Operating Principle

The SITRANS F S system is a transit-time ultrasonic meter that provides exceptional performance using a non-intrusive clamp-on approach. Ultrasonic sensors transmit and receive acoustic signals directly though the existing pipe wall, where the fluid refraction angle is governed by Snell’s law of refraction.

Clamp-on sensor mounted in a reflect configuration

The beam refraction angle is calculated as follows:

sin УЁ = c / V_П†

c = Velocity of sound in fluid

V_П† = Phase velocity (a constant in the pipe wall)

The flowmeter automatically compensates for any change in fluid sound velocity (or beam angle) in response to variations in the average transit time between sensors A and B. By subtracting the computed fixed times (within the sensor and pipe wall) from the measured average transit time, the meter can then infer the required transit time in the fluid (T_Fluid).

The sound waves traveling in the same direction as the flow (T_A,B) arrive earlier than sound waves traveling against the direction of flow (T_B,A). This time difference (О”t) is used to compute the line integrated flow velocity (v) as shown in the equation below:

v = V_П† / 2 · О”t / T_Fluid

Once the raw flow velocity is determined, the fluid Reynolds Number (Re) must be determined to properly correct for fully developed flow profile. This requires the entry of the fluid’s kinematic viscosity (visc) as shown in the equations below, where Q represents the final flow profile compensated volumetric flow rate.

Re = Di · v / visc Q = K(Re) · ( ПЂ / 4 ·Di² ) · v

v = Flow velocity

visc = Ој / ПЃ = (dynamic viscosity / density)

K(Re) = Reynolds flow profile compensation

In wetted type ultrasonic flowmeters the meter constants are configured prior to leaving the factory. As this is not possible with clamp-on meters, the settings must be made by the customer at the time of installation. These settings include pipe diameter, wall thickness, liquid viscosity, etc.

SITRANS clamp-on flowmeters that include temperature sensing can be configured to dynamically infer changes in fluid viscosity for the purpose of computing the most accurate flow profile compensation (K_Re).

Ultrasonic sensor types

Two basic types of clamp-on sensors can be selected for use with the SITRANS F S flowmeter. The lower cost "universal" sensor is the most common type in the industry and is suitable for most single liquid applications where the sound velocity does not vary much. This sensor type can be used on any sonically conductive pipe material (including steel) making it well suited for portable survey applications. Universal sensors are selected based on the pipe diameter range alone, so wall thickness is less important to the selection process.

The second sensor type is the patented "WideBeam" sensor (called high precision), which utilizes the pipe wall as a kind of waveguide to optimize the signal to noise ratio and provide a wider area of vibration. This makes this kind of sensor less sensitive to any change in the fluid medium.

The WideBeam sensor is designed for steel pipes, but can also be used with aluminum and titanium. It is the preferred sensor for HPI applications. Note that unlike the universal type, this sensor selection is dependent only on the pipe’s wall thickness.

Multi-path flowmeters

For improved flow profile averaging, redundancy or better cost per measurement, clamp-on meters can be supplied with 1 or 2 path measurement systems.

In the standard FS230 systems, these can be installed on a single pipe as shown below (two paths on same pipe).

Dual path installation example

SITRANS meter family description

SITRANS FS230 clamp-on flowmeters

The FS230 system is a basic function, permanent (or dedicated) Clamp-on meter that is available with a full range of safety approvals and I/Os. This meter can be used in a wide range of applications.

FST030 transmitter standard flow functions

When configured with standard flow functions, the FST030 transmitter is typically programmed with a fixed viscosity and specific gravity entry, which can limit the mass flow and volumetric flow accuracy when highly variable (multi-product) liquid properties flow through the same pipeline.

It will have the ability to accommodate clamp-on RTDs, or analog input from a temperature transmitter.

FST030 hydrocarbon flow functions

When configured with hydrocarbon functions, the FST030 can be used for applications that will flow a wide range of viscosity with a standard volume (mass) and interface detection functions available. All functions rely on a variable referred to as "LiquIdent (TM)", which is used to infer the liquid’s viscosity and density. This variable represents the measured liquid sonic velocity compensated by the operating temperature and pressure, so for a given liquid product the measured LiquIdent (TM) output will remain constant over a wide range of pressure or temperature.

Standard volume description:

This LiquIdent (TM) variable can also be used to identify the liquid flowing through the pipe as well as it’s physical properties (density, viscosity and compressibility) at base conditions. With this information the meter can be configured to output a temperature and pressure compensated (standard) volume flow rate using the API MPMS chapter 11.2.1 methods as shown below.

Available outputs from this meter include: API, standard density, mass flowrate, standard volume flowrate and liquid identification.

General installation guidelines for transit time clamp-on sensor

• Minimum measuring range: 0 to ± 0.3 m/s velocity (see meter accuracy graph below for more detail)
• Maximum measuring range: 0 to ± 12 m/s (± 30 m/s for high precision sensors). Final flow range determination requires application review

• Pipe must be completely full within the sensor installation volume for accurate flow measurement
• Typical MINIMUM straight pipe requirements are: 10 Diameters upstream / 5 Diameters downstream. Additional straight run is required for double out-of-plane elbows and partially open valves.
• Sensors should be installed at least 20° off vertical for horizontal pipes. This reduces the chance of beam interference from gas buildup at the top of the pipe
• Operation inside the Reynolds transition region, between 1 000 < Re < 5 000 should be avoided for best accuracy
• Submersible and direct burial installations can be accommodated. Consult sales representative for details
• Ultrasonic coupling compound is provided with all sensor orders. Insure that a permanent coupling compound is used for long term installations
• Refer to the "Sensor type selection guide" to insure proper application of the equipment