Titan Discuss The Merits of High Viscosity Flow Measurement Technologies
Titan Enterprises was founded on the belief that the market required a high quality reasonably priced mini-turbine flowmeter. This proved to be correct but we quickly ran into problems with attempting to meter more viscous flow as these turbine flowmeters are Reynolds number dependant. We were therefore forced into looking at alternative low cost, high viscosity flow technologies, but which one? As this was before the advent of our low-flow Atrato ultrasonic flowmeter technology it had to be a mechanical meter.
Below are some of the flow technologies we considered and why we ended up developing an oval gear flow meter, which offered low pressure drop, ease of high volume manufacture, good chemical resistance and a range of detector options.
Nutating Disc Flow Meters
This technique is often used in domestic water meters. Nutating disc flow meters use a flying 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 flowmeter there are a lot of rubbing/bearing surfaces which have to maintain their low friction characteristics. The low flow – high viscosity flow capability is limited, therefore not the type of meter we wanted.
Oscillating Piston Flow Meters
Oscillating piston flow meters are in some ways similar to nutating disc meters but the piston in constrained in one plane and it is therefore more efficient. 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 flow/high viscosity flow applications.
Sliding Vane Flow Meters
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 flow meters can be very accurate but have even more sliding elements, and hence drawbacks, than the previous meters.
Radial Piston Flow Meters
This flow technology typically uses four pistons on a central camshaft fluidically connected with suitable valve arrangements. You can view a radial piston flow meter 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 flow meters are very accurate devices but incorporate complex assemblies and are therefore not possible for a lower cost solution.
Helical Gear Flow Meters
Helical gear flow meters use two inter-meshing helixes to pass a fixed volume of fluid along the length of the helix which are geared together. They are very efficient, but are unfortunately very expensive to produce.
Gear Flow Meters
Gear flow meters operate using two meshed gears in a single close-fitting 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 flow meters also do not lend themselves to a moulded solution which would reduce the cost of manufacture to an acceptable level.
Oval Gear Flow Meters
On investigation, we found that oval gear flowmeter technology combined the benefits of some of the others without compromising the performance, and offered good potential in use as a high viscosity flow and low-flow meter. 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 flow very low.
Comparison of Flow Techniques: High Viscosity Flow Meters
Our decision was to develop an oval gear flowmeter as this technology not only enabled us to handle viscous fluids but also low viscosity and aggressive fluids. Oval gear meters also beneficially offered the option of low cost manufacture for OEM applications.
For further information on oval gear flow meter technology from Titan Enterprises please click here.