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How Multivariable DP Flow Meters Can Improve Performance

There was a TV cooking show a while back, and the host of the show would talk occasionally about implements and appliances around the kitchen. He suggested there is little value in having a device that can only do one thing. If something isn’t capable of multitasking, it isn’t worth the space it takes up. Such a concept is harder to apply in an industrial context than a kitchen, but sometimes it works in a process manufacturing plant. A prime example: multivariable instruments, with DP flow meters probably the most common.

Connor Oberle looks at DP flow meters in his article in the March-April 2019 issue of InTech titled How Multivariable DP Flow Meters Can Improve Performance. He begins with one of the most popular and versatile flow metering technologies and examines how an advanced DP transmitter can extend the capability well beyond a simple flow reading.

When measuring flow, simple DP flow meters provide a reading using just a DP measurement. Using a simple formula, the DP value can be used to determine a flow rate. For many processes, such as liquids where the density and other properties can be reasonably assumed, this is what is required. However, there are many applications where steam or gas is the process fluid, and an uncompensated reading does not deliver much useful information. A compensated flow reading is required for steam or gas, or a mass flow measurement may be required for feeding liquids to critical chemical reactions, for custody transfer and other applications.

A prime example of the kind of flow meter he’s talking about is the Rosemount 3051SFP Integral Orifice Flow Meter, and it has built-in calculations for fully compensated mass flow measurements. It includes an integral sensor to measure the process fluid temperature and use it in the calculations. The ability to add compensation capabilities to simple DP measurements extends the versatility of the flow meter and can improve its accuracy. It can also save money by taking the place of several individual instruments.

The type of DP flow meter described performs the function of several individual instruments: a DP-producing element such as an orifice plate, a DP transmitter, a gauge or absolute transmitter, a temperature transmitter, and a flow computer. Using a single, multivariable DP flow meter with its sophisticated transmitter and ancillary measurements eliminates the need to install all these additional devices, at least in most situations.

It's possible to use these other readings, but is it practical? The answer depends on the specific context. The automation host system has to be able to reach in and pull out the additional data, and every system does not have the capability to do this.

The practicality of using the extra measurements as part of a larger process automation strategy will depend on how they are extracted. If the DP flow meter or any other type of multivariable instrument is used in a Foundation Fieldbus or HART-enabled I/O environment, capturing the additional data is very simple. In a conventional analog I/O environment, accessing the extra functions and variables is more complicated.

So, capturing the additional data is often more about the networking and I/O environment than the instrument itself. There are probably sophisticated flow meters out there unable to deliver their full potential as a result. Fortunately, there is one highly practical solution.

WirelessHART may be the best approach for retrofits in a simple wired I/O environment, or for new installations. Many plants now have WirelessHART networks operating for a variety of purposes and adding an adapter to a DP flow meter is a simple matter. It can then send all its data through the network to any point in larger systems where it needs to be used. No additional wired I/O slots are necessary.

The adapter suggested is the Emerson Wireless 775 THUM Adapter, which can transmit HART measurement and diagnostic information, including secondary variables. It can be added to most HART devices without disturbing the existing wired connection.

You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Flow and WirelessHART Groups and other specialty areas for suggestions and answers.

Posted by Deanna Johnson, Rosemount Measurement & Analytical Global Marcom Manager, Emerson Automation Solutions