- By Connor Oberle
- Automation Basics
Differential pressure (DP) is a common and well-understood technology for measuring the flow of process fluids for several reasons:
- Versatility: It can measure virtually any type of fluid: liquid, gas, or steam.
- Scalability: Installations can be any size.
- Precision: When designed and executed well, DP has very high accuracy.
Another attribute is the ability of DP flowmeters to measure multiple variables using sophisticated transmitters. Let’s unpack this idea and consider what flow is and how it is measured.
Uncompensated versus compensated flow
When measuring flow, simple DP flowmeters 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 a 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.
Some flowmeter technologies, such as Coriolis, measure mass flow natively. This technology can provide a different range of variables than DP flowmeters, so depending on the needs of the process, it might be a more appropriate selection. For example, this technology can calculate specific gravity of a fluid along with solids content.
If a mass flow measurement is required for the process, the flow measurement available from a DP flowmeter (figure 1) can be conditioned by using multivariable technology. Multivariable DP flowmeters are capable of providing additional process measurements and process information, including process temperature, line pressure, fluid properties, and specifics about the pipe geometry and primary element. When this information is available, a compensated flow measurement will correct for changes in density, viscosity, and other dynamic fluid properties, allowing DP flowmeters to be used with more challenging fluids or in critical applications.
As a case in point, if the DP flowmeter using multivariable technology takes input from a temperature sensor, it will be able to improve its flow accuracy by using the temperature value in its flow calculations. If the transmitter has been programmed with the density value versus temperature curve for the process fluid, it can perform all the calculations necessary for a compensated flow reading for any process liquid in real time. We will talk about steam and gases in a moment.
When temperature and other variables are used to compensate the flow value, precision can be improved to ±1 percent or better, depending on the conditions. Because sophisticated DP flowmeters have a high degree of accuracy, this can be a very useful improvement.
Pressure matters too
DP transmitters are designed to measure the difference between two points in a process. In the case of a DP flowmeter, the pressure on the upstream side of the primary element is higher than on the downstream side. For the sake of example, say the pressure drop at a given flow condition is 1.35 psi. This is an accurate reading, but it does not say anything about the pressure in the line. It could be 20 or 200 psi, and there is no way to know without an additional pressure measurement. Or is there? Some DP transmitters can also measure the line pressure in gauge or absolute terms. By using the differential and line pressure measurements together, the pressure on either side of the primary element can be determined. In this example, the high side is 64.92 psig, and the low side is 63.57 psig.
Knowing the high and low line pressures when working with a gas or steam is enormously important for generating an accurate flow measurement, and the DP transmitter can monitor changing conditions and make sophisticated calculations in real time.
Precision and cost
Two elements were mentioned in the opening of this discussion: precision and cost. So far, we have talked about the mechanics of measurement and how they affect precision. A sophisticated DP transmitter provided with the process fluid's characteristics combined with a temperature and line pressure measurement can very precisely measure mass or volume. Such a transmitter can go through the calculation routines 20 times or more per second to ensure a true real-time measurement.
Where does cost come in? The type of DP flowmeter 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 flowmeter with its sophisticated transmitter and ancillary measurements eliminates the need to install all these additional devices, at least in most situations.
Some process engineers may be reluctant to use a secondary variable for a critical measurement. For example, the DP flowmeter can provide its own temperature reading, but perhaps not with an update rate as fast as a stand-alone temperature sensor and transmitter. Of course, it is probably a small number of situations where temperature changes occur so rapidly, and if a critical loop is based on temperature, it will certainly require its own instrument.
Practicality of additional measurements
The practicality of using the extra measurements as part of a larger process automation strategy will depend on how they are extracted. The DP flowmeter has access to the measurements we have already mentioned-along with any additional information, such as fluid density characteristics and line size, embedded in the configuration. The transmitter uses this information constantly for its own internal calculations.
If the DP flowmeter 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. The distributed control system (DCS) simply needs to be programmed to access the data and on how to use it in larger control efforts.
In a conventional analog I/O environment, accessing the extra functions and variables is more complicated. HART multiplexers tend to take a long time to cycle through all the transmitters they service, so the additional readings will not have a fast update rate. A HART interface (figure 3) can work with a single multivariable instrument, breaking out the additional readings and turning them into separate 4-20 mA signals. This works well, but the DCS has to treat them as separate tags just like individual instruments, adding to wiring costs.
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 (figure 4) to a DP flowmeter 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.
Advantages of multivariable instruments
Today's multivariable instruments are possible thanks to advances in transmitter electronics. The little circuit board inside the housing is truly a powerful computer able to perform calculations with remarkable speed. When applied to DP flowmeters, these capabilities provide exceptional accuracy across a huge range of fluid types and characteristics.
Where useful, secondary variables can deliver process information without additional instruments or process penetrations. This double benefit of performance and cost advantages can help optimize the process while reducing the cost of gathering the data necessary for effective decision making
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