Key Components of a Positive Displacement Flowmeter
A positive displacement (PD) flowmeter is composed of several essential parts, each serving a distinct function:
Measuring Section:
This is the core of the flowmeter, consisting of a rotor, housing, and cover plate that form the measurement chamber. When fluid passes through, the rotor spins continuously, with the rotational speed directly proportional to the volume of fluid flowing through.
Seal Coupling:
To transmit the rotor’s speed, a coupling is used. The seal coupling ensures that fluid does not leak, while minimizing resistance. Magnetic couplings are commonly used, as they eliminate the risk of leakage, unlike mechanical seal couplings.
Speed Controller:
The rotor typically rotates at high speeds, so a gear system is used to reduce the speed to an appropriate level for accurate reading and signal transmission.
Display Section:
This part displays the flow measurement. It can either be a pointer indicating the value or a counter recording the total flow.
Signal Transmission:
The signal transmission unit converts the measured flow into output pulses for remote monitoring. These pulses can be voltage or current-based. However, if the flowmeter is only for local display, this transmission section is not required.
Installation Considerations for Positive Displacement Flowmeters
Initial Installation Position:
The rotor shaft must be installed horizontally. If the shaft is not level, the side chamber may bear the weight of the rotor, leading to wear and loss of measurement accuracy.
Ensure that the fluid direction aligns with the arrow marked on the flowmeter. Reversing the flow direction will cause incorrect readings, with the pointer moving in the opposite direction.
Piping Requirements:
Install a bypass line for maintenance or inspection purposes.
A filter should be installed near the flowmeter, especially when the fluid contains impurities. If the fluid has a high concentration of contaminants, the filter size should be one level larger than usual, or use a parallel double-filter system.
When installing vertically, the flowmeter should be placed on the secondary line, as shown in the diagram. If the fluid flows upwards, minimize the vertical pipe length above the flowmeter to reduce the accumulation of debris.
If the fluid contains air bubbles, an air separator should be added.
Starting Up a Positive Displacement Flowmeter
Initial Fluid Flow:
When first introducing fluid, open the bypass valve to flush out any debris in the piping. If there is no bypass valve, remove the flowmeter using a short pipe, clean it, and then reinstall it. Also, clean the filter carefully to avoid damaging the mesh.
Flowmeter Testing:
Slowly open the inlet valve and then the outlet valve, observing the flowmeter’s operation. Finally, close the bypass valve. Ensure that the flow rate, pressure, and temperature do not exceed the specifications listed on the nameplate.
Reusing After Long Idle Periods:
Before restarting a flowmeter that has been unused for a long time, check for any dirt or corrosion inside the flowmeter. Only proceed with operation if it is clean and free of damage.
Zero Reset:
If the flowmeter has a zero-reset feature, ensure it is properly reset at startup.
Temperature Changes:
When the flowmeter starts up, the rotor and housing heat up at different rates. The rotor heats up faster than the housing, and sudden temperature fluctuations could cause the rotor to seize. Therefore, avoid rapid temperature changes during startup.
Usage Considerations for Positive Displacement Flowmeters
Starting and Stopping:
When starting or stopping the flowmeter, especially for rotary or oval gear types, open and close the valves slowly to prevent rotor damage. High-temperature flowmeters require careful temperature management to prevent the rotor from seizing.
Selecting the Appropriate Measurement Range:
Avoid selecting a measurement range that is too small for continuous use. Operating the flowmeter near its upper limit for extended periods can cause excessive rotor speed, leading to wear and reduced service life.
Shutdown Procedures:
For fluids that are prone to solidifying, ensure that the lines are cleared before shutting down the system.
Pressure Drop Monitoring:
Keep an eye on the pressure drop at both ends of the flowmeter. If it suddenly increases (usually above 120 kPa), stop operation and perform maintenance.