Type of Liquid Level Sensor AND ITS APPLICATION

 
 Technorati Tags: Level,Instrument,Process Design

LEVEL MEASUREMENT, which is the detection of the phase split between vapor/liquid, liquid/ liquid, vapor/solid and even liquid/solid, is a key parameter in the operation and control of modern industrial processes. Failure to measure level reliably has resulted in some of the most serious industrial accidents, including those at the Buncefield, U.K., fuel storage depot and BP’s Texas City refinery.
Type of level measurement includes:

Hydrostatic
This continuous indirect method measures the pressure due to liquid level and density plus over-pressure. The sensor measures the difference between this pressure and a reference one, normally atmospheric; so, it’s not well suited for vacuum and pressure service. Instruments come in flanged-mount­ed or rod-insertion styles, the latter not being recom­mended for turbulent conditions. Typical accuracies claimed are ±0.2% of reading but this depends on process fluid properties and conditions.

Float displacer
Suitable for point or continuous applications, it measures the change in buoyancy via a torque tube, lever or servo arrangement. The continu­ous measuring range is set by the displacer length im­mersed in the tank’s external cage, which is preferable for noisy applications, or servo mechanism. The point method uses a float, with the range being limited by the length of the float arm.

Nucleonic
Good for point or continuous duties, this non-contact method, which is independent of fluid density and viscosity, measures the signal strength of a radioactive source beamed across a vessel and has typical ranges of 0.24 m to 3.36 m. Accura­cies generally claimed are ±2% of reading. It’s the preferred method for monitoring level in flash vessels and reboilers under all temperature and pressure conditions.

Radar
Applicable to point or continuous applications, it measures the travel time of an impulse reflected from the liquid surface. Interfer­ence echoes from tank internals, and agitators are suppressed and signals can be characterized to give liquid volume. The sensor doesn’t contact the liquid but is exposed to headspace conditions, which don’t affect the measurement. Reflectivity requires the liquid dielectric constant, εR, to be at least 1.4 (hydrocarbons are 1.9–4.0, organic solvents are 4.0–10 and conductive liquids are over 10). Adjusting the antenna and signal conditions allows tailoring to the particular process, with guided radar used for low εR and turbulent conditions. The method can handle custody transfer because of its claimed accuracy of ±0.5mm.

Capacitance
For point or continuous service, it suits liquids that can act as dielectrics. Sensitivity increases with the difference in dielectric constants, δεR, between the liquid and the vapor space or between the two liquids. Special designs, involving coated and twin probes, are used when δεR is under 1.0, conductivities exceed 100 ʮmho, or to overcome probe build-up effects, and when vessel material is non-conducting. Typical accuracies claimed are ±0.25% of span. However, fluid properties affect mea­surements, so the method isn’t suitable for changing conditions. Maximum conditions are 200°C at 100 bar and 400°C at 10 bar.

Ultrasonic.
Suitable for point or continuous use, it is based on the time-of-flight principle. A sensor emits and detects ultrasonic pulses that are reflected from the surface of the liquid. The method is non-invasive, with some types being non-contact, and isn’t affected by εR, conductivity, density or humidity. Maximum conditions are 150°C at 4 bar.
Load cells.
Appropriate for point and continuous applications, such devices, which can be based on strain gauge or piezoelectric technology, measure the weight of the process vessel plus contents. Individual load cell accuracy of 0.03% of full scale is achiev­able but overall performance depends on correct installation practices to exclude external forces due to associated piping and equipment. For vessels with jackets, agitation and complex piping, it’s difficult to obtain an acceptable accuracy. When the container can be totally isolated, as in final dispensing and filling applications, precision weighing can be achieved.

Tuning fork
This method can detect point liquid level but isn’t suitable for viscous and fouling applica­tions. Maximum conditions are 280°C at 100 bar.

Conductivity
Good for finding point level, it requires a liquid conductivity exceeding 0.1 ʮmho and frequently is used on utility and effluent pump control systems.
Typical compara­tive costs
From lowest to highest, are: conductivity - capacitance - tuning fork - hydrostatic - displacer - ultrasonic - load cell - radar - nucleonic.

APPLICATION CONSIDERATIONS

1. Selection also must consider both the process and its control.

1. Process. It’s essential to understand the physical property variations of the process fluids and the phase changes that may occur within the process during normal and abnormal conditions.

1. Boilers, flash vessels and distillation column bottoms involve boiling liquids, resulting in noisy levels. Displacers in external cages frequently are used on steam generators and flash vessels, provided the process fluids are of low viscosity and relatively clean.

1. Non-contact nucleonic method will prove most reliable for distillation column bottoms, where reproducibility is more important than absolute accuracy. While expensive, it can be more than justified given its value in providing stable column operation and in preventing reboiler fouling due to loss of level.

1. Avoid the use of impulse lines in level systems if the process pressure varies and there’s a tendency for solids’ formation due to freezing, precipitation or polymerization. Purging the lines with inert gas or process compatible fluids will have limited suc­cess and is high maintenance.

1. Nucleonic level detection provides a powerful tool to perform on-line process diagnostics. Typical applications include monitoring level profiles in tray towers, distribution in packed beds, locating level build-up and blockages in vessels, and general flow studies.