- Level and
The importance of level measurement in this context cannot be overstated whether be it for Industrial Process Measurement & Control or in Inventory Tank Gauging.
Significance of Level measurement and control is mainly on account of following factors…
- Safe and efficient control of the process:- A manufacturing process can go haywire, if the levels of relevant liquids and solids are not measured and controlled properly. It can also severely damage production equipment and result in undesirable change in the quality of product or loss of production. (Example – In case of steam generating boilers, measurement & control of water level in the boiler drum is very critical. Loss of water level or excessive level of water are detrimental for the life of boiler and related auxiliaries)
- Monitoring and control of inventory:- Economic efficiency of any operation depends on adequate control of inventory. This is particularly important for high value items, since excess storage of such items adds to the “inventory carrying cost”. However low levels of inventory can starve production activity. Hence accurate measurement of level of liquid / solid inventory is essential.
- Custody transfer:- Accurate measurement of level of liquids and solids facilitates transaction of the items with trust and confidence to ensure faster settlements of deals. Estimation of volume of costly liquids like petroleum products can always be done more accurately by measurement of level rather than by flow metering.
- Human safety:- Spillage of hazardous liquid can cause harm to personnel working in the area and potentially may create safety and environmental hazards resulting in financial losses. Hence overfill protection is very vital. Also disproportionate mixing of two or more ingredients can generate harmful products.
B) Measurement Technology
The technologies deployed for measurement and control of liquid / solid level are broadly classified as follows.
1) Contact Type instruments - These instruments make direct contact with the liquid/solid whose level is being monitored. They are based on various principles such as Archimedes principle, Magnetism, Hydrostatic pressure, Capacitance, Conductivity and Piezo electric vibrations.
While making use of contact type level measuring instruments, compatibility of the Material Of Construction (MOC) of the instrument must be taken into account. Also adequate attention must be paid to the operating pressure and temperature.
1.1 Archimedes principles:-
- A body, whose density is less than that of a liquid, will float in that liquid and the weight of liquid displaced is equal to the weight of the body.
- A body submerged in the liquid will displace volume of liquid equal to the volume of the body and the buoyant force experienced by the submerged body is equal to its loss of weight.
Magnetic force can be used to actuate reed switches and micro-switches without physical contact. Also magnetic force can be transmitted through non-magnetic materials.
The current flows through the conductive liquid to complete the circuit between reference electrode and sensing/control electrode.
1.4 Hydrostatic pressure:-
This method depends on measurement of pressure exerted on pressure sensor by liquid column
when placed at the bottom of the tank. The pressure exerted by the column of liquid is equal to the height of liquid column, h, times the specific gravity, sg, of the same.
(P = h x sg.).
Thus from this we can calculate the level of liquid, ‘h’ in a tank, if we know the pressure of the liquid at the bottom and the specific gravity ‘sg’ of the liquid.
1.5 Capacitance principle:- The capacitance relationship is expressed by the following equation:
C = 0.225 K (A / D)
C = Capacitance in Pico Farads
K = Dielectric constant of material
A = Area of plates in square inches
D = Distance between the plates in inches
Capacitance type switches & transmitter make use of this principle where liquid between two electrode acts as dielectric constant.
1.6 Piezo electric effect:- Piezoelectric Effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress. One of the unique characteristics of the piezoelectric effect is that it is reversible, meaning that materials exhibiting the direct piezoelectric effect (the generation of electricity when stress is applied) also exhibit the converse piezoelectric effect (the generation of stress when an electric field is applied).
All vibrating fork switches make use of piezo electric crystal and works on this principle.
2) Non-contact type instruments – They do not make physical contact with the liquid/solid whose level is being measured. These instruments are mainly based on ultrasonic, radar and laser principles. Also adequate attention must be paid to the operating pressure and temperature.
2.1 Ultrasonic principle:-
Sound travels through air at velocity of 343 meters / sec. at 20 0 C. Echo of ultrasonic pulses at frequencies 30 KHz and above is used to calculate the distance between sensor and target. The range from the sensor to the target is computed by:-
R(t) = ct/2
where: R(t) is the distance to the target
c is the speed of sound
t is the time from the transmission until the echo received.
2.2 Radar principle:-
This non-contact radar technology has two different versions: Pulse radar and Frequency
Modulated Continuous Wave (FMCW) radar.
