Level Measurement in the process industry
This paper touches upon the need for level measurement in the process industry and the commonly used terms related to level measurement. It also provides an overview of the popular technologies used, for level measurement in the process industry and the suitability of these technologies in various application
Why Level measurement
Level measurement is the determination of the linear vertical distance between a fixed reference point (e.g. bottom of a vessel) and the surface of a liquid or the top of a solid. It may also include the measurement of the distance to an interface between two liquids. Level measurement is primarily used for the following
Improving efficiency
Accurate level measurement can ensure that you store only as much as required in a vessel thereby reducing costs and improving productivity
- Consistent supply
Maintaining a certain minimum amount of supply can be achieved using level measurement
- Safety
Filling of vessels above their designed capacities or monitoring leakages are important not only from the safety perspective but also from the adhering to environmental and statutory requirements
- Inventory Management
This is the primary reason for level measurement to keep track of your inventory in terms of volume or weight
- Custody Transfer
Level measurement is commonly used to monitor the amount of material/product that is transferred bought or sold
Commonly used terms
Zero and Span level
Zero level indicated the reference point of vertical measurement while the span is the difference between the minimum and maximum level to be measured
Continuous Vs point level instruments
Continuous level measuring instruments provide output proportional to the level between zero and span. Point level instruments, switches, provide a change in output state at a particular level
Contact Vs Non-Contact measurement instruments
This indicates whether the instruments are in direct contact with the solid or liquid whose level is being measured. The application decides which type of instrument is suitable.
Two wire transmitters
Two wire output is the most commonly used outputs on level transmitters provide a current between 4-20mA corresponding to zero and span levels
TANK GAUGING
Tank gauging is a term for highly accurate liquid level measurement in custody transfer applications used extensively in petroleum and refining industries. It consists of a level measuring instrument, additional hardware units, communication units and specialized software for inventory management. Tank gauging systems are used to track inventory and monitor transfer of product in refineries, petroleum or fuel storage depots and other storage facilities. The tank or vessels used in such applications are between 10-30 m and there is a level accuracy requirement is in the range of 1-3 mm. In a tank gauging system level, temperature, pressure, and density are monitored. These systems also offer correction in volumes and mass on basis of temperature changes. SBEMs TFMS (Tank farm management system) is an example of a tank gauging system which consists of a servo level gauge (139M), tank side indicator (TSIC), data collection unit, (CIU) and tank gauging software (TFMS)
TECHNOLOGIES FOR SWITCHES
These are mainly used for point level detection. The output provide by switches are only ON/OFF i.e. they can be configured or used to detect either the presence or absence of the product.
Float Type
Principle
This switch operates on the principle of a liquid lifting the float due to buoyancy. The float assembly is contains a permanent magnet and it interacts with another magnet in the switch housing. They are mounted on the side or top of the tanks,
Applications
The simple construction and limited components allow these to be used in mostliquid applications including high pressures and temperature. They are available in various materials however;moving parts mean that maintenance may be required especially in viscous environments. SBEMs 137 EA are side mounted float type switches are used in high/low level alarm control systems.
Float type Displacer type
Displacer Type
Principle
A displacer is suspended from the top of the switch using a cable and a spring. The tension in the spring is function of the weight of the displacer. When the displacer is in contact with the liquid the effective weight of the displacer is reduced and the spring tension changes. This change in tension is detected using two permanent magnets similar to the ones used in float switches.
Applications
These are used for liquid duty. They are also used in high pressure vessels. They are available in several materials making them suitable for varied application. SBEMs 137 D/L/Y series of displacer switches are used where the specific gravity of liquid from ranges of 0.5 to 1.2 gm/cc.
Nuclear Type
Principle
This consists of a radioisotope source placed on one side of the vessel and a detector on the opposite side. The amount of radiation/gamma rays are detected at the detector. When product material is present the radiation reduces which is detected and used to then provide the required action.
Applications
Nuclear technologies are non-invasive and do not require any process connections on the tank. They can be used for solids and liquids and are unaffected by temperature, pressure, viscous materials. However this technology is not very popular due to high cost.
Laser/Optical type
Principle
This consists of an optical source at one end which continuously emits light and a light detector. When the light is obstructed by the product appropriate action is taken.
Application
The glass window of the light source and the detector need to be clean at all times for the switch to work satisfactorily.
Capacitance Type
Principle
This switch consists of a metallic electrode that acts as one plate of the capacitor and the vessel wall, if metallic, acts as the other plate of the capacitor. The capacitance changes when the product comes in contact with the electrode. This is sensed using electronics and appropriate action is taken.
Applications
They can be used for solid as well as liquid duty. They are suitable for solids with low dielectric constants like fly ash, cement etc. A variety of electrode types are available depending on the requirement or application. They are not suitable for products with changing dielectrics and products that cause build up on probes. SBEMs 114 MLS/RFS capacitance type switches are available with feature like probe failure alarms, time delayed action, special coat safeelectrodes to avoid problems due to material build up and insulated electrodes for liquid duty.
