Process Level Measurement

Traditional sensors such as displacers and differential pressure transmitters are making way for new technology and in particular two-wire radar level transmitters.

We look at some of the advances in different level technologies and highlight why pulse radar is rapidly becoming the preferred choice for process level measurement.

Capacitive / Admittance level transmitters
Capacitive level transmitters have been used on a variety of liquid and solid level applications for many years. If the process is understood and does not vary, and if a number of basic rules are followed, then capacitive level sensors can prove to be reliable and cost effective.

In the past, applications with very viscous and conductive products have proved to be problematic. However, recent advances such as "phase selective admittance" processing within the transmitter oscillator have allowed automatic compensation for product build-up on the probe. This technology is available with two wire 4 to 20 mA / HART and Profibus PA directly from the transmitter head.


"Phase selective admittance" electronics processing is designed to ignore conductive build-up.
Two-wire transmitters are available with 4-20mA/HART
and Profibus PA.



Hydrostatic level transmitters
Differential pressure transmitters have been used widely in vented storage tanks as well as in pressurised vessels. The use of pressure to measure level has obvious limitations particularly when the density changes between different products. Many older designs have liquid filled chemical seals and capillaries which do not lend themselves to accurate level measurement.

Temperature variations on the liquid fill cause measurement
inaccuracies and the process diaphragm is often thin and prone to mechanical damage.

There have been significant new developments in hydrostatic ensors designed specifically for level measurement. Mechanically robust and chemically resistant Sapphire-Ceramic measurement cells are available with flush, hygienic process connections.These sensors do not have a liquid fill and can be fitted at the very bottom of dished vessels. They have no "dead volume" that can
cause blockages or hygiene problems within food and pharmaceutical processes. High accuracy is achieved with these direct mounted devices.

"Sapphire-Ceramic" hydrostatic level transmitter with flush, hygienic process connection. The design is mechanically robust and abrasion resistant with no "dead" pockets. The sensor can be mounted very low in the vessel. It is ideal for food and pharmaceutical applications.

Ultrasonic level transmitters
The benefits of non-contact level transmitters were first realised with the advent of ultrasonic level transmitters back in the 1970's.

Ultrasonics are suitable for water based liquids and for solids applications where the effects of pneumatic filling are not too extreme.

In recent years, transducer technology has been improved and more importantly echo processing software is more capable than ever before. It is fair to say that ultrasonic level sensors have been pushed to their physical limits. Even the most powerful ultrasonic transducers are affected by vapours, gas composition, excess dust,
steam and condensation as well as filling noise and air movement. Obviously, ultrasonics do not work in a vacuum.

Having said this, ultrasonics still provide a relatively low cost, non-contact level transmitter with ranges from less than one metre to 70 metres and above. Ultrasonic transmitters are available with a variety of analogue and digital output options.

Non-contact ultrasonic level transmitters are used on liquids and solids with measuring ranges up to 70 metres.
Longer ranges generally require larger, lower frequency transducers.

 


Nucleonic or Radiometric level transmitters
Nucleonic (or radiometric) level transmitters are one of the only truly non-invasive forms of level measurement. They are able to measure liquids or solids through the wall of stainless steel or other metal vessels.

This measurement technique, the differential adsorption of Gamma radiation from a radioactive isotope, such as Caesium Cs137 or Cobalt 60, is used to detect the level in the vessel.

This radiation source is specially shielded so that the radiation is only directed towards the product through the vessel wall. The radiation source is mounted near the maximum level on one side of the vessel while radiation detectors are mounted down the opposite side of the vessel. The amount of radiation blocked is proportional to the level.


"FibreFlex" is a unique scintillation detector It uses a special fibre-optic bundle of 7 metres or more that can be wrapped around the contours of a vessel.
In-head smart electronics complete this innovative product.

 


This measurement technique is very capable and is applied in some of the most challenging process level applications in the petrochemical, chemical and offshore industries as well as in heavy solids handling applications typical in the quarrying and cement industries.

Ohmart-VEGA have continued to develop nucleonic measurement into the 21st century. One innovation is the revolutionary "FiberFlex" flexible scintillation detector. Standard ion chamber and Geiger-Mueller tube defectors are heavy with low sensitivity and normal scintillation
tube detectors are rigid and limited in length.