The pulse radar - An electromagnetic wave having frequency between 1 and 100 GHz is sent from the antenna toward the process surface in search of a change in impedance, which will reflect the signal back to the transmitter.
The intensity of pulses reflected back depend upon dielectric constant of the product. The transit time of electromagnetic pulse is used to calculate the distance and in turn the level of process liquid/solid.
The FMCW radar – It uses high frequency RF signal upto 100 GHz and transmitted towards the target and reflected from the target surface at time delayed frequency. The difference in transmitted and received frequency of signal is directly proportional to the distance of target from the antenna which is then converted into level signal of 4-20mA DC.
C) Types of level measuring instruments -
1) Visual indication: – It is achieved using mechanical level gauges which do not depend on electricity for their operation.
- They are used for continuous level indication of liquids. Broadly level gauges are classified as glass gauges and float type gauges.
- The glass gauges are based on principle that “liquid seeks its own level” whereas float type gauges are based on Archimedes principle.
- Level gauges are available in wide variety of materials such as PP, CS, SS304, SS316, PVDF, PTFE lined SS304 or Hastalloy C to suit the type of liquid and operating conditions.
- In several applications, provision of direct reading level gauges is mandatory as statutory requirement.
- ‘Seeing is believing’, hence reliable and do not depend on electricity.
- Glass gauges provide direct indication of liquid level.
- Easy to install and operate.
- Suitable for high temperature and pressure applications.
- They are suitable for free flowing liquids not containing heavy suspended particles.
- Glass gauges have a fragile element like glass tube or flat glass.
Following are the types of level gauges which are commonly used in industry.
1.1 Tubular Level Gauge:–
The gauge is mounted on the side of a tank so as to form close loop with the content of vessel. Liquid level can be observed in the glass tube of the gauge. Guards are provided to protect the glass from accidental breakage.
Suitable for maximum temperature 200 ⁰C and pressure 10 Kg/cm2
Generally these gauges are used for low pressure application, and in general, for ‘open to atmosphere’ tanks.
1.2 Reflex / Transparent Glass Gauge: –
Liquid level is observed in flat reflex or transparent gauge glass. Reflex type glass has prismatic corrugations on its front face by virtue of which liquid portion appears as black and portion above it white.
Transparent gauge is similar to reflex type, except that the gauge glass is smooth finished and a pair of glass is fitted on front and rear side.
Suitable for maximum temperature 400 ⁰C and pressure 200 Kg/cm2
1.3 Magnetic Level Gauge:-
It consists of a magnetic float inside a chamber made up of non-magnetic material and indicator is clamped externally on the chamber. Indicator is available in two types, follower capsule or bicolor flapper.
Float follows liquid level and links with capsule/ flapper to indicate liquid level. This gauge is multi-purpose, as it can be provided with switches and transmitter which are clamped externally on the chamber.
It is suitable for corrosive, toxic and hazardous liquids where glass gauges are considered unsafe. Suitable for max temp 400 ⁰C and pressure 100 Kg/cm2
1.4 Float & Board Gauge:–
It is the simplest gauge consisting of float connected with wire rope and the other end of rope is connected to pointer outside the tank.
Float follows the liquid level and pointer moves on the scale board attached on side of the tank. It is suitable for bulk liquid storage tanks under atmospheric pressure.
1.5 Float & Tape Gauge :–
This is the most accurate gauge, which measures liquid level upto ± 2mm accuracy. It consists of perforated tape wound on drum. Float follows liquid level and tape is wound /unwound on drum through sprocket wheel.
The motion of sprocket wheel is transferred to pointer through gear mechanism to indicate level on the dial.
2) Level detection or control at preset points: -
This is done by using level switch, which provide on/ off contact for operations such as alarm annunciation, start/stop of pumps/auxiliaries, interlocks etc.
- Used to detect liquid / solid level in the tank. Single point or multi point level switches are available.
- Available in wide variety of materials such as PP, CS, SS304, SS316, PVDF, PTFE lined SS304 to suit the type of liquid and operating conditions.
- Weather proof or flameproof protection for outdoor installation and installation in hazardous area are provided.
- The switches, based on Archimedes principle do not require electricity for their operation. But they provide potential free electrical contacts, which can be used for further actions.
