When the physical properties of the fluid are not available, mass measurement devices are used to estimate the flow rate. Mass measurement is significant in streams containing CH4 and C2H6 because of substantial solution mixing effects.
Mass measurement is accomplished by multiplying the volume of the fluid at flowing conditions by the density of the fluid at flowing conditions. This eliminates the need for correction factors for the metered volume. The mass measurement devices utilize the principle that the angular momentum of a mass is directly proportional to the mass velocity.
The resistance of a mass to change its direction is measured by the different devices using combinations of different mechanical and electrical sensors and transmitters that can generate a variety of electronic signals. Unlike some volume flow devices, mass flow meter installations may not require upstream and downstream piping.
Read More: Flow measurement
Coriolis Meters
The Coriolis meter is a mass-measuring device that consists of a sensor, a transmitter, and other peripheral devices to provide monitoring and control functions. The sensor of the Coriolis meter consists of two flow tubes, the drive coil and magnet, two pick-off coils and magnets, and the RTD. The process fluid enters, splits in half, and passes through each flow tube during the meter operation.
The drive coil is energized causing the tubes to oscillate up and down opposite to each other. The pick-off coils are mounted on one tube while the magnets are mounted on the other. Each coil moves through the uniform magnetic field of the adjacent magnet as the two tubes oscillate.
The sine wave is generated that represents the motion of one tube relative to the other. Two sine waves produced are in phase when there is no flow in the tubes. While the induced Coriolis force causes the tubes to twist, resulting in two out-of-phase sine waves when fluid flows through the meter tubes.
The time difference in the sine waves is directly proportional to the mass flow rate through the tubes at given pressure while the density of the fluid is calculated from the frequency of oscillation of the tubes.
Advantages
- A range of (+/-0.1%) and acceptable repeatability.
- It provides multi-variable measurement in one device: mass flow rate, volumetric flow rate, density and temperature.
- Tolerant of the changes in the fluid quality and flow rate.
- Can be used as a bidirectional meter.
- Easy to install and requires low maintenance
- No straight pipe run requirements
Disadvantages
- Significant pressure drop across the meter