MAR-APR 2019

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INTECH MARCH/APRIL 2019 43 AUTOMATION BASICS and other properties can be reasonably as- sumed, 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 informa - tion. 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 Corio- lis, 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 se lection. For example, this technology can cal- culate 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 addi- tional process measurements and process in- formation, 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, al- lowing DP flowmeters to be used with more chal- lenging 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 tem- perature curve for the process fluid, it can per- form all the calculations necessary for a com- pensated 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, de pend - ing 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 transmit- ters can also measure the line pressure in gauge or absolute terms. By using the differential and line pressure measurements together, the pres- sure 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 enor - mously important for generating an accurate flow measurement, and the DP transmitter can monitor changing conditions and make sophis - ticated 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 measure- ment and how they affect precision. A sophisti- cated DP transmitter provided with the process fluid's characteristics combined with a tempera- ture and line pressure measurement can very precisely measure mass or volume. Such a trans- mitter 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 Figure 2. A separate temperature probe (left, mounted from below the pipe) can capture an accurate temperature profile for the pro- cess fluid and send it to the DP transmitter.

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