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Primary Elements – Proven Flow Measurement Technology

Orifice primary elements are used for economical and reliable flow measurement of liquids, gases or steam. During measurement, the medium should be in one pure phase and flow through pipes with circular cross-sections, running full. KOBOLD, represented locally by Instrotech, has on offer their Kobold KPL orifice plates and flanges that require little or no maintenance, as there are no moving parts in the measurement stream.

If a pipeline, through which a medium flows, is reduced at a particular point by a cross-sectional constriction, the flow speed of the measured medium is increased at that point. According to Bernoulli’s energy equation and the law of continuity, the total flow head (dynamic velocity head and static pressure head) is constant.

The increase in speed at the constriction causes a reduction in the static head. The resulting pressure drop is called the differential pressure head; it is a measure of the flow (volume per unit of time or mass per unit of time).
The flow rate q is given by: q = c√∆p

The flow rate (q) is a function of the square root of the differential pressure head (∆p), where c is the coefficient of flow rate determined by the shape of the differential pressure transducer and the operating data. The pipeline may be constricted with orifice plates.

Metering unit design
The entire metering unit comprises the following:
• The transmitter converts the differential pressure signal into a standard output signal. To generate a linear signal, a differential pressure transmitter with internal square-root extraction, e.g. PAD model, is recommended.
• Primary element (e.g. an orifice with flange tapping, corner tapping or D-D/2 pressure tapping etc.)
• Orifice Plates Model KPL-B The orifice plate with welded handle model KPL-B is supplied with plain raised face for fitting between pipeline flanges. It generates and sustains the differential pressure required for flow measurement. The differential pressure head is tapped in the flange or in the pipeline at a particular distance from the orifice plate. This distance is included in the calculations for the plate. The plates are dimensioned to suit prevailing service conditions. The orifice plates have no mounting rings; they are fitted between pipeline flanges.

In the remote version, the differential pressure (DP) transmitter and the primary element are mounted separately and are connected with an appropriate impulse tubing by the user, enabling a flow measurement with maximum temperatures up to +500 °C.

The various designs and process connections allow usage with numerous mounting positions. The calculation of primary elements is according to globally standardised norm ISO 5167. Other standards are also available on request.

• Sizes: DN 50 … DN 600, 2″ … 24″ ASME
• pmax: PN 420 or Class 2500; tmax: +500 °C (930 °F), higher on request
• Material: stainless steel, carbon steel, others on request


For more information on Kobold’s KPL orifice plates and flanges, contact Instrotech on 010 595 1831 or sales@instrotech.co.za

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