After natural gas is extracted from a well and undergoes initial separation, it passes through a gas flow meter to measure the production volume.
These meters are either installed on a separation vessel to monitor production volumes or used for custody transfer, where the gas is measured, and a midstream company takes ownership.
There are four natural gas meter types often used for flow measurement. They are mass flow meters, velocity flow meters, differential pressure, and PD meters.
Flow meters are essential tools for producers to measure the flow rate of gas, with different types selected based on gas volume and fluid properties. While all flow meters serve the primary purpose of measuring flow rate, they differ in operation and measurement techniques.
Flow meters are versatile and used in both upstream production and midstream transportation. In upstream production, they monitor individual well output, facilitate custody transfer at sales meters, and measure the amount of chemicals being injected.
In the midstream sector, flow meters play a crucial role in inventory measurement, process control, fuel management, and regulatory reporting.
The four main types of flow meters include:
1. Mass Flow Meter
2. Velocity Meter
3. Pressure Differential Meter
4. Positive Displacement Meter
Mass flow meters are designed to measure the mass flow rate, which is the amount of liquid passing through the meter per unit of time. This measurement depends on the liquid's density, velocity, and the size of the meter itself.
The Coriolis meter is the most common mass flow meter used in the oil and gas industry. It operates based on motion mechanics: the fluid is divided into two tubes that oscillate in opposite directions. As the fluid flows through these tubes, the Coriolis effect causes a phase shift in their vibration, creating sine waves. The time delay in these waves' motion is directly proportional to the mass flow rate.
Coriolis meters are highly accurate, making them ideal for applications requiring precise measurements. However, they are also quite expensive. Due to their cost, they are not typically used for large flow volumes because producing larger Coriolis meters is not cost-effective.
Another type of mass flow meter is the thermal mass flow meter. This meter measures the mass flow rate based on heat transfer or temperature change from a heated element within the flow stream. As the fluid flows past the heated element, it absorbs heat and cools the element. The rate at which the element cools is directly proportional to the fluid flow volume — the faster it cools, the greater the flow.
Thermal mass flow meters offer excellent accuracy and repeatability, making them reliable for consistent measurements across various applications.
Velocity flow meters measure the flow rate by determining the velocity of the fluid passing through them. This type of meter relies on the principle that the flow velocity is proportional to the volume of the fluid.
The most common velocity meter is the turbine flow meter. This meter operates by having the fluid flow through a rotor that spins freely. As the fluid moves, it causes the rotor to spin. Each revolution of the rotor corresponds to a specific volume of gas or liquid and is detected by a magnetic pickup.
Due to their moving parts, turbine meters are best suited for measuring single-phase fluids, such as dry gas or liquids. For accurate measurements, especially under varying pressure and temperature conditions, these variables must be input into the flow monitor.
Vortex flow meters work based on the vortex shedding principle. When fluid flows past a bluff body or bar positioned in the flow path, it creates alternating vortices, or swirling eddies, on either side of the bar. The meter detects the frequency of these oscillating vortices, which is proportional to the flow velocity. Since vortex meters have no moving parts, they are ideal for applications requiring low maintenance.
Ultrasonic flow meters measure flow using the difference in transit times of ultrasonic pulses traveling upstream and downstream through the fluid. This transit time difference is directly related to the flow velocity. Ultrasonic meters are highly accurate but come with higher costs. They also require additional input for pressure and temperature adjustments to maintain their precision across different applications.
Each type of velocity flow meter offers unique advantages, making them suitable for various fluid measurement scenarios based on the specific needs of the application.
A pressure differential meter measures flow by observing the pressure drop across a fixed obstruction inserted into the fluid flow path. Common types include orifice plates and venturi tubes, both of which rely on the principle of pressure differentials.
Orifice plates and venturi tubes function similarly by creating a restriction in the flow path, leading to a pressure drop. This drop in pressure is measured using pressure transducers installed on the upstream and downstream sides of the meter.
The pressure differential thus created allows the flow meter to determine the flow rate using Bernoulli's Equation. For accurate measurements, it's crucial to account for variations in temperature and pressure within the system.
Pressure differential meters are widely used due to their simplicity and reliability in measuring flow rates across various applications. They provide cost-effective solutions for both liquid and gas flow measurements, depending on the specific needs of the process.
Positive displacement meters measure volumetric flow by using fluid to displace mechanical components. Accurate measurements require compensations for operating temperatures and pressures.
One widely used type is the diaphragm meter, commonly installed outside homes to measure natural gas usage for utility companies.
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