Vibrating fork
Principle
A piezoelectric assembly is used to vibrate a tuning fork at its natural frequency. When the fork is in air, it vibrates at its natural (resonant) frequency, which is being detected by an electronic circuit. However when it comes in contact with any medium (liquid or solid) oscillations dampen and this is detected. Another type uses the principle of variation in frequency of fork when measured in liquid.
Applications
They are not affected by foam, bubbles, turbulence, etc and do not require calibration. They are however not suitable for viscous or sticky media they lead to build up between forks causing false detections. SBEM 135 VL series of switches are suitable for most liquids and free flowing solids and also available in coated versions for corrosive or sticky media.
Conductivity Type
Principle
This switch measures the electrical resistance between two or more electrodes. When the product is in contact with the electrodes the resistance changes and this change in resistance is sensed by the electronics. A single instrument can be used for multiple point level detection.
Applications
They are mainly used for liquid duty and can be used in a variety of applications like water and corrosive liquids like acids and alkalis. SBEMs 130 IC/RC conductivity switches can provide up to 5 detection points and do not require any calibration.
Paddle type
This switch consists of a motor, which drives a paddle through a planetary gearbox arrangement. W hen level of solid reaches the rotating paddle; it tends to stall the latter. The resultant rise in torque is sensed and used to actuate the micro switches. SBEM 135 LC paddle type switch is available with a variety of paddle types and extensions. They are mainly used to control the level in free flowing powders, granules or solids.
Acoustic/Ultrasonic type
They works on the principle of dampening of acoustic waves in a metal rod. The transmitted acoustic waves travels through the sensor (rod) and are reflected back from the bottom of the sensor.
The amplitude of the reflected signal in liquid changes which is detected. SBEMs 136 URS/ARS are used in marine applications for detection of water in various tanks. They can also be used for detection of solids like cement, coal ash and vicious fluids like oils, creams.
TECHNOLOGIES FOR TRANSMITTERS
These are mainly used for continuous level monitoring. They usually provide a 4-20mA output current proportional to the level reading.
Float and Tape type
Principle
This is a mechanical assembly that works on the principle of buoyancy. A level sensing float is connectedto the indicator assembly that contains a mechanical pointer. Usually a fixed scaleor a tape is connected to the float, through a float cable. In a scale type the pointer rolls on the scale when level changes whereas in the tape the tape moves with the level and pointer is stationary.
Applications
They are suitable for all types of non pressurized tanks like cone roof, flat roof, floating roof, underground tanks and for tank in service. These are rugged and simple to use however they are impractical for tanks with larger heights and do not provide any sort of electronics based control e.g. relays. An example of a scale type is SBEMs 131 level indicator that gives high accuracy and is suitable for installation on in service tanks up to 20 m.
Capacitance Type
Principle
The level sensor consists of a metallic electrode which acts like one plate of the capacitor and the vessel wall, if metallic, acts as the other plate of the capacitor. For non-metallic vessels another metallic electrode is used as second plate. The material in vessel acts as dielectric and as the level changes the dielectric changes in turn causing the capacitance to change. The change in capacitance is proportional to the level and is sensed using electronics.
Applications
They are used for liquid duty including high temperature, variable density and viscous products for conductive and non-conductive media. It is an inexpensive technology with accuracies of 2% of span; however they requireon site calibration. As the capacitance is dependent on dielectric constant a change in media dielectric causes errors. It is prone to errors due to product buildup on electrodes and heavy fuming. SBEMs 103 CLT capacitance type transmitter has several electrode variations for different applications, including PTFE sheathed flexible electrodes for corrosive duties.
Magnetostrictive Type
Principle
Magnetostrictive materials have a property creating a mechanical strain pulse when it interacts with two magnetic fields. A magnetostrictive probe consists of a metallic tube that housesa rigid wire made of magnetostrictive material. The sensor emits current pulses through the wire, generating a circular magnetic field. The probe has magnetic float that provides the second magnetic field and runs along the probe length. When the two magnetic fields interact a strain pulse/wave is generated which runs in both directions along the wire. The time is measured between emission of the current pulse and arrival of the wave by the sensor circuitry. This transit time is proportional to the position of float and in turn the level.
Application
They have a high accuracy (1-2mm) and are capable of level, interface and density measurement using the same unit. Longer lengths of probes >4 m are susceptible to errors due to poor handling and installation and turbulence. They are also prone to clogging and sticking of floats due to direct contact with material.SBEMs 134 TM magnetostrictive probe with the 134 H indicator are typically used in LPG and petrol retail outlets.
