Coriolis mass flow meter

·         The measuring principle is based on the controlled generation of coriolis forces. These forces are always present when both translational and rotational movements are superimposed.

·         The amplitude of coriolis force depends on the moving mass.

Notes:- The fluids fights against this rotation because it wants to keep traveling in the straight line. For any given rotational velocity, the amount of fight will be directly proportional to the product of the fluid velocity and fluid mass. This is the basis of a “coriolis mass flowmeter”. The magnitude of the coriolis force will be directly proportional to the fluids mass flow rate.

Measuring principle:

·         When the mass flows through a vibrating pipe coriolies forces exist which bend or twist the pipe.

·         These very small meter tube distortions are measured by optimally located sensors and evaluated electronically

·         This measuring principle is independent of pressure, temperature, density, viscosity and conductivity.

Tube designs:

·         The tube can be curved or straight form.

·         When the design consist of two parallel tubes, flow is divide into streams by a splitter near the meters inlet and outlet its recombined at the exit.

·         The single continuous tube design (or in two tubes joined in series), the flow is not split inside the meter.

·         In either case, drivers vibrate the tube, these drivers consist of a coil contacted to one tube and a magnet connected to the other. The transmitter applies an alternating current to the coil, which cause the magnet to be attracted and repelled by turns.

·         If the electromagnetic sensors are used, the magnet and coil in the sensor change their relative positions as the tube vibrate, causing change in the magnetic field at the coil.

·         The main difference between the force coil and the sensor coil is that the force coil is powered by an AC signal to impart a vibratory force to the tubes, where as the sensor coils are both unpowered so they can detect tube motion by generating AC voltages to be sensed by the electronics module.

·         When there is no flow in the two tubes, the vibration caused by the drive results in identical displacements at the two sensing points.

·         When the flow is present, coriolis force act to produce a secondary twisting vibration, resulting in a small phase difference in the relative motions. This is detected at the sensing points.

·         Meters are available in size from 6mm or less to 200mm or more and for flow ranges from 0-3kg/hr upto 0-680,000kg/hr.

·         Accurate measurement of both liquids & gases.

·         Ranges cover from less than 5g/m to more than 350 ton/hr.

·         Mass flow, density and temperature can be accessed from the one sensor.

·         Many models are affected by vibrations.

·         The upper pipe line diameter is 150mm

Pitot Tube

           In its simplest form the Pitot tube comprises a small tube inserted into a pipe with the head bent so that the mouth of the tube faces into the flow. As a result, a small sample of the flowing medium impinges on the open end of the tube and is brought to rest. Thus, the kinetic energy of the fluid is transformed into potential energy in the form of a head pressure (also called stagnation pressure).
           

            The simplest pitot tube consists of a tube with an impact opening of 3.125 mm to 6.35 mm diameter pointing towards the approaching fluid. This measures the stagnation pressure. An ordinary upstream tap can be used for measuring the line pressure.

            The velocity equation for the pitot tube is given by
            v = Cp 2gh
            Where: Cp is the pitot tube constant.

            Pitot tubes are generally used only in low-to-medium flow gas applications where high accuracy is not required.

Dall Tube

      It is a modified version of venturi tube. It produces large differential pressure with low pressure less than the conventional venturi tube.

      It consists of a flanged spool piece body with a short, straight inlet section terminating in an abrupt decrease in diameter or inlet shoulder. This is followed by a conical restriction and a diverting outlet separated by a narrow annular gap. The high pressure tap is a hole drilled through the body tangent to the inlet shoulder. The low pressure tap is drilled through the body so as to connect with an annular slot in the throat.

      It is not suitable for measuring the flow of fluids containing solids which could settle out in the throat slot. The Dall tube is used for water, sewage, air and steam flow measurement. The Dall tubes are normally cast in gun metal. But for 450 mm and larger sizes, high grade cast iron is used. When it is required to protect the tube from corrosion, it may be lithcote lined.

Advantages

1. Low head loss

2. Short lying length

3. It is available in numerous materials of construction.

Disadvantages

1. Pressure difference is sensitive to up-stream disturbances.

2. More straight pipe required in the approach pipe length.

3. It is not considered for measuring flow of hot feed water.

Positive Displacement Meters

Positive Displacement Liquid Meters:

• Nutating Disk Meter
• Oscillating Circular Piston Meter
• fluted rotor meter or rotating impeller meter or helical rotor meter
• Oval Gear Meter
• Sliding Vane Meters or (rotating vane)
• Reciprocating Piston Meters
• Rotating lobe
• Precision Gear Flow meters

Positive Displacement Gas Meters:

• Wet Gas Meter 
• Diaphragm Meter
• Rotary Positive Displacement Gas Meter

Nutating disk meter:
          In this type of meter, an inlet chamber is formed by the housing, a disk, and a partition between the inlet and outlet port. Water is prevented from leaving the chamber by the disk, which maintains line contact with the upper and lower conical surfaces of the housing. When the pressure is reduced on the outlet side by a demand of water, the difference causes the disk to wobble (but not rotate) about the vertical axis and thus provide a passage for the flow around the partion. The wobble of the disk causes a small pin attached to its spherical mount to trace out a circular path about the vertical axis of the device. This motion of the pin is used to drive the recording mechanical.

