Explore differential pressure, electromagnetic, vortex and swirl, and variable area (rotameters).
Engineers tend to specify volumetric flowmeters to measure liquids when they simply require a measure of the volume of flow per unit of time (gallons/minute, liters/sec, etc.). When corrected for temperature or density, volumetric flowmeters can provide values for mass liquid flow rates. A sampling of volumetric flowmeters includes differential pressure, electromagnetic, vortex and swirl, and variable area (rotameters). Excluded here for reasons of space are turbine, target, ultrasonic and positive displacement meters.
A primary criterion for flowmeter selection is accuracy. Flowmeter accuracy often depends to some degree on application conditions. Pressure, temperature, fluid dynamic influences and external influences can all affect accuracy. Pressure and temperature cause dimensional and physical changes. Fluid flow profiles and pipe roughness can shift flowmeter performance. In other words, a traceable and complete calibration in a lab is no guarantee that a user can expect the same performance and accuracy in the field.
Another significant flowmeter attribute is rangeability, often called turndown ratio. This is the ratio of the maximum to the minimum flow rate capable of being measured at a specified accuracy. In processing industries, extremely high values of rangeability are often meaningless because most processing flow velocities fall within relatively narrow ranges. Liquid pipeline flow rates, for example, typically range from 0.5 to 12 ft/sec, a turndown ratio of 24:1. Accuracy usually falls off for lower flow rates when flow becomes laminar rather than turbulent. Higher flow rates generate high pressure drops, raising pumping energy costs and causing pipe erosion (if solids are present).
Differential Pressure Flowmeters
DP flowmeters are a popular choice in the processing industries, comprisingnearly 30 percent of installations. These flowmeters consist of two components: some element inserted into the flow to increase its velocity and a sensor/transmitter to measure the unrecoverable differential pressure that results. The inserted element is typically an orifice plate, Venturi, nozzle or wedge. The measured differential pressure is a square root function of the volumetric flow rate.
Orifice Plate
These flowmeters have no moving parts, can result in moderate pressure losses and suit extreme temperatures and pressures. Accuracy ranges from 1 to 5 percent. Compensation techniques can improve accuracy to 0.5 to 1.5 percent. Disadvantages include:
- Expensive to install and maintain
- Nonlinear function of flow rate
- DP transmitter required to compute volumetric flow
- Limited rangeability (often less than 5:1)
- Sensitive to changes in temperature, density and viscosity
Characteristics of the inserted elements vary. The orifice plate is the most common, being greatly studied, inexpensive and available in a variety of materials. Rangeability is usually less than 5:1. Accuracy tends to be 2 to 4 percent of full scale. Retaining good accuracy requires maintaining a sharp edge to the upstream side of the orifice plate, which degrades with wear. Pressure loss is high relative to other DP elements.
The Venturi flowmeter is primarily used in water and wastewater applications. Since pressure loss is minimal, it is a good choice when only minimal pressure head is available. It handles dirty fluids and does not require upstream flow profiling (although the Venturi itself has a relatively long length). Rangeability is less than 6:1 and accuracy runs from ± 1 to 2 percent of full scale. Viscosity greatly affects accuracy and flow must be turbulent (Reynolds numbers greater than 10,000).
Venturi
Nozzles, while acting somewhat like Venturis, tend to be shorter for fitting in tighter piping situations. They come in a variety of configurations, some standard and some proprietary, often requiring dependence on manufacturer's data.
The wedge consists of a V-shaped restriction molded into the meter body. In profile the wedge looks like a segmental orifice plate to the incoming fluid, but it is more expensive than an orifice plate. This basic meter has been on the market for more than 40 years, demonstrating its ability to handle tough, dirty fluids. The slanted faces of the wedge provide self-scouring action and minimize damage from impact with secondary phases. Rangeability of 8:1 is relatively high for a DP element and accuracies are possible to ± 0.5 percent of full scale.
Wedge
For especially abrasive or corrosive liquids, the wedge flowmeter can come with remote seals that effectively isolate the metered fluid from the tubes running to the DP transmitter.
Electromagnetic Flowmeters
Often called magmeters, these flowmeters are also a popular choice among instrument engineers, making up about 20 percent of flowmeter installations. Faraday's law says that a conductor moving through a magnetic field produces an electric signal. In this case, the fluid is the conductor and electromagnetic coils surrounding the meter body generate the magnetic field.
Magmeter
Magmeters have no moving parts. They offer high accuracies and wide rangeability as well as an unobstructed flow path. They are ideal for slurries, and perform well with corrosive and erosive fluids. Engineers can pick a non-conductive liner material for the flow tube-Teflon, Tefzel, rubbers, ceramic-to suit the fluid measured. Magmeters require minimum lengths of straight pipe to condition the flow profile. The fluid measured must be conductive, however, which rules out most petroleum-based flows.
The voltage developed across the electrodes within the flow tube is in the millivolt range and should be carried via shielded cable to a nearby converter. The converter boosts the signal to a standard (4-20 mA) or frequency output (0-10,000 Hz) for transmission to a display or controller.
The coils producing the magnetic field may be excited by AC, DC or pulsed DC. DC excitation accounts for most magmeter installations today.
Line frequency AC designs offer fast response to changing flow rates and good signal-to-noise ratios. They tend to be susceptible to zero drift, however, and require frequent manual adjustment of the output at zero flow. Recent advancements to AC designs, along with sophisticated digital signal processing techniques, have incorporated ways to minimize or eliminate zero drift.
With pulsed DC excitation
