The oval gear meter's compatibility for its chosen application is determined by the body and rotor material (wetted components that come into contact with the liquid). The way a rotor is made and cut determines its effectiveness to measure fluids of varying viscosity and temperature. Whilst the material from which it is made determines its compatibility for the application. We'll go through PPS, Aluminium and Stainless Steel rotor features and benefits and how to choose the best material for your application. We'll also discuss the different rotor types and their benefits and disadvantages.
PPS Flow Meter Rotors
Poly(p-phenylene sulfide) (PPS) is a polymer made up of alternating sulfur atoms and phenylene rings in a para substitution pattern. The highly stable chemical bonds of its molecular structure impart a remarkable degree of molecular stability toward both thermal degradation and chemical reactivity. The molecular structure also readily packs into a very thermally stable crystalline lattice, hence PPS is a semicrystalline polymer with a high crystalline melting point of about 285°C (545°F). PPS has not been found to dissolve in any solvent at temperatures below about 200°C (392°F), although swelling can occur with some solvents. When blended with glass fibers and other fillers, PPS is a remarkable engineering composite material which has a unique combination of properties including:
PPS has been used as a manufacturing material in flowmeters for around 30 years, particularly by manufacturers of positive displacement flowmeters, however by far the greatest use of PPS is in the manufacture of automotive components where the mechanical and chemical properties of PPS make it an outstanding material. PPS is an ideal choice for automotive parts due to its excellent performance at high temperatures, its resistance to automotive fluids, and excellent stability and mechanical strength; the same properties that make PPS an ideal automotive material are precisely why it is also ideal as a flowmeter material. In addition to the excellent mechanical properties of PPS, the ability of PPS to mould complex parts to tight tolerances makes a flowmeter containing PPS components an accurate device, but also a very cost effective product to purchase and maintain. Some examples of PPS usage in the automotive industry by companies such as BMW, Toyota, Ford, and Chrysler are; fuel pump impellers, oil pump vanes, hot air ducts, throttle body components, turbocharger components, brake system components, alternator components, electrical connectors, fuel line connectors, fuel injector housings, fuel filter housings, light bulb sockets, water pump impellers, transmission valves components, radiator tanks.
PPS is an excellent rotor material providing a meter with outstanding accuracy, wear life, serviceability, and very favourable cost of ownership. PPS rotors should be chosen whenever the application conditions allow it; factors to consider are compatibility with the measured liquid, and suitability of the operating temperatures.
Aluminium rotors with steel roller bearings provide good performance on all lubricating hydrocarbon liquids. Aluminium rotors can be chosen for any applications where the liquid is compatible with both Aluminium and Carbon Steel, however Aluminium rotors should generally only be chosen over PPS rotors where large and abrupt temperature variations are present, or where operation above 120° is required.
Choose stainless steel whenever the application has large and abrupt temperature variations, or where the chemical/corrosion resistance of 316 stainless steel is required. For many applications where a Stainless Steel flowmeter is required PPS rotors can still be used in place of stainless steel rotors for a significant cost saving without any reduction in meter performance or life span.
When choosing rotor material for an application that is compatible for PPS, Aluminium and/or Stainless, why choose PPS?
Here are some great reasons why one would choose PPS:
Sintering is the process of compacting and forming a solid mass of material by heat [1] and/or pressure[2] without melting it to the point of liquefaction. Sintering happens naturally in mineral deposits or as a manufacturing process used with metals, ceramics, plastics, and other materials. The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. Because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as tungsten and molybdenum. Stainless steel rotors are manufactured from 316 sintered stainless steel which is a bonded process using stainless steel powder.
Keishi The “Keishi Cut” is a terminology for High Viscosity rotors and is a Japanese term. The Keishi cut rotor have every alternate gear tooth on the flat sides of the oval gear machined to provide pressure relief between the gear teeth and can reduce the maximum pressure drop across the meter by approximately 50%. High viscosity rotors is normally used for viscosity above 1000cP. Something to also consider in instances where normal operating viscosity is below 1000cP, remember to factor in the minimum operating temperature. For instance - at start up during winter, there are many places where the minimum winter temperature can get down to below 0°C or around a minimum of 5°C,thus increasing viscosity of the oil or grease significantly, and this could be well over 1000cP.
