Basic Instrumentation Course Prepared by Eng\ Ahmed Mohamed Abdel-Halim EMC, MIDOR Site 2008
3. Level Measurements
3-Level Measurement
Very simple systems employ external sight glasses or tubes to view the height of the fluid. Others utilize floats connected to variable potentiometers or rheostats that will change the resistance according to the amount of motion of the float. This signal is then inputted to transmitters that send a signal to an instrument calibrated to read out the height or volume. So, we have
Sight-type Instruments Pressure-type Instruments Electrical-type Instruments Sonic-type Instruments Radiation-type Instruments
Sight-type Instruments
Glass Gauges The gauges are made of glass, plastic, or a combination of the two materials must be able to withstand the pressure in the vessel.
Sight-type Instruments
Sight-type Instruments
The pressure that the liquid exerts in the tank forces the liquid in the sight glass to rise to the same level as the liquid in the tank. For the pressurized tank, the upper end of the tube is connected to the tank. This creates an equilibrium pressure in both ends of the tube, and the liquid in the tube rises to the same level as the liquid in the vessel. Pressure at the base of a vessel containing liquid is directly proportional to the height of the liquid in the vessel. P=ρgH The level of liquid inside a tank can be determined from the pressure reading if the density of the liquid is constant.
Sight-type Instruments
There are two basic types of flat sight gauges: reflex and transparent. The reflex-type gauge produces a dark area where liquid is present and a light area where vapor is present. The reflex type gauge is normally chosen for liquids that are colorless, clear, and nonviscous. The transparent gauge is generally used when the liquid is colored, viscous, and corrosive. Sight glass gauges are installed with manual shutoff valves at both ends so maintenance can be performed.
Sight-type Instruments
Magnetic Level Gauges
An external float chamber connected to the tank. The float has a magnetic property that makes the indicator goes up and down on a scale according to the level in the tank.
Tape Float
A tape is connected to a float on one end and to a counterweight on the other to keep the tape under constant tension. The float motion makes the counterweight ride up and down a direct-reading gauge board.
Displacers
Displacer level gauges operate on Archimedes’ principle. A body fully or partially immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced. By measuring the buoyancy force produced by a displacer, you can determine a level value. Displacer may used as a level transmitter for single liquid (LT), or differential level transmitter for interface applications (LDT). For empty tank (or filled with a light liquid), there is no (or minimum) buoyancy force Maximum weight of the displacer. For filled tank ( or filled with heavy liquid), maximum buoyancy force generated on the displacer Minimum weight of the displacer. The displacer may be top mounted inside the tank or chambered unit.
Displacers
Displacers Installation
Displacers (LT)
Displacers (LDT)
Pressure-type Instruments
The pressure at the base of a vessel containing liquid is directly proportional to the height of the liquid in the vessel. As the level in the vessel rises, the pressure exerted by the liquid at the base of the vessel will increase linearly. Differential Pressure (DP) capsules are the most commonly used devices to measure the pressure at the base of a tank. When a DP transmitter is used for the purpose of measuring a level, it will be called a level transmitter.
DP Level Transmitter
When using pressure or differential Pressure transmitters to measure tank liquid level, determining the liquid density is important to accurately calculate the level for a given liquid head pressure. Tanks may be open (vented), or they may be closed (pressurized). With closed tanks that may have a pressure above the liquid that is different from atmospheric pressure, a dry leg or wet leg system may be used to port the top-oftank pressure to the opposite side of a differential pressure transmitter that is connected to the tank to measure level.
DP Level Transmitter
If it is a dry leg, then density of vapor or noncondensing gas in dry leg is calculated and used to correct level measurement for vapor density changes. If it is a wet leg, density of liquid in wet leg is calculated and used to correct the liquid level measurement. Transmitter can be mounted at minimum level, or below minimum level. If transmitter is mounted below minimum level, the leg to transmitter from the lower tank connection is assumed to have liquid in it, and its density calculated to correct the liquid level measurement.
