FLOW
Many types of flow meters are used in the refinery for flow measurement. They are as follows
· Restriction type
· Coriollis Mass flow meters
· Thermal Mass flow meters
· Electromagnetic flow meters
· Rotameters
· Vortex flow meters
· Ultrasonic flow meters
Consider a fluid travailing in a pipe cross-section area at a steady rate of flow. If on inserting a restriction in that pipe, since the flow is steady, the same number of molecules should flow through any section of the pipe per unit time. Therefore, the molecules pass through cross-section at a faster rate than the unrestricted pipe-cross-section .In order to bring about this increase in velocity the fluid must decrease its energy content of another form termed as static pressure. Therefore, there exist difference in static pressure between these two points. This is the principal behind flow meter, i.e. a restriction is used to develop a differential pressure. The point at which the minimum fluid area coincides with the minimum pipe diameter is called vena-contracta.
The various devices used in the refinery to create a differential pressure are:
a)Orifice plate
b)Venturimeter
c)Wedge flow meter
d)Pilot tube
a) Orifice Plate
The most widely used primary device is orifice plate and they form the majority of the flow meters. There are different types of orifice plates like concentric orifice plate (a circular disc having a concentric circular base with a sharp 90o corner at the inlet face), eccentric orifice plate (a circular disc having an eccentric circular bore with a sharp 90o corner at the inlet face) and segmental orifice plate (circular disc with a bore which is a segment of the concentric circle and has a sharp 90o corner at the inlet face).
The material of construction of these plates is stainless steels. The concentric orifice is widely used in services. In services where the following medium is sticky or slurry, segmental orifice is preferred. The pressure recovery at the downstream of the orifice plate is about 75 to 80 percent.
b) Venturimeter
Venturimeter is a volume flow-measuring instrument consisting of a convergent – divergent pipe. A standard venturimeter has a 30 degree included angle in the convergent section and 7 degree angle in the divergent section. The diameter ratio “d throat” to “d pipe” is equal to 2. This is used mostly in slurry services. The divergent section allows smooth expansion of the flow back to the flow rate and therefore the pressure recovery is about 87% and so it is best suited in services where the pressures are low.
When compared to orifice meter, this is bulky and costly but the pressure loss is less and also susceptible to less wear and tear.
In the refinery there are only very less number of venturimeters. They are generally used in compressor inlet flow measurement as in the FCCU unit WGC & MAB air inlet flow, acid gas flow in DHDS SRU unit etc.
c) Wedge Flow meter
A wedge-shaped segment is inserted perpendicularly into one side of the pipe while the other side remains unrestricted. The change in cross section area of the flow path creates pressure drops used to calculate flow velocities.
d) Pitot Tube
The pitot tube measures the fluid velocity by converting the kinetic energy of the flow to the potential energy. This indication of velocity combined with the cross-sectional area of the pipe provides an indication of flow rate.
Averaging pitot tube (Annubar) is similar to pitot tube but uses multiple openings as shown in the figure for averaging the pressure.
The main disadvantage is that foreign matter materials can clog the pitot. This is used in large pipelines with averaged out by drilling pre-determined holes along the length of the pipe.
In the refinery annubar is commonly used in very large pipelines for cooling water and raw water services. They are also used in large air ducts used for combustion air to heaters.
There are several types of flow meters that respond to the mass of the flowing fluid rather than volume, area, or velocity. Examples of mass flow meters are thermal mass meters, gyroscopic mass meters, angular momentum mass meters, and Coriollis mass meters.
Coriollis mass meters can measure the density of the fluid in addition to the mass flow. Since volume flow equals mass flow divided by density, the associated electronics can be programmed to output in volume. So these mass flow meters can also be used to measure the volume flow but the accuracy of the volume flow depends both on the accuracy of the mass flow and density.
Principle of operation:
When a particle moves across the surface of a rotating body, it is caused to accelerate by a force called Coriollis force, which acts normally, to both the direction in which the particle moves and the axis of rotation of the body. The magnitude of the coriollis force is directly proportional to the product of the mass of the particle and coriollis acceleration.
Instead of rotating the body about a fixed point, Coriollis force can be generated by vibrating the tube. The smallest amount of energy to vibrate the tube is at the natural frequency of the tube. Ina typical commercial meter, a U tube is supported at free end while the two pipe sections are anchored
A typical construction is shown below:
In addition to measuring the Coriollis force, most meters are capable of utilizing the frequency of vibration of the tube(s) to measure density. Density is related to frequency, though not linearly.
In order to compensate for the effect of pressure and temperature on the sensor, pressure and temperature compensations are provided in certain sensors by using an inbuilt RTD and external pressure transmitter.
The design and construction of the individual mass flow sensor determine the accuracy, sensitivity and repeatability of the meter. These meters are generally factory calibrated against traceable standards and each sensor will have its own calibration factors.
