Differential Pressure Flow Meters: Orifice Plate, Venturi Meter
We continue our series on gas flow meter types. Here we discuss differential pressure flow meters (DP meter), which are inferential meters, and include the orifice plate, Pitot tube, Venturi meter, and V-Cone meters.
Inferential meters measure neither mass nor volume but use a property of the flow stream, such as pressure drop, or the displacement of a float, to infer volumetric flow rate. There are two main types of inferential meters, the differential pressure (DP) meter, and the VA sensor or rotameter.
The differential pressure meter is a ∆P flow meter, where the flowing gas encounters an obstruction in the meter, which creates a differential pressure that, when measured, infers the volumetric flow.
The meter consists of a DP transmitter and an element that places a constriction on the flow, creating a pressure drop. The DP transmitter computes gas flow based on Bernoulli’s theorem, comparing the upstream and downstream pressures.
DP Meter Examples: Orifice Plate, Venturi Meter
Examples of the DP meter include orifice plates, Pitot tube, Venturi meter, and V-Cone meters.
DP flowmeters measure stack gas flow; this use is driven mainly by EPA reporting requirements of greenhouse gases (GHG). Multiple Pitot tubes at various measuring points in the large stacks can measure the stack gas flow in these applications. Other meter types used in this application are the ultrasonic meters and the thermal mass flow meter. DP meters are also used in custody transfer of natural gas. Yet, there is a trend moving toward ultrasonic meters, given that the ultrasonic flowmeter causes a minor pressure drop (compared to the DP meter), and DP meters (mainly orifice plates) are subject to wear. Additionally, ultrasonic meters have more long-term reliability.
The DP meter is approved for custody transfer of natural gas. This low-cost meter is one of the oldest flowmeter technologies, so it is well understood. More recently, multivariable DP flow meters can also provide mass flow measurement.
The meter design intentionally creates a reduced pressure, thus causing an unavoidable pressure drop. The meter’s primary element (e.g., orifice plate) is subject to wear and clogs with gas impurities. The meter’s accuracy depends on its transmitter, which typically degrades at low differential pressure. It has low turndown ratios (low rangeability), and lastly, it is also challenging to have diagnostics for this meter type.
Differential Pressure Flow Meter Manufacturers
There are multiple manufacturers of DP meters as well as the multivariable DP flow meter, including: