Best Practices
in Process
Level
Measurement
Agenda
Introduction: Technology
Trends
Factors Impacting Level
Technology Selection
Review Level
Technologies
– Theory of Operation
– Advantages
– Limitations
– Applying Best Practices
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27-Jun-01, Slide 2
Diverse Technology Choices Exist for Level
Bubbler
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27-Jun-01, Slide 3
Why You are Measuring Level?
Maximize Storage Tank Capacity
Prevent Spills
Process Blending
Custody Transfer
Process Supply
Indication of Fluid Level
Leak Detection
Interface Detection
Liquid or Solids?
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27-Jun-01, Slide 4
Different technologies make different measurements
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27-Jun-01, Slide 5
Selection of the Right Technology
includes knowledge of:
Technology Installation
Application
Conditions
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27-Jun-01, Slide 7
Application Conditions are a Key
Consideration .…
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27-Jun-01, Slide 8
Installation Considerations Go Beyond the
Connection
Outside the tank
Location, diameter and length of
nozzles
Bypass connection
Valve
Inside the tank
Fluid flow
Obstructions, agitators, coils
Overall dimensions
Bottom type
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27-Jun-01, Slide 9
Non-Contact Radar Level Gauging
Principle
A) Microwaves transmitted towards the
surface.
B) Microwaves reflected on the surface.
C) Objects such as tank-walls, agitators
and mechanical structures reflects
disturbing echoes.
D) Reflected microwaves are received in
the gauge.
E) Radar electronics and software
process signals and data to establish
a correct level measuring.
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27-Jun-01, Slide 10
Example
Ultrasonic Radar
Devices are set up under hot, dry conditions
20 m Same level, 10 m
Steam enters vapor space
Ultrasonic Radar
Set up at 100 C 0.0259 sec 0.03334 x 10-6
Travel over 10 m
Water vapor (100 C) 0.0247 sec 0.03343 x 10-6
Difference in travel 0.0012 sec 0.00009 x 10-6
time
Error, (based on set 0.046 m (1.8 inches) 0.0027 m (0.1 inches)
up speed of travel)
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27-Jun-01, Slide 12
Non Contacting Radar Has Some
Limitations
• Heavy Foam:
– Poor Signal-to-Noise Ratio
– No Signal Foam
• Mounting Position Must
Be Considered
• Objects in the Radar
Beam May Interfere
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27-Jun-01, Slide 13
Microwave Frequencies
Frequency determines the wavelength
Frequency and antenna size impact Beamwidth
Different frequencies are good for different applications
6 GHz, 4” 26 GHz, 4”
26 GHz 10 GHz
6 GHz
Beam Beam
width 37° width 9°
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27-Jun-01, Slide 14
Low frequency is less influenced by
6 GHz (C-Band)
Condensation
Vapor
Foam
Turbulence
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27-Jun-01, Slide 15
High frequency and its more narrow beam width
is best suited for:
26 GHz ( K-Band)
Narrow tanks
Tall and narrow nozzles
Nozzles with Valves
Vessels with disturbing obstacles
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27-Jun-01, Slide 16
Guided Wave Radar - Principle of Operation
TDR (Time Domain
Reflectometry)
Microwave Pulses Guided
Down a Probe
Dielectric Change Causes
Microwave Reflection
Distance = Speed X Time of
travel /2
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27-Jun-01, Slide 17
Measurement principle -Level & Interface
FIRST PULSE
– at top level
SECOND PULSE
– Part of pulse
continues until
reflected at lower
product surface
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27-Jun-01, Slide 18
Time Domain Plot - Level and Interface
Interface uses the difference in
dielectric properties
Independent of density changes
Dielectric of medium determines
the speed of the microwave
propagation in a product
Accurate interface level is
dependent on the correct
dielectric value
Dielectric difference needs to
be about 10
Low dielectric fluid needs to be
on top.
