Meter Sizing and Pressure Drop in Oval Gear Meters

Often-times, when selecting a flowmeter for an application, the default is to match the meter to the line size in order to avoid any flow restriction.? While this may seem logical, it is a common misconception that reducing in line size will cause always cause a restriction or high pressure drop.? This can certainly be the case in some applications, but not all.? There are other factors to consider that impact pressure-drop other than the line size, such as viscosity.? In many cases a pipeline can be over-sized relative to the actual process flow rate.? Therefore, simply matching up to the pipe size can be unnecessarily costly when selecting a flowmeter.? An example of this would be selecting a 2” meter simply due to the existing process piping size, when the flow rate in the process can be handled by a ?” meter at a significantly lower cost.

This article will explore pressure drop and pipe size reduction as it pertains to Macnaught’s oval gear flowmeters.

An oval gear meter is a type of positive displacement (PD) flow meter that measures fluid flow by using two oval-shaped gears that rotate within a precision-machined chamber. The principle of operation is fairly simple: as the fluid moves through the meter, it causes the gears to turn, and a known volume of fluid held by the fluid chamber is displaced. This volume represents a fraction of one gallon. Therefore x-amount of rotations equals one gallon. The volume held in the fluid chamber will vary between meter sizes.

Understanding Pressure Drop in an Oval Gear Meter

Pressure drop refers to the loss of pressure as a fluid moves through the meter. In an oval gear meter, this occurs due to factors such as friction, viscosity, and flow resistance. The key aspects affecting pressure drop in an oval gear meter include:

1. Fluid Viscosity Higher viscosity fluids (such as oils or syrups) create greater resistance to flow, increasing the pressure drop. Conversely, low-viscosity fluids (such as water or gasoline) result in a lower pressure drop.? Viscosity relative to flow rate has the greatest impact on pressure drop in a PD meter and should be a key consideration when determining meter size.
2. Flow Rate As flow rate increases, the frictional forces within the meter also increase, leading to a higher pressure drop. However, since oval gear meters are PD meters, the flow rate is largely independent of pressure variations, as long as the fluid remains within operating conditions.
3. Clearances and Gear Design The tightness of the clearances between the gears and the meter body affects resistance to flow. Smaller clearances may enhance accuracy but can contribute to a higher pressure drop. Wear and tear over time may also slightly increase pressure drop due to changes in gear efficiency.
4. Meter Size and Internal Geometry Larger meters with bigger chambers and flow paths typically exhibit a lower pressure drop compared to smaller meters handling the same flow rate. The internal design (including inlet and outlet configurations) influences turbulence and resistance, affecting the overall pressure drop.
5. Operating Pressure and Temperature Temperature variations can affect the viscosity of the fluid, which in turn influences the pressure drop. Higher system pressures may help reduce cavitation and allow smoother flow through the meter.

Managing Pressure Drop

To optimize performance and minimize excessive pressure loss:

• Select the appropriate meter size for the application to balance accuracy and pressure drop.
• Use oval gear meters designed for high-viscosity fluids if needed, as they often have modified designs to reduce resistance.
• Maintain the meter properly to prevent build-up, wear, or mechanical issues that could increase flow resistance.

Reasons for Line Size Reduction:

1. Improved Flow Profile: Some meters require fully developed flow profiles for accuracy, which smaller diameters promote.
2. Enhanced Meter Sensitivity: Higher velocities in a reduced section improve measurement sensitivity in low-flow conditions.
3. Cost Considerations: Smaller flowmeters are less expensive than larger ones while maintaining high accuracy.

Conclusion

Pressure drop in an oval gear meter is a critical factor, especially in applications involving high-viscosity fluids or precise flow measurement. While some pressure drop is inevitable due to mechanical resistance, selecting the right meter and maintaining it properly ensures accurate measurement without excessive energy loss in the system.? The process flow rate relative to the fluid’s viscosity should be the determining factor when selecting a meter size, not necessarily the diameter of the existing piping.? The higher the viscosity of the fluid, the greater the pressure drop will be across various pipe sizes, and thus a meter may need to be over-sized.? Conversely, low viscosity fluids may require a meter to be under-sized, as the pressure drop can be negligible, and a smaller meter size will not cause a flow restriction with such fluids.

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