A wet gas meter measures gas volume by trapping known quantities of gas in rotating chambers submerged in liquid.
At its core, a wet gas meter is a volumetric device that relies on the displacement of liquid by gas to measure flow. Unlike dry gas meters that use bellows, a wet gas meter contains a rotating drum, often called an impeller or rotor, segmented into multiple chambers (typically four). This drum is partially submerged in a liquid, usually water or a specific oil, within a sealed casing.
Here's a breakdown of the process:
The Basic Principle
The fundamental principle is simple: Gas enters the meter, fills chambers of the rotating drum, and as these chambers fill and empty, the drum rotates. Each full or partial rotation corresponds to a precise, known volume of gas measured.
How It Measures Gas Flow
- Gas Entry and Differential Pressure: Gas enters the inlet port of the meter. Inside the casing, the gas pressure is slightly higher than the outlet pressure. According to the reference provided, "When the gas to be measured passes through the wet test gas flow meter, a differential pressure is formed at the inlet and outlet ends of the flow meter."
- Drum Rotation: This differential pressure acts on the surfaces of the submerged drum chambers. This force causes the drum to rotate.
- Chamber Filling: As the drum rotates, the gas fills the chambers one by one, displacing the liquid within those chambers. "The differential pressure forces the drum to rotate, thereby causing the gas to fill the metering chamber one by one."
- Sealing: The liquid (e.g., water) acts as a seal, ensuring that the gas passes through the chambers sequentially and doesn't bypass the measurement mechanism.
- Gas Exit: As a chamber rotates upwards and becomes unsubmerged, the gas exits through the outlet port. The next chamber then begins to fill.
- Volume Registration: The rotation of the drum is mechanically or electronically linked to a counter or index, which registers the total volume of gas that has passed through the meter. Each complete rotation represents a specific, calibrated volume.
Key Components
- Casing: The sealed outer shell that holds the liquid and the internal mechanism.
- Liquid: Typically water, but oil or other liquids may be used depending on the gas and required accuracy. The liquid level is crucial for accurate measurement.
- Rotating Drum: A precisely manufactured, multi-chambered impeller that rotates as gas flows through it.
- Index/Counter: A display (mechanical dials or digital readout) that registers the cumulative volume of gas measured.
Advantages and Applications
- High Accuracy: Wet gas meters are known for their high accuracy, especially at very low flow rates.
- Sensitivity: They are sensitive to small flow rates, making them suitable for laboratory applications, calibration standards, and measuring low gas production or consumption.
- Calibration Standard: Often used as primary standards for calibrating other gas meters (hence "wet test meter").
Considerations
- Liquid Level: The accuracy is highly dependent on maintaining the correct liquid level. Evaporation can affect accuracy, requiring regular checks and refilling.
- Temperature & Pressure: The liquid temperature and pressure can affect the gas volume measured (due to density changes), so these conditions must be controlled or accounted for, especially when used as a standard.
- Maintenance: Requires more maintenance than dry meters (checking/replacing liquid, cleaning).
- Bulk and Weight: Can be larger and heavier than dry meters for the same flow capacity.
In summary, a wet gas meter precisely measures gas volume by leveraging the force of differential pressure to rotate a segmented drum partially submerged in liquid, effectively trapping and releasing known volumes of gas with each turn.
