How to measure the natural gas flow rate ?
To measure flow of natural gas, it can be volume flow rate measurement or mass flow rate measurement. According to the flow meter principle to measure natural gas flow, it can be positive displacement flow meter (PD flow meter), differential pressure flow meter (DP flow meter), and turbine flow meter, vortex shedding flow meter, ultrasonic flow meter, coriolis flow meter or thermal mass flow meter.
Differential pressure flow meter to measure natural gas flow
Differential pressure flow meter is based on Bernoulli equation and fluid continuity equation. When fluids(such as natural gas) passing the DP flow meter, A pressure difference is generated before and after throttling device, and this difference pressure value is proportional to the square of the flow rate. Differential pressure flow meter is including orifice plate flow meter, V-cone flow meter, balanced flow meter, venture flow meter, nozzle flow meter, long radius nozzle flow meter.,etc. For natural gas flow rate measurement, we normally use orifice plate flow meter, V-cone flow meter, balanced flow meter.,etc
Differential pressure flow meters are generally composed of primary element, differential pressure flow meter, 3-way manifold, for natural gas measurement, we also need to equip a multi-variable flow transmitter, pressure sensor, RTD to realize temperature and pressure compensation.
Gas turbine flow meter to measure natural gas flow rate
How gas turbine flow meter (TUF) to measure NG?
When the natural gas flows through the turbine flow sensor, the turbine is forced to rotate under the action of the NG (natural gas) thrust, and its speed is proportional to the average flow velocity of natural gas. The rotation of the turbine periodically changes the magnetic resistance value of the magnetoelectric converter, and the magnetic flux in the detection coil follows it. Periodic changes occur, generating periodic electrical pulse signals. Within a certain flow (Reynolds number) range, the electrical pulse signal is proportional to the volume flow of the natural gas flowing through the turbine flow sensor.
Advantage of natural gas turbine flow meter:
Disadvantages of natural gas turbine flow meter
Gas turbine flow meter for NG has high accuracy: 1.0~ 1.5%
Digital gas turbine flow meter with electronic display, pulse output, frequency output, 4-20mA output
Natural gas turbine flow transmitter has MODBUS or HART protocol options
Compact structure and easy for installation
Can be made into high pressure type natural gas digital flow meter
All stainless steel material gas flow sensor
Periodic verification and calibration is needed after running a period of time to measure natural gas.
Temperature and pressure correction and compensation is needed ,because natural gas density has a great impact on the gas turbine flow sensor accuracy.
Enough straight pipelines is needed in the upstream and downstream when install natural gas turbine flow meter.
Natural gas turbine flow meter is not suitable for the measurement of pulsating flow and mixed-phase flow.
Inline natural gas turbine flow meter cannot be made for large NG pipeline
Vortex Flow meter to measure natural gas
How vortex flow meters work to measure NG?
A non-streamline vortex generator is placed in the vortex shedding flow sensor, and the natural gas is alternately separated on both sides of the vortex generator to release two rows of regular and alternately arranged vortex streets. Within a certain range of flow (Reynolds number), the separation frequency of the vortex is proportional to the volume flow of the natural gas flowing through the vortex flow sensor.
Advantage of Vortex shedding flow meter for natural gas
Disadvantages of vortex flow meter for natural gas
Simple structure, no moving parts, easy to install and maintain
Accuracy of vortex flow meter for natural gas can be 1.5-2 %
Turn down ratio for natural gas vortex flow meter is 10: 1
Low pressure loss for natural gas pipe lines
Digital natural gas vortex flow meter has digital display to indicate natural gas instant flow and total flow
Vortex flow transmitter has frequency, pulse or 4-20mA output
Option with temperature and pressure correction for vortex flow meter
It is not suitable for low Reynolds gas measurement, low flow rate or small diameter natural gas pipe
Long straight pipeline is needed before and after natural gas vortex flow meter.
Inline vortex flow meter only suitable for DN300 (12 inch ) vortex flow sensor
Not goods for pulsating natural gas flow or multiphase natural gas flow
Positive displacement flow meters for natural gas flow measurement
Positive displacement flow meters, or PD flow meters, are the most accurate type flow meters.
The rotating body of positive displacement flow meters, continuously rotates under the action of the natural gas flow pressure difference, and continuously discharges the natural gas from a small space with a known volume. According to the number of rotations of the rotating body within a certain period of time, the volume of natural gas flowing through the PD flow sensor can be obtained.
Roots flow meter for natural gas measurement
Roots flow meter is a kind of typical PD flow meter. It can have very high accuracy when measuring natural gas flow. Heavy duty flow meter with robust constructions can be used for 15 years. As normal type PD flow meters, roots flow meter does not need straight pipe in the upstream and downstream of natural gas pipelines. It can be a direct reading flow meter without any power supply , it can also be digital natural gas PD flow meter with pulse or current outputs
Coriolis mass flowmeter for natural gas flow measurement
Coriolis mass flowmeter, hereinafter referred to as CMF) is a direct mass flow meter that uses the principle of Coriolis force that is proportional to the mass flow when natural gas flows in a vibrating tube.
Advantage of Coriolis flow meter for natural gas
It can directly measure the natural gas mass flow rate, with high accuracy (0.2%, 0.15%), straight pipe sections are not required when installing natural gas flow sensor, the measured value is not sensitive to viscosity, and the change in natural gas density has a slight influence on the measured value. Corioils flow meter can be made into high pressure type natural gas flow meter with pressure rating 600#, 900 LB or even higher.
Disadvantage of Coriolis flow meters for natural gas
Not good for low flow natural gas flow measurement
The max size for Coriolis flow meter is 10”, can be used for larger size.
Coriolis flow meter is currently the most advanced flow meter , the price is not cheap, price starts from 3000$
It has not stable zero point for natural gas measurement
Large pressure loss for natural gas mass flow measurement