WHAT MAKES OVAL GEAR FLOWMETERS OFTEN THE CHOICE FOR MEASUREMENT OF VISCOUS FLUIDS
Titan Enterprises was founded on the belief that the market required a high quality reasonably price mini- turbine fl owmeter. This proved to be correct but we quickly ran into problems with attempting to meter more viscous fl uids as these turbine fl owmeters are Reynolds number dependant and were therefore forced into looking at alternative low cost, high viscosity fl ow technologies, but which one? As this was before the advent of our low-fl ow Atrato ultrasonic fl owmeter technology it had to be a mechanical meter.
Below are some of the fl ow technologies we considered and why we ended up developing an oval gear fl owmeter which offered low pressure drop, ease of high volume manufacture, good chemical resistance and a range of detector options.
Nutating Disc Sliding Vane Gear Meters
Gear fl owmeters operate using two meshed gears in a single close- fi tting chamber. The volume of a single gear tooth is passed through as they rotate. As a result, they offer high resolution but require a fairly high pressure drop before they start working. Gear fl owmeters also do not lend themselves to a moulded solution which would reduce the cost of manufacture to an acceptable level.
Oval Gear
This technique is often used in domestic water meters. Nutating disc fl owmeters use a fl ying saucer or Saturn shaped measuring element, with a slot in the disc and a divider to prevent rotation, wobbling around a circular path. The motion of the nutating disc is up and down but because the spindle is constrained to a small orbit the cyclical movement is circular. It is not a true positive displacement device as there is always clearance around the disc and between the divider plate. In this type of fl owmeter there are a lot of rubbing/bearing surfaces which have to maintain their low friction characteristics. The low fl ow capability is limited, therefore not the type of meter we wanted.
Oscillating Piston
Oscillating piston fl owmeters are in some ways similar to nutating disc meters but the piston in constrained in one plane and it is therefore more effi cient. Oscillating piston meters also rely on a lot of sliding surfaces and have a fairly large constant leak path. This technique can be used for some low fl ow viscous fl uid applications.
This technique uses a rotating hub in an offset orbit in a round chamber. In the hub are a number of spring loaded vanes. These slide in and out as the hub rotates so trapping a volume of liquid and transferring it from the inlet to the outlet. Sliding vane fl owmeters can be very accurate but have even more sliding elements, and hence drawbacks, than the previous meters.
Radial Piston
This fl ow technology typically uses four pistons on a central camshaft fl uidically connected with suitable valve arrangements. You can view a radial piston fl owmeter as a circular four cylinder two-stroke aero engine without the spark and running with an incompressible liquid instead of a gas mixture. Radial piston fl owmeters are very accurate devices but incorporate complex assemblies and are therefore not possible for a lower cost solution.
Helical Gear
Helical gear fl owmeters use two inter-meshing helixes to pass a fi xed volume of fl uid along the length of the helix which are geared together. They are very effi cient, but are unfortunately very expensive to produce.
On investigation, we found that oval gear fl owmeter technology combined the benefi ts of some of the others without compromising the performance. The rubbing/bearing areas are small and controlled. The clearances can be easily maintained in manufacture. The gears themselves can be moulded in chemically resistant materials with excellent bearing properties and precision machined afterwards to ensure closer tolerances if required. The pressure drop is relatively small and the start-up fl ow very low.
Comparison of Flow Techniques for Viscous Fluids
Technology Volumetric effi ciency
Nutating disc
Oscilating piston
Author Details
Trevor Forster, Managing Director, Titan Enterprises His experience in fl uid handling dates back to the mid 1960’s when he started working on rotating seals and fl owmeter design for third party clients. Trevor draws upon over 40 years of using innovative design and production techniques to produce elegant fl ow metering solutions for organisations around the globe.
Email: trevor@fl
owmeters.co.uk • Web: www.fl
owmeters.co.uk
Sliding vane Radial piston Helical gear Gear meter Oval gear
Our decision was to develop an oval gear fl owmeter as this technology not only enabled us to handle viscous fl uids but also low viscosity and aggressive fl uids. Oval gear meters also benefi cially offered the option of low cost manufacture for OEM applications.
Bearing surfaces
Chemical resistance
Moulding capability
Sale price
OCTOBER / NOVEMBER •
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