Level measurement
Involves the continuous or single-point measurement of a level of a liquid or solid in a tank or vessel.
Point meas. Devices referred to as level “switches” while continuous level meas. Instruments are indicators/detectors
Two classes of basic level measurement:
- Direct - level determined by direct measurement
- Inferential - level inferred by measurement of another variable
Direct methods
-dipstick
-sight glass
-constant displacement float systems
Inferential methods
- hydrostatic head level measurement
-open tank hydrostatic level measurement
-bubblers?
-ultra sonic level measurement
-microwave or radar level measurement
-capacitance level measurement
-nuclear level measurement
Hydrostatic head level measurement
Liquid level can be inferred by measuring a D/P caused by the pressure or “head” exerted by a fluid column in a vessel.
For open tanks, high side is connected to bottom of vessel, low side open
For closed tanks, high side connected to bottom, low side connected to vapour space (top) of vessel
When would you use extended diaphragm and chemical seals?
Extended for viscous, slurry, or other plugging type applications
Chemical seals (diaphragm seals) used with D/P level devices where process is corrosive or toxic
Advantages and limitations of hydrostatic head level measurement
Pros
-economical
-Easy to use and well understood
-uninfluenced by factors such as dust, foam, vapour, build up, etc
-unaffected by physical characteristics such as conductivity, dielectric constant, etc
Cons
-accurate measurement requires fluid with constant density
Bubblers (air purge systems)
Purge systems isolate the transmitter from the process liquid by using an intermediate gas or fluid such as air or water.
Ultrasonic level measurement
A pulse of ultrasonic sound energy is emitted by a transmitter. The sound travels outward until it is reflected back by striking a surface or interface such as air/liquid, oil/water, etc.
Distance between transmitter and surface of liquid can be calculated as:
d= v*t/2
Pros and cons of an ultrasonic level measurement
Pros
-No moving parts
-non contacting
-maintenance free
-unaffected by changes in liquid, viscosity, etc
Limitations
-relies on the pulse being unaffected during its flight time
-foaming, liquid turbulence, blocking of signals by other objects, etc affects the measurement
Microwave or radar level measurement (unguided)
Unguided is similar to ultrasonic. A pulse of high frequency electromagnetic energy is emitted by a transmitter. The receiver listens for the reflected energy and measures the time difference between sending the pulse and receiving the reflection
Guided Wave Radar
Emits a burst of uwave energy, creating an electromagnetic field around a guide, typically a rod or cable. Field travels down the length of the guide and when it strikes an interface surface, energy is reflected back to the receiver
Pros and cons of guided wave radar
Pros
-accurate and reliable
-can be used in a wide variety of applications
-no impact on measurement
-no moving parts
Cons
-precautions need to be taken with respect to probe choice
Pros and cons of unguided
Pros
- used with liquids, slurries & some solids
-no impact on measurement
-no moving parts
- non contact
Cons
-needs a clear view of the surface with a smooth, unobstructed, unrestricted mounting nozzle
-energy tends to not be reflected by light and airy foam while a dense and heavy foam typically reflects energy.
Capacitance level measurement
Capacitance probes can determine level of liquid and some solids. The liquid acts as a dielectric between the two plates of a capacitor formed by a single rod and the sides of a metallic tank. If the tank is non-metallic, a two probe system can be used.
Pros and cons of capacitance probes
Pros
-tolerates a variety of process conditions, such as density, high temp, high pressure, viscous products, slurries, foams etc
-used to measure interfaces
-inexpensive, no moving parts, easy to clean, simple
Cons
-adversely affected by heavy foams
- a change in dielectric creates errors in the reading, as does a coating on the probe
-calibration can be difficult since bench calibration is not possible
Nuclear level measurement
Can be used to measure level of solids in a vessel. Nuclear source emits a beam of gamma radiation that is focused on a radiation detector. As the vessel fills with solid material,the material absorbs the gamma radiation, preventing it from reaching the detector.
Pros and cons of nuclear level measurement
Pros
-non invasive (no instrument connections required on the tank)
- non contacting
-unaffected by high temps, high pressure, corrosive materials, abrasive materials, etc
-can be used for both point and continuous level measurements in solid, liquid, & interface applications
Cons
-relatively high cost
-licensing & leak checks are required as well as a high degree of health & safety checks & care over source handling & disposal