Ultrasonic type
Principle
This is non-contact type of level measuring technology that works on the principle of transit time. The ultrasonic transmitter is mounted on top of tank and transmits an ultrasonic pulse (in KHz) into the tank which travels at the speed of sound. This pulse is reflected back from the surface of tank and then detected by the electronics. The time required by the pulse to travel to surface and back detected by the electronics and is proportional to level.
Distance = (Speed of sound x time delay)/2
Applications
Theyare used for liquid duty with limited applications for solids. It is a non-contact technology and with accuracy of 0.2% of span. It is also largely unaffected by liquid density, viscosity and dielectric. It is affected by temperature as the velocity of sound changes with temperature. It is also not suitable for very high pressure (3 bar) and temperatures (70 C) due to materials of construction. Excessive fumes and turbulence causes signal weakening. SBEMs 136 ULTRAMATE transmitters are available in ranges of 4,8, 10,15 and 25 meters and have a built in temperature transmitter to compensate for velocity changes due to temperature.
Pulsed Radar type
Principle
This is non-contact type of level measuring technology that works on the principle of transit time. The pulsed radar is mounted at the top of the tank and sends out a high frequency
microwave(in GHz) that travels at the speed of light. This pulse is reflected back from the surface of tank and then detected by the electronics. The time required by the pulse to travel to surface and back detected by the electronics and is proportional to level
Distance = (Speed of light x time delay)/2
Applications
The pulsed radar can be used in liquids as well as solids and has better accuracy and range than the ultrasonic transmitters. They are It is also unaffected by liquid density, viscosity.The dielectric constant (K) of the medium however does play a role in the signal strength. The larger the K the better is reflected energy. Excessive foaming may also reduce the energy of the reflection. SBEM 138 pulsed radars is available in frequencies of 6 and 26 GHz and can be used in corrosive, fuming, turbulent applications and in extreme conditions of temperature and pressure
FMCW Radar Type
Principle
FMCW mean frequency modulated continuous wave. In this, the transmitter sends a microwave of continuously varying frequency to the liquid surface. The reflected wave is then analysed for change in frequency due to Doppler Effect. This change in frequency is directly proportional to the distance of the liquid.
Application
These are mainly used in tank gauging systems and provide a high level of accuracy (< 1mm>
Guided Wave Radar type
Principle
The guided wave radar works on the principle of time domain reflectrometry.It consists of probe mounted at the top of the tank which extends to the bottom. A low energy microwave pulse is transmitted down the probe. This pulse travel at the speed of light and a part of the energy is reflected back at any interface point i.e. air and liquid or between two liquids (oil and water). This time delay between the transmitted signal and received echo is measured by the transmitter and is proportional to level
It is capable of both level and interface measurement and can be used in a variety of applications like liquids, slurries, sludge’s as well as some solids. It is unaffected by changes in liquid density, dielectric or conductivity, Changes in pressure, temperature do not affect accuracy. Specialized probes are also available for measurement under high temperature and pressure. SBEM 132 GWR series are available in flexible or rigid probe designs with lengths up to 30m and accuracies of less than 15 mm.
Servo Based
Principle
The servo level gauge uses a bi-directional motor attached to a displacer and a cable. The cable is wound on a calibrated measuring drum. The motor is controlled by an electronic load cell which senses the buoyancy of the displacer. In equilibrium the weight of the displacer is balanced however when the level changes the buoyancy changes and this is sensed and the motor is moved to restore equilibrium. The movement of motor is measured and gives the level. The gauge also measures interface and density.
Applications
They are mainly used in tank gauging systems and provide a high level of accuracy and are capable of accurate interface and density measurement. SBEMs 139 M servo gauge is a capable of measurements of upto 37 meter with an accuracy of 1 mm.It is a multipurpose instrument, capable of level, interface, density and temperature measurement, popularly used in tank gauging. It provides a single instrument for any tank.
Hydrostatic type
Principle
Level measurement using differential pressure measurement is a commonly understood and proven technology. In case of open tank the reference pressure is atmospheric while in closed tanks a reference pressure equivalent to vessel pressure must be provided. The level is also dependent on the specific gravity of the liquid.
Level = Differential pressure / Specific Gravity
Application
Level measurement using pressure is economical and well accepted. It can handle a wide range of pressure, temperature and also foam and turbulence. A major drawback of the technology is that it is affected by specific gravity or density. SBEM 103 HPT series offer reliable level measurement for ranges up to 20 m with accuracy of 0.1% of span.
Conclusion:
There are a number of level measuring technologies that can be used in the process industry. Each technology comes with its own set of advantages and limitations and the final selection must done based on the medium being measured, process conditions, fitment, control requirement and cost.
Author is Managing Director of SBEM Pvt. Ltd, India
SBEM is a pioneer in the level and flow instrumentation in India and has been in the business of delivering value to its customers by manufacturing world class instruments for the past 35 years. SBEM is also the only Indian manufacturer of the servo gauge and also a solution provider to the Indian Defense.