Oscillating circular piston meter:
           It is similar to the nutating disk meter. Its center is constrained to move in a circle by the transmission so that the radius of the cavity is essentially the sum of the radii of the rotary piston and the circle on which its center moves. When the center of the rotary piston is at the top of its travel, the piston forms a closed compartment with the cavity. One rotation of the shaft will cause the rotary piston to return to its starting place and so to discharge the volume of one compartment.

Fluted rotor meter or rotating impeller meter or helical rotor meter:
         The axial and radial fluted rotor meters work on the same principal. The axial fluted rotor meter makes use of two aluminium spiral fluted rotors working within the same measuring chamber — with the rotors maintained in a properly timed relationship with one another by helical gears. As the product enters the intake of the measuring unit chamber, the two rotors divide the volume being measured into segments; momentarily separating each segment from the flowing inlet stream and then returning them to the outlet of the measuring unit chamber. During this ‘liquid transition’, the segments of flow are counted and the results are transferred to a totalising counter or other flow recording device by means of a gear train.

Oval gear meter:
            The oval gear meter is a special form of a multiple rotor meter in which each oval rotor is toothed, and sealing between the rotors is enhanced by the resulting labyrinth. Each rotor transmits fluid from inlet to outlet and forms a closed compartment when its major axis is aligned with the flow direction. The volume passed per revolution of each rotor is four times the volume between the rotor and the oval housing when the rotor is confining liquid [Figure 9.4(a)]. In place of the leakage paths between the rotors of multi rotor meters, there will be, for this meter, extremely small leakage where the rotors mesh, and the tolerances for the other surfaces are likely to be to a high standard, giving a very small value for the overall leakage.

Sliding vane:
            Consists of a cylindrical rotor from retractable vanes protrude. The fluid flow against the vanes causes the rotor to rotate. As the rotor rotates, the trapped fluid between vanes is swept around and out of the chamber. The number of revolution of the rotor is thus a measure of the amount of fluid that has been passed through the meter. Accuracy is high, about 0.1%.

Reciprocating piston meter:
            Reciprocating piston meters are probably the oldest PD meter designs. They are available with multiple pistons, double-acting pistons, or rotary pistons. As in a reciprocating piston engine, fluid is drawn into one piston chamber as it is discharged from the opposed piston in the meter. Typically, either a crankshaft or a horizontal slide is used to control the opening and closing of the proper orifices in the meter. These meters are usually smaller (available in sizes down to 1/10-in diameter) and are used for measuring very low flows of viscous liquids.

Lobed impeller or Rotating lobe:
              In the rotating lobe design, two impellers rotate in opposite directions within the ovoid housing (Figure 3-3B). As they rotate, a fixed volume of liquid is entrapped and then transported toward the outlet. Because the lobe gears remain in a fixed relative position, it is only necessary to measure the rotational velocity of one of them. The impeller is either geared to a register or is magnetically coupled to a transmitter. Lobe meters can be furnished in 2-in to 24-in line sizes. Flow capacity is 8-10 gpm to 18,000 gpm in the larger sizes. They provide good repeatability (better than 0.015% AR) at high flows and can be used at high operating pressures (to 1,200 psig) and temperatures (to 400¡F). The lobe gear meter is available in a wide range of materials of construction, from thermoplastics to highly corrosion-resistant metals. Disadvantages of this design include a loss of accuracy at low flows. Also, the maximum flow through this meter is less than for the same size oscillatory piston or nutating disc meter.

Precision Gear Flow meters:
           The spaces between the gears and the chamber wall form the fluid transfer compartments. In this version, rotation is sensed by two electromagnetic sensors operating through a pressure-resistant and nonmagnetic element in the housing. Two sensors can be arranged to allow better resolution than one and to determine flow direction. Measurement uncertainty of ±0.1% rate is claimed. It is also claimed that rapid flow reversal can be followed (e.g., 801/min in about 0.01 s). Meters for flow range of as low as 0.001 1/min and as high as 1,000 1/min may be available with temperature ranges of -30 to 150°C and pressure up to 300 bars or more

Wet Gas Meter:
            The wet-type gas meter (Figure 2.11) comprises a gas-tight casing containing a measuring drum, with four separate compartments, mounted on a spindle that is free to revolve. The casing is filled to approximately 60% of it’s of volume with water or light oil. Under normal operation the gas passes through the measuring drum so that each compartment of the drum must, in turn, be emptied of water and filled with gas ⎯ thus forcing the drum to rotate. In an alternative arrangement the gas is introduced into the space above the water in the outer casing and then passes through the drum to the outlet of the meter. The spindle on which the measuring drum is mounted is connected through gears to record the quantity of gas passing through the meter. Such meters are available in capacities ranging in size from 0.25 to 100 dm3 with accuracy down to ±0.25%.