One of the most common categories of positive displacement flow-meters are the ‘Gear Meters’, but there are many different types of gears used in flow-meters; in some applications any old Gear Meter will do, but not all applications! Read on to learn what type of Gear Meter you should be choosing!
Spur gears or circular gears are the most common type of gears used in engineering, and as a result when most people think of a ‘gear’ they are picturing a spur gear. Spur gears have straight teeth located around a circular hub, and are mounted on parallel shafts; they are most often used to transmit power or motion in products ranging from an automotive transmission to a wind-up alarm clock, however they are also one of the more simple gear forms used in positive displacement flowmeters.
Spur gear flow-meters are typically well suited to high viscosity fluids, such as oils, so are often used extensively in the hydraulic market where they can produce an accurate measuring system with the benefit of the very thick hydraulic fluids. One of the disadvantages of spur gear flowmeters is their accuracy on medium or low viscosity fluids; which is why most spur gear manufacturers will specify the accuracy statement of the flow-meter for liquids thicker than 20-30cSt.
One of the most commonly used Gear Meters is the Oval Gear meter; these have the same straight gear tooth as the spur gear, however the teeth are located around an oval/elliptical shaped hub, rather than a circular hub.
The reasons that Oval gear meters are so common, and why they have been so successful since their introduction into the market in the 1930’s is that they are one of the most utilitarian types of flow-meter, providing exceptional performance across the vast majority of applications. While some flow-meter technologies will really excel in certain specific applications, the Oval Gear meter will function well in just about every situation. The benefit of this ‘one technology fits all’ characteristic of the Oval Gear is that with a little bit of knowledge most people will find it easy to choose the right meter for the job.
Oval gear meters larger than ½” in size will generally provide around 0.3% linearity or 0.5% accuracy (when calibration accuracy is taken into account) whenmeasuring any liquid which is 3cP or thicker, and in many cases can produce equivalent accuracy with liquids much thinner than this by using a narrower flowrate range than is specified for 3cP. For thicker liquids accuracy can be significantly better than 0.5%; with up to 0.1% accuracy achievable in some high viscosity applications.
Oval Gear meters are often considered to be not suitable for low viscosity applications, such as solvents, and while there is some truth in this, quite often it simply means that more care is required in selecting the right size of oval gear meter to suit your low viscosity liquid. Often with correct meter selection an oval gear metering system can be built for a low viscosity liquid which out-performs other ‘low viscosity’ meter technologies (such as turbines).
In the world of Gear flowmeters, the oval and the circular gear are by far the most commonly used, but there are many other shapes that can be used in a Gear Meter design. Other gear shapes such as trilobed gears, or even quad-lobed.
gears are possible, and there are already some flow-meter manufacturers manufacturing flow-meters using these types of gears. In fact a Chinese company (which can be found by searching for ‘Super Oval Flowmeter’ on Youtube.com) already holds patents on such a design and manufactures small capacity flowmeters using the principle of Tri-lobed gears and Quad-lobed gears.
For a number of years, FLOMEC experimented extensively with Tri-lobed and Quad-lobed gear meter designs, developing Tri-lobed gear meters from ¼” up to 4” in size and having these tested by the Australian National Measurement Institute (NMI). After extensive development and testing of the Tri-lobed gear meter concept, no real advantage could be found over the Oval gear design. The main discovery during testing was a slightly higher pressure loss through these meter designs due to the effect of splitting up the liquid into 6 (or 8) pockets per revolution of the flowmeter – as opposed to the 4 pockets formed by an Oval Gear flowmeter.
Understanding rotors technology and material benefits and uses is benefical for seleting the correct flow measurement solution for your application. For assistance with anything discussed in this article or flow measurement in general, we have plenty of resources to assist you. Contact us for more information.
Rotor basics in Oval Gear Flow Meters The oval gear meter's compatibility for its chosen application is determined by the body and rotor material (wetted components that come into contact with the liquid). The way a rotor is made and...
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