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Flow, level & control


Benefits and trade-offs of flow meter miniaturisation


For over 40 years, Titan Enterprises has strived to make smaller and better flow meters, ranging from miniature turbines and gear meters through to more recently ultrasonic devices. As with any product development when considering miniaturising a flow device there will be benefits as well as trade- offs to consider. In this article Titan looks at the considerations for turbine, oval gear, ultrasonic and thermal flow meters


A: TurbInE Flow mETErs Making a very low flow, high performance turbine meter is always going to be a challenge. There are a whole range of physical issues preventing you from producing small devices. Generally radial flow turbines are better for low flow applications than axial flow turbines. In no particular order, the main problems include:


i. Friction


Stiction, which can be defined as the friction on two surfaces which prevent the parts being in motion is the first problem. This effect on any bearing will determine the point at which a turbine starts rotating. With a large turbine it is generally not a problem as you are likely to have a large driving torque relative to the stiction. On a miniature device the available driving torque compared to the stiction is greatly reduced. Just getting the bearing spinning freely can be a challenge. This is often overcome with sapphire bearings but again the contact area of these highly polished surfaces can still be relatively high. A point contact offers the lowest friction and is possible with ball or cone bearings. The problem then becomes bearing load. A single point contact equates to an infinitely high bearing load which reduces bearing life. Stiction itself will also depend on the mass of the turbine.


ii. weight


The mass of the turbine in the fluid not only effects the stiction it also has a consequence on the response time. The greater the mass the slower the response time. A turbine with minimal mass could be of a similar density to the fluid that is being metered so that it possesses neutral buoyancy. There is also a trade-off here between turbine diameter, blade width / thickness, materials of construction and detection method. The larger the moment on the turbine from offset of the incoming jet, the stronger the driving force. But if the mass and rotational resistance is higher then so is the drag on the other non-driven blades. Very fine blades can be hard to detect optically and the blade width is restricted to the diameter of the incoming jet plus some incoming jet “spreading” distance within the fluid. Too narrow, and much of


34


the fluid will spill around the side, but too wide and unnecessary drag will occur.


iii. Detection system


A no-drag system is essential and optical is ideal providing you have a fluid which transmits the light effectively. It can be reflective or more often some type of beam cutting arrangement. Adding magnetic material to the turbine for a zero drag inductive, magnetic or Hall Effect detector increases the mass but enables the flowmeter to handle opaque fluids such as emulsions. For such applications there is a trade-off between magnetic elements, their size and mass.


iv. Fluidics


As flow rates become lower, the more liquids look and behave like treacle. Turbines become viscosity sensitive as they are primarily Reynolds number devices which are happier with turbulent flow. We try to negate some of these effects with fluidic tricks such as the induced secondary vortices which behave like “roller” bearing which reduce viscous drag and extend the linear flow range.


v. General mechanics


This is a catch-all section and hard to quantify. There is a relationship between turbine diameter, jet offset and size as well as turbine and chamber thickness and clearances. It is a delicate compromise between these elements.


b: GEAr Flow mETErs Titan Enterprises opted to incorporate the oval gear design into its gear meter products, as there is a much greater driving torque for a given differential pressure which is derived from fluidic asymmetry of the gears. There are fewer subtleties in the design options on oval gear flowmeters, in comparison to radial turbines, because of their basic operating principles. Oval gear flow meters operate on the principle of positive displacement, by taking a parcel of fluid and transferring it from the inlet side to the outlet side without any leakage. This sounds very easy but there is a trade-


off between gear clearance, leakage, friction and fluid viscosity.


a. Clearance and leakage


Make the clearances small and you can meter low viscosity fluids very accurately over a large flow range. Make them too large and the flow meter will not work well until the liquid is highly viscous. As an oval gear flow meter is made smaller, the leak path becomes proportionately larger. If we take one of our smaller oval gear flowmeters and straighten out the leak path it can be defined as a strip some 60mm long by the clearance between the gear and the chamber. We would normally run that clearance at 0.03mm which gives a potential leak area of 1.8mm2


This is equivalent to a round


hole nearly 1.5mm diameter. If you halved the size of the gears in all directions the leak path would become essentially half so still nearly a 1mm hole for a square-law reduction in gear size and efficiency. Bigger oval gears are easier to manufacture as they are more efficient at, relatively speaking, lower flows.


b. Friction


If there is stiction an oval gear flow meter will not start. If there is running friction the linearity will not be good. Like mini-turbines, friction must be kept to a minimum. However, there are fewer options with oval gear flow meters as the bearings loads can be quite high. This is because the load induced by the meter’s pressure drop (which is necessary to work the meter) becomes problematic at higher flows.


January 2021 Instrumentation Monthly


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