Differential pressure (DP) flowmeters

Endress+Hauser UK has been manufacturing differential pressure (DP) flow devices for 25 years. The tried-and-tested technology for measuring flow in a closed pipe remains popular due to its low cost and simple design.

We manufacture a range of primary DP flow devices from our Centre of Competence in Manchester, including averaging Pitot tubes, orifice plates, cone meters and Venturi tubes. As they are manufactured in-house, each device can be tailored to the end user’s exact requirements.

The measuring principle is based on Bernoulli’s equation which establishes the direct relationship between the pressure and speed of a fluid flowing in a pipe. Differential pressure flow meters introduce a restriction in the flow path causing a larger flow velocity than in the rest of the pipe. According to the Bernoulli equation this results in a reduction of the static pressure.

The pressure difference between the static pressures upstream and downstream of the orifice plate is measured by a differential pressure transmitter. 

The value of the differential pressure is very much depending on the diameter ratio (β) of the internal diameter of the restriction (d) to the internal diameter of the pipe (D). 

The relationship between flow rate (Q) and differential pressure (Δp) is a square root function. 

Different primary elements are used for different measurements, including the orifice plate, flow nozzle, wedge, cone, and Venturi tube.  

Averaging Pitot tubes are based on the same principle, except the upstream side of the probe is exposed to both static pressure and dynamic pressure and the downstream side is exposed only to the static pressure.

The resulting differential pressure is measured and can be used to calculate the flow rate. Downstream of the Pitot tube the static pressure is reduced by the permanent pressure loss. With Pitot tubes this pressure loss is much less significant than with other primary elements. 

• Universal measuring principle for all fluids: gases, liquids and steam 

• Measurement method globally standardised according to ISO 5167 

• Purely mechanical, no moving parts 

• High pressure and high temperature capabilities 

• Operational compact versions available to minimise installation costs 

A wide range of primary elements is used in liquid, gas and steam flow measurement up to 420bar (6,091psi) and 1,000°C (1,832°F). A wealth of experience contained in numerous standards makes this type of measurement well accepted and widespread all over the world. 

One of the main areas of application is still measurement in hot water and cooling cycles as well as steam and condensate in secondary circuits at very high temperatures. Venturi tubes and pitot tubes offer an alternative to orifice plates wherever keeping pressure losses low is demanded or in large pipelines with diameters of up to several meters. Differential pressure measuring systems permit the exchange of transmitters at any time without interrupting the process.

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Orifice plates are the most popular differential pressure primary elements as they provide a low-cost flow measurement solution. They are used for general purpose flow measurement of liquids, gases, and steam where permanent pressure loss is not a significant factor. They provide an economical, easy to install and versatile solution.

The concentric sharp-edged orifice plate is considered the standard orifice plate design. Alternative orifice plate designs are also available, which are commonly used for applications with highly viscous or contaminated media.

The scope of supply can include the plate, carrier, flanges, accessories, or the full metering run. The type of pressure tapping has a crucial influence on the mechanical construction of the orifice and on the mounting into the pipe. The Deltatop product family comprises all types of pressure tapping described in ISO5167.

Endress+Hauser also supply Compact Orifice Carrier Assemblies which allow for directly mounting the transmitter onto a welded integral manifold. The simplified design eliminates the need for connecting a remote mount transmitter using impulse piping, saving time, and reducing installation costs.

As well as measuring flow, orifice plates can be used to restrict the flowrate to a certain value or to reduce the pressure at a certain flowrate. Restriction orifice plates are very similar to orifice plates, but they do not need an additional transmitter as in the case of flow measurement. Restriction orifices are therefore a purely mechanical way of reducing the pipe cross-sectional area.

Due to the pressure reduction requirements, restriction orifice plates are generally subject to a much higher mechanical load than orifice plates. For this reason, the plate thickness must be calculated individually for each specific application.

Benefits

• Universally suitable for measuring liquids, gases and steam

• Orifice plate bore designs available to cover challenging applications

• Cost-effective solution suitable for various

• Transmitter mounting selectable according to the application: operational compact version for minimising installation costs, and remote version for demanding process conditions (high temperature, high pressure) and difficult installation conditions

Where flow measurement is required for high velocity steam and gas applications, nozzles are a proven technology that offer long term accuracy. Unlike Orifice plates, the design does not rely on a sharp edge, making them less susceptible to abrasion and can maintain performance with no maintenance or inspection required. Nozzles are standardised internationally in accordance with ISO 5167-3:2003.

There are three types of nozzle according to ISO 5167:

• ISA 1932 nozzle

• Long-radius nozzle

• Venturi nozzle

The shape of the inlet profile constitutes the main difference between ISA 1932 nozzles and long-radius nozzles. ISA 1932 nozzles comprise of two circular segments, whereas long-radius nozzles comprising a quarter ellipse, wherein two different shapes can be used.

