Measuring flow via differential pressure is a reliable, time-proven method with a large install base in the industry. Differential pressure flowmeters consist of two key components. One is a restriction having known characteristics, such as an orifice, venturi, cone, nozzle, or wedge. These are sometimes referred to as primary elements. The second component in this type of system is a differential pressure transmitter which measures the differential pressure across the primary element and then sends the readings to a control system. Advantages of the differential pressure flow meter include reduced maintenance, improved particle tolerance, and higher temperature operation versus mechanical flow meters such as turbine and positive displacement type.
Differential Pressure flow meters, also known as DP flow meters, make use of a flow body that is called a primary element which is installed inside of a pipe in order to cause a constriction in the flow path which in turn creates a pressure drop. When the primary element is used in conjunction with a differential pressure transmitter, the volumetric flow rate may be calculated as a function of the differential pressure across the obstruction; hence the term “differential pressure flowmeter”.
DP flow measurement in one of the oldest flow measurement technologies and dates back to at least the 17th century — though the measurement of flow using nozzles actually goes all the way back to Roman times. In 1738, Daniel Bernoulli developed his famous equation for flow rate calculation which shows that increasing velocity by constricting a flow path increases fluid pressure. Restricting the flow path creates an increased pressure and if you can measure the upstream and downstream pressure, you can calculate the flow rate.
The most basic differential pressure flow measurement technique available today makes use of an orifice plate as the primary element. This type of flowmeter first appeared in 1909, and was used to measure steam flow. Shortly thereafter, the oil and gas industries began using orifice plate flowmeters due to ease of standardization and low maintenance. Until other flow technologies came into the market, primary elements were the most popular.
While DP flowmeters are among the least expensive methods of measuring flow, they are also among the most problematic. The primary concern for this type of measurement system is that it requires a second instrument in order to provide the flow rate reading, which is the Differential Pressure (DP) transmitter. Pressure drops are also higher compared to other types flowmeters, which translates to increased energy costs in order to to maintain the flow in the process. SmartMeasurement has developed a comparison exhibit of more advanced technologies versus DP flow elements such as an Orifice Plate: Download
When properly sized, Differential Pressure flow meters are excellent for very low flows or low Reynolds numbers (Re. < 500), high-viscosity fluids which may contain dust or solid particles, or applications for measuring flows of liquid cement, crude oil, asphaltic oil, coal-tar oil, wastewater, hard pulp, carbon black solutions, as well as two phase flows. For a discussion of various DP flow technologies please go to: DP Flow Measuring Principle
Since pressure drops are one of the the biggest concerns for DP flow measurement, SmartMeasurement only offers one type of flow element, which is the style that has demonstrated the lowest pressure drop among all primary flow elements. This element is known in the industry as a cone type element and is offered as the SmartMeasurement ACONE. In addition to lower pressure drops, the ACONE also offers dramatically reduced straight run requirements versus other primary elements such as the orifice plate or venturi. When paired with the ALDPT differential pressure transmitter, the SmartMeasurement ACONE provides a complete flow measurement package for liquid, gas, or steam applications. When used with the ALDPT-MV and an RTD temperature sensor, the ACONE is part of a complete system which is capable of calculating both mass and volumetric flow rates. SmartMeasurement Pressure Transmitters
The SmartMeasurement ACONE differential pressure flowmeter is designed for the harshest operating environments and for the widest variety of fluids. This type of element consistently out-performs traditional ΔP devices and other major flow technologies. The ACONE’s enhanced performance is due to the shape and position of the cone-shaped obstruction inside the flow body in relation to the ΔP measurement ports. The cone interacts with the flow, reshaping the fluid’s velocity profile and creating a region of lower pressure immediately downstream of itself. Two pressure taps are designed to measure the differential pressure between the static line pressure and the low-pressure zone created downstream of the cone. Flow is calculated by the Bernoulli equation of conservation of energy. The cone element also acts as its own flow conditioner, fully conditioning any turbulence in flow due to limited upstream and downstream straight runs.