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27-Jun-01, Slide 19
Three Measurement Variations Are Possible
Level Only
Level and Interface
Interface w/Immersed Probe
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27-Jun-01, Slide 20
Guided Wave Radar has Many of the Same Advantages
as Non-contacting, but Has Others As Well
Non-contacting Easy swap (small openings)
Top-down Solids, powders, granules
Handles changing density, dielectrics, Interface
conductivity, temperature, pressure,
pH and viscosity
Handles a variety of tanks from difficult
geometry to storage vessels
Virtually unaffected by dust, vapor and
turbulence
Direct measurement
Can be isolated from process by using
barriers such as Teflon windows or
valves
No moving parts
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27-Jun-01, Slide 21
GWR can offer significant stability over technologies
affected by density, turbulence or other factors
For a 20 inch span, a
density change of 20% can
yield a 4” error. A 0.2
change in SG is common
in many oil separators
Output Signal Plots
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27-Jun-01, Slide 22
Guided Wave Radar has Some
Limitations
Contacting technology
Agitators, blades, baffles may
interfere with probe installation
Foam is unpredictable
Caution is needed for highly
viscous fluids and heavy
coating
For interface
measurements,the upper fluid
dielectric needs to be stable
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27-Jun-01, Slide 23
Probe Choices can Impact Success of
Measurement
Sticky, coating,
crystallizing
products
Installation
Fluid movement or
turbulence
Pull forces in solids
applications
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27-Jun-01, Slide 24
Signal strength and form vary with
probe style
Coaxial
Twin Leads
Single Leads
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27-Jun-01, Slide 25
Heavy coating can lead to bridging
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27-Jun-01, Slide 28
Single lead probes can handle coating
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27-Jun-01, Slide 30
Application: Pump Sump Pit at Refinery
Application:
Sump pit, separate oil and
water
Consistent and reliable
measurement needed to
avoid level upsets and
subsequent overflow.
Solution;
3302 and a 2.35 m long rigid
twin lead probe
Reliable measurement of
both oil and water layer
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27-Jun-01, Slide 31
Application Example: Turpentine Storage
A mixture of water and
turpentine is pumped into
storage tank and separates
Turpentine is removed from
vessel by pumping water into er = 2.2
the tank to push out turpentine
Probe is completely immersed;
distance to the water interface
level is measured
Previous method: visual
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27-Jun-01, Slide 32
Application Example: 3300 and 3051:Interface of High
Dielectric Foam and Liquid
Black Liquor and Soap mixture
often has up to 10 ft of foam;
overspills were common
DP measured liquid only; Foam
was measured manually
Foam has higher dielectric and
is too sticky for a good interface
measurement with GWR
Radar measures top of foam
Foam height – liquid height =
interface of foam and liquid
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27-Jun-01, Slide 33
DP Level Theory of Operation: Simple
and Well Understood
Level in a tank is proportional to
the head pressure exerted by the
process fluid
Changes in working pressure of
tank accommodated by referencing
low pressure side of DP to head
space of tank
‘Remote Seals’ often used on level
measurements to handle severe
operating conditions or special
process connection needs
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27-Jun-01, Slide 34
Pressure Has Many Advantages as a
Level device
Reliable, Simple, Well Understood
Flexible Uses:
– Level, density, interface, mass
Unaffected by Agitation or Foam
Unaffected by internal tank equipment
No moving parts
Diaphragm Seals extend limitations due to
process conditions such as:
– High Temperatures
– Corrosive processes
– Dirty or Viscous mediums
– Sanitary Connections
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27-Jun-01, Slide 35
Pressure Has Some Limitations
Often requires 2 taps
Variable density creates errors
Temperatures beyond 660 F/ 350 C
High vacuum applications are tricky
Highly corrosive processes limit life
Abrasive processes can damage diaphragms
Liquids Only
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27-Jun-01, Slide 36
Questions?
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27-Jun-01, Slide 37