Swirl flow meters (vortex precession)

• The swirl meter is based on the principle known as vortex precession.
• The swirl meter can be used with both gases and liquids, its main application as a gas flow meter.
• The major advantage of the vortex precision technique over that of vortex shedding is, only three diameter of straight line required upstream of the meter (3D/1D)
• A inlet guide body whose shape is similar to a stationary turbine rotor is located in the inlet of the measuring device.
• The fluid entering the meter to spin about the center line this swirling flow passes through a venture, where it is accelerated and then expanded in an expansion chamber.
• The expansion changes the direction of the axis about which the swirl is spinning- moving the axis from a straight to a helical path. This spiraling vortex is called vortex precision.
• A flow straighter is used at the outlet from the meter.
• The rotation (swirl) is measured with pizeo sensor.
• The vortex shedding frequency is between 1 and 2000Hz, the higher.

High integrity pressure protection system (HIPPS)

           

               A High Integrity Pressure Protection System (HIPPS) is a Safety Instrumented System (SIS) designed to prevent an unsafe condition caused by pressure arising (e.g. due to separator outlet blocked in the choke valve downstream, blocked pipeline, etc).

               The decision to utilize a HIPPS in addition of utilize a PSV shall be based on the study of risk. The aim of this study is to determine a certain SIL requirement.

               This study will conclude whether some process condition need to have a HIPPS or its ok to protect it by a PSV valve only.

               A High Integrity Pressure Protection System typically is a complete functional loop consisting of:

• The initiators which detect the high pressure.
• A Logic Solver, which processes the input from the initiators and transmits an output to the final elements.
• The Final Elements, which consists of a valve actuated by solenoids.

                In general, the HIPPS required a minimum SIL 3 certification for a complete loop above (sensor, logic solver, and final element). Therefore the HIPPS architecture of the initiators, logic solver, and final elements shall be determined by using a SIL calculation. But in typical the architecture of the HIPPS will consist of 3 pressure transmitter, redundant logic solver, and 2 shutdown valve to achieve SIL 3 requirement. See VOTING LOGIC for more detail information about loop architecture selection to achieve certain SIL requirement.

                Another unique design of the HIPPS valve is the closing time of the valve that must be as fast as possible. In general application the HIPPS valve closure time requirement is calculated and simulated by using a dynamic simulation. The aim of the fast closure time in the HIPPS valve is to protect the downstream of the valve from a very high pressure condition that can happen in a very fast time.

                As an example and illustration, let’s say we have a separator downstream of the production manifold that needs to be protected from a high pressure due to separator outlet blocked. The separator design pressure is 800 psig, and the pressure downstream of the choke valve through the production manifold can be as higher as 1500 psig. With this condition and known flow rate to separator, the pressure arising time can be calculated. If the normal operating pressure of the separator is 600 psig, then time pressure arising from 600 psig to 800 psig must be calculated. Lets say, after some calculation the time needed by the separator to arise it pressure from 600 psig to 800 psig is 4 second, then this 4 second will be the minimum closing time of the HIPPS valve. If the closing time of the HIPPS valve is more than 4 second, the pressure inside the separator will arise more than its design pressure when the outlet is blocked and will endanger the platform (event though there will always be a PSV installed in the separator).

source:- instreng

Emergency Shutdown (ESD) System Philosophy

There are several purposes of the ESD System which is:

• Protection of personnel
• Protection of the environment
• Minimize loss of production and damage to plant assets

Typically, the ESD System could fulfill the above objective by the following implementation:

• Monitoring of an operational or equipment condition
• Automatic action in case of process hazardous conditions is exist by de-energizing electrical equipment, shutting down and/or isolating process equipment and, isolating and depressurizing the installation.
• Enabling manual initiation of ESD actions through ESD push button all around the plant.
• Monitoring the Fire & Gas conditions (F&G) by the F&G System
• Automatic action in case of F&G hazardous conditions exist by providing audible and visual alarms for personnel.

Typical ESD System Component

• Dedicated process transmitters
• Logic Solver
• Shut-Down valves (SDV), Fail to Close type, the purpose of this valve is to isolate.
• Blowdown valves (BDV), Fail to Open type. The purpose of this valve is to depressurize.

In practice the plant is usually divided into several isolable units that can be depressurized and isolated.