weirs.

Application and Performance :

                As with most flow meters, the spool piece or pipe section must always be full to assure proper operation and volumetric flow indication. Most manufacturers will specify the minimum distances from the valves, tees, elbows and pump, etc. that will ensure accurate flow meter performance. Typically, 10 to 20 diameters upstream and 5 diameters downstream are required. The flow meter relies upon a ultrasonic signal traversing across the pipe; therefore, the liquid must be relatively free of solids and air bubbles. Bubbles in the flow stream generally cause more attenuation of the acoustic signals than solids do and therefore can be tolerated less. The flow meter can tolerate a larger percentage of solids than bubbles.

         Depending on the process fluid, proper transducer materials and protection must be chosen to prevent transducer damage due to chemical action. Process temperature limitations must also be considered for proper flow meter application.

            Accuracy is usually specified as a percent of rate. Typically for a single path flow meter it is around 1 to 2% of rate, depending upon design, velocity, pipe size, and process. This accuracy can be expected only of calibrated flow meters and only within their range of calibration.

                Repeatability is usually specified as a percent of rate, typically about 0.5% depending upon velocity range and calibration.

             To improve performance and accuracy for larger pipe sizes, some suppliers offer flow meters with two, four, or more pairs of transducers arranged to interrogate multiple acoustic paths. The cost of such units is higher than that of a single path flow meter. The inaccuracy of multiple-path flow meters can reach 0.5% of actual reading within a narrower range, if the flow velocity exceeds 1 ft/s (0.3 m/s).