Open Tank (Vented) or Closed Tank With Dry Leg
Assume dP cell below bottom tap we make zero suppression calculation. The span points for the dP cell are calculated as follows DP0% = d ρ g DP100% = (Lmax+d) ρ g The DP transmitter must be calibrated from DP0% to DP100% Any condensation in the leg will create an error in the level measurement. We can prevent condensation in most cases by doing steam tracing of the reference leg.
Closed Tank With Wet Leg
Assume the leg filled with liquid that ( ρw > ρ ) to prevent the serviced liquid goes to the transmitter cell. Assume dP cell below bottom tap we make zero elevation calculation. The span points for the dP cell are calculated as follows DP0% = d ρ g – h ρw g DP100% = (Lmax+ d) ρ g – (hρwg) Any change in the height or density of the wet leg fluid influences the accuracy of the level measurement.
Bubbler Level Measurement System
Used If the process liquid contains suspended solids or is chemically corrosive or radioactive. It is desirable to prevent it from coming into direct contact with the level transmitter. A bubbler tube is immersed to the bottom of the vessel in which the liquid level is to be measured. A gas (called purge gas) is allowed to pass through the bubbler tube. Consider that the tank is empty, so, the gas will escape freely at the end of the tube and therefore the gas pressure inside the bubbler tube (called back pressure) will be at atmospheric pressure. As the liquid level inside the tank increases, pressure exerted by the liquid at the base of the tank (and at the opening of the bubbler tube) increases.
Bubbler Level Measurement System
As a result, the gas pressure in the bubbler tube will continue to increase until it just balances the pressure of the liquid & any excess supply pressure will escape as bubbles through the liquid. A level transmitter (DP cell) used to monitor this backpressure. The bubbler tube is connected to the high-pressure side of the transmitter, while the low pressure side is vented to atmosphere. The output of the transmitter will be proportional to the tank level. Note that bubbling action has to be continuous or the measurement signal will not be accurate. The purge supply pressure should be at least 10 psi higher than the highest hydrostatic pressure the process will encounter. You should keep the purge rate small so no significant pressure drop occurs in the dip tube. The purge medium is air or nitrogen, although you can use liquids.
Bubbler Level Measurement System
Diaphragm Level Detectors
Diaphragm detectors operate by the simple principle of detecting the pressure that the process material exerts against the diaphragm. The diaphragm box instrument consists of an air-filled diaphragm that is connected to a pressure detector via air tubing. As the level rises above the diaphragm, the liquid head pressure compresses the captive air inside. A differential pressure element senses the air pressure, which is displayed as level.
Diaphragm Level Detectors
Servo Tank Gauge
Its level measuring principle is based on the detection of variations in the weight of a displacer suspended in the process fluid. The displacer on level is partly immersed in the liquid. The apparent weight is the weight of the displacer minus the weight of the displaced product. The hart of the servo gauge is an accurate force transducer that continuously measures the apparent weight of the displacer. A weight corresponding with the apparent weight of the displacer on level is programmed by software settings. In equilibrium condition, the weight of the partly immersed displacer balances against the weight programmed to be measured by the force transducer.
Servo Tank Gauge
Servo Tank Gauge
When emptying the tank, the liquid level starts moving downwards. The force transducer will experience an increasing weight, as upward force is no longer acting on the displacer. Smart communication between the force transducer and the servo controller will ensure that the displacer is lowered. The servo motor drives the measuring drum to unwind measuring wire until the displacer is partly immersed in liquid again until the measured or apparent weight equals the programmed weight. The smart processing unit then can detect the level of the liquid inside the tank corresponding to the drum apparent weight.
Servo Tank Gauge
Electrical-type Instruments
Capacitance Probes one side of the process tank acts as one plate and an immersion electrode is used as the other. The dielectric is either air or the material in the vessel & the dielectric varies with the level in the vessel. With the tank empty, the insulating medium between the two conductors is air. With the tank full, the insulating material is the process liquid or solid. As the level rises in the tank to start covering the probe, some of the insulating effect from air changes into that from the process material, producing a change in capacitance between the sensing probe and ground. This capacitance is measured to provide a direct and linear measurement of tank level. Noting that this type of instrument used only with NON-conducting service to act as a dielectric between capacitance plat.
