Source: Siemens 2023 | Author: Daniel Henningsen
In addition to inline flow meters, Siemens also offers clamp-on ultrasonic flow measurement. The following measurement systems are available in the portfolio:
- SITRANS FS230
- SITRANS FS220
Operating Principle
The operating principle, the transit-time difference method, is used in both inline and clamp-on measuring devices. The significant difference is that, unlike inline measurement, which is permanently installed in the pipe and in contact with the medium, the clamp-on measurement is installed entirely outside the pipe and without media contact.
The transit time method measures the time between two ultrasonic signals sent through a liquid or gaseous medium. The ultrasonic signal sent with the flow has a shorter transit time than the signal sent against the flow.
The flow velocity is determined from the transit time difference and the speed of sound. The internal pipe diameter is calculated based on the wall thickness and the outer pipe diameter. Additionally, the Reynolds number must be determined to correct the developed flow profile. The flow velocity, the internal pipe area, and the Reynolds flow profile compensation then result in the volumetric flow rate.
Installation
When installing the sensors, which operate as both transmitter and receiver, two different methods are available:
Direct Installation:
- Diagonal installation of the sensors on opposite sides of the pipe
- Both signals are sent DIRECTLY from one sensor to the other
Reflective Mounting:
- Mounting on one side of a pipe
- Both signals are REFLECTED on the opposite inner wall of the pipe
- Flow profile can be "cut" up to four times
- Reflective mounting only works on materials that can reflect the ultrasonic signal
Multipath Measurement:
Both direct and reflection mounting can be implemented with more than one sensor pair.
The use of multiple measurement paths increases the accuracy of the measurement. Multipath measurement is particularly used when measuring gases.
Application Area
Clamp-on measurement can be used for both liquids and gases. The relevant industries include water and wastewater management, heating, ventilation, and air conditioning technology, the energy sector, the oil and gas industry, and manufacturing.
Depending on the application, different sensors are used. Two types of sensors are defined:
- Universal Sensors (Shear Waves): Applicable to almost all wall materials and liquids, not functional with gases!
- High-Precision Sensors (Lamb Waves): Specifically for use with steel pipes, high accuracy, and suitable for gas measurements
Measurement in Explosive Areas
Especially in the chemical, oil, and gas industries, sensors may need to be installed in explosive areas.
This is achieved with a cable length of over 20 m using an external digital sensor link. The external DSL can be installed in Zone 0, the sensors in Zone 0, and the transmitter in Zone 2.
Anomaly Compensation
The flow profile is crucial for the functionality and resulting accuracy of the measurement, especially in clamp-on measurements.
When the flow profile is disturbed, such as due to suboptimal installation conditions, the highest flow velocity in the pipe is outside the pipe center, causing a measurement error. This means the flow profile is disturbed and no longer symmetrical.
Poor inlet conditions, such as pipe bends, spatial curves, or inflows, negatively affect the flow profile.
To account for these measurement anomalies, the Siemens SITRANS transmitter offers a patented anomaly compensation. The type of disturbance is selected from a table and considered with the help of a dynamic correction factor adapted to the flow profile.
Gas Measurement
Determining the transit time difference is significantly more complex in gas measurement.
When measuring media with low density, the signal-to-noise ratio is negatively affected.
The Wide Beam® method counteracts this by using the pipe material as an ultrasonic acoustic amplifier to optimize the signal-to-noise ratio.
Additionally, due to the sometimes very high flow velocities, the so-called “beam-blowing” effect occurs, where the ultrasonic signals are blown away and distorted. This effect is compensated by the internal algorithm known as the High-Flow Setting, allowing the measurement of gases up to 40 m/s.
Due to the low density of gases, pipe noises generated by the sensors become more noticeable. To minimize pipe noises, damping films are applied.
Thus, the useful signal can be clearly distinguished from pipe noises.