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Coriolis Flow Meter Applications & Installation

Coriolis flow meter application Coriolis mass flow meters can detect the flow of all liquids, including Newtonian and non-Newtonian, as well as that of moderately dense gases. Self-draining designs are available for sanitary applications that meet clean-in-place requirements.

Most meters are provided with intrinsically safe circuits between the flow tube and the transmitter. Therefore, the amount of driving power that can be delivered to the flow tube is limited.

When fluid is unloaded from tank trucks, drums, or railroad cars, slug flow can occur, making the meter output unpredictable. If a slug-flow recovery feature is provided in the transmitter, it will stop the measurement when slug flow is detected by the excessive drive power drawn or by the drop in process density (reduction in sensor output amplitude).

The amount of air in the process fluid that can be tolerated by a meter varies with the viscosity of the fluid. Liquids with viscosities as high as 300,000 centipoise can be metered with Coriolis meters. Gas content in such highly viscous liquids can be as high as 20% with the small bubbles still remaining homogeneously dispersed. Gas content in low viscosity fluids, like milk, will separate at concentrations as low as 1%.

The cost of an average-sized (under 2 in.) Coriolis flow meter is between $4,000 and $5,000. These mass flow meters provide short payback periods on applications where measurement accuracy lowers production costs (bathing, billing) or where multiple measurements (including density, temperature, pressure) are needed. On the other hand, they may not be competitive when used in simple flow measurement applications where volumetric sensors are sufficient and where repeatability is more important than precision. The ability to extract data on total mass charged, solids rate, percent solids, and viscosity from a single instrument does lower the total cost of measurement, improves process control, and provides redundancy for other instruments.

Continuous tube designs are generally preferred for slurry and other multi-phase fluid applications. The total flow is divided by splitters in split-tube designs, and the resulting two streams do not have to be at exactly the same mass flow rate to maintain accuracy (they do, however, need to have the same density). Different densities in the two parallel tubes imbalance the system and create measurement errors. Therefore, if there is a secondary phase in the stream, a simple flow splitter may not evenly distribute the flow between the two tubes.

Continuous tube designs are also preferred for measuring fluids that can coat and/or clog the meter.

Continuous tubes, if sized to pass the largest solid particles in the process fluid, are less likely to clog and are easier to clean.

Installation Recommendations

There are no Reynolds number limitations associated with Coriolis meters. They are also insensitive to velocity profile distortion and swirl. Therefore, there is no requirement for straight runs of relaxation piping upstream or downstream of the meter to condition the flow.

The meter should be installed so that it will remain full of liquid and so air cannot get trapped inside the tubes. In sanitary installations, the meter must also drain completely. The most desirable installation is in vertical upward flow pipes (Figure 5-6B), but installations in horizontal lines (Figure 5-6A) are also acceptable. Installations where the flow is downward in a vertical pipe are not recommended.

In newer Coriolis designs, normal pipe vibration should not affect the performance of the Coriolis meter if it is properly supported by the process piping (Figure 5-6C). No special supports or pads are needed for the flow tube, but standard piping supports should be located on either side of the meter. If the installation instructions require special hardware or supports, the particular meter design is likely to be sensitive to vibration, and the pulsation dampeners, flexible connectors, and mounting/clamping attachments recommended by the manufacturer should be carefully installed.
A
Figure 5-6A: Horizontal Lines
B
Figure 5-6B: Vertical Lines
C
Figure 5-6C: Support by Process Piping
If your application requires that you install two Coriolis flow meters in series or mount two Coriolis meters near each other, the manufacturer should be consulted to prevent crosstalk between the two units.

If air bubbles are likely to be present in the process fluid, it is recommended to install an air release upstream of the meter. System design characteristics that can result in the presence of air (and which can often be eliminated at the design stage) include:
  • Common piping used for pumping into and out of storage tanks
  • Allowing the formation of a vortex in stirred vessels under low-level conditions
  • Allowing air leakage through packing glands of pumps that develop high vacuums on the suction side (this can occur when pumping from underground storage)
  • Vaporization of stagnant process fluid in pipes exposed to the sun
  • High valve pressure drops causing vaporization and flashing
  • Allowing the pipe to drain for any reason, including lack of check valves
  • Allowing storage tanks, trucks, or railroad cars to drain completely
  • Using the same pipe for pumping different materials at different times
  • Allowing foam formation by high turbulence in high velocity fluids
It is recommended to install (upstream of the meter) strainers, filters or air/vapor eliminators as required to remove all undesirable secondary phases. Figure 5-7C illustrates an air eliminator installation. Its function is to slow the velocity of the liquid, thereby allowing time for the entrained air to separate and be removed by venting. The rise and fall of the liquid level in the eliminator due to the accumulation of free air closes and opens the vent valve and discharges the air (Figure 5-7A&B).

Prior to zeroing the meter, all air should be removed. This can be accomplished by circulating the process fluid through the meter for several minutes at a velocity of approximately 2-6 ft/sec. On batching or other intermittent flow applications, the meter should stay flooded so that it does not need to be repurged. All meters should be installed so they can be zeroed while filled with liquid.
Closed Vent Lines
Figure 5-7A: Closed Vent Lines
Open Vent Lines
Figure 5-7B: Open Vent Lines
Open Vent Lines
Figure 5-7C: Typical Installation
When zeroing the meter, any associated pumps or other equipment should be running so that their noise can be zeroed out. This can be achieved in most cases by locating a shut-off value downstream of the meter and either operating the pump with its discharge blocked, which is acceptable with centrifugal pumps for a short period, or by opening the pump bypass on positive displacement pumps. Valves used in zeroing the meter should provide tight shut-off; double-seated valves are preferred.

Meters that are expected to be calibrated in-line must be provided with block and bypass valves so that the reference standard (master) meter can be installed and disconnected without interrupting the process. The requirements for in-line calibration (for ISO 9000 verification) consist of comparing the output of the meter against a reference standard of higher accuracy, such as a dead-weight calibrated weigh tank. Before Coriolis meters, the reference standard was expected to be an order of magnitude more accurate than the meter being calibrated; however, due to the high accuracy of Coriolis meters, this is rare.

In less critical installations (where weigh tanks are not used), volumetric provers or master meters (typically another Coriolis or a turbine meter calibrated at a flow laboratory) are used. When a volumetric reference is used in calibrating a mass flow meter, the fluid density must be very precisely determined.

Control valves should be installed downstream of the meter to increase the back-pressure on the meter and lower the probability of cavitation or flashing.

When the process fluid must be held at higher temperatures, some Coriolis meters can be supplied with steam jackets. As an alternative, electrical heating tape can be added to the housing. Jackets or heating tapes must be installed by the manufacturer.

When flow-metering is not required, the Coriolis meter can be used solely as a densitometer. In that case, to minimize cost, usually a small (1/2 in.) meter is installed in a by-pass line. Such a configuration is acceptable only in clean services that will not clog the small bore of the meter. In addition, a restriction must be placed in the main piping (between the by-pass taps) to ensure a flow through the meter.

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