Capacitance Probes
Resistance Tapes
In these devices, resistive material is spirally wound around a steel tape & mounted vertically from top to bottom on a process tank. The pressure of the fluid in the tank causes the resistive tape to be short-circuited, thus changing the total resistance of the measuring tape. This resistance is measured by an electronic circuit and is directly related to the liquid level in the tank.
Ultrasonic Type Instruments
These system consist of a transmitter mounted on top of the tank and receiver as well as associated electronics equipment. The transmitter antenna radiates the signal toward the surface of the process liquid in the tank. A portion is reflected back to the antenna, where it is collected and routed to the receiver. The strength of the reflected signal is directly related to the dielectric constant of the liquid. The instrument measures the time that elapses between the transmitted burst and the echo signal which is proportional to distance between the transducers and the surface of the liquid inside the tank.
Ultrasonic Type Instruments
Ultrasonic Type Instruments
Ultrasonic level measurement works on a "time of flight" principle. Ultrasonic Level Measurement Instruments use sound waves to determine level. A piezoelectric crystal inside the transducer mounted on top of the tank converts electrical pulses into sound energy that travels in the form of a wave at the established frequency and at a constant speed in a given medium. The sound waves are emitted in bursts and received back at the transducer as echoes. The Ultrasonic Level Measurement Instrument measures the time for the bursts to travel down to the reflecting surface and return. This time will be proportional to the distance from the transducer to the surface and can be used to determine the level in the tank. Sound waves are a form of mechanical energy that uses the molecules in the atmosphere to propagate, so, any changes in the chemical makeup of the atmosphere cause the speed of sound to vary.
Ultrasonic Type Instruments
Affected by powders, heavy vapors, surface turbulence and foam. Cannot operate in vacuum or high pressure. Limited temperature range More position sensitive than other technologies
Ultrasonic Type Instruments
Radar And Microwaves Instrument
Radar and Microwaves instruments are similar to the Sonic-type, but have advantage that they don’t affected by the molecules in the atmosphere to propagate unlike the sonic-type. Both radar signals and microwaves travel at the speed of light, but are distinguished by their frequencies and by their power levels. The transmitter is a microwave oscillator and directional antenna (a parabolic dish, horn-type antenna or rod antenna). The receiver consists of an antenna, a high gain, a pulse-decoding circuit, and an output circuit. Microwave signals are absorbed almost entirely by water and to varying degrees by water-based liquids or by products that have a high moisture content. Microwave signals cannot penetrate metals but are reflected by them, so metal storage tanks or hoppers must have a detector window that is transparent to the microwave signals.
Radar And Microwaves Instrument
Radar And Microwaves Instrument
Radar And Microwaves Instrument
There are two types of radar level instruments: noncontact and guided wave. In the noncontact type the output electromagnetic energy of the radar antenna is very weak, typically about 1 mW. On the return to the instrument, this weak signal loses more energy. Liquid turbulence and some foams can further complicate the measurement by scattering or absorbing the radar pulse. The guided-wave radar unit can overcome these problems by using pulses of electromagnetic energy that are transmitted down the probe tube. The wave guided offers a highly efficient path for the signal to travel down to reach the surface of the liquid and then bounce back to the receiver.
Radar And Microwaves Instrument
Nuclear Level Measurement
Nuclear radiation systems have the ability to “see” through tank walls, and thus they can be mounted on the outside of process equipment. Suitable for liquid or solid material detection. Composed of a radioactive source material and a radiation detector, the two are mounted across the diameter of a storage vessel for either solid or liquid material. The product to be measured is attenuating the radiation coming from the radioactive source and according to the height of the product in the vessel, more or less of the original radiation is reaching the detector. This measuring signal is then transferred to an output signal which directly correlates to the actual Level of the product. Two typical nuclear level instruments: Using a single low-level gamma-ray source on one side of the process vessel and a radiation detector on the other side of the tank. Using several gamma sources at different heights on the tank.
