Flow Rate Measurement in Full Pipes

The relevance of water volume measurement is growing, especially in large diameters. The challenge associated with fresh water or cooling water processes is to create accurate flow recordings or to document and regulate individual consumptions and withdrawal volumes. However, flow detection especially in large diameters is highly challenging. In such situations, the transit time measurement – due to its high flexibility as a cost-efficient and reliable measurement system – is literally made for permanent flow metering.

Figure 1.

Securing the Water Supply

The requirement for many processes is to feed as little fresh water as possible. Water feed and withdrawal quantities also need to be monitored constantly. All these tasks mean that the flow rates have to be permanently investigated and verified. Integration into higher systems such as SCADA systems is crucial as the systems are typically used within large areas.

Conception and Selecting the Measurement System

In order to ensure constant flow recording, a measurement system that can determine the medium velocity covering the entire wetted area needs to be used. This is especially important with varying flow conditions. The majority of measurement systems that are generally used either provide the required penetration depth or feature only the spot velocity measurement. Quite simply, when it comes to installation, some measurement systems either require too many employees or cause too many costs. Measuring high medium velocities to many systems is also an equally impossible task.

One cost-efficient way to acquire reliable information on the prevailing flow/discharge is the measurement using the ultrasonic transit time difference principle. Such types of systems stand out for high operational safety and low maintenance expenses. They can be flexibly used with all the required media and sizes. When compared to other methods, the measurement system also has the advantage to be largely independent of the media’s properties to measure such as viscosity, fluctuating temperatures or electrical conductivity.

This measurement principle is based on directly determining the transit time of an acoustic signal between two ultrasonic sensors. Such types of sensors are also described as hydro-acoustic converters (A and B in the illustration given below). Two sonic impulses are transmitted sequentially after each other, followed by measuring the different transit times between the receiver and transmitter. The impulse heading downstream (t2) reaches the receiver sooner than the impulse heading upstream (t1).

Highly accurate time measurements and a signal correlation were used to measure the required times. This signal correlation compares the signal transmitted with the signal received by the opposite converter. Therefore, the comparison enables the determination of the accurate moments of transmission and reception of the measurement signal. The variation between both determined times is proportional to the average flow velocity within the measurement path.

Figure 2. Schematic illustration of transit time difference principle.

     t1 = Impulse time against flow direction
     t2 = Impulse time in flow direction
     L = Transit time / distance between sensors

Formula: Average transit time difference in a measurement path

The average cross-sectional velocity can be determined by using this formula, and therefore the flow rate from the measured average velocities within the individual layers is related to the velocity coefficients accordingly.

    Q = flow rate
     k = measurement place-specific correction factor
     A = wetted area
     vg = average velocity

Formula: General flow calculation

The more measurement paths are used, the more data on the flow profile prevailing at the measurement spot can be obtained. In this case, the total flow rate is the total of the individual flow rates. Therefore, the accuracy of the flow rate determination will be increased using multiple measurement paths. The effects of disturbing flows crossing the main flow direction can be reduced by arranging the sensors of a multi-path measurement crosswise. Cross flows may lead to measurement errors. If a multi-path measurement setup is used, it may also reduce the length of intake and discharge sections needed to calm down the flow profile at the measurement point.

Figure 3. Schematic illustration of multiple measurement paths.

Due to new Computational Fluid Dynamics (CFD) models and in-depth testing at leading institutes, behavior and effect of flow profiles downstream of standard disturbances could be investigated. Based on the results, flow profiles can be integrated downstream of elbows and other disturbances into calculation models directly in the measurement system's transmitter. Only the distance to the measurement spot and the type of disturbance need to be specified. From these specifications, the correction factors to be used are automatically determined by the measurement. Therefore, the result of the flow measurement is highly accurate and can also be used together with shorter calming and intake sections.

Figure 4. CFD-model of a disturbance (elbow).

The new NIVUS GmbH device types allow for the transit time method to be used both as a clamp-on system and invasive measurement. Here, the type of sensor used must be chosen based on the situation on the measurement place. Using a multi-path system with wetted sensors in a defined arrangement can help achieve the highest measurement accuracy. If the sensor cannot be inserted into the process (corrosive, abrasive, or other problematic media), they can be installed on the pipeline from the outside without interrupting the process (clamp-on measurement system).

Implementing a Measurement

Figure 5. Transmitter and various sensor types.

Tasks of the example depicted: long-term recording of flow velocity and flow rate for archiving in a drinking water supply pipeline of a distribution system operator.

In this case, accuracy requirements are quite high because the measurement place has to be used for billing purposes.

A NivuFlow 600 system with invasive sensors by NIVUS GmbH was employed. By using tapping nozzles, the sensors were inserted into the pipe. This is how the readings can be provided to the following SCADA system with the desired level of accuracy through data connection. The wide range of sensors and installation material enables picking up readings at different measurement spots.

A very minimalistic method can be followed in terms of spare parts stock:

There is no need to stock diameter-specific parts, there is one measurement system for nearly all pipe diameters and measurement places.

Summary

Based on the ultrasonic transit time difference principle, flow measurements have undergone many years of elaborate testing. They have also shown to be successful in practical use and stand out for a high level of flexibility and accuracy in terms of applicability in different measurement places. Due to ease of maintenance and robustness, the ultrasonic transit time principle is ideal for measuring in pipes with smaller diameters (such as cooling water or process water) and also for permanent measurements on demanding measurement sites (such as hydro-electric plants, large pipe diameters, high process water volumes and varying media).

However, with new-generation devices, the method’s advantages have been considerably extended. Among other things, accuracies of flow measurements and measurement ranges have been significantly increased.

This information has been sourced, reviewed and adapted from materials provided by NIVUS GmbH.

For more information on this source, please visit NIVUS GmbH.