Author: Trevor Forster, Managing Director of Titan Enterprises Ltd
A whole raft of additives are required in order to guarantee the successful transportation and refining of crude oils. These vary from simple surfactants through to complex blended scale and corrosion inhibitors. These chemicals are injected in small quantities at high pressure and are critical to the whole refining process. Consequently, careful monitoring of their addition to any process is essential, and often this is best done using petrochemical flow meters.
Petrochemical additive injection fluids vary in both viscosity and density, and any petrochemical flow meter installed into a plant should be able to cope with a wide range of physical and chemical properties. The choice of flow measurement solutions is still quite narrow however, with only a few technologies offering acceptable measurement resolution. Traditionally positive displacement flow meters have been the choice for petrochemical flow meters, for additive injection fluids.
These petrochemical flow meters are typically small with tight clearances between their moving components. Being small helps with the high pressure rating but because of the tight manufacturing tolerances they are susceptible to contamination, which can inhibit efficient operation or stopping the meter working all together - an inconvenience at the best of times, but very expensive if the meter is installed sub-sea.
Non-invasive petrochemical flow meters with no moving parts are the most desirable but are also traditionally the most expensive. This article reviews some of the currently used flowmeter technologies for this application and looks ahead to new advances on the development horizon.
Rotary Piston Petrochemical Flow Meters
Rotary piston meters use a circular disc which "rotates" in a circular cavity. Each rotation displaces a known amount of fluid. Although the spindle circulates a central boss, the actual piston motion could be referred to as "nodding" as the circular element merely describes an oscillation bounded by the circular spindle track and the linear divider that causes the piston to slide in a "circular" motion within a round chamber. Fixed volumes of fluid are transferred both inside and outside the piston from the inlet to the outlet. Manufacturers of rotary piston meters take great care in choosing materials which have low coefficients of friction as well as limited sliding areas. These techniques improve the flow meters linearity as well as extending the operating range. By definition these are typically low resolution meters. They normally have a single magnet in the central spindle but some devices will have multiple magnets in an oval pattern to increase the meters resolution. Unfortunately, metering of petrochemical additive injection fluids means that rotary piston meters have a lot of sliding surfaces and are extremely sensitive to contamination and wear.
Petrochemical Flow Meters: Spur Gear & Oval Gear Meters
These two types of gear meter are superficially the same but operate in very different ways. A standard gear meter usually has a few very large gear teeth which are meshed in a chamber with close clearances on all surfaces. The teeth themselves form a seal along their length so 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 gears to rotate displacing a volume of fluid approximately equal to one gear tooth volume. Usually a sensor is used to count the passing of each tooth generating a high resolution pulse train.
Spur Gear & Oval Gear Petrochemical Flow Meters
Oval gear meters rely on an entirely different theory. The teeth in an Oval gear meter are still used to drive the gear and seal the central path but the differential force is developed by the shape of the ovals not the gear teeth on the lobe. Oval Gear meters from different manufacturers include gears of varying oval shapes depending on the resolution and flow requirement of the target application. By using an oval shape a much greater driving pressure can be generated resulting in a wider flow range and lower pressure drop compared to a standard gear meter. This also permits the meters to operate satisfactorily with lower viscosity fluids. The displaced volume is a product of the oval shape not the gear profile shown in dark blue above. The sensor is usually magnetic with a detector at the face of the gear. The resolution is lower than the standard gear meter although multiple magnets can sometimes be incorporated.
Helical Petrochemical Flow Meters
Helical flow meters use a pair of helical gears rather like two Archimedes screws intermeshed. The chamber cross section is therefore similar to a figure of eight. The two rotors are "meshed" along their length and synchronised using a pair of ordinary gears at one end. As the fluid passes own the chamber it rotates the gears. They are very accurate devices and due the detection of motion taking place on the meshing gears, offer high resolution. They are, however, sensitive to contamination.
Thermal Petrochemical Flow Meters
Accurate low flow thermal meters use two or even three elements. One element is used for reference temperature measurement. The second is a heat source, and the third measures the heat dissipation and so the flow rate. These are mass flow devices and they are capable of measuring very low flows although the thermal characteristics of the liquid must be known for precise measurement. Like the other electronic flowmeters, Coriolis and ultrasonic, there are no moving parts and they therefore offer good long term reliability.
Coriolis Petrochemical Flow Meters
Coriolis 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 proportional to the mass of fluid flowing in the tube. The faster the flow the greater the reaction. Coriolis meters detect the reaction of mass flow so they are inherently mass flow meters and will meter both volume and density. 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 extremely accurately - however they typically are also relatively expensive.
Ultrasonic Petrochemical Flow Meters
Ultrasonic flow meters have yet to join the armoury of petrochemical low flow metering tools in any appreciable way, but very low-flow high-pressure products are currently under development which will handle the flow rate as well as the very high pressure. Ultrasonic flow meters offer a very promising prospect as their manufacturing costs should permit a much lower installed cost than the desirable Coriolis flow meters although without the mass flow and density outputs which are not always required. These will be the preferred time of flight devices as Doppler shift meters are unlikely to ever attain he required performance. The prospect of a simple, straight through construction with high pressure capability and no moving parts at a competitive price should soon see a meter of this principle being available for these low flow applications.
Thermal Petrochemical Flow Meters - Conclusions
Each one of above petrochemical flow meter technologies has its own strengths and weaknesses and a choice will largely depend on financial constraints and personal experience. The rotary piston meter has a lot of sliding surfaces and is extremely sensitive to contamination and wear. The standard gear meter has a relatively high pressure drop and requires a fully lubricating fluid. Oval gear meters have relatively low resolution although this may not be an issue where the consumption and control of a fluid over 24 hours is important. Helical meters are more expensive than the other gear meters, require lubricating fluids and are also bulky. Thermal meters can be accurate but are typically setup just to the fluid being metered so are not very versatile. Coriolis meters would appear to be the panacea for additive injection but their price is often prohibitive permitting a compromise inflow meter choice. I believe future developments in ultrasonic metering will bridge the gap with acceptable performance at an acceptable price. Ultrasonic flow meters will, no doubt, be a welcome addition to the low flow metering armoury.
