- Metering skid are installed at CGS & DRS for analyzing & measuring the gas flow quantity.
- Metering skid comprises of a flow measurement device, which is of utmost importance in CGS as natural gas custody transfer takes place.
- The system parameters like pressure, flow & temperatures govern the selection & sizing of a gas meter.
- Gas flow meter are either volumetric or mass flow meter. There are many types of meters such as Coriolis meters, Diaphragm gas meter, Rotary positive displacement meters, Orifice type meters, Turbine meters & Ultrasonic gas flow meters.
- An orifice meter is a conduit & also a restriction to create a pressure drop. A nozzle or a thin sharp edged orifice is used for flow restriction.
- As the gas approaches the orifice, the pressure increases slightly and then drops suddenly as the gas passes through the orifice. The decrease in pressure as the gas passes through the orifice is a result of the increased velocity of the gas passing through the reduced area of the orifice. When the velocity decreases, as the gas leaves the orifice the pressure increases, and tends to its original level. All the pressure loss is not recovered because of the friction and turbulence losses in the stream. The pressure drop across the orifice increases when the rate of flow increases. When there is no flow, there is no differential pressure. The differential pressure is proportional to the square of the velocity & is proportional to the square of the rate of flow.
- The turbine meter is a device used to measure the volumetric flow of a fluid.
- Turbine flow meters use the mechanical energy of the gas to rotate a rotor in the flow stream. The rotor is designed with 6-10 blades which are angled to transform energy from the flowing gas into rotational energy. The rotor shaft spins on bearings when gas flows across it. The rotor’s rotation frequency is proportionate to the flow velocity. In this way, the gas volume is calculated based on the rotation of the rotor.
- The shaft rotation can be sensed mechanically or by detecting the movement of the blades. As the rotor spins, the blade tips pass through the magnetic field generated by a permanent magnet mounted in close proximity to the rotor. Each blade generates a pulse & with increase in gas flow, more pulse are generated. A transmitter processes the pulse signal to determine the flow of gas.
- The main advantage of this meter is the ability to handle large gas quantities, high accuracy & turndown. Turbine meters are used for custody transfer metering of gas.
- There are two concepts which are used in the ultrasonic flow meter viz. Doppler effect & Transit Time Differential method.
- In the Doppler method, reflectors are needed in the processes gas. The frequency based method was used in the 70’s but due to malfunctioning, was stopped & is now used in few special applications.
- The Transit-Time Differential method is of importance in present scenario. It measures the volumetric flow rate & also characterizes the type of fluid. It is based on the principle of velocity measurement- dependency of velocity on frequency. The velocity in methane medium, which is a principle constituent Of natural gas is 430 m/sec. In this meter, an emitter sends ultrasonic wave frequency (1-5MHz) at an angle into the flowing gas. The wave strike particles moving through the sound field.
- In turn, the receivers now seize the reflector frequency out of the flowing gas line, which is if a different frequency than that of the emitted frequency. The difference of frequency is therefore a linear measure of the rate of motion of a gas particle. The measuring method needs a sufficient number of reflecting particles in a medium on a continuous basis. The particle must be large enough to provide sufficiently good reflections and the reflected wavelength should be at least greater than λ/4, where λ is the incident wavelength.
- The ultrasonic field extends only to the peripheral flow & these meters are widely used for custody transfer metering.
Positive Displacement meters:
- There are two types of positive displacement meters viz. reciprocating & rotary. Positive displacement meters measure the volumes of gas, flowing through by taking a note of feeding & discharging of fixed volume. These type of meters consist of a chamber which obstruct the flow. Inside the chamber, a reciprocating unit is placed to create discreet parcels of fixed volume from the passing fluids. The volume of fluid passing through the mechanical chamber is obtained by taking note of the parcel passing through the chamber. The volumetric flow is calculated from the revolution rate of the mechanical device. The accuracy depends upon the integrity of capillary seal, which separates incoming fluid into discreet parcels. A filtration system is required to remove particle larger than 100 micro meters.
- A reciprocating meter consists of reciprocating pistons operating within their respective measuring chambers, where each piston works in a manner similar to an automobile engine. The pistons are fitted to an oval plate which has a shaft, extending from its upper surface. The valve pivot drives a sliding valve from piston to piston as known amount of product flows through each cylinder bore, controlling the sequence of events in a smooth mechanical motion. This smooth mechanical motion is then transferred through the calibrator to register the reading.
- Rotary meter are available in various configurations such as lobe type, vane type, screw type etc.
- A rotary lobe type meter consists of two “8” shaped rotating impellers that move opposite to one another, when gas flows through them. These impeller are not affected by the gas density, turbulence & pulsation in line. These meters are mainly used for gas flow ranges from 0.2 to 650 m3/hr & the pressure range up to 20 bar.
- The rotor, cylinder & head plates form a measurement chamber through which gas flows. This ensures that a known volume of gas is trapped and discharged through the meter.
- The volume of gas closed between the measuring chamber walls and rotors is transported four times in one full cycle from the meter inlet to the outlet. The displaced gas per revolution is multiplied by the number of impeller revolution to determine the volume of gas passed by the meter. The actual volume of gas is indicated on the mechanical counter.
- These meters are popular in industry because of their high accuracy, compact size & long service life. These meters are capable of handling higher gas volumes & pressure & are widely used in utility billing & sub-metering. The major drawback is that this meter has moving parts & is limited to metering clean, dry gases. So in this case, filtration becomes essential before passing gas through the meter.
- A diaphragm meter is physically composed of a pressure container which forms a part of chambers that measure gas. These chambers are filled & emptied periodically & the diaphragm separates the chamber so that the gas pressure fluctuates on either sides. The third component which is important is the valve & seats that controls the flow of gas into the side of the diaphragm. The valve & the index register the revolution of the entire mechanism.
- A diaphragm meter is like a double acting piston where there are two phase systems. If the piston is in phase then in next operation it will be 90 degree out of phase.
- Each stroke of diaphragm displaces a fixed volume of gas, so that at one full stroke a fixed volume of gas is displaced & the meter will record the amount of displacement. When the demand for gas is made on the downstream side of the meter, a pressure drop is created across the meter & its diaphragms. The differential provides the force to drive the meter. In addition to the capacity of the meters the body must be physically constructed to withstand the internal pressure of the gas. Modern diaphragm meters have sufficient structural integrity to withstand a minimum shell test of 10 psi. The meter should always be shipped, stored & installed in an upright position. Dust caps on the inlet & outlet connections should be left in place until the meter is installed. Caution should be used with meters while maintenance, as gas may be present within the diaphragm chambers.
- Diaphragm meter is the most widely used for domestic & commercial gas metering & is usually installed at the customer premises. When the meter is used in the areas like domestic & commercial, ventilation should be provided & meter should be accessible for examination, reading, replacement or maintenance.
- Coriolis meters work on the concept of Coriolis effect. Coriolis effect is described as the deflection of moving objects when the motion is described relative to a rotating reference frame. Coriolis meters measure the force resulting from the acceleration caused by mass moving toward a centre of rotation. These meter measure the mass flow rate of gas directly.
- In Coriolis flow meter, no compensation is needed for temperature, pressure & density variation. A U-shaped flow tube with inlet on one side & outlet on the other enclosed in sensor housing is connected to an electronic unit. The gas flows through one or more oscillating tubes. The flow rate of gas affects the oscillation of the tubes, which can be used to determine the gas flow rate.
- These meter have higher accuracy than most flow meter & can be used in a wide range of flow conditions. These meters are also suitable for bi-directional flow.