Typical measurement accuracy is between 0.15 % and 0.25% of rate plus a zero shift, which is also termed zero stability. The zero shift error is due to small varying offsets in both the sensor and electronics. Due to zero error, accuracy is degraded as the flow approaches zero. To obtain the best possible low-end performance, coriollis flow meters offer the ability to check and adjust zero at actual line operating conditions.
These meters can be checked / verified using master meters or other volumetric / gravimetric methods and the reading can be adjusted by adjusting a particular factor generally called the meter factor.
In most of the cases a separate transmitter is connected to the sensor, which powers the sensor and also converts the sensor data to useful digital or analog outputs. The calibration constants, meter factors and other factors, which determine the type of output, are configured in this transmitter.
Advantages & Disadvantages:
Advantages
§ A clear tube provides a fundamental means of measuring mass flow.
§ No moving components and requires less maintenance.
§ Available in a range of corrosion – resistant materials.
§ Calibration independent of viscosity and flow profile.
§ Accuracy degraded at low flow rates due to zero shifts.
§ Performance is affected by the presence of air or gas pockets.
§ Signal will be corrupted due to slug flow conditions caused by the presence of large void fractions and hence not suitable for two phase flow.
§ Sensitive to vibration
§ High-pressure drop at full in some designs.
§ They are bulky in some designs.
§ Maximum size is only 150mm
§ Extremely expensive.
In the refinery there are a large number of mass flow meters of different make (Micro motion, Rheonik, Bopp & Reuther, Krohne Marshall) are used in various units because of its specific advantage over other conventional volume meters. A large number of them are used in Truck loading area for custody transfer (sales) of products, fuel oil meters to heaters and very few in process units (DHDS).
Thermal Mass Flow Meters:
Earlier few thermal mass flow meters were used in the flare flow measurement purpose, which are now replaced with ultrasonic type instruments. The thermal mass flow measurement principle are now limited to few flow switches used in combustion air to heaters. They are of M/s. Fluid Components Inc. (FCI) make using thermal dispersion principle.
Thermal Dispersion technology uses the principal of measuring the heat loss, or cooling effect, of a fluid flowing across a heated cylinder. A typical flow element configuration uses two RTDs, sheathed in thermowells, separated by a gap. Heat is applied internally to one RTD relative to the other, creating a differential temperature between the two. This differential temperature is greatest at no flow conditions and decreases as flow increases, cooling the heated RTD.
Changes in flow velocity or immersion of the flow element into a liquid directly affect the extent to which heat is dissipated and, in turn the magnitude of the temperature differential between the RTDs. This differential is electronically converted into an electrical signal that can be used to trip a relay.
Since the relationship between flow rate and cooling effect is directly related to mass in gas applications, Thermal Dispersion technology, combined with advanced signal linearizing circuitry, is used to provide a highly repeatable and accurate measurement of gas or air mass flow rates.
Electromagnetic Flow Meter
Electromagnetic flow meters measure the volume flow of electrically conductive liquids and slurries. An electric conductor, in this case the electrically conductive medium, passes through a magnetic field.
The voltage U induced in this medium is directly proportional to the mean flow velocity v.
The induced voltage signal is picked up either by two measuring electrodes in conductive contact with the medium or indirectly by capacitive coupling.
A signal converter amplifies the signal and converts it into a standard analog signal (e.g. 4 to 20 mA) and a frequency signal (e.g. 1 pulse for every US gallon or cubic meter of medium flowing through the measuring tube).
To ensure that the voltage is not short-circuited by the pipe wall, the measuring tube is made of an electrically insulating material or fitted with an insulating liner.
Measurement is largely independent of the flow profile and other properties of the medium, such as pressure, temperature, viscosity, density, consistency, electrical conductivity, and electrode contamination.
The electromagnetic flowmeter consists of a primary head that is installed in the pipeline, and a signal converter. The compact design has the signal converter mounted directly on the primary head.
Only few instruments of these types is used in the refinery. In LOCAT SRU unit M/s. Krohne make electromagnetic flow meter is used.
Rotameter
Rotameters are the most widely used type of variable-area (VA) flowmeter. In these devices, the falling and rising action of a float in a tapered tube provides a measure of flow rate (see Figure). Rotameters are known as gravity-type flowmeters because they are based on the opposition between the downward force of gravity and the upward force of the flowing fluid. When the flow is constant, the float stays in one position that can be related to the volumetric flow rate. That position is indicated on a graduated scale. Note that to keep the full force of gravity in effect, this dynamic balancing act requires a vertical measuring tube.
Figure: - The rotameter's operating principle is based on a float of given density's establishing an equilibrium position where, with a given flow rate, the upward force of the flowing fluid equals the downward force of gravity. It does this, for example, by rising in the tapered tube with an increase in flow until the increased annular area around it creates a new equilibrium position. By design, the rotameter operates in accordance with formula for all variable-area meters, directly relating flow rate to area for flow.
The term rotameter derives from early versions of the floats, which had slots to help stabilize and center them and which caused them to rotate. Today’s floats take a variety of shapes, including a spherical configuration used primarily in purge meters. The materials of construction include stainless steel, glass, metal, and plastic.
