Flow, level & control


Differential pressure meters are versatile instruments which employ proven, well- understood measuring technology


Turbine Turbine meters contain a freely suspended rotor, and the flow against its vanes causes the device to rotate at a rate proportional to flow velocity. A sensor/transmitter is used to detect the rotational rate of the rotor; when the fluid moves faster, more pulses are generated. The transmitter processes the pulse signal to determine the flow of the fluid in either forward or reverse direction. Turbine meters incorporate a time-tested


measuring principle, and are known for high- accuracy, wide turndown and repeatable measurements. They produce a high-resolution pulse rate output signal proportional to fluid velocity, and hence, to volumetric flow rate Turbine meters are limited to use with clean fluids only. Bearing wear, a common concern with this type of device, was largely addressed by the development of ceramic journal bearings. As a mechanical meter, turbines require periodic recalibration and service.


Impeller Impeller meters are frequently used in large diameter water distribution systems. The device consists of a paddle wheel inserted perpendicularly into a process stream. The number of rotations of the paddlewheel is directly proportional to the velocity of the process. Impeller meter attributes include: direct


volumetric flow measurement (often with visual indication), universal mounting, fast response with good repeatability, and relatively low cost. Note their performance suffers in applications with low fluid velocity. The meters are also sensitive to flow profile. They can only be used in clean, low-viscosity media.


Variable Area Variable Area meters are inferential measurement devices consisting of two main components: a tapered metering tube and a float that rides within the tube. The float position — a balance of upward flow and float weight — is a linear function of flow rate. Operators can take direct readings based on the float position with transparent glass and plastic tubes. Simple, inexpensive and reliable, Variable Area


meters provide practical flow measurement solutions for many applications. Be advised most of these meters must be mounted perfectly vertical. They also need to be calibrated for viscous liquids and compressed gases. Furthermore, their turndown is limited and accuracy relatively low.


Instrumentation Monthly October 2019


Ultrasonic There are two types of ultrasonic meters: transit time and Doppler. Both designs will detect and measure bi-directional flow rates without invading the flow stream. Ultrasonic meters are ideal for troubleshooting, diagnostics and leak detection. They can be used with all types of corrosive fluids, as well as gases, and are insensitive to changes in temperature, viscosity, density or pressure. Ultrasonic meters have no moving or wetted


parts, suffer no pressure loss, offer a large turndown ratio, and provide maintenance-free operation— important advantages over conventional mechanical meters. Conversely, the precision of these meters becomes much less dependable at low flow rates Unknown internal piping variables can shift the flow signal and create inaccuracies.


Vortex Vortex meters make use of a principle called the von Kármán effect, whereby flow will alternately generate vortices when passing by a bluff body. A bluff body is a piece of material with a broad, flat front that extends vertically into the flow stream. Flow velocity is proportional to the frequency of the vortices. Flow rate is calculated by multiplying the area of the pipe times the velocity of the flow. Vortex meters have no moving parts that are subject to wear, and thus regular maintenance is not necessary. Only clean liquids can be measured with this type of instrument. They are particularly well suited for measurement of gas emissions produced by wastewater. Vortex meters may introduce pressure drop due to obstructions in the flow path.


Oval Gear Oval Gear meters utilise a positive displacement meter design, whereby fluid enters the inlet port and then passes through the metering chamber. Inside the chamber, fluid forces the internal gears to rotate before exiting through the outlet port. Each rotation of the gears displaces a specific volume of fluid. As the gears rotate, a magnet on each end of the gear passes a reed switch, which send pulses to the microprocessor in the register to change the LED display segments. The latest breed of Oval Gear meters directly


measures actual volume. It features a wide flow range, minimal pressure drop and extended viscosity range. This design offers easy installation and high accuracy, and measures high temperature, viscous and caustic liquids with simple calibration.


Nutating Disc Nutating Disc meters are most commonly used in water-metering applications. A disc attached to a sphere is mounted inside a spherical chamber. As fluid flows through the chamber, the disc and sphere unit wobble or “nutate”. This effect causes a pin, mounted on the sphere perpendicular to the disc, to rock. Each revolution of the pin indicates a fixed volume of liquid has passed. Nutating Disc meters have a reputation for high


accuracy and repeatability, but viscosities below their designated threshold adversely affect performance. Meters made with aluminium or bronze discs can be used to meter hot oil and chemicals.


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KEY FACTORS IN FLOW METER SELECTION In a typical chemical production facility, fluid characteristics eg. single or double phase, viscosity, turbidity, flow profile (laminar, transitional, turbulent, etc.), flow range, and accuracy requirements are all important considerations in determining the right flow meter for a particular measurement task. Additional considerations such as mechanical restrictions and output-connectivity options also impact the user’s choice. For most general industrial applications, the key factors in flow meter selection are:


Process Media Different flow meters are designed to operate best in different fluids/gases and under different operating conditions. That is why it is important to understand the limitations inherent to each style of instrument. The most important difference between these two types of media lies in their relative compressibility (i.e, gases can be compressed much more easily than liquids). Consequently, any change that involves significant pressure variations is generally accompanied by much larger changes in mass density in the case of a gas than in the case of a liquid.


Type of Measurement Industrial flow measurements fall under one of two categories: mass or volumetric. Volumetric flow rate is the volume of fluid passing at a given volume per unit time eg. Litres per minute. Mass flow rate is the movement of mass per time unit eg. Kilograms per minute. It can be calculated from the density of the liquid (or gas), its velocity, and the cross sectional area of flow. Volumetric measuring devices, like variable area meters or turbine flow meters, are unable to distinguish temperature or pressure changes. Mass flow measurement would require additional sensors for these parameters and a flow computer to compensate for the variations in these process conditions. Thermal mass flow meters are virtually insensitive to variations in temperature or pressure.


Flow Rate Information A crucial aspect of flow meter selection is determining whether flow rate data should be instantaneous or continuous. A flow rate has to do with the quantity of a gas or liquid moving through a pipe or channel within a given or standard period of time. A typical continuous flow measurement system consists of a primary flow device, flow sensor, transmitter, flow recorder, and totaliser.


A Nutating Disc meter


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