Venturi nozzles combine the benefits of both nozzle and Venturi meters. The front face and rounded inlet are identical to ISA 1932 nozzles. However, the Venturi nozzle has an additional conical divergent section to reduce the permanent pressure loss.

Benefits

• Offers accurate measurement for high pressure gas and steam applications.

• Offers a robust design, minimum maintenance and upkeep required.

• Long service life, no abrasion or corrosion from the fluid

• Designs available for high temperatures and pressures

• Calibration available to offer reduced uncertainty

Venturi tubes are commonly used for a wide range of standard and demanding applications across different industries, providing accurate flow measurement with significantly less permanent pressure loss compared to other primary elements. They offer an excellent resistance to corrosion and wear, meaning that once they are installed, they provide a long service life with virtually no maintenance required.

The design generally comprises of four main sections: an entrance cylinder, a conical convergent inlet, a cylindrical constriction or throat and a conical divergent section with variable angle.

Venturi tubes can be installed horizontally or vertically and have shorter upstream piping requirements to simplify installation and reduce costs. Many versions of Venturi tubes are available and standardised in accordance with a range of international or national standards, practically all of which can be provided by Endress+Hauser.

Benefits

• Universally suitable for measuring liquids, gases and steam

• Offers lower permanent pressure loss

• No moving parts, minimum maintenance required

• Designs available for high temperatures and pressures

• Calibration available to offer reduced uncertainty

• Bidirectional version possible

Cone meters are designed to give accurate and reliable flow measurement even in more challenging conditions. The cone element can reshape and change the flow profile, therefore acting as its own flow conditioner. As a result, the required straight run is minimal, making it an excellent solution when straight pipe lengths are limited.

The cone is a primary element composed of a conical restriction and is assembled so that its axis is concentric with the axis of the section of the pipe in which it is fixed. The cone meter described here consists of a section of pipe with a diameter D in which the cone is fixed and of the upstream and downstream pressure tap connections necessary for the measurement.

Cone meters are standardised internationally in accordance with ISO 5167-5:2016, which means that they offer a ±5% measurement accuracy (uncalibrated). Optional calibration would further improve the measurement accuracy to within ±0.5% of the measured value.

Benefits

• Universally suitable for measuring liquids, gases and steam

• Offers lower permanent pressure loss

• Minimal straight length requirement

• No moving parts, minimum maintenance required

• Designs available for high temperatures and pressures

• Calibration available to offer reduced uncertainty

Due to their robust design, Wedge meters offer a reliable solution to measuring dirty, viscous, or abrasive flows including those with a risk of fouling. The profile is virtually immune to wear or erosion, therefore little maintenance and inspection is required. Various tapping designs are available that can accept remote seal elements, which avoids the blockage of impulse lines and response time problems that other devices can encounter when metering slurries and viscous products.

The welded v-shaped Wedge element means that they can be designed for use in extreme pressure and temperature applications, as well as bi-directional flow measurement. Endress+Hauser design and manufacture Wedge meters according to ISO-5167:6, which offer a ±4% uncalibrated accuracy, which is relatively high compared to other primary elements. For applications where higher accuracy is required, calibration can be performed making it comparable to other elements like Venturi tubes and nozzles.

Venturi tubes use the Venturi effect, which is the reduction of fluid pressure that results when a fluid runs through a constricted section of pipe. The resulting pressure difference is measured.  After the constricted area, the fluid is passed through a pressure recovery exit section where up to 80% of the differential pressure generated at the constricted area is recovered.

The design generally comprises of four main sections: an entrance cylinder, a conical convergent inlet, a cylindrical constriction or throat and a conical divergent section with variable angle.

Benefits

• Norm ISO 5167 is available

• Suitable for liquids, gases and steam

• Low requirements for straight upstream and downstream pipes

• Bi-directional flow capability

• Durable – excellent long-term stability

• No moving parts to replace, so offers low maintenance effort

Averaging Pitot tubes provide cost-effective flow measurement of liquids, gases and vapours in a wide range of applications. This simple device works by generating a differential pressure when placed in the path of a flowing medium. They are ideally suited to larger pipework, where they offer significant savings over other flow measurement devices.

The differential pressure generated is sensed via the high pressure and low pressure ports, which are connected to a suitable transmitter. The resulting flow rate can then be derived from the square root of the differential pressure signal.

Endress+Hauser’s Averaging Pitot Tubes have an internal high pressure tube that further averages any distortion in the flow profile and helps to provide excellent performance, even when it is installed in the plane of a close upstream bend.

The sensor can be easily retrofitted into existing pipelines with minimum disruption - installation is achieved with only one or two small entry holes, with different connections & orientations available. Retractable versions are also available which allow insertion and retraction of the sensor from the pipework without the need for shutting down the process.

Benefits

• Transmitter mounting selectable according to the application: operational compact version for minimising installation costs, and remote version for demanding process conditions (high temperature, high pressure) and difficult installation conditions

• Low permanent pressure loss

• Robust design; no moving parts

• Bidirectional measurement available