The tapered tube’s gradually increasing diameter provides a related increase in the annular area around the float, and is designed in accordance with the basic equation for volumetric flow rate:
Where:
Q | = volumetric flow rate, e.g., gallons per minute |
k | = a constant |
A | = annular area between the float and the tube wall |
g | = force of gravity |
h | = pressure drop (head) across the float |
With h being constant in a VA meter, we have A as a direct function of flow rate Q. Thus, the rotameter designer can determine the tube taper so that the height of the float in the tube is a measure of flow rate.
These types of meters are generally used in low size and low flow application and where local indication alone is required (In certain cases using magnetic coupling and suitable electronics, 4-20 mA output for remote indication is also taken).
Vortex Flow meter:
The vortex flowmeter is used for measuring the flow velocity of gases and liquids in pipelines flowing full. The measuring principle is based on the development of a Karman vortex shedding street in the wake of a body built into the pipeline where Re > 20,000.
The periodic shedding of eddies occurs first from one side and then from the other side of a bluff body (vortex-shedding body) installed perpendicular to the pipe axis. Vortex shedding generates a so-called “Karman vortex street” with alternating pressure conditions whose frequency is proportional to the flow velocity. The non-dimensional Strouhal number S (primary head constant) describes the relationship between vortex shedding frequency F, width b of the body, and mean flow velocity v. These alternating pressures are measured using piezoelectric sensors, which gives the current signal to the signal convertor.
F = S * v / b
These type of meters are installed in air compressor discharge (M/s. Krohe make) line and are capable of indicating the Free Air Discharge (FAD) since they also measure temperature (using RTD) and pressure (strain gauge).
Ultrasonic Flow meters:
a) Milltronics OCM 111 Flow meter:
The Milltronics OCM 111 open channel flow meter is an ultrasonic instrument to measure flow in Outlet C. As a system it is used in conjunction with a remote ultrasonic transducer and a temperature sensor. The OCM 111 transmits a pulse signal to the transducer, which is then emitted as ultrasonic pulses. The pulses echo off the water surface, which are then sensed by the transducer. The time for a pulse to echo back from the water surface is temperature compensated and converted into a measurement of head. The OCM 111 converts the head measurement into flow rate. The flow rate is totalized and stored in a data log to provide flow analysis. An infrared programmer is used to communicate with the OCM 111 transducer.
b) Panametrics Mass Flowmeter for Flare lines
Flow rate in flare systems are characterised by unsteady flow, pulsating pressure, varying composition, varying temperature at wide flow ranges and sometimes bi-directional
The Digital Flow GF868 Mass Flowmeters have been installed in Flare gas lines which measures mass flow rate by using a patented algorithm to determine the instantaneous average molecular weight of the gas being flared and the flare gas flow rate is measured using patented Correlation Transit-Time technique.
Principle of measurement
It uses two ultrasonic transducers located in the gas flow, each of which is capable of both sending and receiving ultrasonic pulses. The electronic circuits measure the time it takes for an ultrasonic pulse to travel from one transducer to the other. A pulse traveling in the direction of the flow arrives at the opposite transducer in a shorter period of time than a pulse traveling against the flow. From the time difference, The GF868 calculates the flow velocity of gas at the transducer installation point in the flare system. Once the sound speed of the gas has been determined by the GF868, its on-board computer uses temperature and pressure inputs in conjunction with the sound speed to calculate instantaneous average molecular weight and mass flow rate of the gas. Hence a Pressure transmitter and Temperature transmitter also is provided on the line, near the transducer location and their outputs are connected to the GF868 system.
Groups of time measurements are repeated rapidly. Measurements are averaged to give a steady meter reading and smooth output.
The configuration data like Pipe parameters such as Transducer path length, Pipe dia, etc and details of analog I/O Pressure transmitter and temperature ranges are entered from the instrument keypad.
The wide range ability of this instrument allows measurement of flow rate from 0.03 up to 85 meters/second.
These meters have dual channel option, which gives it the ability to measure two paths in one pipe for maximum accuracy, or two flow rates in separate pipes with one installation.
Each analog input card in the GF868 has two analog channels to which the Pressure and temperature transmitters are connected in 2 - Wire mode. Each analog out put card has 4 analog channels, which is used to indicate Mass Flow, Sp. Gravity etc in DCS.
In KRL, we have 4 Nos. GF868 Dual channel flowmeters, and 1 No. Single channel Flowmeter at the following locations.
Tag No. | Location | Remarks |
YFE - 335 | FCCU LEFPU | Dual Channel |
YFE -336 YFE - 337 | PRI H2 PRI HC | Dual Channel |
YFE –1604 YFE - 1605 | ARU CDU2 | Dual Channel |
YFE -1606 YFE - 1607 | SRU1 SRU2 | Dual Channel |
YFE - 338 | CPP | Single Channel |
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