"FiberFlex" utilizes a special fibre-optic bundle that can follow the contours of the process vessel. It is light in weight, easy to transport and install and it can be longer than standard detectors at over 7 metres.

Advances in detector technology include in-head HART / 4 to 20 mA transmiller, Foundation Fieldbus transmitters for scintillation detectors including "FiberFlex".

The nucleonic technique can also be used for interface level measurement and for density measurement.

Guided microwave level transmitters
The guided microwave level measurement technique is a variation of radar. Also known as Time Domain Reflectometry, or TDR, extremely short microwave pulses are transmitted down the outside of a cable or rod and reflected off the surface of the product that is being measured. The measurement possibilities include
level measurement in liquids and solids plus interface measurement between low dielectric non-conductive liquids and conductive liquids.

In solids applications, guided microwave sensors look similar to cable capacitive probes. However, they have the advantage of being unaffected by changing product characteristics and they can be calibrated without filling the vessel. Also, they are independent of the internal structure of the silo or hopper and many different powders and granular products can be measured.

Guided microwave or TDR level transmitters for different powder and granular solids.
Guided microwave or 'radar on a rope'. It can be calibrated without filling the silo and is independent of changes in the characteristics of the product being measured.


Different designs are used on liquids applications. This includes dual rod and concentric tube/rod designs. The advantage of these designs is that the electrical field around the probe is concentrated in the tube or between the rods and there is little external influence from the vessel structure. However, these designs are susceptible
to build up of product between the rods or within the tube. This can lead to measurement problems with false echoes caused by the build-up. They are better suited for clean liquids.

Two-wire, loop powered guided microwave level transmitters for liquids and solids are available with HART /4 to 20 mA and Profibus PA.

Radar level measurement
The benefits of radar as a level measurement technique are compelling and clear. Radar provides a non-contact sensor that is virtually unaffected by the following process conditions

* Changes in process temperature,
* Changes in process pressure
* Vacuum
* Variations in the gas and vapour composition.
* Air movement between the sensor and product surface
* Density of the product
* Conductivity of the product
* Dielectric constant of the product

Pulse radar in particular, presents additional capabilities and benefits in process vessel level applications.

Using time of flight measurement, pulse radar does not need the expensive and power consuming processors that enable the alternative FM-CW (frequency modulated continuous wave) radar technique to function.

There are no Fast Fourier Transform (FF1) algorithms to calculate. Instead, the echoes derived from a pulse radar are discrete and separated in time. All of the processing is dedicated to echo analysis alone.

This means that pulse radar is better equipped to handle multiple echoes and false echoes that are common in process vessels. Some FM-CW radar transmitters cannot cope with multiple echoes that occur in simple horizontal cylindrical vessels due to the parabolic effect of the vessel top. This does not trouble pulse radar. The low power consumption requirements of pulse have ensured
that a very capable iwo-wire intrinsically safe radar has been realised.

The averaging of the pulse technique reduces the noise curve to allow smaller echoes to be detected. Well designed circuits containing good quality electronic components ensure that pulse radar can detect echoes over a wide dynamic range of about 80 dB. This can make the difference between reliable and unreliable measurement.

VEGA produced the world's very first iwo-wire, loop powered radar level transmitter in the summer of 1997. The VEGAPULS 50 series set the standard in performance for process radar. Overnight, two-wire technology reduced the unit cost of radar sensors and ensured that radar became an affordable first choice level measurement technique.

These two-wire radar level transmitters have been proved on some of the most arduous applications from agitated chemical and pharmaceutical process reactors to solids applications including fly ash and cement clinker silos.

The still unrivalled, "ECHOFOX" echo processing software is
at the heart of this success.

5.8 GHz radar
Example: VEGAPULS 54 6" horn is optimum size.




Choice of radar frequency
Process radar level transmitters operate at microwave frequencies between 5.8 GHz and 26 GHz. Manufacturers have chosen frequencies for different reasons rangin from licensing considerations, the availability of microwave components and perceived technical advantages.

There are arguments extolling the virtues of high frequency radar, low frequency radar and every frequency radar in between. In reality, no single radar frequency is ideal for all radar level applications.

It is a question of physics. Which frequency is best for the particular process conditions or vessel characteristics Low frequency: 58 GHz 5.8 GHz radar is ideally suited to difficult process vessel applications such as chemical reactors and solids level applications. The lower
frequency is less affected by condensation, build-up, agitation and foam.

