Industrial Instrumentation: Flow Measurement

What is flow measurement ?

 

Let's first define what flow rate is!

Flow rate refers to the amount of fluid that passes through a specific point over a given period of time and a flow measurement instrument is used to precisely measure the flow.
This measurement can also be integrated into a flow measurement system, enabling it to count, display, register, monitor, control, balance, dose, and fill. Flow measurement instruments are widely used across industries in their daily operations.

Referring to my previous post on Steam Turbines, flowmeters play a crucial role in determining the amount of liquid, gas, and steam flowing through a cross-sectional area. By doing so, they can also help calculate other important variables, such as energy flow.

However, no single technology can address the needs of all applications. When it comes to steam, whether wet, saturated, or superheated, there are two key working principles that are particularly useful for accurate measurement:

• Vortex (the chosen principle for this post☝😀)
• Coriolis

Figure 1: A vortex-flowmeter.

Vortex Principle

Volume flow is proportional to vortex frequency (Theodor von Kármán). In other words, turbulence forms in the flow downstream of obstacles, such as bridge piers. In a vortex meter, a bluff body is placed in the middle of the pipe to disrupt the fluid flow. Downstream of the bluff body, a mechanical sensor detects pressure differences in the flowing fluid.

Figure 2: Inlet flow and turbulence formation.

As the flow rate reaches a specific threshold, alternating vortices form on both sides of the bluff body, creating regions of high and low pressure known as the Kármán vortex street. The frequency of these pressure changes corresponds to the frequency of the passing vortices, which the sensor records. The distance between consecutive vortices is directly related to the flow volume, enabling the calculation of total flow by counting the vortices.

Main components:

• Transmitter: Amplifies and filters signals, computes relevant data, generates interface signals. It is the human machine interface (HMI) and display.
• Sensor: Contains all parts and functions of the device which are mounted in the pipe.
• Process connection: Provides the interface between the flowmeter and the customer’s process piping.
• Bluff Body: Positioned inside the flow, the bluff body causes vortices to shed as the fluid passes by.
• DSC Sensor: Positioned downstream of the bluff body, the DSC sensor registers the frequency of the shedding vortices.

Figure 2: Components and working animation.

Advantages:

• This principle is well-suited for gases, steam, and liquids, with particular advantages for steam, making it ideal for power generation processes.
• Highly robust against water hammer, insensitive to pipe-borne vibrations (up to 1g), and resistant to temperature shocks.

Disadvantages:

• Long inlet runs required.
• The vortex principle is optimal when the Reynolds number exceeds 10,000. Below this threshold, vortices may not shed properly from the bluff body.

Next time you visit a power plant, see if you can spot a vortex flowmeter!
Source: Endress+Hauser
Model: Prowirl F 200

Watch this video to fully grasp the working principle of the #Vortexflowmeter!