SITRANS FP230/FPS200 primary elements (ISO 5167)

Primary differential pressure devices are standardized mechanical flow sensors, often also referred to as differential pressure transducers. The primary differential pressure devices are calculated and manufactured according to EN ISO 5167.

Through constriction of the line diameter in the pressure device, the flow rate creates a differential pressure that is converted with the help of a differential pressure transmitter into a proportional current signal or flow value. The assignment of differential pressure to flow is created by a calculation of the primary differential pressure device.

Primary differential pressure devices are suitable for single-phase media such as gas, steam and liquids without solid components.

• Suitable for universal use across the globe and widely accepted in all industries
• Very robust and can be used in a wide range of nominal diameters
• Suitable for high temperature and pressure ranges
• Low uncertainty of measurement
• No wet calibration required as they use an internationally standardized flow rate measurement procedure
• Differential pressure transmitter can be used over a long distance from the measuring location
• Differential pressure method is well known and has a large installed base
• SITRANS P differential pressure transmitter is easy to parameterize again if process data change. They are adapted by recalculating and assigning new parameters to the transmitter or, in case of an orifice plate with annular chamber, by using a new orifice plate

• Technical gases
• Compressed air
• Fresh and combustion air
• Steam/Heat quantities
• Heat transfer fluids
• Water

Basics: Orifices for flow measurement

Orifice plates are usually differentiated by their type of installation, type of differential pressure tapping and the shape of the orifice.

The characteristic differential pressure is created by the orifice bore which is the defined circular opening. It is usually of square edged concentric type according to ISO 5167-2 positioned in the middle of the pipe.

The main features are a sharp edge, a cylindrical bore of a certain length and a conical bevel tapering to the rear. Alternatively, the relevant standards provide for deviating designs, which are used for applications with highly viscous (e.g. quarter-circle nozzles) or contaminated media (e.g. segment orifices).

The standard design is permitted by the standard for an inner diameter of the pipe between 50 mm and 1000 mm. For pipes with smaller inside diameters, standards such as ISO 15377-TR or ASME MFC-14M which go beyond these standards must be taken into account. Orifice plates for pipes with small inside diameters are usually designed as meter tubes.

In order to reduce the uncertainties of these meter tubes, the devices can be calibrated on a flow test bench if required on request.

Types of differential pressure tappings

The differential pressure can be tapped in different ways:

Corner tapping

Directly in front of and behind the orifice plate an opening is placed in the corner of a carrier ring to measure upstream and downstream pressure. Both pressure signals are routed through these openings to the outside.

Corner tapping with annular chamber

The orifice plated is held by an annular chamber. Upstream and downstream pressure are measured through an annular gap opening between carrier ring and orifice plate. Both pressure signals are averaged over the entire circumference and routed outside.

Flange tapping

The orifice plate is held between two so-called orifice flanges. Upstream and downstream pressure signals are measured through flange taps which are drilled into the flanges.

Tapping with distance D, D/2

The orifice plate is held between regular flanges. Upstream and downstream pressure signals are measured through taps in the pipe with distance of D (upstream) and D/2 (downstream) to the orifice plate.

Designs

• Standard orifice plate with corner pressure tappings (7ME171)
• Standard orifice plate with annular chamber (7ME172)
• Orifice meter run (7ME173)
• Orifice plate (7ME174)
• Orifice plate with orifice flanges (7ME175)

Mounting arrangements

For more information on installation position and piping, please see the Operating Instructions "SITRANS FPS200" on SIOS.

The orifice plate is installed between two flanges in the pipeline. Using condensation pots (for steam) and initial shut-off valves, the differential pressure of the high-pressure side and low-pressure side is directed through differential pressure lines to a manifold and to the differential pressure transmitter. For fluids with pressure and temperature fluctuations it makes sense to take an additional measurement of the pressure and temperature in order to correct the flow signal of the transmitter in a subsequent correction computer.

Selection of mounting point

The flow measuring regulations EN ISO 5167 not only consider the design of primary differential pressure devices, but also assume that their installation is in accordance with the standard so that the specified tolerances can be retained. The required inlet and outlet pipe sections according to ISO 5167 can be found in the calculation protocol of the respective orifice plate. Configuration of the pipeline should allow for standardized installation (required inlet and outlet pipe section). Particular attention must be paid to ensure that the primary device can be fitted in a sufficiently long straight section of pipe. Bends, valves and similar should be fitted sufficiently far upstream of the primary device to prevent them having a detrimental effect. Primary devices with a large diameter ratio are particularly sensitive to interferences.

Design of measuring point

The design of the measuring point depends on the medium and on the spatial conditions. The designs for gas and water only differ with regard to the position of the tapping sockets (see section "Tapping sockets"); condensation pots are provided for steam applications.

Orifice meter runs

On lines with small nominal diameters (DN 10 to DN 50) the measurements are influenced by the wall roughness and diameter tolerances of the pipes, more than measurements with larger nominal diameters. These influences are counteracted by using orifice meter runs with fitted inlet and outlet pipe sections made of precision pipes. For exact measurements with orifice meter runs, the flow coefficient C can be determined by means of calibration.

Z-Options for cable glands, plugs, labeling, approvals, blanking plugs, flanges seals, device settings, etc. according to SITRANS P320

Further versions that are available on request:

• Other types of primary differential pressure device: nozzles, venturi nozzles, classic venturi tubes etc.
• Other nominal diameters and nominal pressures to EN, ASME and other standards
• Other lengths, special lengths
• Other materials
• Sealing face with recess or groove
• Flushing rings
• Other tapping sockets, multiple tappings
• Material acceptance test certificates or cold water pressure tests

For more information please see the Installation Instructions and the Instruction Manuals SITRANS P on SIOS.