Flow Measurement & Control


PD meters - design offers improved performance

February 2004 Flow Measurement & Control

The VSE gear-type positive displacement flowmeters are designed for use with fluids having lubricity and viscosities up to 100 000 cSt. Performance characteristics that surround these meters include an excellent rangeability of 2000:1 within a ±0,03% of rate measurement accuracy and a repeatability of ±0,05% of flow rate.

Based on the meshing gear-principle, two specially designed gear wheels developed by VSE are precisely matched and enclosed in a very accurately machined housing. Depending on the application, ball or wear resistant sleeve bearings of hardened steel or tungsten carbide located within the gears offer an extremely low resistance to the inertial forces that manifest themselves at start-up or at very low flow rates, thereby contributing to the exceptional rangeability of these meters.

Fluid flow initiates rotation of the almost frictionless gears and the measuring chambers that are constituted by the space between the gear teeth and the housing, define a known discrete volume of liquid that pass through the meters. The rotation of the gear wheels is such that exactly one tooth pitch defines that volume.

This rotation is detected by a system of two non-contacting magnetic field coils that are offset by a quarter of a tooth pitch. This phase shift in gear rotation recognises the flow direction as well as provides for doubling or quadrupling of impulses that lends the meter an excellent capability for resolution of the generated flow signals.

The square waves emanating from the preamplifier can be processed by any suitable electronic system for the indication and totalisation of flow-rate including volume batching, mixing, proportioning and cylinder speed and position measurement in typical test stand facilities.

The patented gear mechanism has overcome the constraints usually imposed by the conventional gear design. By virtue of the enlarged measuring chambers and therefore the relatively low lateral velocity of the expelled fluid, the unique VSE construction has contributed to an almost 60% reduction in the pressure drop across these meters. A further 20 dBa reduction in noise level and an overall improvement in the meters' performance is also attributed to this patented concept.

The meter bodies may be constructed in stainless steel, grey cast iron or aluminium, with each material to suit fluid compatibility and process pressures and temperatures up to 1000 bar and 150°C respectively.

Installation of the meters on a subplate assembly supplied with each meter for incorporation into the process piping, facilitates for simplicity in mounting, servicing and the periodic flushing of the process where necessary. The VSE precision gear meters are available for supply voltages of 12 or 24 V d.c. and are also available in explosion-proof and intrinsically safe models for location within hazardous areas.



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