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 support, 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.
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
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.
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 a 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 proves 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, an 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 bypass 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.