Differential Pressure Flow

Differential Pressure Flow

Differential pressure (DP) flow measurement determines flow by creating a controlled pressure drop with a primary element and measuring the resulting differential pressure with a transmitter. Common primary elements include orifices, Pitot tubes, and Venturi tubes, which generate a DP signal that is converted to flow in the transmitter. The approach is proven across gases, vapors, and liquids, including services with elevated process pressures and temperatures.

The measuring principle is rooted in the relationship between velocity and pressure in a restriction: as flow increases, the generated differential pressure rises and the transmitter calculates flow rate from the measured DP. In practice, square-root extraction is applied for volumetric flow, and compensation (pressure/temperature) is commonly added when mass flow of gases or steam is required. Because the primary element and transmitter are modular, the same DP platform can be adapted from utility monitoring to critical process metering.

Benefits include broad applicability across media, familiarity of the engineering method, and the ability to extend the same transmitter family to adjacent tasks such as level and filter monitoring. Packaged manifolds and valve blocks can reduce installation variability and support safer commissioning and maintenance by standardizing impulse-line isolation and equalization practices.

Typical applications include steam and boiler systems, compressed-air and fuel-gas distribution, flare and vent lines (where appropriate primary elements are selected), chemical feeds, and cooling or service-water measurement. DP-based monitoring is also widely used for filter condition tracking and for tank level measurement via hydrostatic head or remote seals, leveraging the same pressure platform for multiple measurement objectives.

Engineering considerations are primarily driven by the chosen primary element: accuracy and rangeability, required straight-pipe runs, susceptibility to fouling or plugging, and permanent pressure loss. Impulse line design, condensate management for steam, and material compatibility often dominate long-term reliability. Selecting the restriction type (orifice vs. Venturi vs. Pitot) and the transmitter configuration should therefore be treated as a complete measurement system decision rather than a transmitter-only selection.

Field Instruments & Controls, Inc. an exclusive authorized representative of sales and service for Endress+Hauser.