To successfully transport and refine crude oils often large amounts of additives are required. These range from simple surfactants through to complex blended scale and corrosion inhibitors. Crucial to the entire refining process, these chemicals are injected in small quantities at high pressure. As a result, it is important to carefully monitor their addition to any process, and often this is best done using petrochemical flow meters.
Petrochemical additive injection fluids differ in density as well as viscosity, and any petrochemical flow meter installed into a plant should be able to manage an extensive range of chemical and physical properties. The selection of flow measurement solution is still quite narrow, as only a few technologies provide a satisfactory measurement resolution.
Conventionally, positive displacement flow meters have been the option for petrochemical flow meters, for additive injection fluids. These petrochemical flow meters are usually small and have tight clearances between their moving components. The small size helps with the high-pressure rating, but due to the tight manufacturing tolerances they are prone to contamination, which can prevent efficient operation or stop the meter working completely – an inconvenience at the best of times, but quite expensive if the meter is installed sub-sea.
Non-invasive petrochemical flow meters with no moving parts are the most preferred choice; however, they are also the most expensive. This article reviews some of the presently used flow meter technologies for this application and looks forward to the latest advances on the development horizon.
Rotary Piston Petrochemical Flow Meters
Rotary piston meters contain a circular cavity inside which “rotates” in a circular disc. With every rotation, a known amount of fluid is displaced. While a spindle circulates a central boss, the actual piston motion could be referred to as “nodding” as the circular element only describes an oscillation bounded by the circular spindle track and the linear divider that makes the piston to slide in a “circular” motion within a round chamber.
A fluid of fixed volumes is transferred both inside and outside the piston, from the inlet to the outlet. Rotary piston meter manufacturers carefully choose materials that have both low friction coefficients and limited sliding areas. Besides enhancing the linearity of flow meters, these techniques also extend the operating range. By definition, these are characteristically low-resolution meters. Their central spindle usually has a single magnet, but certain devices will have multiple magnets in an oval pattern to boost the resolution of the meter. Unfortunately, metering of petrochemical additive injection fluids implies that the rotary piston meters are extremely sensitive to wear and contamination and have a lot of sliding surfaces.
Petrochemical Flow Meters: Spur Gear and Oval Gear Meters
Although these two types of gear meter are superficially the same, they operate in very different ways. Typically, a standard gear meter has a few very large gear teeth meshed in a chamber with close clearances on all surfaces. As the teeth themselves form a seal along their length, the only possible leakage path is around the outside of the meshed cogs to the chamber walls. The pressure imbalance across the gears causes the rotation of the gears, displacing a volume of fluid about equal to one gear tooth volume. A sensor is normally used for counting the passing of each tooth, producing a high-resolution pulse train.
Spur Gear and Oval Gear Petrochemical Flow Meters
Oval gear meters work based on a completely different theory. The teeth in an Oval gear meter are still used for driving the gear and sealing the central path but the differential force is developed not by the shape of the gear teeth on the lobe, but that of the ovals. Oval Gear meters from different manufacturers include gears of varying oval shapes according to the flow requirement and resolution of the target application. It is possible to generate a much greater driving pressure by using an oval shape which results in lower pressure drop and in a wider flow range compared to a standard gear meter.
This also allows adequate operation of the meters with lower viscosity fluids. The volume which is displaced is a product of the oval shape, not the gear profile shown in dark blue above. Generally, the sensor is magnetic and has a detector at the face of the gear. While the resolution is lower than the standard gear meter, multiple magnets can sometimes be integrated.
Helical Petrochemical Flow Meters
Helical flow meters use a pair of helical gears rather like two Archimedes screws intermeshed. Hence, the cross-section of the chamber is similar to a figure of eight. The two rotors are “meshed” along their length and synchronized using a pair of ordinary gears at one end. As the fluid passes down the chamber, it rotates the gears. These devices are very accurate, and thanks to the detection of motion taking place on the meshing gears, provide high resolution. However, they are sensitive to contamination.
Thermal Petrochemical Flow Meters
Coriolis Petrochemical Flow Meters
Coriolis petrochemical flow meters still use moving parts but only minutely and on the outside of the flow tube. They use the fact that if a tube full of moving fluid is vibrated, it will cause a reaction to the fluids’ movement that is proportional to the mass of fluid flowing in the tube; the faster the flow, the greater the reaction. Since Coriolis meters detect the reaction of mass flow, they are inherently mass flow meters and will meter both density and volume. It is rather like trying to rotate a gyroscope at 90° to the spin axis where the external force will induce a reaction at 90° to the applied force. Such meters are very sensitive and will meter very low flows even with some contaminants very accurately – however, they typically are also relatively expensive.
Ultrasonic Petrochemical Flow Meters
Ultrasonic flow meters are yet to join the armoury of petrochemical low flow metering tools in a substantial way, but very low-flow high-pressure products, which will handle both the flow rate and very high pressure, are presently under development. Ultrasonic flow meters show a very promising prospect, as their manufacturing costs should allow a much lower installed cost than the sought-after Coriolis flow meters although without the mass flow and density outputs which are not always necessary.
These will be the preferred time of flight devices as Doppler shift meters are not likely to ever achieve the required performance. The prospect of a simple, straight through construction that has high pressure capability without moving parts at a reasonable price may lead to a meter of this principle being available for these low flow applications.
With each one of the aforementioned petrochemical flow meter technologies having its own advantages and disadvantages, the selection will greatly depend on personal experience and financial constraints. The rotary piston meter has plenty of sliding surfaces and is very susceptible to contamination and wear. The standard gear meter has a comparatively high pressure drop and demands a completely lubricating fluid. Although oval gear meters have a relatively low resolution, this may not be a problem where the consumption and control of a fluid over 24 hours is essential.
When compared to the other gear meters, helical meters are bulky, more expensive, and require lubricating fluids. Despite being accurate, thermal meters are not very versatile as they are typically setup just to the fluid being metered. Coriolis meters, which are apparently the panacea for additive injection, are prohibitively expensive allowing a compromise in flow meter choice.
It is believed that future developments in ultrasonic metering will close the gap with an acceptable performance at an acceptable price. Ultrasonic flow meters will certainly be a welcome addition to the low flow metering armoury.
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