In general terms, 5.8 GHz is more "forgiving" than higher frequencies. In addition, 5.8 GHz is capable of measuring liquid ammonia where higher frequencies signals are damped.

5.8 0Hz radar
Example: VEGAPULS 53 PTFE dielectric rod antenna.


High frequency: 26 GHz
26 GHz is ideally suited for bulk storage tanks, horizontal cylinders, mixing tanks, small vessels such as receivers and also bypass tubes and stilling tubes.

The high frequency radar has better focusing for a given antenna size. As a consequence, a higher accuracy can be achieved and smaller horn antennas can be used. The focusing of high frequency radar avoids many false echoes and there is very little influence from standard vessel nozzles.

When combined with the pulse technique, there is virtually no near-range dead band thus ensuring the maximum vessel volume is usable.

26 GHz radar
Example: VEGAPULS 42
Smallest horn antenna radar with
a diameter of 40mm.



Non-contact process radar has developed at a rapid pace.
Antenna designs for more and more arduous process environments; echo processing software that can cope with the false echoes from the internal structure of a vessel; a choice of radar frequencies to ensure that the correct radar is chosen for the myriad of possible
applications; very capable and affordable two-wire loop powered pulse radar transmitters.

All of these factors lead to the conclusion that the future of process level measurement lies in non-contact pulse radar.

Waveguide radar
With very low dielectric constant liquids, such as butane (dielectric constant of 1 .4) and propane (dielectric constant of 1 .6), a stilling tube is required to enable measurement with radar.

Essentially, the stilling tube is a wave-guide that concentrates the microwaves and ensures that the signal amplitude is high enough for reliable measurement. A high frequency radar design for liquefied gases and other clean liquid hydrocarbons uses an integral stilling tube. The design is optimised for maximum accuracy and has the benefit of being pre-calibrated.

Radar applications
Two-wire, intrinsically safe 5.8 GHz
radar on a small process reactor vessel
application.



Ultimate chemical resistance
A special enamel (glass) coated metallic horn antenna is aimed at the chemical and pharmaceutical industries. This has all the benefits of a horn antenna with excellent materials compatibility. The enamel coating is the same material as the glass lined vessels for which they are designed.

The only other "wetted" material is the PTFE waveguide inside the enamel coated steel horn. The 5.8 GHz frequency makes this radar ideal for chemical reactor applications.

 

High frequency pulse radar measuring
hot oil in a stirred process vessel.



High temperature and pressure
Pressurized process vessels in the chemical and petrochemical industries are required to withstand constant high temperatures at continuous high pressures.

These arduous process conditions are met by using radar with a chemically and thermally stable ceramic waveguide. The whole assembly in laser welded to ensure that the transmitter is gas tight and that differential thermal expansion is negligible. The electronics housing are mechanically isolated from the high process temperatures by a temperature extension tube. The design is capable of withstanding 400 °C at a constant pressure of 160 Bar.

___________________________________________________

The author of this article, Peter Devine has published a book called "Radar level measurement - the user's guide".

The guide is a full and comprehensive publication in full colour hardback A5 format covering the following:

Part I
1. History of radar
2. Physics of radar
3. Types of radar.

Part II
4. Radar level measurement
5. Radar antennas
6. Radar level installations
a. Mechanical installation
b. Electrical installation

Part III
7. Other level techniques:
Amongst others, DP transmitters, Capactive transmitters, Electromechanical
transmitters and Ultrasonic.
8. Applications:
Covering applications in the following industries: Brewing & Distilling, Cement, Chemical, Food, Metals, Offshore, Pharmaceutical, Power and Water & Waste water industries.

Part IV
9. VEGA radar
Two wire loop powered radar
Vegapuls 50 - technical specification
Vegapuls 40 - technical specification
Vegapuls 40 & 50 options
Echofox software

The book also has additional appendices, which cover the following:
Glossary of terms, Radar power & radar power density, Dielectric constants
Symbols, Photograph acknowledgements.

__________________________________________________

Article by...
Peter Devine - Technical Manager
VEGA Controls Limited

Telephone: +44(0) 1444 870055
Fax: +44(0) 1444 870080
Website:
www.vega-controls.co.uk





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