- Since the electrical switch is magnetically coupled with the float, there is complete isolation between the liquid and the electrical portion of the instrument.
- ·Some switches may not be found suitable for direct operation of higher loads.
In such cases, use of intermediate relays/contactors may be required.
Following are the different types of level switches generally used in industry.
2.1 Float Guided Switch –
It can provide single or multi point switching. It consists of one or more hermetically sealed reed switches located inside the guide tube and a magnetic float moves freely along it. Float follows the liquid level and actuate the reed switch.
The output reed contacts have low power rating (40, 60, 120 VA) but nevertheless can be directly connected PLC/DCS. Suitable for max. temp. 150 ⁰C & pressure 10 kg/cm2
2.2 Float Pivoted Switch –
It is single point switch generally side mounted to the tank wall. The switch operates through a glandless magnetic coupling to provide micro switch changeover contacts of 5A, 230VAC.
The switch is ideally suitable for tanks which are inaccessible from top & bottom. Suitable for max. temp. 300 ⁰C & pressure 20 kg/cm2
2.3 Displacer Switch –
It is single or multipoint switch. The displacer, is suspended from a wire rope is connected to actuator rod. With rising level, when displacer is submerged in the liquid, it experiences a buoyant force and actuates the micro switch of 5A, 230VAC rating.
The level set points are site adjustable. Suitable for maximum temperature 300 ⁰C and pressure of 100 Kg/cm2
2.4 Conductivity Switch –
As the name suggest, it is suitable for conductive liquids only. In this, a small AC signal is passed through a reference electrode and same is detected at control electrodes at various set points.
The current flowing from reference electrode to control electrode is used to operate relay which can handle more power. Suitable for max. Temp. 100 ⁰C & pressure 5 kg/cm2
2.5 Capacitance Switch –
It measures capacitance formed by two electrodes of the switch where liquid forms the dielectric. Variation of level results in change in capacitance which is measured and relay is operated. Suitable for max. temp. 60 ⁰C & pressure 5 kg/cm2
2.6 Vibrating Fork Switch –
In air its fork vibrates at its resonance frequency created by means of piezo electric crystal. When material touches to the fork, its damped vibrations are sensed in electronics to operate relay.
It is single point switch and is available for liquid and free flowing granular /powder. Suitable for max. temp. 250 ⁰C & pressure 10 kg/cm2
2.7 Rotating Paddle Switch –
It is solid level detection switch. It consists of synchronous motor which rotates at 1 RPM. The shaft of the motor is extended to the required length and paddle is attached to it.
In absence of material paddle rotates and when solid level reaches to the paddle it stops rotating and operates a micro-switch lever to provide changeover contacts.
3) Continuous level measurement and monitoring: –
· Level transmitters are used for quantitative measurement of liquids/solids which help in inventory control and custody transfer. They generally provide an electrical output of 4 to 20 mA DC corresponding to 0 to 100 % of the level, which in turn can be expressed in terms of various units of measurement including volume/weight.
Transmitters are available in material of PP, SS304, SS316, PVDF coated SS to suit process liquids. Inventory control and custody transfer can be effected by using the transmitter.
- Exact quantitative measurement facilitates continuous level monitoring.
- Output is also available in RS485 MODBUS or HART. Fieldbus compatible.
- Multiple alarm points can be assigned from indicator PLC/ SCADA system.
- Limitations -
- Contact type level transmitters are not suitable solid applications
- Indicator/SCADA system is needed to display level in meaningful unit.
Following are the main types of transmitters available for different applications in the industry.
3.1 Float guided transmitter :-
It consists of magnetic float moving along the guide tube, containing a chain of resisters & reed switches placed across it. Float moves on the guide tube according to liquid level to actuates reed switch and corresponding voltage across the resister is converted into current output of 4-20mA.
It can be used for max. temp. 150 ⁰C & pressure 10 kg/cm2
3.2 Hydrostatic transmitter:-
10 meters of pure water column corresponds to 98.1 Kpa of pressure. Piezo-resistive sensor fitted at bottom of the probe measures liquid column pressure at the bottom of the tank.