Nuclear Level Measurement
Level Measurement Errors
Over ranging Damage to the D/P Cell Faulty Sensing Lines Loss of Loop Electrical Power Connections: with an incorrectly connected DP cell the indicated level would go down while the true tank level increases. Extra weighted displacer: The sludge formed on the displacer makes faulty indication of the level. Blocked bubbler tube: gives higher level reading than the present one.
Level Switches
A level switch is a device that senses the level of a liquid in a process tank. They are also used to control valves or pumps in order to maintain fluid level at a set value or to prevent tanks from being overfilled. Level switch can be Float type switch. Ultrasonic type switch. Rotating Paddle type switch. Displacer type switch. Capacitance type switch.
Float Type Switch
The buoyant force is the operating principle of float level switch. Floats should always be lighter than the minimum expected specific gravity of the process fluid. As the level rises and falls, the float tilts up and down, thus opening and closing its electric contact. The free length of the cable determines the actuation level. In most cases, magnetic coupling transfers the float motion to the electric contact or indicator mechanism. The switch itself can be mercury or snap-action type switch. The float, which contains an annular magnet, rises or falls with liquid level and is guided by the tube. The switch assembly can be either inserted directly into the tank or side-mounted in a separate chamber. The side-mounted switches are completely sealed and well suited for heavy duty industrial applications
Float Type Switch
Top mounted switch (directly in the tank)
Float Type Switch
Side-mounted
Displacer Type Switch
A displacer remains partially or completely submerged. The apparent weight of the displacer is reduced as it becomes covered by more liquid. When the weight drops below the spring tension, the switch is actuated. Displacer switches are more reliable than regular floats on turbulent, frothy, or foamy applications.
Displacer Type Switch
Ultrasonic Level Switches
Echo transmitter-type level switches contain transmitter and receiver units. The transmitter generates pulses in the ultrasonic range, which the receiver detects. The transmitter and receiver can be located in the same probe or on opposite sides of the tank Sound waves are easily transmitted across a transducer gap in the presence of a liquid medium, but are severely attenuated when the gap is dry. The level switch will be actuated when the sound beam is interrupted . This type of switch is effective for both solid and liquid material applications. Noting that the probes of Ultrasonic switch installation has two types. Contact type probe: which the probe itself is inserted inside the tank and surrounded by the media. Non-Contact type probe: the probe is installed externally of the tank.
Ultrasonic Level Switches
Ultrasonic Level Switches
Ultrasonic Level Switches
Nuclear Level Switch
The radioactive source and the radiation detector are mounted across the diameter of a storage vessel for either solid or liquid material. Any height of material beyond the level of the source/detector arrangement will attenuate the strength of radiation reaching the detector. This decrease in radiation at the detector can provide a switch contact for measurement, alarm point, or even control of the vessel level.
Nuclear Level Switch
Rotating Paddle Type Switch
Used to detect the presence or absence of solids in a process tank. A Synchronous motor keeps the paddle in motion at very low speed when no solids are present, there is very low torque on the motor drive. When the level in the tank rises to the paddle, torque is applied to the motor drive and the paddle stops. The level instrument detects the torque and actuates a switch or set of switches.
Rotating Paddle Type Switch
Capacitance Type Switches
As media rises and falls in the tank, the amount of capacitance developed between the sensing probe and the ground also rises and falls. This change in capacitance is sensed by the electronic circuitry. At the predefined set point, the electronic circuit gives the alarm or trip signal.
Capacitance Type Switches
Thermal level switch
The sensor head employs two temperature sensors with a constant heating source physically attached to one of the temperature sensors. The second temperature sensor is isolated from the heating source and provides compensation for changing process temperatures. The probe operates by sensing the thermal conductivity of the product surrounding the probe. All liquids that form an interface will have a difference in thermal conductivity that can be sensed.
Thermal level switch