Sensed pressure is converted into mv output which is processed to provide 4-20mA output. Suitable for atmospheric pressure and temp. 70⁰C
3.3 Capacitance level transmitter:-
The construction and working is similar to the capacitance switch except the change in capacitance is converted into 4-20 mA output signal. Suitable for max. temperature 60⁰C and pressure of 5 kg/cm2
3.4 Magnetostrictive level transmitter:-
It is high accuracy (< 1mm) float type level transmitter A pulse of current sent over the wave guide wire, creates a circular magnetic field which interacts with magnetic field generated by the float, to produce twist in the wire.
Time delay between starting of pulse and its return is measured and converted into 4-20mA o/p, which corresponds to float position / liquid level. It can be used for max temp 100⁰C & pressure 10 kg/cm2
3.5 Guided wave radar level transmitter:-
This is a contact type transmitter based on Time Domain Reflectometry (TDR) principle. Low power electromagnetic pulses are sent along the probe length down to the product surface and partially reflected back to the electronics.
Transit time of emitted and reflected pulse is proportional to distance, which is then converted 4-20 mA signal. This transmitter is suitable for max temp. 250⁰C & pressure of 10 kg/cm2
3.6 Displacer type level transmitter:-
It consists of a displacer suspended from an operating lever, the other end of which is connected to a torque tube and further to a lever assembly fitted with magnetic system.
The change in liquid level causes the operating lever to rotate torque tube proportionately to the change in buoyant force acting on displacer.
This rotation is transferred to lever assembly causing angular displacement to change the magnetic field, which is sensed by sensor and converted into 4-20 mA o/p through signal processing electronics. Suitable for max temp. 300 ⁰C & pressure 60 kg/cm2
3.7 Differential Pressure Transmitter :-
These transmitters are based on piezo-resistive principle or capacitance principle and are extensively used for level and flow measurement applications.
They are ideally suitable for level measurement in highly pressurised vessels like boiler drum. Transmitter can be connected to the vessel by means of impulse pipes. Remote diaphragm seal type transmitters are also available for hazardous, viscous fluids.
3.8 Non-contact Ultrasonic level transmitter:-
It transmits a short burst of ultrasonic sound toward a target, which reflects the sound back to the sensor. The system then measures the transit time of transmitted & reflected signal which is proportional to the distance to the target.
The same transducer can be used to both transmit and receive the sound, or separate transducers can be used to transmit and receive. Suitable for max. temp. 70⁰C & pressure < 1bar
3.9 Non-contact radar level transmitter:-
As explained earlier, these transmitters can be either “pulse type” or based on “FMCW principle”. Transmitters are programmable at site. Higher the frequency of operation, lower is the range of the transmitter but better is the resolution and vice-versa.
Different types of antennas such as Horn Type, Wave Guide, Parabola Type, Patch Antenna or Drop Antenna are available to suit the applications. Suitable for max temp. 300⁰C & pressure of 35 kg/cm2
Combinations of level gauge, switch and transmitter can also be used. All float type gauges can be provided with switches and transmitters. Magnetic level gauge can be provided with twin chamber i.e. one for level gauging and other for radar transmitter.
D) Selection Criteria
Level measuring instrument is selected with reference to following criteria/conditions.
- Chemical compatibility with liquid
- Physical property of service material – Density, conductivity, dielectric constant, viscosity & suspended particles
- Operating temperature & pressure
- Foaming /fuming conditions
- Agitation /turbulence in liquid
- Angle of repose and particle size
- Area classification as hazardous/non-hazardous
- Mounting position & process connection on the tank/vessel
- Desired accuracy
Not only selection but also proper installation and commissioning of the instrument is equally important for achieving accuracy in level measurement.
There is not universal instrument for level measurement of liquids/ solids. However appropriate instrument is selected with given criteria as above.
E) Advent of Digital Era:-
Similar to other walks of life, advancements in digital technology has beneficially impacted level measurement technology as well. In addition to the conventional 4 to 20 mA DC output from the transmitters, MODBUS RTU output is also available which can be directly monitored from the computer. ‘Tank Farm Management System’ which is essentially a SCADA system, facilitates centralised monitoring and inventory control of chemicals, solvents and petroleum products.
Also several functions related to monitoring of level, such as on/off operations of pumps, open/close operations of valves, alarm annunciation etc. can be executed from remote by making use of GSM/GPRS network.
Graphic display and animation of level indications on the DCS/SCADA screens makes it convenient for the operator to monitor several parameters at a glance.
Recently level measurement & monitoring is done using wireless technology for various IIoT applications